the llamas de cabrera gold district a new discovery in the variscan basement of northwest spain a fluid inclusion and stable isotope study the northwest of the iberian massif is well known to host numerous gold mineralizations which were exploited during roman times this study presents a multidisciplinary approach leading to the mineralogical fluid inclusions and geochemical characterization of the recently rediscovered roman gold district of llamas de cabrera gold occurs in extensional quartz veins hosted in the upper layers of the lower to middle ordovician armorican quartzite and occasionally in the overlaying middle ordovician slates from luarca formation the veins are characterized by multistage ore deposition developed along three hydrothermal stages as fe i as fe ii and au zn cu pb followed by late supergene alteration processes the fluid inclusion study undertaken on mineralized quartz revealed the presence of three types of fluids 1 co2 ch4 fluid inclusions 2 aqueous carbonic fluid inclusions dominated by co2 and subordinate ch4 related to quartz arsenopyrite pyrite deposition and 3 aqueous fluid inclusions related to the au sulfide deposition in conjunction with arsenopyrite geothermometry and fluid inclusion data an attempt was made to determine the p t conditions of ore formation changes occurred in the p t conditions fromarsenopyrite and pyrite deposition in quartz veins from aqueous carbonic fluids at 300 390 c and 200 220 mpa towards 180 310 c and b200 mpa at the stage of gold from aqueous fluids mixing of two aqueous fluids of relatively contrasted salinity were favorable factors for decreasing gold solubility and could allow gold deposition the 34s values are similar for the two as fe stages 8 0 to 16 3 and 9 0 to 19 5 respectively and for the post s1 pyrites from siliciclastic luarca formation suggesting a comparable sulfur source combining calculated 18o values of fluids of 6 7 to 9 4 with fluid inclusion data indicates that fluids of unknown origin are largely equilibrated withmetamorphic lithologies at medium high temperatures the ascent of some late variscan magmatic bodies not exposed at the present day erosion level provides the heat source for convective water circulation gold quartz veins from llamas de cabrera display similar features and processes to related deposits of similar type elsewhere in the variscan belt of western europe northwest and central western iberia french massif central bohemianmassif however llamas de cabrera shows certain differences such as the non appearance of igneous rocks in the area and the absence of sb rich minerals compared to some variscan sb au deposits including northern central and western iberia this last could probably be due to a greater depth of formation for the as au veins in comparison to sb au veins

collapse of the thickened variscan crust in the french massif central mont pilat extensional shear zone and st etienne late carboniferous basin abstract a large scale extensional shear zone is described from the mont pilat area in the northeastern part of the french massif central this shear zone is about 2 km thick and is characterized by well developed n dipping foliation and stretching lineations deformation occurred under conditions of retrograde metamorphism from high temperature conditions to greenschist facies it is superimposed on a composite foliation related to the earlier variscan 380 350 ma compressional events associated with southward emplacement of nappes and hp mt metamorphism regional n s lineations and shear criteria observed at a regional scale indicate a northward displacement of the hanging wall during extension large strains locally recorded in thick mylonitic zones suggest that displacement was substantial some tens of kilometres late superimposed extensional brittle deformation and pseudotachylites show that northward displacement on normal faults continued at a shallower depth a large granitic pluton granite du velay intruded late in the the variscan orogeny was associated with extension some small leucogranitic syntectonic melts were emplaced within the shear zone and are strongly deformed geochronological rb sr data on these leucogranite bodies give an age of 320 ma and 40 ar 39 ar dating of synkinematic biotites from gneissose rocks in the shear zone yields a plateau age of 313 ma these data show that extensional ductile deformation and magmatism were still active during the latest late carboniferous thus extension occurred immediately after the well known late palaeozoic collision type compressional tectonics which had significantly thickened the crust we propose that the extensional tectonism described here is due to the gravitational collapse of the initially thick variscan crust the stephanian intracontinental sedimentary basin of st etienne was formed along the hanging wall of the normal shear zone during the late stages of this post orogenic crustal extension

scouring of gold from the lower crust quartz carbonate lodes have produced a major part of the world9s gold the source for the lodes which were emplaced along shear zones in the upper crust has been enigmatic the bamble belt of south norway is depleted in gold by more than an order of magnitude relative to its crustal abundance other elements present in gold deposits have also been lost gold dissolved and was removed in oxidized sulfur rich fluids rising through this broad ductile shear zone during high grade metamorphism deformation synchronous with metamorphism created the permeability that permitted a pervasive flow of fluids

late visean thermal event in the northern part of the french massif central new 40ar 39ar and rb sr isotopic constraints on the hercynian syn orogenic extension he french massif central belongs to the central metamorphic domain of the european hercynian belt 40ar 39ar mica and amphibole dates have been obtained on magmatic and metamorphic rocks from the northern part of the french massif central sioule and brevenne violay series the 336 to 337 ma dates recorded by a granodiorite the st gervais pluton and its mylonitic border along the ste christine fault and the 337 5 ma muscovite ages from the brevenne series are consistent with the age of wrenching related to the late stage of the hercynian compression the remaining age spectra from the sioule area provide ages ranging from 331 to 335 ma that are interpreted as cooling ages related to rock exhumation after the main stages of metamorphism and deformation in the studied area late visean dates widespread in north massif central correspond to a huge magmatic event characterized by the emplacement of red granites granophyres and sub aerial acidic volcanics known as the tufs anthraciferes formation rb sr isotopic data from volcanic and hypovolcanic rocks from the sioule and montagne bourbonnaise areas place constraints on the source of the magmas simple metapelite crustal melting or a major contribution from depleted mantle sources are ruled out and instead mantle derived magmas were probably involved at least as a heat source during the late visean magmatism exhumation and cooling of metamorphic rocks below 300 c occurred contemporaneously with this late visean magmatism in the geodynamic evolution of the hercynian belt the late visean ca 335 330 ma magmatism corresponds to a major break marking the onset of post collisional extension in the inner zone which had been thickened by collision in latest devonian earliest carboniferous this late visean extension in the inner zone which is synchronous to compression in the northern and southern forelands of the belt appears as a syn orogenic process

the moldanubian zone in the french massif central vosges schwarzwald and bohemian massif revisited differences and similarities in order to portray the main differences and similarities between the northeastern variscan segments french massif central fmc vosges black forest and bohemian massif bm we review their crustal scale architectures the specific rock associations and lithotectonic sequences as well as the ages of the main magmatic and metamorphic events this review demonstrates significant differences between the moldanubian domains in thebmand the fmc on this basis we propose distinguishing between the eastern and western moldanubian zones while the vosges black forest mountains are an intermediate section between the bm and the fmc the observed differences are the result of first the presence in the french segment of an early large scale accretionary system prior to the main variscan collision and second the duration of saxothuringian armorican subduction which generated long lived magmatic arc and back arc systems in the bohemian segment while the magmatic activity in the fmc was comparably short lived

dynamics of shallow late variscan gold mineralization the le châtelet au arsenopyrite quartz veins massif central france the le ch telet gold deposit is unique among the mined deposits of the french variscan basement in that the gold is not free but caught up in the arsenopyrite crystal lattice microstructural analysis shows that the au arsenopyrite quartz veins are hosted by north trending dextral strike slip faults resulting from nne ssw compression during the early stephanian around 300 ma textural study indicates that vein emplacement was due to sequential dynamics related to spasmodic strike slip movements along these brittle structures seismic regime the classic sequence begins with the slow crystallization of sulphide free comb quartz followed by a co deposition of microcrystalline quartz and needles of au arsenopyrite the rapid crystallization of the au arsenopyrite bearing microcrystalline quartz was due to a sudden drop in fluid pressure the veins correspond to dilational jogs formed by a suction pump mechanism during a late phase of the variscan orogeny they were trapped in migmatite about 1 km below surface

late variscan tectonic evolution by thinning of earlier thickened crust an 40ar39ar study of the montagne noire southern massif central france abstract 40ar 39ar age determinations on samples selected on deformational criteria form the basis of a proposal of a new detailed tectonometamorphic history of the montagne noire the southern part of the massif central france this range is classically divided into a cover and an axial gneissic and migmatitic core which was metamorphosed under high t medium p conditions the basement is structurally overlain by epimetamorphic paleozoic sediments in the north in the south equivalent rocks form a well known pile of recumbent nappes the variscan history of this area comprises early shortening and thickening of the continental crust the climax of this event took place around 320 ma ago as is shown by previous rb sr data and by 40ar 39ar measurements new developments in structural analyses have led to a better understanding of the late evolution of the area in terms of diapiric uplift of the core south southwestwards verging recumbent nappes have been emplaced and were weakly metamorphosed in their lower parts this movement was accompanied and followed by dextral extensional wrenching of the paleozoic cover along the northern and southern margins of the axial zone attendantly ductile shear zones were formed in order to decipher the complex history three structural levels have been dated with the 40ar 39ar method these levels are 1 the migmatitic and gneissic series in the axial core where micas yield ages of c 315 ma 2 mylonites from the northern and southern shear zones where biotites and muscovites yield ages of c 310 ma 3 muscovites and biotites from the epimetamorphic nappe structures yielding ages of c 297 ma the later ages may represent a younger thermal pulse these data are compared and discussed in the light of previous radiometric studies of the area and precise biostratigraphic constraints a detailed geodynamic evolution of the studied segment is proposed

structural controls on the auriferous shear zones of the saint yrieix district massif central france evidence from the le bourneix and laurieras gold deposits at least 2 000 occurrences of gold mineralization including four economic deposits occur in the saint yrieix district in the massif central france two of these the le bourneix and laurieras deposits are currently being mined the mineralization consists of quartz lenses with sulfides and native gold the lenses are several meters thick and 300 m in length and occur within a shear zone several kilometers long in para and orthogneisses of the hercynian basement metallogenic and structural investigations in the saint yrieix district and in le bourneix mine have made it possible to relate the various episodes of mineralization to the sequence of tectonic events the lenses of vein quartz were emplaced during phases of extension their distribution being controlled by anastomosing mylonitic bands formed during a preceding tectonic phase the mylonitization was accompanied by sericitization and the introduction of disseminated pyrite arsenopyrite rutile and organic matter along the planes of mylonitic foliation gold fine grained galena and sulfosalts were introduced later impregnating the microsaccharoidal quartz resulting from cataclasis of initially barren vein quartz subsequent partial remobilization of the gold bearing lenses during phases of extension caused coarsening of the gold with the appearance of argentiferous nuggets the emplacement of the quartz lenses in brittle ductile shear zones and the diachronism between the introduction of the vein quartz and that of the gold are characteristic of this mineralization and make it possible to propose guides for exploration

uranium migration and retention during weathering of a granitic waste rock pile abstract this study investigates the post mining evolution of s type granitic waste rocks around a former uranium mine vieilles sagnes haute vienne nw massif central france this mine was operated between 1957 and 1965 in the la crouzille former world class uranium mining district and is representative of intra granitic vein type deposits 50 years after mine closure and the construction and subsequent re vegetation of the granitic waste rock pile we evaluate the environmental evolution of the rock pile including rock alteration neo formation of u bearing phases during weathering and u migration vertical trenches have been excavated through the rock pile down to an underlying paleo soil allowing the investigation of the vertical differentiation of the rock pile and its influence on water pathways weathering processes and u migration and retention arenization dominantly drives liberation of u by dissolution of uraninite inclusions in the most alterable granitic minerals i e k feldspar and biotite retention of u in the matrix at the base of the waste rock pile and in the underlying paleo soil most likely occurs by precipitation of nano uranyl phosphates or a combination of co precipitation and adsorption reactions of u onto fe oxy hydroxides and or clay minerals even though u migration was observed u is retained in stable secondary mineral phases provided the current conditions will not be modified

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

east asian gold deciphering the anomaly of phanerozoic gold in precambrian cratons early cretaceous orogenic gold deposits in eastern asia are globally unique in that large phanerozoic lode gold deposits occur in archean paleoproterozoic cratons in the northern pacific region ca 125 ma orogenic gold deposits in the north china yangzte and siberian craton margins as well as in young terranes in california may ultimately relate to the giant cretaceous mantle plume in the southern pacific basin and the relatively rapid tectonic consequences along both continental margins from resulting pacific plate reconfigurations in eastern asia such consequences include reactivation of and fluid flow along major fault systems with fluid focusing into simultaneously forming isolated core complexes of uncertain genesis deposition of gold ores in previously devolatilized high grade precambrian metamorphic rocks requires an exotic source of ore fluid most likely subducted mesozoic oceanic crust and or overlying sediment an implication is that phanerozoic metamorphic core complexes in other destabilized craton margins could host large gold resources

spatial and temporal distribution of the orogenic gold deposits in the late palaeozoic variscides and southern tianshan how orogenic are they abstract a principal uncertainty in models of orogenic ore deposits concerns their ages relative to orogenic processes the yardstick of the relation has resided loosely in the peak of metamorphism age estimates in the variscides and tianshan indicate that most orogenic ore deposits were formed in the course of the late carboniferous to middle triassic with a peak between 305 and 280 ma their locations settings and ages suggest an association with coeval lithosphere scale strike slip deformation which played a role in the destruction of the orogenic system following crustal shortening this destruction tends to invalidate the orogenic association of the orogenic ore deposits observation of a mantle signature in some of these ore deposits and in coeval magmatic rocks suggests that the strike slip belts tapped into sub crustal levels here lithological composition had been modified by subduction during the stage of construction consequently any orogenic element in the ore deposit models is apart from location only an element of inheritance of subduction modified lithosphere features acquired during construction of the orogen these features may however well have been crucial for the formation of the ore deposits in view of the translithospheric extent of the strike slip belts fluids which contributed to the ore deposits may have come from at least the entire in part metasomatized lithosphere column the ore deposits could probably not have been formed without these deep reaching strike slip systems a model is proposed for the geological setting of the orogenic gold deposits in the variscides and the southern tianshan it consists of plutono metamorphic elements in the mantle in the lower and in the middle crust within a transpressional to transtensional tectonic framework of translithospheric faults and associated domes and pull apart basins with shallow marine to terrestrial sediments and interlayered felsic and mafic volcanics the thermal engines of these systems were probably in localized upwellings of the asthenosphere prompted and controlled by the lithosphere scale deformation at the time at an even larger scale oroclinal bending of the old orogenic backbone may have played yet another role in the localization of the ore deposits the thesis of specific localized tectono thermal engines is at odds with the original proposals of a relation between orogenic ore deposits and regional metamorphism it suggests that the late palaeozoic gold antimony mercury ore deposits in the variscides and the southern tianshan are more akin to the intrusion related ore systems

5 1 salsigne example of a late variscan gold deposit southern french massif central lat 43 20 n long 2 21 e

the krasna hora milesov and přicovy sb au ore deposits bohemian massif mineralogy fluid inclusions and stable isotope constraints on the deposit formation the krasna hora milesov and p icovy districts czech republic are the unique examples of sb au subtype orogenic gold deposits in the bohemian massif they are represented by quartz stibnite veins and massive stibnite lenses grading into low grade disseminated ores in altered host rocks gold postdates the stibnite and is often replaced by aurostibite the ore zones are hosted by hydrothermally altered dikes of lamprophyres krasna hora milesov or are associated with local strike slip faults p icovy formation of sb au deposits probably occurred shortly after the main gold bearing event 348 338 ma au only deposits in the central part of the bohemian massif fluid inclusion analyses suggest that stibnite precipitated at 250 to 130 c and gold at 200 to 130 c from low salinity aqueous fluids the main quartz gangue hosting the ore precipitated from the same type of fluid at about 300 c early quartz arsenopyrite veins are not associated with the sb au deposition and formed from low salinity aqueous carbonic fluid at higher pressure and temperature 250 mpa 400 c the estimated oxygen isotope composition of the ore bearing fluid 4 1 smow based on post ore calcite suggests its metamorphic or mixed magmatic metamorphic origin and excludes the involvement of meteoric water rapid cooling of warm hydrothermal fluids reacting with cold host rock was probably the most important factor in the formation of both stibnite and gold

near solidus delta 18 o depletion in a ta nb bearing albite granite the beauvoir granite france the beauvoir albite topaz lepidolite granite has been intersected through an interval of 700 m in a fully cored drill hole at echassieres in the massif central france the beauvoir granite is a southerly inclined sheet bearing late hercynian hololeucocratic ta nb li be mineralization that intrudes mica schists the drill hole intersects the schists from the surface to a 100 m depth and again at 800 m at the bottom of the hole three facies of the granite have been defined on the basis of petrographic and geochemical studies b1 100 500 m b2 500 750 m and b3 750 790 m trace element and sr isotope geochemistry indicate the cogenetic nature of the three facies ta contents increase fairly regularly from asymp 30 to asymp 250 ppm from the base to the top of the sheet previously analyzed samples from exploration drill holes in the apical varieties of the granite b1 b2 gave scattered delta 18 o values ranging from 4 6 to 8 6 per mil the delta 18 o value was measured in whole rocks and mineral separates from the deep drill hole samples these samples were carefully selected to be unaffected by hydrothermal and low temperature alteration the b1 facies and the upper part of the b2 facies are characterized by the presence of two generations of quartz an early globular form q g and a late interstitial quartz q i the results show a decrease in delta 18 o values from depth upward in the b3 granite the average values are normal for leucogranites delta 18 o of interstitial quartz 11 6 per mil and of mica 9 3 per mil whereas in the b1 granite they are 5 3 per mil for interstitial quartz and 4 5 per mil for mica the values for the b2 granite are intermediate between those for the b1 and b3 facies oxygen isotope temperatures are 570 degrees or 50 degrees c similar to experimentally obtained solidus temperatures the delta d values of lepidolite and whole rock from fresh samples from the drill hole range respectively from 91 to 66 and 76 to 60 per mil compared with smow the wt percent of h 2 o in whole rock ranges from 0 23 to 0 76 no correlation exists between the delta d values of lepidolite or whole rock and the delta 18 o values of minerals or the depth or wt percent of h 2 o these results are interpreted as evidence of interaction in the liquidus solidus interval between the magmatic system and external water of meteoric origin this water probably originated in the convective system of the hydrothermal fluids related to the penecontemporaneous emplacement of the nearby colettes granite globular quartz from the b1 facies shows delta 18 o values similar to quartz in the lower part of the drill hole delta 18 o 8 5 11ppm this similarity confirms that the three granitic facies are cogenetic and constrains the time of interaction with the external fluid to a period after the crystallization of the early globular quartz topaz is also an early crystallizing mineral that contains fluid inclusions of magmatic origin a variation of 5 0 to 10 1 per mil in the delta 18 o values of topaz downward in the borehole agrees with a high temperature isotopic equilibrium between quartz and lepidolite and provides evidence of early interaction between the upper part of the beauvoir granite with external fluids during the magmatic and late magmatic stages this early interaction shown by the isotopic characteristics of magmatic mineralogy and by the isotopic temperatures took place across the liquidus solidus interval

a comparison of near surface codar and vacm measurements in the strait of juan de fuca august 1978 three days of simultaneous near surface current measurements made by coastal ocean dynamics applications radar codar and surface moored vector averaging current meters vacm in the strait of juan de fuca are compared the study focuses on the mean flow and tidal components that contained approximately 79 of the kinetic energy the mean speed and direction differences were less than 7 cm s and 10 respectively the k1 and m2 tidal constituents agreed to within 14 cm s with a mean difference of 5 2 cm s this study demonstrates that codar yields estimates of the near surface currents that are comparable with conventional vacm arrays to 10 15 cm s

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

5 2 châtaigneraie example of a late variscan tungsten district southern french massif central lat 44 40 n long 2 35 e

alteration of as bearing phases in a small watershed located on a high grade arsenic geochemical anomaly french massif central at a watershed scale sediments and soil weathering exerts a control on solid and dissolved transport of trace elements in surface waters and it can be considered as a source of pollution the studied subwatershed 1 5 km sup 2 was located on an as geochemical anomaly the studied soil profile showed a significant decrease of as content from 1500 mg kg sup 1 in the 135 165 cm deepest soil layer to 385 mg kg sup 1 in the upper 0 5 cm soil layer directly in the stream suspended matter and the 63 micro m fraction of bed sediments had as concentrations greater than 400 mg kg sup 1 in all these solid fractions the main representative as bearing phases were determined at two different observation scales bulk analyses using x ray absorption structure spectroscopy xas and microanalyses using scanning electron microscope sem and associated electron probe microanalyses epma as well as micro raman spectroscopy and synchrotron based micro scanning x ray diffraction micro sxrd characterization three main as bearing phases were identified i arsenates mostly pharmacosiderite the most concentrated phases as in both the coherent weathered bedrock and the 135 165 cm soil layer but not observed in the river solid fraction ii fe oxyhydroxides with in situ as content up to 15 4 wt in themore deepest soil layer and iii aluminosilicates the least concentrated as carriers the mineralogical evolution of as bearing phases in the soil profile coupled with the decrease of bulk as content may be related to pedogenesis processes suggesting an evolution of arsenates into as rich fe oxyhydroxides therefore weathering and mineralogical evolution of these as rich phases may release as to surface waters less

crustal thinning recorded by the shape of the namurian westphalian leucogranite in the variscan belt of the northwest massif central france in the western part of the variscan belt of the french massif central the gueret massif is an extensional allochthon emplaced to the southeast during namurian westphalian time the internal structure of the coeval leucogranites surrounding the western edge of the gueret massif consists of the plateau d9aigurande brame st sylvestre and st goussaud massifs these plutons are asymmetric diapirs rooted in the marche bussiere madeleine and arrenes faults respectively the plutons indicate diverging extravasation from the gueret massif whatever their shapes the plutons have a northwest trending stretching lineation that is also marked in the granite host rocks by retrograde metamorphic minerals the same extension direction is also inferred from namurian westphalian leucogranites and dikes intruding the gueret massif extension related granitoids present typical structural patterns such as asymmetric shapes extravasation from the collapsing area and persistence of a stretching lineation parallel to the regional extension direction these features may be used to diagnose crustal thinning

linking mesozoic lode gold deposits to metal fertilized lower continental crust in the north china craton evidence from pb isotope systematics abstract the source s of lode gold deposits formed in precambrian cratons related to accretion collision and cratonic reactivation formerly attributed to either supercrustal volcano sedimentary sequences or deep crustal sub crustal origin remain controversial largely because gold deposits are spatially related to metamorphosed rocks but geochemical data somewhat indicate a poorly understood deep source reconciling such conflicts is important to better understand the main factor controlling the formation of ore deposits and their genetic link with specific tectonic settings giant late mesozoic lode gold provinces in the north china craton ncc were formed ca 1 7 ga later than cratonization metamorphism and contemporaneous with intensively felsic to mafic magmatism related to cratonic reactivation in this study we conduct a comprehensive pb isotope study on major gold deposits from the eastern yanshan belt northern margin of the ncc in order to constrain the source s of gold we attempt to map regional pb isotope variations of lower continental crust lcc and develop a two stage quantitative model punctuated by three prominent geological events at 2 80 ga 1 85 ga and 0 16 ga to reproduce time integrated pb isotopic signatures of deep seated lithospheric reservoirs to make a comparison with pb isotopic signatures of the gold mineralization gold bearing pyrites within different types of host rocks have relatively uniform pb isotopic ratios which are significantly different to high grade metamorphosed host rocks but similar to those of spatially associated late mesozoic granitic rocks our data show that the pb isotopic signatures of gold deposits vary consistently with presumed regional pb isotopes of the lcc during the late mesozoic lead isotopic heterogeneity of the lcc was likely caused by underplating of mafic magmas derived from mantle sources during underplating highly chalcophile elements e g au ag and cu were concentrated at the base of the lcc due to sulfide saturation from mafic magmas integrating petrological geochemical geochronological and considering chalcophile element solubility we propose a new genetic model to describe the formation of late mesozoic gold deposits in the ncc 1 early formation of sulfide bearing cumulates with high au cu ratios during magma differentiation at the base of the lcc 2 subsequent fluid fluxed remelting of these cumulates at the onset of lithospheric extension and release and ascent of ore forming fluids to the site of precipitation considering the metallogenic characteristics of other au ore forming systems we suggest that metal fertilization in deep seated reservoirs and subsequent tectonic decompression are important factors controlling the development of au rich ore deposits worldwide this study demonstrates how pb isotope can be employed to trace source s of gold deposits

ore deposits of the french massif central insight into the metallogenesis of the variscan collision belt the french massif central fmc represents the whole west european variscan wev belt in terms of both the geodynamic evolution and the metallic content thus a study of the metallogenic evolution of the fmc may elucidate the conditions that allow the mineralisation of a collision belt since recent collision belts e g the himalayas or the alps show that mineralisation does not necessarily result from the collision process the palaeozoic history of the fmc is divided into three geodynamic stages unevenly involved from the metallogenic view point the eo variscan stage cambrian to silurian was not important the meso variscan stage devonian early carboniferous was of limited importance and most of the mineralisations formed during the neo variscan stage late carboniferous early permian in addition some more mineralisation was produced during the mesozoic because of the thermal reactivation linked with the alpine orogenies

polyphase wrench tectonics in the southern french massif central kinematic inferences from pre and syntectonic granitoids in the variscan french massif central the south limousin area consists of low to medium grade metamorphic rocks intruded by two granitic bodies the structural and textural analyses of these plutons undertaken in parallel with the structural analysis of their host rocks allow us to characterize and to date different stages in the tectonic evolution of this area this study shows that the south limousin area experienced successivelly two strike slip events along two geographically distinct shear zones from north to south the left lateral estivaux and the right lateral south limousin strike slip faults respectively these ductile faults subdivide the south limousin into three structural units from north to south they are the upper gneiss unit thiviers payzac unit and genis unit the two granitic bodies intrude the thiviers payzac unit only the younger estivaux granite is a syntectonic pluton which emplaced during left lateral wrenching 40ar 39ar dates from biotites indicate an early carboniferous age 346 3 ma the older granite is a pretectonic body it is the ordovician saut du saumon augen orthogneiss in which detailed structural analyses show the polyphase nature of the solid state deformation our microtectonic data indicate that the right lateral motions overprint the left lateral ones and produce apparently symmetrical fabrics

error analysis for tidal coefficients with application to hf radar surface current measurements abstract simulated hf radar data from a coastal dynamics applications radar codar system is used to predict errors in the mathematically recovered fourier tidal coefficients the model consists of a fourier series with prominent components of 12 24 and 25 8 hr plus white gaussian noise taken at hourly intervals this model demonstrated recovery of the 12 hr component with only data for 1 day and all three components recovery with data for 4 days with

hydrothermal fluids assisted crustal scale strike slip on the argentat fault zone abstract the role of hydrothermal fluids in assisting the activity of strike slip faults is investigated using a range of new geological geophysical and geochemical data obtained on the argentat fault massif central france this fault zone 180 km long and 6 to 8 km width has experienced coeval intense channeling of hydrothermal fluids and brittle deformation during a short time span 300 295 ma according to seismic data the fault core is a 4 km wide vertical zone of high fracture density that rooted in the middle crust 13 km and that involved fluids in its deeper parts 9 13 km depth if stress analyses in the fault core and strain analyses in the damage zone both support a left lateral movement along the fault zone it is inferred that hydrothermal fluids have strongly influenced fault development and the resulting fault has influenced fluid flow fluid pressure made easier fracturing and faulting in zones of competent rocks units and along rheological boundaries repeated cycles of increase of fault fracture permeability then overpressure of hydrothermal fluids at fault extremity favored strong and fast development of the crustal scale strike slip fault the high permeability obtained along the fault zone permitted a decrease of coupling across the weak fault core connections between shallower and lower crustal fluids reservoirs precipitate the decrease of fault activity by quartz precipitation and sulfides deposition the zones of intense hydrothermal alteration at shallows crustal levels and the zones of fluid overpressure at the base of the upper crust both controlled the final geometry of the crustal scale fault zone

metal mobility during hydrothermal breakdown of fe ti oxides insights from sb au mineralizing event variscan armorican massif france hydrothermal alteration related to sb au mineralization is widespread in the variscan armorican massif but mineral replacement reactions are not well characterized in particular the hydrothermal breakdown of ilmenite titanohematite based on petrography electron probe micro analyzer and laser ablation inductively coupled plasma mass spectrometer analyses we document mineralogical change at rock and mineral scale and the redistribution of sb and others trace elements during the recrystallization of ilmenite titanohematite to hydrothermal rutile hydrothermal alteration is mainly potassic with associated carbonation the replacement mechanism is interpreted to be an interface coupled dissolution reprecipitation process results show that mn zn co ni sn mo and u are released during hydrothermal alteration whereas sb and w are incorporated in newly formed hydrothermal rutile from the hydrothermal fluid furthermore the concentration of sb evolves through time suggesting a change in fluid composition likely related to an enrichment of fluid in sb during rutile crystallization considering that fe ti oxides breakdown during hydrothermal alteration is common within epithermal and mesothermal orogenic au sb mineralizing systems results report in this study yield important constraints about metal mobility and exchanges in hydrothermal gold systems

9 processes of tectonism magmatism and mineralization lessons from europe abstract metallogenic provinces in europe range in age from the archaean to the neogene deposit types include porphyry copper and epithermal cu au volcanic hosted massive sulphide vms orogenic gold fe oxide cu au anorthosite fe ti oxide and sediment hosted base metal deposits most of them formed during short lived magmatic events in a wide range of tectonic settings many can be related to specific tectonic processes such as subduction hinge retreat accretion of island arcs continental collision lithosphere delamination or slab tear in contrast most sediment hosted deposits in europe evolved in extensional continental settings over significant periods of time in europe as elsewhere ore formation is an integral part of the geodynamic evolution of the earth s crust and mantle many tectonic settings create conditions conducive to the generation of water rich magma but the generation of ore deposits appears to be restricted to locations and short periods of change in temperature and stress imposed by transitory plate motions crustal influence is evident in the strong structural controls on the location and morphology of many ore deposits in europe crustal scale fault fracture systems many involving strike slip elements have provided the fabric for major plumbing systems rapid uplift as in metamorphic core complexes and hydraulic fracturing can generate or focus magmatic hydrothermal fluid flow that may be active for time spans significantly less than a million years once a hydrologically stable flow is established ore formation is strongly dependent on the steep temperature and pressure gradients experienced by the fluid particularly within the upper crust in europe significant fracture porosity deep in the crystalline basement 1 is not only important for magmatic hydrothermal systems but allows brines to circulate down through sedimentary basins and then episodically upward expelled seismically to produce sediment hosted base metal deposits and kupferschiefer copper deposits emerging research stimulated by geode can improve the predicting power of numerical simulations of ore forming processes and help discover the presence of orebodies beneath barren overburden

superimposed tectonic and hydrothermal events during the late orogenic extension in the western french massif central a structural and 40ar 39ar study in the western french massif central the argentat fault is a major structure through which as au fluids percolated in the late carboniferous along brittle fractures new petrostructural investigations show that an early ductile normal dextral faulting coeval to leucogranite emplacement took place during the late visean syncollisional extension of the belt and was accompanied by a hydrothermal event marked by the growth of muscovites whose 40ar 39ar ages cluster around 335 ma this early fluid channelling is associated with brittle deformation only in the hangingwall of the argentat fault whereas ductile deformation is restricted to the footwall these results provide new evidence for the upper crust implication during the syncollisional extension in the french massif central this study stresses the interest of a detailed multimethod analysis to characterize hydrothermal processes especially in basement areas where the tectonic plutonic and metamorphic evolution is polyphased

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

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt

metallogenesis of the french part of the variscan orogen part ii time space relationships between u au and snw ore deposition and geodynamic events mineralogical and upb data abstract mineralogical geochemical structural and isotopic u pb studies were carried out on representative uranium and gold deposits from the french part of the variscan orogen results are compared with complementary data from the literature in order to propose a reconstruction of the metallogenesis in the variscan terranes with a particular emphasis on u deposits the data show that the early metal concentrations or pre enrichments in the terranes are affected by successive metallogenic stages which caused significant metal transport and deposition thus the significant geodynamic events which affected the variscan terranes were responsible for multistage metal deposition or alteration of the early concentrations and for the superimposed features of the metal deposits u pb geochronological results complemented by a compilation of geochronological data on u and pb zn ba f deposits show that three major stages of u deposition took place in western europe in the periods 325 300 ma 290 260 ma and 190 170 ma in addition small uranium mineralization events have been dated at 425 ma 90 110 ma and as tertiary about 40 ma the permo stephanian stage is associated with late brittle deformation associated with major shear zone movements and constitutes the major stage of u concentration the jurassic phase has a prominent part in the hydrothermal remobilization of uranium and in f ba pb zn deposition throughout western europe especially in the eastern part of the massif central other stages correspond to subordinate uranium migration within variscan terranes during earlier or later geodynamic phases the silurian metamorphism in vendee and the mesozoic to tertiary tectonic movements related to the pyrenean or alpine orogenies in the south eastern part of the massif central and in the alps data available on au and sn w as sb deposits have been considered for comparison with the u metallogenesis stages related to the deposition of such metals display significant similarities with those related to uranium deposition they occurred mostly during visean to permian events related to the intrusion of the late visean sn w and late carboniferous sn w li f granitoids and to the late brittle deformation stages which affected the hercynian basement in the vicinity of major shear zones au as sb however the significant discrepancies between the physical chemical conditions of solubilization transport and deposition of the metal led to distinct deposit location independently of the metal source location within the terranes

origin of the atypical puy les vignes w breccia pipe massif central france constrained by trace element and boron isotopic composition of tourmaline abstract the puy les vignes w deposit is an atypical wolframite bearing hydrothermal breccia pipe hosted in migmatitic biotite sillimanite gneisses in the northwestern french massif central the deposit is characterized by volumetrically important tourmaline alteration allowing to investigate the fluid evolution of the ore forming hydrothermal system four generations of hydrothermal tourmaline tur 1 tur 4 formed during pre syn and post mineralization stages were identified based on detailed petrographic observations and were analyzed in situ for their chemical and boron isotopic compositions at the grain scale tourmaline commonly shows oscillatory zoning and dissolution textures resulting from a multi stage crystallization in a fluid dominated system the different generations of hydrothermal tourmaline have dravite schorl compositions and show similar major and trace element contents falling into the field of metamorphic rocks high concentrations of v cr sr and low concentrations of li sn in tourmaline suggest a metamorphic dominated origin of these elements the boron isotopic compositions of tourmaline range between 13 3 and 7 8 and cannot unambiguously distinguish between a magmatic and a metamorphic fluid origin our data indicate that the chemical and boron isotopic composition of tourmaline was dominantly controlled by high temperature fluid rock interactions between the metamorphic basement and boron rich reduced and low salinity hydrothermal fluids based on these results we propose a fluid evolution scenario for the puy les vignes ore forming hydrothermal system release of magmatic hydrothermal fluids from an unexposed peraluminous leucogranite at ca 324 ma is proposed as the main mechanism responsible for early greisenization and formation of disseminated tur 1 this episode was followed by massive tourmalinization and hydraulic brecciation of the overlying gneisses producing a tourmaline rich tur 2 crackle breccia by interactions with boron rich hydrothermal fluids fluid assisted reopening and collapse of the former tourmalinite at ca 318 ma yielded a matrix supported quartz tourmaline breccia pipe and formation of wolframite bearing quartz veins accompanied by deposition of tur 3 finally post ore hydraulic fracturing at ca 300 ma led to formation of tourmaline rich microbreccias tur 4 possibly from metamorphic fluids

sources of hercynian granitoids from the french massif central inferences from nd isotopes and consequences for crustal evolution the french massif central is characterized by a great abundance and variety of granitoids emplaced during hercynian times 360 280 ma distinct nd i and 87sr86sr i are not associated with particular petrographic types however lower nd i values 8 5 occur among the younger leucogranites from the western part of the massif intermediate nd i 6 correlate with aluminous monzogranites and higher values 6 to 3 with calc alkaline granodiorites in the eastern massif central nd i are homogeneous at the regional scale in the western massif central the leucogranites and the monzogranite granodiorites display different trends of nd i with emplacement ages from these results extreme models involving a depleted mantle or an old archean or early proterozoic source may be ruled out the trend of increasing nd i with decreasing age observed in the monzogranite granodiorite group suggests either an increasing contribution of a mantle component or the progressive melting of more mafic more refractory crustal sources or both the reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile the large amount of granites may be the result of favourable processes during the collision tectonics combined with the unusual abundance of fertile materials enriched in water and heat producing elements in the pre carboniferous crust

hydrothermal processes and ore forming fluids in the le bourneix gold deposit central france the le bourneix gold deposit in the french massif central occurs within a shear zone in gneisses of the limousin hercynian basement gold bearing quartz lenses formed during extension phases after cataclasis of initially barren quartz postdating mylonites bouchot et al 1989 the economic mineralization arsenopyrite gold pyrite galena and pb sulfosalts hosted by microsaccharoidal quartz formed after crystallization of arsenopyrite and pyrite synchronous with white milky quartz a later paragenesis associated with hyaline quartz comprises ag rich gold sphalerite galena tetrahedrite and sulfosalts arsenopyrites have a range of compositions with a mean as content equivalent to an equilibrium temperature with pyrite of about 400 degrees or 30 degrees c hydrothermal alterations synchronous with ore deposition resulted in a phengite chlorite association and then a propylitic chlorite carbonate assemblage from fluid inclusion studies it appears that the general evolution of the ore fluids is 1 circulation under high pressure 2 kbars at 400 degrees or 50 degrees c of a low salinity solution with minor volatile contents 2 mixing of this solution with a co 2 rich vapor induced by a drop in pressure down to 0 6 kbars producing a heterogeneous fluid at 320 degrees c and 3 circulation of volatile free aqueous solutions at decreasing temperature 300 degrees 120 degrees c and salinity most of the economic ore probably formed at the transition from 1 to 2 lead isotope data suggest a local source for metals with mixing of poorly radiogenic lead from the basement orthogneiss with radiogenic lead from a supracrustal source

the variscan kasperske hory orogenic gold deposit bohemian massif czech republic kasperske hory one of the largest gold deposits in the bohemian massif is characterised by shear zone related gold bearing quartz veins at the tectonic boundary of the two metamorphic terranes of the moldanubian unit studies of the geology mineral paragenesis fluid inclusions and oxygen geothermometry have been carried out to determine the mineralisation events and alternative p t t model of this deposit the pre mineralisation stage is characterised by geochemical evidence of pegmatites 700 c and 1 1 0 45 gpa and metamorphic structures resulting from deformation dx 1 at the regional metamorphic peak 650 c and 0 5 gpa the mineralisation in dx 2 began to develop from the formation of the q1 and q2 quartz veins stage i under brittle ductile conditions with the n s extension these quartz veins were filled mainly by apatite plagioclase 1 muscovite and arsenopyrite 1 revised and reinterpreted gechronological data indicate that the earliest quartz veins can be dated at 344 ma the q3 veins were formed later under brittle to brittle ductile conditions with crystallisation of plagioclase 2 arsenopyrite 2 scheelite chlorite pyrite and calcite stage ii the corresponding fluids of stages i and ii belong to the c h o n system the p t conditions correspond to 590 520 c and to 0 44 0 25 gpa for stage i and 480 390 c and 0 2 gpa for stage ii stage iii is characterised by the precipitation of younger quartz chlorite molybdenite calcite pyrrhotite 1 galena 1 native gold bismuth and other bi te minerals the associated aqueous fluids salinity 1 5 8 wt nacl equiv follow fluid inclusion trails and were trapped at relatively low pressures and temperatures 290 180 c and 0 1 gpa stage iv is mainly dominated by carbonate and fluorite fillings of the open fractures and cracks arsenopyrite 1 and 2 and molybdenite are the main gold bearing and gold carrying ore phases at this deposit the means of microanalysis indicate the presence of both micron sized and probably submicron sized particles of gold 920 1000 in the fractures and fissures of the arsenopyrite 1 and structurally bound invisible gold in the arsenopyrite 2 crystal lattice e g gold as au i economic gold mineralisation native free gold flakes 930 1000 is linked to stage iii

comparative behavior of sr nd and hf isotopic systems during fluid related deformation at middle crust levels abstract we have carried out a comparative rb sr sm nd and lu hf isotopic study of a progressively deformed hercynian leucogranite from the french massif central belonging to the la marche ductile shear zone in order to investigate the respective perturbation of these geochronometers with fluid induced deformation the one meter wide outcrop presents a strongly deformed and mylonitized zone at the center and an asymmetric deformation pattern with a higher deformation gradient on the northern side of the zone ten samples have been carefully collected every 10 cm north and south away from the strongest deformed mylonitic zone they have been analyzed for a complete major trace element data set oxygen isotopes rb sr sm nd and lu hf isotopic systematics we show that most of major and trace elements except sio2 alkaline elements k2o rb and some metal transition elements cu are progressively depleted with increasing deformation this depletion includes ree y but also hfs elements ti hf zr nb which are commonly considered as immobile elements during upper level processes variations in elemental ratios with deformation e g decrease in lree mree hree nd hf th sr increase in rb sr u th and constant sr nd lead to propose the following order of element mobility u th sr nd hf hree we conclude in agreement with previous tectonic and metallogenic studies that trace element patterns across the shear zone result from circulation of oxidizing f rich hydrothermal fluids associated with deformation a temperature of the fluid of 470 480 c can be deduced from the 18o equilibrium between quartz muscovite pairs elemental fractionation induces perturbation of the rb sr geochronometer the well defined 87rb 86sr 87sr 86sr correlation gives an apparent age of 294 19 ma slightly younger than the 323 4 ma age of leucogranites in this area this apparent age is interpreted as dating event of intense deformation and fluid circulation associated with mass transfer and exhumation of the ductile crust shortly after the leucogranite emplacement sm nd and lu hf isochron type diagrams do not define any correlation because of the low fractionated sm nd and lu hf ratios isotopic data demonstrate that only the lu hf geochronometer system is not affected by fluid circulation and gives reliable tdm age 1 29 0 03 ga and ehf signatures by contrast the sm nd geochronometer system gives erroneous old tdm ages of 2 84 4 ga there is no positive end ehf correlation because of decreasing end values with deformation at constant ehf values however end ehf values remain in the broad end ehf terrestrial array which strongly indicates that fluid induced fractionation can contribute to the width of the terrestrial array the strong ehf negative values of the leucogranite are similar to metasedimentary granulitic xenoliths from the french massif central and confirm the generation of the leucogranite by several episodes of reworking of the lower crust

4 transpressional tectonics lower crust decoupling and intrusion of deep mafic sills a model for the unusual metallogenesis of sw iberia abstract sw iberia is interpreted as an accretionary magmatic belt resulting from the collision between the south portuguese zone and the autochthonous iberian terrane in variscan times 350 to 330 ma in the south portuguese zone pull apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the iberian pyrite belt massive sulphides precipitated in highly efficient geochemical traps where metal rich but sulphur depleted fluids of dominant basinal derivation mixed with sulphide rich modified seawater massive sulphides formed either in porous reactive volcanic rocks by sub seafloor replacement or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks while magmatic fluids probably had a minor influence on the observed geochemical signatures the ossa morena zone was a coeval calc alkaline magmatic arc it was the site for unusual mineralization particularly magmatic ni cu and hydrothermal fe oxide cu au ores iocg most magmatism and mineralization took place at local extensional zones along first order strike slip faults and thrusts the source of magmas and iocg and ni cu deposits probably lay in a large mafic ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust here juvenile melts extensively interacted with low grade metamorphic rocks inducing widespread anatexis magma contamination and further exsolution of hydrothermal fluids hypersaline fluids 18 o fluid 5 4 to 12 were focused upward into thrusts and faults leading to early magnetite mineralization associated with a high temperature 500 c albite actinolite salite alteration and subsequent copper gold bearing vein mineralization at somewhat lower temperatures assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex further injection of both basic and sulphide rich magmas into the upper crust led to the formation of ni cu rich breccia pipes younger 330 to 280 ma peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis these granitoids have w sn and pb zn related mineralization that also shows geochemical evidence of major mantle crust interaction late epithermal hg cu sb and pb zn ag mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex in all cases most of the sulphur seems to have been derived from leaching of the host sedimentary rocks 34 s 7 to 20 with only limited mixing with sulphur of magmatic derivation the metallogenic characteristics of the two terranes are quite different in the ossa morena zone juvenile magmatism played a major role as the source of metals and controlled the styles of mineralization in the south portuguese zone magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids which are dominated by crustal signatures most of the magmatic and tectonic features related to the variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs

palaeozoic oolitic ironstone of the french armorican massif a chemical and structural trap for orogenic base metal as sb au mineralisation during hercynian strike slip deformation in the saint aubin des ch teaux quarry armorican hercynian belt western france an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the lower ordovician gres armoricain formation the hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritised lenticular bodies within the oolitic ironstone layer these sulphide lenses are spatially associated with strike slip faults and extend laterally from them after the massive sulphidation stage fe as stage 1 subsequent fracturing allowed the deposition of base metals stage 2 and pb sb au stage 3 parageneses in veins the dominant brittle structures are vertical extension veins conjugate shear veins and strike slip faults of various orders all these structures are filled with the same paragenetic sequence deformation analysis allows the identification of structures that developed incrementally via right lateral simple shear compatible with bulk strain affecting the central armorican domain each increment corresponds to a fracture set filled with specific parageneses successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids maximum of 390 350 c during the main sulphide stage 1 the subsequent stages present a marked temperature drop 300 275 c lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying neo proterozoic basement lead isotope data relative ages of deformation and comparison with neighbouring deposits suggest that large scale fluid pulses occurred during the whole hercynian orogeny rather than pulses restricted to the late hercynian period the vicinity of the hercynian internal domain appears as a key control for deformation and fluid flow in the oolitic ironstones which acted as a chemical and structural trap for the hydrothermal fluids the epigenetic mineralisation of saint aubin des ch teaux appears to be very similar to epigenetic sulphidation described in banded iron formation hosted gold deposits

the timing of w sn rare metals mineral deposit formation in the western variscan chain in their orogenic setting the case of the limousin area massif central france new 40ar 39ar dating performed on rare metal granites and w sn deposits in the northern limousin has provided evidence of two metallogenic episodes an early namurian episode c 325 ma was contemporaneous with the emplacement of the large peraluminous leucogranite bodies which are associated with small w sn deposits but also with some larger deposits at puy les vignes 323 4 0 9 ma and moulin barret 323 7 0 8 ma formed at a shallower level above cryptic granite plutons these new data indicate that the metallogenic potential of the namurian leucogranites might have been underestimated most other w sn deposits in the northern limousin area are attributed to a mid westphalian episode c 310 ma and are contemporaneous with the emplacement of all the rare metal granites both episodes were related to leucogranite emplacement and associated fluid circulations but in two different geodynamic contexts the early namurian episode may be related to syncollisional extension of the variscan belt whereas the mid westphalian one occurs during generalized extension and rapid exhumation of the belt associated with the granulite facies metamorphism of the lower lithosphere probably related to the delamination of the lower lithosphere thus w sn and rare metals ta nb be li deposits are clearly temporally and probably genetically related to leucogranitic magmatism

the variscan french massif central a new addition to the ultra high pressure metamorphic club exhumation processes and geodynamic consequences abstract this paper presents the first documentation of coesite bearing eclogites in the eastern french massif central monts du lyonnais unit and discusses the exhumation processes for these very high pressure rocks a combination of mineralogical and geochronological datasets allows us to quantify a depth time path and related exhumation rates high pressure metamorphism is constrained to 400 420 ma minimum 28 kbar or ca 90 km by 360 380 ma the rocks were exhumed to 30 km depth these kinetic results conform to the geological constraints extracted from the tectonic and sedimentary record of the eastern french massif central these multidisciplinary approaches provide new information on paleozoic orogeny and allow us to discuss the relative roles of subduction and collision in exhumation of very high pressure rocks we suggest that a significant amount of exhumation of these rocks occurred during subduction prior to continental collision continental collision itself was responsible only for the final stages of exhumation under a transpressive regime

cooling pattern and mineralization history of the saint sylvestre and western marche leucogranite pluton french massif central i 40ar 39ar isotopic constraints abstract the saint sylvestre western marche leucogranite complex lc is a composite pluton emplaced 324 m y ago in the variscan continental crust of the northwestern limousin french massif central petrographic structural and new isotopic data are used to constrain the cooling pattern of the pluton in the temperature range for ar retention in muscovite part i and the genetic relationships with the subsequent uranium mineralization postdating the time of intrusion by 40 50 m y part ii a suite of thirty seven muscovite and nine biotite concentrates selected over the entire plutonic complex were analyzed by the 40 ar 39 ar technique muscovite is characterized by well defined plateau ages ranging between 314 and 301 ma while biotite gives less well behaved age spectra with apparent ages varying between 320 and 300 ma the muscovite age variations are systematic and clearly associated with regional trends at the scale of the pluton field petrographic gravimetric and structural relationships indicate that the age pattern is controlled by the intrusion shape and the regional post cooling fault network cutting through the laccolith when restored to their approximate pre faulting geometry the age variations are shown to correlate positively with sample elevation with a top to base difference as large as 10 14 m y over a mean intrusion thickness of 3 km this pattern is interpreted to reflect cooling driven by erosion and exhumation of the thickened variscan crust during the westphalian and is confirmed by two dimensional thermal modelling part ii the data show that 40 ar 39 ar mica ages can postdate the time of crystallization of the host pluton by up to 23 ma depending on the level sampled at the surface they warn against interpreting k ar mica ages from similar low relief igneous settings as closely postdating intrusive events without a precise knowledge of the thermal structure cooling mechanism and post cooling faulting of the pluton in turn provided that the pre faulting geometry of the intrusion is properly assessed the vertical cooling pattern can reveal fine scale variations in the thermal structure of old eroded intrusions that may be extremely useful for recovering quantitative information on the mechanisms and rates of cooling in ancient orogens

40 ma of hydrothermal w mineralization during the variscan orogenic evolution of the french massif central revealed by u pb dating of wolframite we present u pb thermal ionization mass spectrometer tims ages of wolframite from several granite related hydrothermal w sn deposits in the french massif central fmc located in the internal zone of the variscan belt the studied wolframite samples are characterized by variable u and pb contents typically 10 ppm and show significant variations in their radiogenic pb isotopic compositions the obtained u pb ages define three distinct geochronological groups related to three contrasting geodynamic settings i visean to namurian mineralization 333 327 ma coeval with syn orogenic compression and emplacement of large peraluminous leucogranites ca 335 325 ma ii namurian to westphalian mineralization 317 315 ma synchronous with the onset of late orogenic extension and emplacement of syn tectonic granites ca 315 310 ma and iii stephanian to permian mineralization 298 274 ma formed during post orogenic extension contemporaneous with the permian volcanism in the entire variscan belt the youngest ages 276 274 ma likely reflect the reopening of the u pb isotopic system after wolframite crystallization and may correspond to late hydrothermal alteration e g ferberitization our results demonstrate that w sn mineralization in the fmc formed during at least three distinct hydrothermal events in different tectono metamorphic settings over a time range of 40 ma

antimony deposits in the variscan armorican belt a link with mafic intrusives the variscan armorican belt was the world leading producer of antimony at the beginning of the twentieth century however geological controls on the deposits remain unconstrained here we illustrate the field setting of mineralisations and perform a statistical analysis using geographical information system software geophysical and geological data the analysis shows that sb deposits are spatially correlated with high density and magnetic zones this may reflect the presence of mafic ultramafic bodies at depth a suggestion that is further supported by numerous occurrences of doleritic dykes throughout the region future antimony prospecting should focus on the high density and high magnetic zones which comprise about 18 of the armorican massif links between sb deposits and mafic intrusions are recognised elsewhere in the west european variscan belt emphasising that these may be key for antimony prospecting in deformation belts

collapse of a hercynian tibetan plateau into a late palaeozoic european basin and range province the large width the importance of crustal shortening and the likelihood of thick crust in the late palaeozoic hercynian or variscan belt of southern europe suggest a tectonic evolution analogous to the evolution of the tibetan plateau and its neighbouring himalaya1 and of the altiplano and other high plateaux of the andes here we extend dewey and burke s analogy1 of the hercynian chain with tibet to include the subsequent stage of pervasive normal faulting and crustal extension and thinning which characterize the active tectonics of tibet parts of the high andes and the basin and range province of western north america it appears that normal faulting and rifting also characterize the late palaeozoic tectonic activity of the hercynian chain

the beauvoir topaz lepidolite albite granite massif central france the disseminated magmatic sn li ta nb be mineralization the beauvoir topaz lepidolite albite granite french massif central is the latest intrusion in a variscan peraluminous granitic complex composed of three units successively emplaced the concealed la bosse granite the colettes two mica granite and the beauvoir granite the 900 m deep drill hole of the geologie profonde de la france deep drilling program of the continental crust has provided a continuously cored section of the beauvoir granite the exposed section of the beauvoir granite is presently mined for kaolin in the upper 200 m it also represents a huge subeconomic disseminated li sn ta be deposit sharing many characteristics with rare metal bearing pegmatites compared to similar li f rich igneous bodies the beauvoir granite is strongly enriched in sn 200 1 400 ppm ta 20 400 ppm and be 20 300 ppm li is located in lepidolite and amblygonite ta and nb mainly in columbo tantalite and uranium rich microlite and sn in cassiterite be is located in lepidolite crystals and in unidentified mineral s the major element composition of the beauvoir granite is similar to that of macusani volcanic glasses of peru and ongonite subvolcanic rocks of mongolia the main structural textural mineralogical and geochemical characteristics of the beauvoir granite can be explained by extreme fractionation of f and li rich magmas and associated fluid phases most trace elements show a strong upward enrichment the high f sub o 2 low solidus temperature and high fluorine content of the beauvoir magma have considerably reduced sn and w fractionation in the expelled magmatic fluid despite its high chlorine content 25 30 wt nacl equiv however f and li enriched geochemical halos have developed in the enclosing mica schist in response to the boiling of the magmatic fluid the relatively deep emplacement of the granite 3 km has limited the development of hydraulic fracturing and subsequent hydrothermal circulation and mineralization thus despite the very high specialization of the beauvoir granite no significant vein type mineralization is directly related to it the la bosse quartz ferberite stockwork was developed before the emplacement of the colettes and beauvoir granites and is related to the earlier concealed la bosse granite

applications of dual doppler hf radar measurements of ocean surface currents abstract hf doppler radar measurements of near shore sea surface currents is a relatively new technique it has the advantage of covering a wide area of the sea surface simultaneously while being shore based in this paper we compute the m 2 k 1 and mean currents over an area of the easter strait of juan de fuca from hf radar measurements made during the summer of 1979 from these components we computed trajectories of simulated continuous leaks from a proposed oil pipeline in this area by computing the trajectory over one day we estimated the shoreline locations that would be impacted by the oil

identification of fluid inclusions in relation to their host microstructural domains in quartz by cathodoluminescence abstract the geometry of fluid trapping in relation to microfracture healing or filling was investigated using massive quartz vein samples characterized by multistage deformation and fluid trapping the multidisciplinary approach used in this study includes 1 mapping of oriented thin sections of quartz using the cathodoluminescence cl mode of a scanning electron microscope sem sem cl images display areas of different intensities from dark to white which correspond to the intensity of the cl and are caused by trace elements present in the quartz microfracture healing by newly formed quartz crystallization may occur under physical chemical conditions different from those which characterized the early quartz matrix formation thus evidences of linear cathodoluminescent markers have been searched and identified by paralleling sem cl images and transmitted light microphotographs data show that cathodoluminescence makes clearly visible the healed crack networks which are only detected in part by transmitted light microscopy the technique is essential to establish the microcrack chronology and to estimate the width affected by healing along a fluid inclusion trail such data are useful for the estimation of the microfracture permeability changes throughout the history of fluid migration 2 systematic and statistical analysis of the microstructural markers in horizontal and oriented planes using an interactive videographic analyzer which gives the geometry of the paleofluid pathways orientation dip and width of the veinlets fluid inclusion trails and cathodoluminescent microdomains 3 microthermometric and raman analyses of fluid inclusions in selected fluid inclusions for the determination of the p v t x conditions of fluid migration in a specific microcrack network studied quartz samples come from au mineralized quartz veins from the french massif central and some quartz samples from the cassiar mountains in british columbia have been observed for comparison in all samples the quartz matrix exhibits intense microfracturing suggesting strong brittle deformation which postdates the main stage of quartz lens formation the quartz veins are characterized by multistage quartz crystallization healed microfractures clear quartz bands as well as quartz comb veinlets were studied as functions of the physicochemical conditions in relation to stages of deformation and ore deposition this study shows that systematic measurements of microstructural marker orientations together with detailed fluid inclusion and mineral petrography and microthermometry may lead to a precise determination of ore fluid pathways and chronology

a rare phanerozoic amphibolite hosted gold deposit at danba yangtze craton china significance to fluid and metal sources for orogenic gold systems the danba gold deposit is located in a poorly documented gold province on the north western margin of the yangtze craton it is sited in devonian sequences in a high grade metamorphic terrane that includes an extensional metamorphic core complex around the deposit peak metamorphic conditions of 6 0 5 kbar and 650 50 c at ca 193 ma were followed by retrograde sillimanite grade conditions of 4 5 0 5 kbar and 550 50 c the deposit is hosted in a broadly strata bound ductile brittle shear zone with high t proximal alteration assemblages of biotite amphibole plagioclase and ore assemblages dominated by pyrrhotite but with a strong association between gold and bismuth tellurides alteration mineral thermobarometers together with heating freezing studies of low salinity h2o co2 ch4 fluid inclusions indicate p t conditions of early ore deposition of approximately 4 5 kbar and 500 650 c at around 185 9 ma indicated by re os geochronology on ore related molybdenite in conjunction all data demonstrate that danba represents a lower jurassic hypozonal orogenic gold deposit that formed during post peak metamorphic retrogression the primary high p t nature of the deposit combined with its late metamorphic timing negate that the ore fluid was sourced via devolatilization of the hosting supracrustal sequences a deep externally derived ore fluid source is required the most likely source is the k h2o co2 and ore metal fertilized lithospheric mantle that was metasomatized during neoproterozoic subduction it is proposed that transition from lithospheric transpression to extension in the jurassic triggered the devolatilization of this metasomatized lithosphere to cause the formation of this rare phanerozoic amphibolite hosted gold deposit at danba

observations of a summertime reversal in circulation in the strait of juan de fuca while measuring currents in the strait of juan de fuca we recorded an unusual 3 day reversal in the surface current from seaward to landward in apparent response to the southerly winds of a coastal storm the reversal was mointiored by four surface drifters and three moored current meter arrays that recorded the current at 20 to 130 m depths spatial details of the reversal patterns mapped by an hf current sensing radar at the boundary between the inward and outward flow showed a complex surface circulation pattern with a strong southerly flow toward new dungeness split although such reversals have been previously recorded in nonsummer months their appearance in the summer when coastal cyclonic storms are uncommon and river discharge is high is unusual

geochemical constraints on the genesis of hercynian two mica leucogranites from the massif central france abstract late hercynian two mica leucogranites dated at 340 280 3a old occur across the entire massif central france and form dykes and stocks up to 130 km in length they are peraluminous molecular al 2 o 3 na 2 o k 2 o cao asi 1 16 1 25 and have low initial nd 8 2 to 6 1 indicating derivation from the metasedimentary lower crust as sampled in crustal xenoliths nd 340 8 6 to 4 3 nd 280 9 0 to 4 8 their high sio 2 70 78 wt p 2 o 5 up to 0 5 rb and ga and low ba sr and ree are mainly a result of partial melting processes rather than fractionation partial melting was initiated by underplating of the lower crust by mantle derived basic magmas between 360 and 270 ma and was intensified by granulite facies metamorphism between 300 and 280 ma from this model there is little similarity between the genesis of two mica leucogranite in the massif central and that in the himalaya with which it has previously been compared

tracing metal sources in peribatholitic hydrothermal w deposits based on the chemical composition of wolframite the example of the variscan french massif central abstract we present a complete dataset for major minor and trace elements in wolframite fe mn wo4 based on the example of several peribatholitic hydrothermal w deposits located in the variscan french massif central fmc the wolframite samples were characterized down to the micrometer scale by combining scanning electron microscopy observations and in situ chemical analysis by electron probe microanalyzer and laser ablation inductively coupled plasma mass spectrometry most samples are fe dominated wolframites and are characterized by variable fe fe mn values for the studied deposits from 0 36 to 0 86 without significant alteration features wolframites present intracrystalline variations for concentrations of several minor and trace elements principally for nb ta and sn which reflect growth zoning the minor and trace element compositions of the different wolframite samples show at first order similar enrichments in ta nb sn zn and in as well as depletions in lree li th ti zr hf pb v co and mg compared to the upper continental crust second order compositional variations are observed between the studied deposits which can be separated in four main regional geochemical groups this work highlights that the chemical composition of wolframite is controlled by both the crystallochemical parameters and the composition of the primary hydrothermal fluid the regional variations observed in the geochemical signatures of wolframite reflect local variations in the composition of the hydrothermal fluid i e variable metal sources therefore this suggests that the minor and trace element contents in wolframite likely represent direct markers of the source of the hydrothermal fluid and ultimately of the metals the comparison between the trace element compositions of wolframite with the compositions of regional granitic rocks and worldwide shales suggests that evolved peraluminous granites represent the main source of metals in peribatholitic hydrothermal w deposits from the fmc whereas metasedimentary rocks have a limited to no influence on the metal budget

ductile structures and instabilities their implication for variscan tectonics in the ardennes abstract increasing consideration of instabilities may explain the development of structures at all scales in naturally deformed rocks this address is a review of geometric scale free concepts applied to the tectonic study of ductile structures the notion of instabilities is first given to introduce non equilibrium dynamics such as strain localisation and single deformation events producing superimposed structures these concepts lead to the idea that orogens are regions of accumulated strain in zones dominated by non coaxial deformation consequently stretching lineations depend on finite strain and do not represent the plate motions repeated phases of deformation represent local perturbations during a kinematic and dynamic event rather than being correlatable as regional phases the fold and thrust belt of the northern variscan belt in the ardennes is taken as a natural example where superposed structures represent the instable response to forces among which plate tectonic forces play a subsidiary role

lower crustal rejuvenation and growth during post thickening collapse insights from a crustal cross section through a variscan metamorphic core complex the variscan lower crust of europe acquired its main features seismic layering intrusion of mantle derived magmas granulite grade metamorphism when the upper crust underwent post thickening extension through gravitational collapse we suggest that post thickening collapse is a geodynamic process that contributes to the growth and differentiation of continental lithosphere in collapsed mountain belts

microfracturing and fluid mixing in granites w sn ore deposition at vaulry nw french massif central abstract the vaulry w sn mineralisation located at the eastern boundary of the blond rare metal leucogranite is contained in a set of subvertical quartz veins locally with muscovite and minor quartz selvages the sequence of deposition was 1 milky quartz predominantly as fracture filling generally affected by subsequent ductile deformation 2 hyaline quartz wolframite cassiterite 3 minor sulphides other sets of quartz veinlets although generally barren are observed in the blond massif fluid migration at the microscopic scale within the granite and in the vicinity of quartz fractures was constrained by studying the geometry of fluid inclusion planes and fluid inclusion chemistry in and outside the mineralised area three major sets of subvertical fluid inclusion planes are recognised a n050 060 e set mostly developed in the veins and in the immediate vicinity a n110 130 e set regionally developed in the granite and a n140 160 e set of local extent as a whole the density of fip decreases from the mineralised zones toward the barren part of the pluton except for the n140 160 e set these are locally abundant around quartz veinlets with similar orientations that form a broad n s band near the blond locality mineralising fluids observed as primary inclusions in cassiterite and in undeformed hyaline quartz are mostly aqueous with moderate salinity and a minor volatile component at variance with many other w sn deposits in the variscan belt ore deposition occurred around 315 c at an estimated depth of 5 5 km under hydrostatic to slightly suprahydrostatic pressures it resulted from fluid mixing in the central part of a large hydrothermal system between two end members i a hot 425 430 c moderately saline fluid that contained a diluted volatile component and although na dominated minor amounts of li and ca the estimated 18o indicates that this fluid was completely equilibrated with the tectono magmatic pile pseudo metamorphic fluid ii a cold 230 c low salinity fluid evolved meteoric water that mixed with and eventually overprinted the early moderately saline fluid responsible for granite muscovitization at 425 430 c later a second hydrothermal system was initiated by the percolation of heated meteoric water with very low salinity the system was by then at least 3 5 km deep and the fluid was heated to 300 c these characteristics are reminiscent of the ca 305 ma episyenitic hydrothermal system known elsewhere in the n limousin

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

the velay dome french massif central melt generation and granite emplacement during orogenic evolution this paper is a synthesis of available data on the velay dome that include both small and large scale lithologic and structural mapping strain analysis isotope geochemistry geochronology and pressure temperature estimates the velay dome one of the largest granite migmatite domes of the variscan belt formed during orogenic collapse at around 300 ma its study allows an assessment of the thermal and geodynamic context leading to voluminous crustal anatexis of the variscan orogenic crust a first melting stage developed in connection with south verging thrust zones during the early carboniferous leading to a crustal thickening estimated at 20 km minimum the involvement of fertile lithologies and the intrusion of plutons of deep origin contributed to the development of water saturated melts the volume of biotite granite extracted from melt during this period was limited the second phase of melting corresponded to generalized melting of gneiss achieved by biotite dehydration melting reactions and accompanied by the generation of cordierite bearing granites at this stage crustal scale detachment faults were activeandpartiallyobliteratedtheearlierstructures thenewstructureswereprogressivelytiltedtotheverticalatthemarginofthe velay dome due to the southward and lateral ballooning of the granitic dome the reconstructed p t path indicate that the large volume ofmelt produced was a consequence ofa significant increase in temperature at the onset of biotite dehydration melting at the base of the crust this melting event is coeval with granulite facies metamorphism associated to underplating of mantle derived magmas as suggested by the geochemical signature of late paleozoic lower crustal xenoliths sampled by cenozoic volcanoes and withtheisotopicsignatureofthelategraniticintrusions accordingly itisproposedthatasthenosphericupwellingwasresponsible for the temperature increase favoring melting of hydrous minerals d 2001 published by elsevier science b v

auriferous mineralization in some shear zones a three stage model of metallogenesis studies of numerous shear zones hosting auriferous quartz veins suggest a three stage model of formation for such veins these stages are indicative of the progressive increase of the gold concentration with the evolution of the shear zone and comprise an early stage with invisible gold an intermediate stage with finegrained gold and a late stage with gold nuggets the early stage consisting of two consecutive episodes comprises the development of the shear zone sensu stricto which results in the formation of a structure characterized by mylonite this structure subsequently acts as a drain for hydrothermal fluids under the combined action of deformation and hydrothermal circulation the rocks in this structure undergo mineralogical transformations that depend on the initial composition of the rock these phenomena occur with increasing intensity towards the cores of the structures the sites of substantial silicification and sulfidation gold is first fixed in the crystal lattice of pyrrhotite up to 30 ppm of gold disseminated throughout the structure as a whole in the core it is destabilized to pyrite marcasite during the second episode of this early stage and the gold thus freed is trapped in the lattice of locally abundant ferriferous sulfides pyrite arsenopyrite up to 1 6 wt gold and berthierite at the intermediate stage again consisting of two consecutive episodes the shear zone can generate openings allowing the emplacement of lenses and veins of milky white quartz when these various veins are in turn crushed by continuing tectonic activity microsaccharoidal quartz forms by cataclasis and acts as a receptacle for the gold mineralization therefore textbook auriferous quartz veins are only one particular aspect of this stage characterized by the appearance of visible native gold caused by destabilization of the auriferous sulfides of the early stage this gold is generally very fine grained several micrometres and relatively silverfree the late stage i e the fifth episode in a brittle domain can occur within a structure of the early or intermediate stage it corresponds to a stage of opening during which stockworks of quartz or of quartz and carbonate form causing in situ remobilization of the auriferous mineralization from the earlier stages this phenomenon is accompanied by the appearance of gold nuggets several millimetres in size very often rich in silver electrum

metallogenesis in the french part of the variscan orogen part i u preconcentrations in pre variscan and variscan formations a comparison with sn w and au abstract in the french part of the variscan orogen the pre variscan metamorphic rocks and the variscan granites resulting from pure crustal recycling are enriched in uranium and tin data on tungsten and gold are too scarce for a reasonable estimation of the average concentrations of these elements to be obtained the spatial distribution of uranium preconcentrations is transverse to the main variscan structural domains and the distribution of the sn w mineralizations prompts the same conclusions u sn w and au metal enrichment in crustal domains in europe seems to result from late proterozoic to early palaeozoic basic and acid magmatism for uranium at least two main stages of partial melting early palaeozoic and late carboniferous lead to enrichment of the metal in peraluminous leucogranites these leucogranites represent the main source of late variscan uranium and tin within the peraluminous granitic complexes the main metal source is related to the late intrusion of small granitic cupolas strongly enriched in metals in the case of uranium the efficiency of the hydrothermal remobilization depends on the proportion of metal located in uraninite which is easily leachable by hydrothermal solutions the emplacement of these cupolas is structurally controlled in well defined areas by large shear zones these shear zones are continuously active during the successive emplacement of the granitic magmas most of the hydrothermal uranium mineralization is related to the permian reactivation of the shear zones in a brittle stage these zones then channel the hydrothermal fluids involved in the last metal concentration step leading to the formation of the orebodies the efficiency of metal remobilization is dependent on a close spatial relationship between magmatic structures enriched in uranium and brittle structures channeling the hydrothermal fluids

zoned hydrothermal alteration and genesis of the gold deposit at le chatelet french massif central hydrothermal alteration envelopes in the walls of the le chatelet au arsenopyrite deposit are superimposed on a regional chlorite phengite assemblage the outer parts of the envelopes consist of a kaolinite li bearing tosudite siderite alteration facies whereas the inner parts comprise an illite siderite and or pyrite pyrrhotite alteration facies accompanied by quartz near the feeder conduit transition between the two facies is gradational the economic gold bearing arsenopyrite associated with rare pyrrhotite loellingite pyrite and marcasite is concentrated in siliceous veins and disseminated in illitized and silicified wall rock analyses of fluid inclusions in quartz show two distinct fluid generations early primary to secondary volatile rich co 2 ch 4 fluids mainly in metasomatic quartz and migmatite quartz included in the mineralized veins and late secondary aqueous and slightly saline 5 4 wt nacl equiv solutions distributed widely throughout the hydrothermal facies spatial relationships between the volatile rich fluids and the hydrothermal facies indicate that the fluids were contemporaneous with the first stages of hydrothermal activity their presence as primary inclusions within arsenopyrite bearing metasomatic quartz suggests that they were also involved at a later stage with arsenopyrite deposition the variable co 2 ch 4 ratios of these early fluids reflect possible effervescence and trapping at a temperature lower than or equal to 270 degrees c the stability of the alteration minerals and fluid inclusions indicates that the temperature of hydrothermal activity at le chatelet ranged between 280 degrees and 180 degrees c the arsenopyrite association with pyrrhotite and loellingite or pyrite indicates that oxygen fugacities during sulfide deposition were very low log fo 2 45 to 39 at 250 degrees c compatible with the estimate made from the co 2 ch 4 ratio of the early inclusions logf sub o 2 38 5 the presence of illite reflects neutral to slightly alkaline ph conditions thermodynamic modeling carried out between 100 degrees and 250 degrees c shows that a single stage zoned alteration in both the outer and inner parts of the envelopes facies 2 o and 2 i would result from differential dissolution of the protolith minerals in particular a rapid dissolution of plagioclase due to temperature variations in the hydrothermal system and flow gradient comparison between the deduced age of mineralization stephanian a b and that of the regional silico albitic to albitic and lithium rich granite magmatism suggests the latter as a possible heat and fluid source indicated by abundant li at le chatelet nonequilibrium deposition of the au arsenopyrite mineralization during an abrupt opening phase along brittle faults is supported by well developed breccia in conduits cemented by fine grained arsenopyrite bearing quartz and a well developed patchy zoning of the au arsenopyrite the new data obtained from fluid inclusions suggest two possibilities for the hydrothermal activity at le chatelet single fluid evolution with a late overprint of the hydrothermal alteration by aqueous fluids or two fluid evolution with introduction of the aqueous fluid during a phase of opening

the biards sb au bearing shear zone massif central france an indicator of crustal scale transcurrent tectonics guiding late variscan collapse structural textural mineralogic microthermometric and isotopic data obtained on the biards sb au district located in the south limousin area massif central france provide new evidence for the tectonic setting of stibnite and gold deposition during the late orogenic variscan collapse in western europe a variety of geologic and geophysical data define a major crustal scale left lateral strike slip shear zone that acted as a trap for sb au mineralization during these movements fault fill veins and then extensional veins of limited extent were created along rheologic heterogeneities that focused brittle deformation geometry and orientation of both vein sets indicate two main episodes of emplacement of ore related to a progressive wrench movement in response to an counterclockwise rotation of the regional stress axes both veins sets are filled by quartz and sulfides in three stages an early arsenopyrite pyrite stage with fine grained quartz a sphalerite pb sb sulfosalt native gold stage with comb quartz and a late stibnite berthierite stage with comb quartz combined fluid inclusion and stable isotope studies on quartz muscovite and sulfides reveal that the three stages result from the continuous evolution of a deep seated hydrothermal system in which the latest stages mixed with meteoric water these data argue for cooling and a pressure drop during ore deposition and indicate a shallow level of emplacement for sb au veins a comparison of the biards deposits with other upper paleozoic gold and antimony deposits in the massif central the western variscides and elsewhere in the world illustrates their common association with crustal scale strike slip shear zones the upper carboniferous age 305 300 ma of the sb au deposits indicates that they formed during the early stages of the postthickening extension of the variscan lithosphere sb au bearing strike slip shear zones resulted from the late variscan crustal collapse guided by the very large scale dextral shearing between central europe northern africa and the northern appalachians and associated with the opening of the paleo tethys and the closing of the theic ocean during the late paleozoic gondwana laurasia translation

seasonal and regional characterization of horizontal stirring in the global ocean 1 recent work on lagrangian descriptors has shown that lyapunov exponents can be applied to observed or simulated data to characterize the horizontal stirring and transport properties of the oceanic flow however a more detailed analysis of regional dependence and seasonal variability was still lacking in this paper we analyze the near surface velocity field obtained from the ocean general circulation model for the earth simulator ofes using finite size lyapunov exponents fsle we have characterized regional and seasonal variability our results show that horizontal stirring as measured by fsles is seasonally varying with maximum values in summer time fsles also strongly vary depending on the region we have first characterized the stirring properties of northern and southern hemispheres then the main oceanic basins and currents we have finally studied the relation between averages of fsle and some eulerian descriptors such as eddy kinetic energy eke and vorticity over the different regions

geochronological and geochemical characterization of magmatic hydrothermal events within the southern variscan external domain cevennes area france geochronological mineralogical and geochemical analyses have been focussed on the mont lozere borne plutonic complex and surrounding rocks cevennes french massif central in which b w sn and as au sb mineralization is encountered two main results are highlighted 1 the existence of a 301 306 ma magmato hydrothermal event unrelated to the emplacement of the pont de montvert borne plutonic body at 316 ma 2 the magmatic and hydrothermal features are strongly associated both in time and in space thus demonstrating an intimate connection between mineralizing processes and magmatism in this part of the french massif central we also show that mineralization and associated hydrothermal occurrences do not correspond to a simple and single geochemical signature and that a contamination model must be invoked in order to account for the complexity of isotopic results this study demonstrates that the application of the o and h isotopic signatures as tracers of the source and nature of fluids in an orogenic context requires some specific care finally a model of the tectonic magmatic hydrothermal evolution of the study area is suggested in which we discuss two alternative scenarios the first one implies the existence of two different hydrothermal mineralizing events bo w sn and as au sb ones the second one suggests the same source for all hydrothermal and mineralized structures

the p v t x f o2 evolution of h2o co2 ch4 bearing fluid in a wolframite vein reconstruction from fluid inclusion studies aqueous carbonaceous and later pure aqueous fluid inclusions in quartz from a ferberite fe 95mn 05 wo4 vein within the low grade metamorphic aureole of the borne granite french massif central have been studied by microthermometry and raman spectrometry the bulk v x properties of the aqueous carbonaceous inclusions have been derived using the equation of state of heyenet al 1982 for the low temperature co2 ch4 system a p t path has been proposed for their trapping using the equations of state of jacobs and kerrick 1981a for the h2o co2 ch4 system two main episodes were reconstructed for the history of the aqueous carbonaceous fluid 1 primary h2o co2 ch4 vapourrich inclusions in quartz indicated the early circulation of a low density fluid 65 mole h2o 34 mole co2 1 mole ch4 and traces of n2 d 0 35 gcm 3 at around 550 50 c and 700 100 bar fluid cooled approximately isobarically to 450 400 c and was progressively diluted by h2o with a concomitant increase in density the fo2 of the h2oco2 ch4 fluid estimated from the equilibrium co2 2h2o ch4 2o2 first ranged from 10 22 to 10 27 bar close to the q f m buffer within analytical errors these values were consistent with the presence of graphite in equilibrium with the fluid 2 a drop in pco2 and therefore a drop in fo2 was recorded by the secondary liquid rich inclusions in quartz the inclusions formed at and below 400 c were composed of h2o and ch4 only and fo2 at that stage was below that fixed by the graphite fluid equilibrium this second episode in the fluid rock system could be explained by the drop of temperature below the blocking temperature of the graphite fluid equilibrium according to this interpretation the blocking of the graphite fluid equilibrium occurred at t 370 c and probably at 400 c on account of the pressure correction mass spectrometric data show that ferberite contains h2o co2 and ch4 in fluid inclusions which lie in the gap of the v x properties of the aqueouscarbonaceous fluid in quartz deposition of ferberite probably occurred at around 400 c the previously inferred blocking temperature resulting from either the drop in pco2 the drop fo2 and or the related ph increase it is concluded that the existence of a blocking temperature for the graphite fluid chemical equilibrium may be a critical factor for maintaining a stable fluid pressure gradient in geothermal systems occurring under greenschist facies conditions in graphite bearing rocks

geochemical evidence for a multistage magmatic genesis of ta sn li mineralization in the granite at beauvoir french massif central the beauvoir rare metal granite at the southern edge of the echassieres granite massif french massif central is mineralized by disseminated ta sn and li the granite body formed from an independent melt which was emplaced soon after the main granite facies of the massif the geochemistry of the beauvoir granite relates it to the phosphorus rich type of ta bearing granites and geochemical and mineralogical data establish its similarity to pegmatite melts the beauvoir granite results built up from two distinct intrusions both intrusions are cogenetic and were emplaced within a short time interval a geochemical study of a deep drill hole bored down to 900 m under the surface to the floor of the granite sheet with selected key zones observed in greater detail has allowed the recreation step by step of a long process leading to the economic concentration of ta sn and li the comparison of major element trace element and isotopic geochemistry with the petrology makes it possible to unravel the numerous processes contributing to the ore formation local events need to be determined before main events can be identified constraints on the interpretation of the ore genesis are provided by a careful observation of data interpretations of these observations are strongly backed up by experimental data e g keppler 1993 the source rock was most likely enriched in sn and w at about 10 ppm precluding a slightly enriched granitic melt by partial melting it is not known whether the melt was already enriched in ta but this is not a prerequisite subsequently deep seated fractional crystallization produced granitic magma similar in composition ta nearly equal 9 m to the adjacent colettes granite and further fractionation processes led to a melt already strongly enriched in ta ca 40 ppm strongly peraluminous and enriched in the depolymerizing elements f and p the viscosity was therefore very low allowing easy ascent during transport small scale fluctuations in the viscosity were amplified by upward movement leading to an f and p rich melt preceding the magma batch this less viscous melt again underwent crystallization while moving through the upper

a two stage exhumation in western french massif central new geochronological evidences of syn collisional extension abstract systematic 40ar 39ar dating of granitoids and surrounding metamorphic rocks has been performed in the limousin area w of french massif central in order to provide new constrains concerning its late orogenic thermal and tectonic evolution the same 40ar 39ar age of ca 335 338 ma is found on the present day outcrop surface for most of the limousin metamorphic rocks hence indicating that the entire massif was uniformly cooled down at this period these ages are very close to the 40ar 39ar ages proposed in the sioule and brevenne areas in the literature but also to the coupled tectonic and magmatic activity in the south limousin these results help to document the existence of two exhumation stages each one associated with an extensional stage a first exhumation stage has been active at ca 335 340 ma available data indicate that this extension and associated exhumation and cooling occurred over a distance of more than 200 km in the internal zones of the belt whereas compression was still active in the northern and southern parts of the belt a second exhumation stage may be related to the late orogenic extension and collapse of the belt at ca 310 305 ma and is characterized by the uplift of the st sylvestre complex by unroofing of the overlying metamorphic and granitoidic formations

uranium mobilization from the variscan questembert syntectonic granite during fluid rock interaction at depth leaching of uranium fertile granites represents a major source of uranium as uraninite is easily dissolved in oxygenated aqueous solutions this phenomenon is well documented at surface conditions but remains poorly documented for granites at depth in this study we propose that surface derived oxidized hydrothermal fluids leached uranium from uraninite in the questembert peraluminous granite at temperatures greater than 70 to 160 c this variscan synkinematic granite is characterized by widespread and pervasive development of vertical and permeable c s structures these structures likely facilitated the infiltration of oxidized hydrothermal fluids from the surface their circulation at depth and the subsequent fluid rock interaction in the granite published oxygen isotope data shows that it has undergone subsolidus fluid rock interaction dated between 312 and 303 ma by 40ar 39ar analyses on muscovite these interactions were responsible for the concomitant decrease of the feldspar 18o values together with uranium leaching mass balance calculations suggest that this hydrothermal event could have liberated several hundred thousand tonnes of uranium from the questembert granite the liberated uranium may have been dispersed in early permian intramountainous basins and therefore disseminated over large areas this study emphasizes that the efficiency of uranium leaching may be directly related to the extent of subvertical structure development in granites emplaced along strike slip shear zones which allow for downward infiltration of oxidized surface derived fluids a specific and systematic sampling is required to better constrain the proposed model

late orogenic carboniferous extensions in the variscan french massif central the variscan french massif central experienced two successive stages of extension from middle carboniferous to early permian in the northern massif central the first stage began in the late visean immediately after nappe stacking and is well recorded by namurian westphalian synkinematic plutonism the middle carboniferous leucogranites widespread in the nw massif central limousin and sioule area were emplaced within a crust extending along a ne sw direction at the same time the hanging wall or gueret extensional allochton moved toward the se several examples of the synextensional plutonism are also recognized in central limousin saint mathieu dome la porcherie and cornil leucogranites these examples illustrate the relationship between granite emplacement and crustal scale deformation characterized by nw se stretching and ne sw shortening in the central and southern massif central cevennes ch taigneraie and margeride areas plutonism is dominantly granodioritic and exhibits the same structural features nw se maximum stretching and overturning to the se middle carboniferous namurian westphalian extension was parallel to the variscan belt both in the massif central and southern armorican area this extensional regime was active from the late visean in the north while compression dominated in the southernmost domains montagne noire and pyrenees the second extensional stage occurred from late carboniferous to early permian this event was responsible for the opening of intramontane coal basins brittle deformation in the upper crust and ductile normal faulting localized on the margin of cordierite granite migmatite domes data from the coal basins show that the half graben is the dominant structural style except for basins located along submeridianal left lateral faults which have pull apart geometries late carboniferous extension occurred along the ne sw direction the ne sw maximum stretching direction can be found in the whole massif central but is more developed in the eastern part the extensional direction is transverse to the general trend of the belt and top to the ne shearing is dominant correlations of these two extension directions with neighboring variscan massifs are discussed

conditions of gold bearing arsenopyrite crystallization in the villeranges basin marche combrailles shear zone france a mineralogical and fluid inclusion study the au as sb deposits from the marche combrailles northwestern part of the french massif central district are spatially associated to the marche combrailles shear zone host rocks are either visean volcano sedimentary tuffs or formations of the paleozoic basement presenting a greenschist facies assemblage which resulted from extended hydrothermal metamorphism the strong macro and microfracturing that affected the villeranges basin as well as the surrounding formations is related to late tectonic reactivation of the marche combrailles shear zone around 300 315 ma which favored intense circulation of hydrothermal fluids solubilization transport and deposition of au as and sb occurred together with specific rock alteration along faulted zones on distances ranging from several centimeters to a few decimeters wall rocks are affected by complete bleaching resulting from chlorite replacement by phengite t 240 degrees or 30 degrees c followed by complete replacement of the previous minerals by an illite quartz ankerite pyrite assemblage gold is carried in a combined state with arsenopyrite occurring either within a diffuse network of quartz ankerite veinlets or as isolated crystals in the altered tuff arsenopyrite crystals are characterized by important enrichments either at their periphery or within microcrack networks affecting their whole mass mass balance calculations on fresh and altered rocks show that during the ore stage hydrothermal fluids contributed h 2 o co 2 and s au sb to authigenic minerals and leached almost completely na ba sr si fe ca mg al ti and k exhibit similar contents in fresh and altered rocks and are redistributed among the authigenic minerals ore fluids are characterized by an aqueous composition a rather low salinity and temperatures ranging from 160 degrees to 200 degrees c ore deposition occurred at a low ph indicated by the complete alteration of the host rocks into a quartz illite association and at a low f sub o 2 intermediate between the f sub o 2 fixed by the hematite magnetite and ni nio oxygen buffers gold deposition probably resulted from a destabilization of au bisulfide or thioarsenide complexes during the sulfide crystallization thus the au as sb ores in the villeranges area result from a single hydrothermal stage and occur during a late stage of brittle deformation significantly posterior to the continental basin formation and the acid volcanism ore fluid circulation is linked mostly to the fluid convection produced by the abnormal heat flow characterizing the marche combrailles shear zone

5 late variscan mineralizing systems related to orogenic processes the french massif central abstract the french massif central constitutes an exceptional study area due to the diversity of its metallic deposits its internal position in the variscan belt and the abundance of available geological geophysical and metallogenic data obtained within the geofrance 3d programme the deposits formed towards the end of the orogenic evolution represent the economic products of two distinct mineralizing systems a au sb hydrothermal system and a w sn and rare metals magmatic hydrothermal system which were simultaneously active during a short time span between ca 310 and 300 ma two types of gold deposit can be distinguished on the basis of their depth of emplacement deep seated gold deposits developed under lithostatic to hydrostatic pressure during rapid exhumation and shallow gold deposits emplaced under hydrostatic pressure with no significant uplift deposits of w sn and rare metals were emplaced in the upper crust during final crystallization of specialized magmas after their rapid ascent perhaps enhanced by simultaneous regional uplift the gold bearing systems are associated with a complex network of re activated crustal scale faults initially active during the period between 335 and 315 ma normal motion along the faults coeval with 335 to 315 ma granite migmatite domes played a major role in the 3d distribution of the hydrothermal plumbing system gold and related metals were carried within huge hydrothermal cells which reached ca 100 km by 10 km in area and 30 km in depth in contrast granites rich in magmatophile elements w sn rare metals generated smaller hydrothermal cells 10 km by 10 km in area and

late visean hidden basins in the internal zones of the variscan belt u pb zircon evidence from the french massif central u pb zircon ages from volcanic rocks interbedded in two upper carboniferous basins of the variscan massif central france document the presence of an unexpected late visean sedimentary sequence zircons extracted from a fireclay at the base of the sedimentary pile in the bosmoreau basin provide an age of 332 4 ma identical to the 333 2 ma age obtained from a rhyolitic lava sampled in the decazeville basin in addition both volcanic rocks contain inherited or xenocrystic zircons reflecting the ages of the basement and or source components the bosmoreau fireclay contains a limited amount of xenocrystic zircons having ages of ca 390 400 ma whereas inheritance is much more abundant in the decazeville rhyolite which includes a mesoproterozoic source component 1 0 ga the ca 330 ma ages demonstrate that these two basins with mainly stephanian sedimentary infill were actually initiated during the late visean and are the first known upper crustal evidence of the visean westphalian extensional event recognized previously only in midcrustal plutonic bodies moreover the location of these basins in the hanging wall blocks of two major fault systems i e the argentat and sillon houiller faults suggests a genetic link between fault motion and basin opening as early as the late visean these results indicate that magmatism and associated volcanism block movement along major crustal scale faults erosion and basin opening were contemporaneous and shed new light on the visean westphalian synconvergent extension that was previously understood only as a deep seated event

multistage deformation of au quartz veins laurieras french massif central evidence for late gold introduction from microstructural isotopic and fluid inclusion studies abstract the relative chronology of fluid migration quartz and au deposition in a silicified fault from the main au district laurieras st yrieix district from northern french massif central has been determined from microstructural fluid inclusion isotopic and ore mineral evidences three main stages of fluid circulation microfracturing and quartz crystallization and ore deposition were distinguished on the basis of textural relationships and the pressure temperature and composition of the palaeo fluids 1 a series of early fluid events was responsible for the localized drainage of retrograde metamorphic fluids along the main fault and the subsequent sealing by milky and microcrystalline quartz preceeded the main au ore stages early fluids were aqueous carbonic trapped under lithostatic to sublithostatic pressures at temperatures in the range 350 500 c subsequently several types of microstructures were developed in the early quartz matrix 2 ns microfractures filled by clear quartz arsenopyrite and boulangerite i contain significant refractory gold concentrations clear quartz formed from aqueous carbonic fluids of lower densities than those of the earlier fluids significant pressure drops down to pressures around 55 mpa were responsible for a local immiscibility of the aqueous carbonic fluids at temperatures of 340 20 c 3 the main ore stage is characterized by the formation of dense sets of sub vertical ew microfractures healed fluid inclusion planes in quartz and filled by ore minerals native gold galena and boulangerite ii when they crosscut earlier sulfides the fluids are aqueous with low and decreasing salinity and probable trapping temperatures around 230 c isotopic data obtained on microfissured quartz indicate these dilute aqueous fluids may be considered as meteoric waters that deeply infiltrated the crust late microfissuring of a mesothermal quartz vein originally barren only with pyrite and arsenopyrite appears to be the main factor controlling gold enrichment it can be related to late hercynian deformational stages disconnected from the early fault formation and silicification these late stages which affected the hercynian basement during its uplift are of critical importance for the formation of au ores we concluded that this type of au ore formed under rather shallow conditions is distinct from those generally described in most mesothermal au veins