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

base metal mineralization in the kanmantoo group south australia a regional sulfur isotope study a number of base metal sulfide deposits are hosted by multiply deformed metagraywackes of the kanmantoo group south australia sulfide mineralization is of three main types 1 copper deposits which include the kanmantoo and bremer mines south hill prospect and several minor occurrences 2 lead zinc deposits at the aclare wheal ellen and strathalbyn mines 3 pyrite pyrrhotite mineralization of the nairne pyrite deposit and other pyritic schists within the kanmantoo group copper deposits are discordant and pipelike and contain chalcopyrite pyrrhotite magnetite and pyrite as the major opaque minerals lead zinc deposits lack magnetite and are apparently concordant beds of pyritic schist reach an overall thickness of 100 m and persist for as much as 32 km sulfur isotope ratios for ore shoots from the kanmantoo mine excluding one sample range from 3 5 to 12 4 per mil and average 8 0 or 2 3 per mil average values for individual ore shoots differ by as much as 4 per mil 34 s enrichment is noted at kanmantoo among some low grade sulfide mineralization delta 34 s 16ppm and supergene pyrite from the main orebody delta 34 s 39 6ppm the minor deposits of copper and lead zinc all show a similarity in sulfur isotope composition the delta 34 s values for sulfides range from 4 7 to 6 5 per mil and averages for these deposits lie close to zero per mil the pyritic schists are strongly depleted in 34 s and contain biogenic sulfur formed in an anoxic marine environment individual sulfides range from 19 9 to 11 9 per mil isotopic disequilibrium is a characteristic feature of coexisting sulfide pairs from many of the deposits despite subsequent amphibolite facies metamorphism 3 8 kb and 500 degrees 600 degrees c sphalerite galena pairs at the wheal ellen mine yield consistent calculated temperatures 379 degrees c and may have reequilibrated below the metamorphic peak base metal mineralization is accounted for by subsurface copper and sea floor lead zinc accumulation from hydrothermal fluids circulating to or expelled from depths of at least 3 to 4 km within the sedimentary basin this minimum depth estimate for fluid extraction corresponds to the stratigraphic interval occupied by pyritic schists which underlies the horizon of base metal mineralization sulfur isotope evidence suggests that sulfur was extracted by hydrothermal leaching from the pyritic schists sulfur within the base metal mineralization shows a mixed cambrian seawater and biogenic pyritic schist source variations in mixing proportions of the two forms of sulfur are responsible for differences among ore lenses at the kanmantoo mine and individual deposits within the region

continental growth and convergence related arc plutonism in the mesoarchaean evidence from the barberton granitoid greenstone terrain south africa u pb zircon ages from trondhjemitic gneisses of the badplaas domain located in the southwestern parts of the mesoarchaean barberton granitoid greenstone terrain of south africa document a hitherto largely unrecognized plutonic event the geochronological results indicate the addition of juvenile felsic crust over a period of 60 myr between ca 3290 and 3230ma and prior to the main collisional event in the granitoid greenstone terrain at 3230 ma the timing and duration of plutonism together with the structural and compositional heterogeneity of the badplaas domain suggest that the badplaas domain represents part of a convergence related magmatic arc on a regional scale the spatial and temporal relationships between plutonism metamorphism and deformation are interpreted to preserve an almost complete inventory of a mesoarchean arc trench system this includes 1 the 3290 3230ma convergence related magmatic arc 2 a largely coeval deposition of the back arc type volcano sedimentary 3260 3225ma fig tree group of the barberton greenstone belt 3 relics of the underplated crust preserved in high p low t rocks to the immediate east of the magmatic arc juxtaposed against 4 high t medium p rocks of the overlying plate along the major crustal structure of the inyoni shear zone these findings corroborate the notion that thermal and rheological conditions on the early earth supported at least locally crustal growth and the extraction of buoyant felsic crust along convergence related magmatic arcs

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

phase relationships in buchan facies series pelitic assemblages calculations with application to andalusite staurolite parageneses in the mount lofty ranges south australia low pressure medium to high temperature buchan type regional metamorphism of pelitic rocks in the mount lofty ranges south australia is defined by the development of biotite staurolite andalusite fibrolite prismatic sillimanite and migmatite zones k feldspar makes its first appearance in the prismatic sillimanite zone and here we restrict our discussion to lower grade assemblages containing prograde muscovite concentrating particularly on well developed andalusitestaurolite parageneses in general the spatial distribution and mineral chemical variation of these assemblages accord with the predictions of petrogenetic grids and p t and t xfe pseudo sections constructed from the internally consistent data set of holland and powell 1990 in the system kfmash assuming a h2o 1 although analysed white mica compositions are systematically more aluminous than predicted importantly the stability ranges of most critical assemblages predicted by these grids and pseudo sections coincide closely with p t estimates calculated using the data set of holland and powell 1990 from the mount lofty ranges assemblages with the exception of mn in garnet and zn in one staurolite cordierite muscovite assemblage non kfmash components do not significantly appear to have affected the stability ranges of the observed assemblages an apparent local reversal in isograd zonation in which andalusite first appears down grade of staurolite suggests a metamorphic field gradient concave towards the temperature axis and together with evidence for essentially isobaric heating of individual rocks is consistent with the exposures representing an oblique profile through a terrain in which heat was dissipated from intrusive bodies at discrete structural levels

prolonged magmatism and time constraints for sediment deposition in the early archean barberton greenstone belt evidence from the upper onverwacht and fig tree groups the single zircon evaporation shrimp ion microprobe and conventional dissolution techniques were used to determine 207pb 206pb and u pb ages on samples from the upper onverwacht and fig tree groups of the early archean barberton greenstone belt south africa zircons from dacitic rocks of the upper hooggenoeg formation yield ages of 3445 3452 ma a tuff in the basal kromberg formation has a mean age of 3416 5 ma a tuffaceous band 5 cm thick in the uppermost kromberg formation contains igneous zircons with a mean age of 3334 3 ma the 1700 m section of kromberg formation between these two samples is composed of basaltic lavas minor komatiites and cherty metasediments the overlying mendon formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m a cherty stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 3 ma the basal fig tree group has units as old as 3259 3 ma and upper units in the fig tree are as young as 3225 3 ma xenocrystic zircons in the upper onverwacht and overlying fig tree group samples suggest that successive igneous units inherited zircons from underlying units and that over several hundred million years episodes of intermediate to felsic igneous activity took place at 20 40 ma intervals structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the barberton greenstone belt

precise u pb zircon dating of a post d2 meta dolerite constraints for rapid tectonic development of the southern adelaide fold belt during the cambrian numerous morb like mafic dykes plugs and sills intruded the kanmantoo group and older meta sediments of the southern adelaide fold belt from pre d1 to post d2 during the delamerian orogeny a post d2 meta dolerite yields a precise u pb zircon age of 510 2 ma this age combined with a published shrimp zircon age of 526 4 ma reveals a rapid tectonic evolution of the fold belt during the cambrian events including the thick kanmantoo group9s deposition major mafic magmatism low p high t regional metamorphism and multiple deformations all took place from about 526 to 510 ma this rapid evolution supports the suggestion that the heat source for the metamorphism originated from continental extension associated with mantle upwelling and mafic magmatism

early palaeozoic foreland thrusting and basin reactivation at the palaeo pacific margin of the southeastern australian precambrian craton a reappraisal of the structural evolution of the southern adelaide fold thrust belt abstract regional and detailed structural mapping kinematic analysis and balancing and restoration of cross sections has lead to a re interpretation of the adelaide fold belt in south australia in this belt sedimentary rocks of the late proterozoic adelaidean and early cambrian normanville and kanmantoo sequences were deposited during at least two episodes of subsidence related to major crustal attenuation contraction and crustal thickening accompanied by granitoid intrusions are related to the cambro ordovician delamerian orogeny during this event both basins were reactivated inverted and the sedimentary rocks are now incorporated in a wnw verging foreland fold and thrust belt at the margin of the proterozoic southeast australian craton maximal shortening of the orogen was around 55 within the adelaidean basin shortening is dominantly accommodated by major mylonitic shear zones and reverse faults in the cambrian kanmantoo basin several thrusts are demonstrably reactivated growth faults across which thickness changes of cambrian sedimentary rocks are revealed by balancing and restoration of cross sections owing to the steep easterly dips of these faults in the western part of the kanmantoo basin lateral shortening of up to 58 is largely accommodated by intense folding and fold axial planar flattening strain further east strain magnitudes wane and the overall shortening of around 30 is accommodated by both discrete thrust zones and regional folds the basin reactivation along the australian part of gondwana s palaeo pacific craton margin reflects the rapid transition from passive to active tectonism which possibly is a consequence of the disintegration of the formerly conjugate margins of gondwana and laurentia

decoupled geochemical behavior of as and cu in hydrothermal systems cu rich and as rich growth zones in pyrite provide new insights into the composition of late stage magmatic fluids and their host hydrothermal ore deposits the pyrite is from the pueblo viejo dominican republic and yanacocha peru high sulfidation gold silver deposits which are thought to form from hydrothermal systems that interacted with magmatic vapor plumes electron microprobe analysis secondary ion mass spectrometry and elemental maps show that pyrite the most common sulfide mineral in both deposits contains three different types of growth zones 1 as rich zones that are enriched in au ag sb te and pb 2 cu rich zones with significantly lower concentrations of these elements and 3 barren pyrite zones with no other elements these zones are interpreted to result from mixing between the pyrite forming fluid and vapors that invaded the main hydrothermal system episodically comparison to experimental studies of elemental partitioning and analyses of fumaroles and fluid inclusions from magmatic hydrothermal systems suggests that the as rich vapor formed at high and possibly magmatic temperatures whereas the cu rich vapor formed at lower temperatures possibly during migration of the original magmatic vapor the presence of finely spaced multiple growth zones in pyrite suggests that the composition of at least high sulfidation hydrothermal systems can be affected intermittently and repetitively by vapors probably from underlying magmas

a model for low pressure facies metamorphism during crustal thickening low pressure metamorphic facies i e high t p ratios are widespread in a wide range of tectonic settings explanations offered for the occurrence of these facies include extensional and or magmatic models however these fail to explain that the low p facies metamorphism is commonly coeval with a phase of pervasive crustal thickening with t p ratios increasing during or slightly lagging behind the thickening we propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion thinning of the mantle lithosphere

stress analysis post orogenic extension and 3 01 ga gold mineralisation in the barberton greenstone belt south africa gold deposits in the barberton greenstone belt bgb have formed along a common network of brittle ductile shear zones that developed during extension and emplacement of felsic porphyry dykes at 3015 ma at a deposit scale mineralised shears occur in en echelon arrays distributed as riedel p and anti riedel shears with envelopes that connect groups of mine workings along nnw ene ne and ese trends the dip of the mineralised shear envelopes is generally moderate to steep 50 80 the existence of the directional alignment of gold deposits at the scale of the greenstone belt has been demonstrated with fry analysis stress inversion for mineralised shear zones from workings near 28 mines using a bingham axial distribution and an optimized right dihedron approach provide near identical results with 1 vertical and 3 horizontal nw the stress regime is generally radial extensional or pure extensional at the scale of the greenstone belt the mineralised shear envelopes comprise two sets of near orthogonal trends positioned symmetrically with respect the measured far field stress 1 a nw and ne trending pair with the latter accommodating pure normal slip and the former both dextral or sinistral strike slip and 2 a nnw and ese trending pair accommodating dextral transtensional and sinistral transtensional movements respectively the fact that all mineralised shears developed systematically in response to a similar far field stress suggests that mineralisation formed during a single tectonic stage pb pb ages of 3017 1 17 8 ma and 3008 8 16 ma from a syn mineralisation porphyry dyke near golden quarry indicates gold deposition at 3015 ma i e 85 ma after the craton had stabilised older porphyry dykes intruded near sheba and consort mines have been dated at 3100 ma and 3260 ma respectively and provide a minimum and maximum age constraint on accretionary deformation in the area we propose that the main phase of gold enrichment in the bgb occurred after tectonic and thermal stabilisation of the crust at a time the craton was acting as a stable terrane mineralised brittle ductile shear zones allowed ingress of deeply sourced fluids during cratonic extension mine camps display a cluster spacing of 12 km which suggests a regular distribution of controlling shears possibly as a function of the elastic rigidity of the lithosphere rather than linking mineralisation to orogenic processes during greenstone belt accretion we link gold mineralisation to the opening of fault bounded intracratonic basins including the pongola and witwatersrand basins and concomitant igneous activity

field geochemical and u pb isotopic constraints from hypabyssal felsic intrusions within the barberton greenstone belt south africa implications for tectonics and the timing of gold mineralization abstract field mapping geochemical and u pb zircon isotopic data are reported for two undeformed feldspar quartz porphyries from within the central region of the barberton greenstone belt 3450 ma and from a porphyry at the deepest level of one of the largest gold mines in the barberton area fairview mine one porphyry the mudpools porphyry has an age of circa 3230 ma and provides a minimum age on unusual bif ferruginous shale hosted mudpool structures field mapping constraints and similar geochemical signatures between the mudpools porphyry and a second porphyry the ironstone pod porphyry suggest that 3230 ma can also be considered the minimum age of the equally unusual ironstone pods the two porphyries cross cut fig tree sediments which are regionally deformed d 2 and intrude up a thrust fault s suggesting pre 3230 ma ages for d 2 deformation and thrusting geochemical data trace elements ree indicate that these two porphyries are high level co magmatic equivalents of the nearby 3227 ma kaap valley tonalite which in part intrudes the lower sequences of the greenstone bell since the edge of the kaap valley pluton is also deformed by the d 2 deformational event either d 2 is diachronous or d 2 in the center of the belt is not equal to d 2 at the edge of the greenstone belt zircon from the fairview mine porphyry indicates a maximum age of 3126 21 ma 2 a gold mineralized shear which cross cuts the fairview porphyryl is responsible for hydrothermal alteration of the porphyry u pb analysis of hydrothermal rutile associated with gold from the porphyry gives an age of 3084 18 ma 2 and may represent the age of gold mineralization associated with this late shearing it appears that 3230 ma is a important age in early barberton greenstone belt history signifying intrusion of the kaap valley tonalite adjacent to the greenstone belt together with the emplacement of high level co magmatic equivalents porphyries of the kaap valley tonalite along regional thrust faults shear zones within the greenstone belt gold bearing hydrothermal fluids have utilized similar shear zones 150 ma after emplacement of both the porphyries in the central region of the greenstone belt and the kaap valley tonalite

ti diffusion in quartz inclusions implications for metamorphic time scales quartz inclusions in garnet from samples collected from the staurolite zone in central new england are zoned in cathodoluminescence cl the cl intensity is interpreted to be a proxy for ti concentration and the zoning attributed to ti diffusion into the quartz grains driven by ti exchange between quartz and enclosing garnet as a function of changing temperature the cl zoning has been interpreted using a numerical diffusion model to constrain the time scales over which the diffusion has occurred temperature time histories are sensitive to the presumed peak temperature but not to other model parameters the total time of the metamorphic heating and cooling cycle from around 450 c to the peak temperature 550 600 c back to 450 c is surprisingly short and encompasses only 0 2 2 million years for peak temperatures of 600 550 c the metamorphism was accompanied by large scale nappe and dome formation and it is suggested that this occurred as a consequence of in sequence thrusting resulting in a mid crustal ductile duplex structure

gold mineralization in as rich mesothermal gold ores of the bogosu prestea mining district of the ashanti gold belt ghana remobilization of invisible gold the bogosu prestea mining district of southwestern ghana is a 33 km section of the early proterozoic ashanti gold belt greenschist facies carbonaceous and carbonate bearing turbidites and greywackes and mafic dikes host numerous economic mesothermal gold deposits structurally higher ores in the bogosu concession have brittle deformation and consist of disseminated sulphide lodes in tectonically disrupted sedimentary rocks and carbonate altered mafic dikes most gold occurs as micrometre size particles in arsenian pyrite and as invisible gold in arsenian pyrite and arsenopyrite the structurally deeper ores of the adjoining prestea concession are associated with brittle ductile deformation and consist of extensive crack seal quartz veins and graphitic shear zones only minor amounts of invisible gold were detected in these deeper lodes gold occurs dominantly as abundant microscopic and larger particles in sulphide arsenide minerals and in gangue the gold distribution patterns revealed by sims microprobe analysis and ion maps emp and colour staining suggest that most of the primary gold in the bogosu prestea system precipitated in solid solution with sulphide arsenide minerals however post depositional concentration and redistribution occurred in increasing degree with 1 increase in metamorphic hydrothermal gradients in the gold system depth 2 decrease in the refractory properties of the host mineral and 3 increase in the amount of post depositional host mineral recrystallization and deformation gold evolved from primary solid solution within sulphide arsenide minerals to colloidal and micrometre size particles concentrated in voids fractures and internal grain boundaries and finally to microscopic and larger particles at sulphide arsenide grain margins and in the gangue assemblage the general conclusions presented here are applicable to as rich gold deposits of all ages worldwide the presence of gold in late fractures is insufficient evidence for late stage introduction of gold in mesothermal gold systems

silver isotopes as a source and transport tracer for gold a reconnaissance study at the sheba and new consort gold mines in the barberton greenstone belt kaapvaal craton south africa

low pressure regional metamorphism in the omeo metamorphic complex victoria australia the omeo metamorphic complex forms the southern end of the wagga metamorphic belt which is the main locus of palaeozoic low pressure metamorphism in the lachlan fold belt south eastern australia it comprises metamorphosed ordovician quartz rich turbidites originally derived from precambrian cratonic rocks prograde regional metamorphism occurred in the early silurian very soon after sedimentation had ceased the sequence of metamorphic zones with increasing grade is chlorite biotite cordierite andalusite k feldspar and sillimanite k feldspar migmatites occur in the sillimanite k feldspar zone but large bodies of s type granite were derived from rocks underlying the exposed ordovician sequence p and t estimates for the highest grade rocks are t 700 c and p 3 5 kbar indicating a very high p t gradient of 65 c km the high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre metamorphic back arc basin environment and intrusion of hot mantle derived magmas into the lower and middle crust regional retrograde metamorphism coincided with a general reheating of the crust in the siluro devonian accompanied by intrusion of many i type plutons and resetting of the k ar dates of some earlier plutons the omeo metamorphic complex was exposed to erosion at this time

pyrite in a sulfate poor paleoarchean basin was derived predominantly from elemental sulfur evidence from 3 2 ga sediments in the barberton greenstone belt kaapvaal craton abstract multiple sulfur isotope variability in archean sedimentary rocks provides constraints on the composition of the earth s earliest atmosphere the magnitude and sign of mass independent anomalies reflect not only atmospheric processes but also transformations due to the archean marine sulfur cycle prior to preservation into sedimentary pyrite the processes affecting the archean marine sulfur cycle and the role of microbial or abiotic redox reactions during pyrite formation remain unclear here we combine iron fe and multiple sulfur s isotope data in individual pyrite grains with petrographic information and a one dimensional reactive transport model to investigate the sources of fe and s in pyrite formed in a paleoarchean sedimentary basin pyrites were selected from mudstones sandstones and chert obtained from a drill core in the ca 3 2 ga mapepe and mendon formations of the fig tree and onverwacht groups respectively in the barberton greenstone belt kaapvaal craton south africa pyrite textures and 56 fe distinguish early diagenetic pyrite formed with pore water ferrous iron disseminated grains with average 56 fe pyrite 0 from late diagenetic pyrite formed through sulfidation of iron oxide minerals layered and aggregate forms with average 56 fe pyrite 1 mass dependent s isotope variability in pyrite was small 34 s pyrite ranged from 1 1 to 3 3 with a correspondingly minor spread in 33 s pyrite ranging from 0 3 to 2 1 and 36 s pyrite ranging from 3 08 to 0 27 that indicates a lack of post depositional re working with other distinct sulfur sources our combined fe and s isotope data are most readily explained with pyrite sulfide derived from microbial reworking of solid elemental s iron oxide minerals were necessary to buffer sulfide concentrations and provide favorable conditions for microbial sulfur disproportionation to proceed the lack of a negative 33 s signal indicates that pyrite from relatively deep marine diagenetic environments only partially records the products of atmospheric photolysis consistent with low sulfate concentrations in the paleoarchean ocean

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

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

a multidisciplinary study of invisible gold in arsenopyrite from four mesothermal gold deposits in siberia russian federation characterization of gold in four siberian mesothermal gold deposits was done through a multidisciplinary investigation including ore microscopy inaa sims epma and 197 au moessbauer spectroscopy the deposits are olympiada and veduga in the enisei mountain range and nezhdaninskoye and sentachan in the verkhoyansk kolyma area in which vein and disseminated orebodies are hosted by sedimentary sequences metamorphosed to amphibolite olympiada and greenschist facies ore minerals include arsenopyrite and pyrrhotite or pyrite as major constituents with minor scheelite and sphalerite chalcopyrite and native gold as minor constituents with or aurostibite galena gudmundite tetrahedrite stibnite and berthierite some of the gold mineralization is metallurgically refractory in all four deposits largely due to the presence of invisible gold in arsenopyrite and to a lesser extent in pyrite several generations of arsenopyrite are recognized in the auriferous zones of the veduga nezhdaninskoye and sentachan deposits textural observations indicate late deposition of native gold with respect to arsenopyrite no evidence was found of arsenopyrite recrystallization and the crystallization of native gold is considered to result from an evolution of fluids rather than redeposition of gold wide variations in the gold contents of arsenopyrite occur within deposits within samples and within individual grains inna analyses of individual arsenopyrite grains 30 70 mu m in size show that the gold concentration ranges from 197 au moessbauer spectra of arsenopyrite concentrates from the veduga nezhdaninskoye and sentachan deposits exhibit mainly a component with isomer shifts between 3 3 and 3 6 mm s which is interpreted as strong evidence of the presence of chemically bound gold in the arsenopyrite crystal structure the spectra of samples from the veduga deposit also contain minor amounts of native gold for which negative isomer shifts near 1 2 mm s are expected metallic gold dominates in the 197 au moessbauer spectra of samples from the olympiada deposit but varying amounts of chemically bound gold are also visible in the spectra some attributed to aurostibite it is concluded that the ores from these deposits are refractory partly because the gold occurs in arsenopyrite as chemically bound gold as well as submicroscopic metallic particles probably as small as 2 nm this study also shows that au bearing arsenopyrite is usually depleted in as with respect to its stoichiometric composition but correlation of au enrichment to fe deficiency was not found it is inferred from 197 au moessbauer spectroscopy that substitution of fe super 3 for au super 3 is most likely the temporal change in different modes of gold occurrence i e as native gold or invisibly in sulfides in the mineralized zones and veins is interpreted to result from an evolution of fluids rather than to be a recrystallization phenomenon dissemination of au bearing sulfides within the altered halos infers that a pervasive fluid rock interaction was an effective mechanism for deposition of minerals and gold absorption of gold complexes onto sulfide surfaces is also suggested to have played an important role in the deposition of native gold during the latest episodes in the formation of the deposits

mineralogy and geochemistry of sulfosalts from baia sprie ore deposit romania new bismuth minerals occurrence abstract the baia sprie epithermal ore deposit is one of the best known deposits in romania and europe it consists mainly of a vein system with two major veins 1 principal vein and 2 southern vein along fractures that delimitate an andesitic block and a subvolcanic intrusion the principal vein is one of the longest hydrothermal veins in europe with more than 5 km in length a thickness that varies between 0 5 and 22 m and it extends vertically on more than 800 m samples belonging to different levels of principal vein have been analyzed using electron microprobe a new occurrence of bismuth minerals was identified within the deeper part of the deposit bismuthinite lillianite gustavite disordered intergrowths of lillianite homologues heyrovskyite and cosalite the presence of bi sulfosalts suggests temperature of 350 c while the as content in arsenopyrite indicates formation temperatures between 320 and 360 c the oscillatory zoning bands of pyrite and tetrahedrite tennantite members indicate an increased sb and as concentration in hydrothermal solutions the zoning textures suggest a high s and low as concentration in the early fluids followed by multiple pulses of as rich solutions in the late fluids the high content of as in pyrite and the presence of bismuth minerals at baia sprie deposit can represent indicators for significant au concentrations within the lower level of the mineralization from the type locality baia sprie new analytical data for semseyite were presented and for the first time a new occurrence of geocronite jordanite series was identified in the intermediate and upper part of the deposit

application of cathodoluminescence to magmatic quartz in a tin granite case study from the schellerhau granite complex eastern erzgebirge germany a model of the cooling history of tin bearing granitic magma forming the schellerhau granites eastern erzgebirge germany is shown on the basis of quartz textures similar grain size similar grain habit and correlatable growth textures of phenocrysts in different granite varieties give proof of a common crystallization history before the melts of the schellerhau granite varieties were intruded four nucleation events occurred during crystallization in different crustal levels between about 20 and 1 km depth the parental melt of the schellerhau granites is interpreted to have contained 2 5 wt h2o originally the water content of the melt during the subvolcanic intrusion stage amounted to more than 5 wt and characterizes highly evolved residual melts that enable the formation of tin deposits this study contributes to a better understanding of the development and behaviour of fractionated tin bearing granitic melts and links quartz cathodoluminescence cl with microanalytical studies

structural controls on hydrothermal alteration and gold antimony mineralisation in the hillgrove area nsw australia the hillgrove gold antimony deposit is hosted in late palaeozoic biotite grade metasedimentary rocks and permo carboniferous granitoid intrusions of the new england orogen mineralisation occurred at a range of structural levels during rapid uplift in the orogen at 255 245 ma hydrothermal fluids were controlled by extensional faults in a regional scale sinistral strike slip fault system principal faults in this system were developed in and possibly evolved from mylonite zones which were active during late permian tectonics earliest mineralisation formed scheelite bearing quartz veins and these were followed by auriferous arsenopyrite pyrite quartz carbonate veins with minor base metal sulphides this latter type was accompanied by sericitisation and carbonation of the host rock with addition of sulphur arsenic and gold in zones up to 20 m from veins quartz stibnite veins with electrum gold aurostibite and arsenopyrite form a prominent and economically important hydrothermal type with little wall rock alteration but extensive hydrothermal breccia formation and local open space filling textures below a mining depth of 300 500 m this type passes over a short distance downwards into stibnite poor gold bearing veins late stage carbonate stibnite veins with gold and silver sulphosalts cut all earlier veins and have open space filling textures aspects of the hillgrove deposit have similarities to many other orogenic gold deposits in the sw pacific which have been formed at different structural levels hillgrove is distinctive in having evidence for mineralisation at this wide range of structural levels in the one deposit formed progressively during syn orogenic uplift

bimodal distribution of gold in pyrite and arsenopyrite examples from the archean boorara and bardoc shear systems yilgarn craton western australia this study investigates the microstructures geochemistry and hydrothermal evolution of gold bearing pyrite and arsenopyrite from six orogenic gold deposits in the archean eastern goldfields province western australia scanning electron microscope sem electron microprobe emp and laser ablation inductively coupled plasma mass spectroscopy la icp ms analyses show that the gold bearing minerals possess a number of similar textural features including the occurrence of invisible gold within initial phases of growth and later stage visible gold associated with alteration rims the alteration rims are characterized by a higher than average atomic mass mainly owing to arsenic enrichment and are preferentially located along fractures and grain boundaries in the pyrite and arsenopyrite these observations suggest that visible gold formation is associated with hydrothermal alteration of preexisting pyrite and arsenopyrite textural observations and la icp ms data suggest that some invisible gold was remobilized from early formed pyrite and arsenopyrite to form visible gold during development of these alteration rims gold may also have been added by hydrothermal fluids during a later stage of mineralization in situ geochemistry and phase relationships of alteration rims are used to further constrain the hydrothermal process responsible for formation of alteration rims and visible gold in fractures based on sulfide stability relations our data indicate that development of arsenopyrite alteration rims associated with late stage visible gold formation was related to an increase in temperature maximum increase from 310 to 415 c and up to of sic orders of magnitude increase in sulfur fugacity whereas changes in oxygen fugacity were less important la icp ms analyses show that the relative and absolute variations in selected trace element au ag sb bi ba te pb co and mo concentrations can also be used to distinguish between unaltered and altered pyrite and arsenopyrite in general trace elements within pyrite and arsenopyrite have a relatively uniform distribution whereas later stage alteration rims have more variable trace element distributions although the observed textures are typical of prograde metamorphic coronae we suggest that they are the consequence of variations in fluid conditions and chemistry and that mineralization occurred in response to syn and or postpeak metamorphic fluid infiltration

cathodoluminescence and trace element zoning in quartz phenocrysts and xenocrysts quartz phenocrysts and xenocrysts are a common component of volcanic rocks but rarely contribute to our understanding of magmatic or eruptive processes due to an apparent internal chemical and structural homogeneity scanning electron microscopy cathodoluminescence imaging sem cl and secondary ion mass spectroscopy ion microprobe analysis sims are used here to demonstrate that quartz crystals in volcanic rocks show hitherto unrecognised internal structural and chemical zoning two contrasting crystal morphologies have been studied porphyritic euhedral bipyramidal quartz crystals in dacite from bonne nuit bay jersey uk show fine scale growth zoning in sem cl zones are narrow 1 40 m wide planar parallel to crystal faces and overgrow more broadly zoned rounded cores these crystals are interpreted to have formed at small degrees of undercooling by diffusion controlled growth quiescent magma conditions are indicated by the preservation of an al rich diffusive boundary layer which has not been removed by melt shearing during convection in contrast rounded and embayed porphyritic quartz grains in a quartz dacite from the esperanza mine e1 salvador northern chile show broad internal sem cl zones with irregular boundaries some of which are truncated by grain margins indicating dissolution crystal rounding may occur by dissolution or melting due to thermal or thermal chemical disequilibrium with surrounding melt and embayments by gas bubble enhanced convective dissolution are consistent with the development of incipient ocelli textures strings of amphibole inclusions adjacent to quartz rims several episodes of rounding can be recognised in some xenocrysts ion microprobe sims analysis of quartz from both samples shows that a1 contents are higher in areas of bright cl emission previous studies of cl in quartz have suggested that several factors control luminescence although a1 substitution for si in sio4 tetrahedra may be partly responsible for sem cl zoning recorded in volcanic quartz additional mechanisms including the effect of growth rate on defect density probably also play a major role

trace elements in hydrothermal quartz relationships to cathodoluminescent textures and insights into vein formation high resolution electron microprobe maps show the distribution of ti al ca k and fe among quartz growth zones revealed by scanning electron microscope cathodoluminescence sem cl from 12 hydrothermal ore deposits formed between 100 and 750 c the maps clearly show the relationships between trace elements and cl intensity in quartz among all samples no single trace element consistently correlates with variations in cl intensity however in vein quartz from five porphyry cu mo au deposits cl intensity always correlates positively with ti concentrations suggesting that ti is a cl activator in quartz formed at 400 c ti concentrations in most rutile bearing vein quartz from porphyry copper deposits indicate reasonable formation temperatures of 750 c using the titaniq geothermometer titanium concentrations of 10 ppm in all veins that formed at temperatures 350 c suggest a broad correlation between ti concentrations and temperature of quartz precipitation in quartz from most deposits formed at 2000 ppm but in high temperature quartz al concentrations are consistently in the range of several hundred ppm aluminum concentrations in quartz reflect the al solubility in hydrothermal fluids which is strongly dependent on ph aluminum concentrations in quartz therefore reflect fluctuations in ph that may drive metal sulfide precipitation in hydrothermal systems

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

mineralogical siting and distribution of gold in quartz veins and sulfide ores of the ashanti mine and other deposits in the ashanti belt of ghana genetic implications the ashanti belt of ghana constitutes a gold province which has produced a total of about 1500 t of gold historically gold mineralization is found in steep nne ssw to ne sw trending shear zones predominantly transecting metasediments of the palaeoproterozoic birimian supergroup 2 2 2 1 ga disseminated in ca 2 1 ga granitoids in paleo conglomerates of the tarkwaian group 2135 ma and in recent placers the distribution of gold its chemistry paragenesis and mineralogical siting in the mesothermal ores of the major mines in the ashanti belt namely konongo ashanti bogosu and prestea mine are the subject of this study at the localities studied gold is present in two main types of ores 1 quartz veins with free milling gold the gold is relatively silver rich true fineness values from 730 to 954 and is accompanied by a distinct suite of cu pb sb sulfides 2 sulfide ores consisting of arsenopyrite pyrite and rarer pyrrhotite and marcasite with refractory gold the ores have apparent fineness values larger than 910 arsenopyrite and locally at bogosu pyrite were identified as the hosts of submicroscopic gold mean concentrations of gold in arsenopyrite in various samples from the different mines obtained by secondary ion mass spectrometry sims range from 67 to 314 ppm au gold concentration mapping in individual arsenopyrite crystals from the different deposits revealed similar patterns of gold distribution the grains possess a gold poor core and elevated gold contents are present along distinct crystal growth zones towards their rims the outermost crystal layer is usually gold poor the well preserved distribution patterns also indicate that remobilization of gold from the sulfides played an insignificant role in the ores of the ashanti belt multiple quartz veining and growth zoning of the sulfides are interpreted as manifestations of multiple episodes of fluid infiltration fluid flow and mineral deposition the bimodal occurrence of gold in spatially closely associated quartz vein and sulfide ores indicates a genetic link between these ore types a model implying a grossly coeval formation of the ores from mesothermal fluids is proposed

synmetamorphic lode gold deposits in high grade archean settings lode gold deposits that formed under conditions of the middle to upper amphibolite and granulite facies have been identified in the yilgarn block of western australia these deposits which form a continuum with lower metamorphic grade mesothermal gold depos its show that at least some of the fluids responsible for mesothermal gold deposits originated in or below the middle crust studies of 18 o suggest that fluid compositions were controlled by the wall rocks at least for some elements these more proximal gold deposits provide more information about deep fluid sources than do investigations of higher level mineralization deposited from fluids strongly modified by wall rock interaction during the early part of their ascent

a paleozoic suture in eastern gondwanaland a new tectonic interpretation of southeastern australia tasmania and north victoria land antarctica is given in which a major n s trending lineament representing a suture zone is recognized separating essentially shallow water shelf and continental deposits of latest cambrian or early ordovician age from deeper water ordovician flysch deposits lateral offset of this lineament which is interpreted as a suture along which thrust telescoping and major strike slip faulting occurred could adequately explain the present distribution of major rock types in the three areas and also removes a number of problems previously encountered in correlation

silica alteration zones in the barberton greenstone belt a window into subseafloor processes 3 5 3 3 ga ago abstract silicification of volcanic rocks ranging from komatiitic to dacitic in composition is a common phenomenon of the 3 5 to 3 2 ga old volcano sedimentary succession of the barberton greenstone belt silica enrichment occurs in zones several tens of metres thick at the top of volcanic sequences that are capped by sedimentary chert horizons sio2 contents increase upsection from the original igneous value up to 92 silicification is associated with a depletion of most elements commonly mobile during water rock interaction while k2o rb and ba are enriched in these zones some transition metals sc v and cr were immobile whereas ni and co and some other metals cu zn were mobile and are depleted the 18o values of volcanic rocks show a positive linear relationship with silica content and range from 9 0 to 17 3 the element depletion enrichment patterns and oxygen isotope data indicate low temperature c 100 150 c hydrothermal processes for the origin of the alteration zones the loss of most elements coupled with the addition of sio2 as well as ree systematics indicate high water rock ratios and a fluid ree composition typical of archaean seawater that had a 18o value of 0 hydrothermal activity is attributed to heat derived from cooling of volcanic rocks and a high regional heat flow in an oceanic plateau like setting and resulted in the establishment of shallow subseafloor convection cells diffuse venting of hydrothermal fluids over broad areas of the ancient ocean floor provided an ideal environment for both seafloor and subsurface biota 3 5 ga ago

hydrothermal alteration zoning and gold concentration at the kerr addison archean lode gold deposit kirkland lake ontario kerr addison a major archcan lode gold deposit of the abitibi belt with a production of approximately 10 x 10 6 oz au approximately 320 metric tons up to 1983 is localized in a zone of talc carbonate and chlorite carbonate rocks immediately south of a regional structural break the kirkland lake larder lake fault zone relict volcanic textures taken in conjunction with the ratios of diag nostic relatively less mobile elements collectively indicate that most of the wall rocks of the mine were originally mafic and ultramafic rocks along with their clastic derivatives regional greenschist facies metamorphism is locally superposed by a distinct zonation of gold associated hydrothermal alteration characterized by massive introduction of co 2 coupled with variable enrichments of alkali metals and calcium which produced systematic trends of alteration mineral assemblages alteration types of the initial stage contain chlorite which is replaced by muscovite or albite in the advanced stage of alteration with a concomitant increase in the proportion of carbonate minerals numerous generations of crosscutting veins are enveloped by alteration borders in which the sequence of the al bearing minerals namely albite mus covite and chlorite develop from the vein to the wall rocks always in a systematic sequence the same sequence is observed at a larger scale from the ore zone to the periphery of the mine area the intensity of alteration can be monitored by saturation indices co 2 ca fe mg for carbonates and 3k na al 3k al and na al for muscovite albite muscovite and albite respectively molar ratios plots of the moving averages of these indices across the 3 850 ft level of the mine reveal a remarkable correspondence to variations of gold abundance suggesting a genetic relationship of gold enrichment to hydrothermal alteration on a broad scale gold is concentrated to the south of the kirkland lake larder lake fault zone and at a more detailed scale in the transition zone between carbonate muscovite and carbonate albite alteration types during advanced alteration fluid x sub co 2 was approximately 0 1 and na k asymp 10 volcanics of the kinojevis and larder lake groups are 18 o enriched relative to pristine tholeiites and possess distinct mineral delta 18 o values probably due to different conditions of marine water alteration during or shortly after extrusion in domains of advanced alteration these are all shifted down to a common narrow bracket here 18 o 16 o and d h data indicate fairly constant temperatures 270 degrees 300 degrees c and a common isotopic composition of hydro thermal fluids delta 18 o 5ppm delta d 30 to 50ppm across the alteration zoning these results are in common with many other archcan lode gold deposits fe dolomite has a uniform delta 13 c value of 2 8 or 0 42 per mil 1sigma throughout the footwall and ore zones but it is enriched by approximately 1 per mil in the graphite free hanging wall rocks fluid delta 13 c value was 4 or 1 per mil reflecting either a magmatic carbon reservoir or mixtures of marine carbonate carbonaceous and magmatic sources the enrichment of gold at one side of the kirkland lake fault zone is interpreted to be the result of development of a metasomatic zoning induced by gradients in oxidation state the precious metal being concentrated in the reducing side zonation of alteration types south of the kirkland lake larder lake fault zone represents various steps of a continuous process of incremental changes of the rocks to adapt to the composition of an externally generated fluid successive generations of fracture arrays and veins have the same alteration patterns and thus the composition of the incoming hydrothermal fluid seems to have remained essentially unchanged through time gold mineralization at the kerr addison mine and its contiguous alteration halo are interpreted to be the result of repeated cycles of hydraulic fracturing fluid penetration and local reaction with rocks fluid rock interaction produced lateral variations of fluid composition which led to gold concentration probably due to gradients in ph preferentially at the interface between the carbonate muscovite and carbonate albite alteration types which is also the site of pyrite concentration the alteration assemblages and specific site of gold pyrite concentration at kerr addison are common to several other described archcan lode gold deposits

fluid inclusion petrography abstract a procedure of fluid inclusion studies is proposed with emphasis on the criteria of selecting fluid inclusions for detailed microthermometry and spectroscopic analysis an overview of descriptive and genetic classifications of fluid inclusions in single crystals and in massive rocks is given with the intention of further differentiating the commonly used terms primary and secondary fluid inclusions some principles of fluid inclusion modification are explained cathodoluminescence cl studies of quartz with the optical high power cl microscope and the electron microprobe provided with a cl detector are an important help in fluid petrography cl textures are subdivided in primary growth textures and a wide variety of secondary microtextures which are in part induced by fluid inclusions the latter is grouped in textures indicative of local lower crystal order increasing defect structures and microtextures indicative of local quartz healing reduction of the defect structures microtextures showing the genetic relationship between fluid inclusions and the host mineral provide information about the possible post entrapment changes of fluid inclusions and therewith testify their geological relevance

reassessment of archean crustal development in the barberton mountain land south africa based on u pb dating this paper presents u pb ages for 23 samples from the barberton mountain land an archean granite greenstone terrain in the kaapvaal craton of south africa a reexamination of the geology of this area in the light of previous u pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short lived compressional event within the time span 3230 3215 ma which was synchronous with deposition of the moodies group and the upper part of the fig tree group sediments this event may have been responsible for thrusting of much older sequences in the onverwacht group including slivers of 3 54 ga old gneiss circa 3 47 ga old komatiitic and tholeiitic sequences and circa 3 45 ga old felsic sequences key constraints on the timing of deformation are previously determined 3227 3225 ma ages for detrital zircons in thrusted units of the fig tree group a 3229 4 3 ma age for a deformed porphyry which is pre or synthrusting a previously determined 3227 3 ma age for an undeformed porphyry in an inferred thrust fault 3227 1 ma ages for both the kaap valley pluton and an ignimbrite in the stolzburg syncline both of which predate regional upright folding and a 3216 2 ma age for the dalmein pluton which appears to cut regional upright ne sw trending folds in the kromberg syncline part of the final phase of deformation associated with regional compression areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the barberton region at about 3105 ma the linear configuration of the syenogranitic plutons which are also aligned with the gabbroic usushwana complex as well as the geometry of the granitoid sheets suggests that the circa 3105 ma magmatism was caused by incipient rifting around a nw se trending crustal fracture this episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt some of the above interpretations are in conflict with previous models for early development of the barberton greenstone belt a 3352 6 ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m y younger than the probable age of volcanic rocks which host the dikes this conflicts with the suggestion that much of the onverwacht group comprises an ophiolitic section obducted in an intraoceanic setting at circa 3 45 ga that partially melted to form trondhjemitic magmas which intruded along active thrust faults present data suggest that despite its antiquity and the diversity of ages and lithologies found within it the barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins

new u pb and 40ar 39ar ages from the northern margin of the barberton greenstone belt south africa implications for the formation of mesoarchaean gold deposits gold mineralization in the mesoarchaean new consort gold mine in the barberton greenstone belt south africa is situated in the immediate hanging wall of an exhumed composite granitoid gneiss dome known as the stentor pluton the granitoid greenstone contact is characterized by a contact parallel high strain zone that separates the mid crustal gneisses from the greenschist facies greenstone belt in contrast to the lower greenschist facies deposits in the central parts of the greenstone belt the mineralized shear zones at new consort are hosted by distinctly higher grade metamorphic rocks ranging from upper greenschist to upper amphibolite facies conditions a distinct change of ore and alteration assemblages with increasing metamorphic grade points to a temporal and spatial relationship between gold mineralization and the juxtaposition of the stentor gneiss dome against the overlying greenstone belt examination of the structural and petrologic evolution of the mineralized high strain zone combined with new u pb and ar ar data and previously published ages indicates a multistage tectono metamorphic evolution early high t medium p metamorphism ca 600 700 c and 5 1 kbar in the structurally deeper parts of the mine is interpreted to be related to an early period of orogen parallel ne sw extension and associated detachment shearing during the main d2 accretionary event in the centre of the greenstone belt between ca 3260 and 3225 ma recent work has shown that these early high t fabrics were reactivated at ca 3105ma during the subsequent d3 tectonism and associated emplacement of large potassic granite batholiths the gold mineralization was coeval with ne sw extensional exhumation and solid state doming of the stentor gneiss dome during the subsequent d4 deformation this late stage deformation and the gold mineralization postdate the emplacement of the d3 related potassic granites by more than 50 million years as indicated by 1 a u pb age of 3046 27ma for a retrograde titanite generation in a d2 related amphibolite sample 113 and 2 a 3027 7 5ma age for hydrothermal titanite from the so called footwall lens mineralization which temperature has been estimated at 590 40 c this high t event is also recorded by a gcorrected h 3040 4ma hornblende ar ar age from the same sample suggesting that cooling of the stentor gneiss dome shortly followed the exhumation a considerably younger ar ar age 2966 4 ma has been obtained for an amphibolite from the contact parallel high strain zone pointing to non uniform cooling following the mineralization the gold mineralization was largely contemporaneous with the emplacement of nnw trending syn kinematic granitic pegmatites and strain localization along the pegmatite swarm at ca 3030 3040 ma the mineralization is also considerably younger than the gold mineralization in the greenschist facies gold deposits in the barberton greenstone belt which has previously been dated at ca 3080 ma in conjunction with previously published ages the data indicate the au mineralization in the barberton greenstone belt to have occurred during at least two mineralization episodes at ca 3080 and 3040 ma the main phase of au mineralization was introduced during the end of an either protracted or episodic structural and thermal evolution of the granitoid greenstone terrain that may have lasted some 200 ma

cathodoluminescence and micro structural evidence for crystallisation and deformation processes of granites in the eastern lachlan fold belt se australia trace elements al k ti fe growth and deformation pattern in quartz of the multiple deformed carcoar barry and sunset hills granites were investi gated by electron micro probe and cathodolumines cence zoned quartz phenocrysts with high ti concentrations 70 ppm that show blue cathodolu minescence originated from the early stage of magma crystallisation multiple deformation of quartz causes the redistribution of al and k in the quartz lattice which results in the accumulation of these elements in submicroscopic inclusions 0 5 lm of muscovite like composition in contrast structural ti in quartz is mostly kept in the lattice common halos of defect poor secondary quartz around fluid inclusions result from re equilibration of fluid inclusions because of differences between fluid pressure and lithostatic pressure e g duringuplift isothermal decompression and or a b transition of quartz duringhealing defect poor sec ondary quartz grows at the cost of the host quartz and releases or replaces defect centres the results of micro structural investigations combined with al in horn blende thermobarometry allow the reconstruction of regional processes carcoar and barry granodiorites and sunset hills granite were intruded in the late ordovi cian early silurian at depths of 4 8 6 and 10 12 km respectively in contrast to the continuous crystallisation of the granodiorite magmas the magma of the sunset hills granite ascended in a stepwise fashion causing multiple quartz nucleation the two granodiorites were multiple post magmatically deformed first during early devonian under more brittle conditions at temperatures of 350 400 c whereas the sunset hills granite experienced more ductile deformation at tem peratures of around 550 c

the genesis and tectonic control on archaean gold deposits of the western australian shield a metamorphic replacement model abstract gold deposits occur in greenstone belts world wide and contribute to anomalously high gold production from archaean terranes as in other cratons archaean gold mineralization of western australia represents a complex array of deposit styles despite this most deposits are clearly epigenetic and large deposits have a number of features in common including their strong structural controls distinctive wallrock alteration fe sulphide k mica albite ca mg fe carbonates consistent metal associations au ag as sb w b low base metals commonly fe rich host rocks great depth extension and lack of appreciable vertical zonation these shared characteristics combined with their ubiquitous occurrence indicate that archaean gold deposits had a common origin related to the tectonic evolution of greenstone belts auriferous hydrothermal systems were broadly synchronous with regional metamorphism and emplacement of synkinematic granitoids and felsic porphyry intrusions although these gold systems involved low salinity lowdensity reduced near neutral h 2 o co 2 fluids carrying gold as reduced sulphur complexes the origin of the fluids is equivocal most timing evidence and stable isotope data cannot distinguish metamorphic from magmatic granitoid or felsic porphyry orggins but the lack of consistent spatial relationships between specific volumetrically significant intrusive phases and large gold deposits in a number of cratons strongly favours metamorphic derivation of fluids the metamorphic replacement model for gold mineralization involves devolatilization of the lower portions of the greenstone pile with high geothermal gradients inhibiting significant melting co 2 possibly formed by the decarbonation of early alteration related to mantle degassing along crustal scale synbasinal fault zones auriferous fluids were channelled along greenstone scale faults in part developed during reactivation of crustal scale faults in a strike slip regime gold deposition occurred largely under greenschist facies conditions about 300 400 c 1 2 kb in response to decreasing gold solubility with declining temperature however a major control on gold deposition was fluid wallrock interaction many large deposits formed by sulphidation of fe rich host rocks with synchronous deposition of fe sulphides and gold however the variable nature of gold depositing reactions including lowering of fo 2 and ph allowed a multitude of small and some large deposits to form wherever that fluid circulation occurred in consequence several of the relatively small deposits currently worked from open pit are hosted by ultramafic and felsic rocks there are few constraints on the source of components au s k co 2 added to gold deposits but even giant deposits such as the golden mile kalgoorlie could have formed from a realistic greenstone source volume ca 8 8 5 km convective circulation of fluids could have contributed to the generation of high fluid rock ratios on the regional scale the markedly heterogeneous distribution of large gold deposits gold productivity and host rocks to deposits can be accommodated by the metamorphic replacement model the most favourable conditions for development of auriferous hydrothermal systems operated in younger ca 2 7 0 1 ga rift phase greenstones where greatest extension and crustal thinning produced high geothermal gradients crustal scale synbasinal faults and rapid extrusion and burial of volcanics including abundant komatiites iron rich tholeiitic basalts and dolerites were preferred host rocks for large gold deposits the least favourable conditions existed in older ca 3 5 3 4 ga platformphase greenstones where gentle sagging on submerged continental crust produced eruption of mainly mafic volcanics with few komatiites commonly in very shallow water environments this allowed intense synvolcanic alteration of both gold source rocks and potential host rocks the generally smaller gold deposits formed mainly in ultramafic or greywacke hosts younger ca 3 0 ga platform phase greenstones appear intermediate in nature but unlike other greenstones have significant epigenetic gold deposits in originally oxide facies bif which were deposited on relatively deep water platforms similar controls appear to exist on a world scale with gold mineralization peaking at ca 2 7 0 1 ga in response to development of major rift zones in thickened relatively mature continental crust interestingly the giant witwatersrand goldfield formed at about the same time

invisible gold in arsenian pyrite and arsenopyrite from a multistage archaean gold deposit sunrise dam eastern goldfields province western australia the sunrise dam gold mine 11 1 moz au is the largest deposit in the archaean laverton greenstone belt eastern goldfields province yilgarn craton western australia the deposit is characterized by multiple events of fluid flow leading to repeated alteration and mineralization next to a major crustal scale structure the au content of arsenian pyrite and arsenopyrite from four mineralizing stages d1 d3 d4a and d4b and from different structural and lithostratigraphic environments was measured using in situ laser ablation inductively coupled plasma mass spectrometry pyrite contains up to 3 067 ppm au n 224 whereas arsenopyrite contains up to 5 767 ppm n 19 gold in arsenopyrite d4a stage was coprecipitated and remained as invisible gold nanoparticles and or lattice bound during subsequent deformation events in contrast gold in pyrite is present not only as invisible gold but also as micrometer size inclusions of native gold electrum and au ag tellurides pristine d1 and d3 arsenian pyrite contains relatively low au concentrations 26 ppm the highest au concentrations occur in d4a arsenian rich pyrite that has recrystallized from d3 pyrite textures show that this recrystallization proceeded via a coupled dissolution reprecipitation process and this process may have contributed to upgrading au grades during d4a in contrast au in d4b pyrite shows grain scale redistribution of invisible gold resulting in the formation of micrometer scale inclusions of au minerals the speciation of au at sunrise dam and the exceptional size of the deposit at province scale result from multiple fluid flow and multiple au precipitating mechanisms within a single plumbing system

regional folding low angle thrusting and permeability networks structural controls of gold mineralization in the hope reef at fairview mine barberton greenstone belt south africa abstract the hope reef forms part of a number of high grade mineralized but hitherto undocumented auriferous shear zones at fairview mine in the barberton greenstone belt in south africa collectively referred to as the hanging wall reefs the reefs are hosted by steeply dipping greywacke units of the fig tree group on the western limb of the regional scale ulundi syncline the currently mined shallow hope reef represents a top to the northwest low angle thrust that formed as a secondary accommodation structure during continued nw subhorizontal shortening after the lock up of the ulundi syncline northwest shortening strains correspond to the main phase of upright regional folding d2 3 in the greenstone belt the formation of shear and extensional vein sets documents fault valve behaviour along the low angle thrust driven by close to lithostatic fluid pressures and under very low differential stresses the hanging wall reefs at fairview mine underline the formation of mineralized reefs in the barberton gold district with vastly different orientations structural and lithological controls either in sequence or broadly contemporaneous but during the late stage shortening of the greenstone belt the absolute timing of this deformation and associated hydrothermal events remain unclear

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

gold and trace element zonation in pyrite using a laser imaging technique implications for the timing of gold in orogenic and carlin style sediment hosted deposits laser ablation icp ms imaging of gold and other trace elements in pyrite from four different sedimenthosted gold arsenic deposits has revealed two distinct episodes of gold enrichment in each deposit an early synsedimentary stage where invisible gold is concentrated in arsenian diagenetic pyrite along with other trace elements in particular as ni pb zn ag mo te v and se and a later hydrothermal stage where gold forms as either free gold grains in cracks in overgrowth metamorphic and or hydrothermal pyrite or as narrow goldarsenic rims on the outermost parts of the overgrowth hydrothermal pyrite compared to the diagenetic pyrites the hydrothermal pyrites are commonly depleted in ni v zn pb and ag with cyclic zones of co ni and as concentration the outermost hydrothermal pyrite rims are either as au rich as in moderate to highgrade deposits such as carlin and bendigo or co ni rich and as au poor as in moderate to low grade deposits such as sukhoi log and spanish mountain the early enrichment of gold in arsenic bearing syngenetic to diagenetic pyrite within black shale facies of sedimentary basins is proposed as a critical requirement for the later development of carlin style and orogenic gold deposits in sedimentary environments the best grade sediment hosted deposits appear to have the gold climax event toward the final stages of deformation related hydrothermal pyrite growth and fluid flow

climate simulations of the permian triassic boundary ocean acidification and the extinction event 1 the causes for the permian triassic boundary ptb extinction the largest mass extinction on record remain enigmatic the period is marked by large scale volcanic eruptions and evidence for widespread ocean anoxia which have led to suggestions that these events generated or played a part in the extinction furthermore hypercapnia and ocean acidification caused by volcanic emissions of co2 and ch4 have been put forward as potential kill mechanisms we present the first ptb climate simulations in which ocean acidity is evaluated directly with a coupled climate carbon cycle model the experiments also address the sensitivity of ocean circulation and oxygen levels to uncertainties in paleogeography and to different bottom topographies modeled temperature and precipitation evaporation are in good agreement with reconstructions and climate sensitive sediments there is also good agreement between modeled vegetation and reconstructed biomes the reduction in ocean ph brought about by the increase in atmospheric co2 is biologically significant aragonite saturation levels are low enough to make the whole ocean unsuitable to aragonitic species and large areas of the ocean become unsaturated in relationship to calcite no general bottom anoxia is reproduced modeled deep ocean o2 concentrations are not significantly impacted by changes in paleogeography and bathymetry an indication that in our model a change in ocean dynamics resulting from climate warming is not sufficient by itself to generate widespread anoxic conditions during the period

the coupled geochemistry of au and as in pyrite from hydrothermal ore deposits the ubiquity of au bearing arsenian pyrite in hydrothermal ore deposits suggests that the coupled geochemical behaviour of au and as in this sulfide occurs under a wide range of physico chemical conditions despite significant advances in the last 20years fundamental factors controlling au and as ratios in pyrite from ore deposits remain poorly known here we explore these constraints using new and previously published empa la icp ms sims and pixe analyses of as and au in pyrite from carlin type au epithermal au porphyry cu cu au and orogenic au deposits volcanogenic massive sulfide vhms witwatersrand au iron oxide copper gold iocg and coal deposits pyrite included in the data compilation formed under temperatures from 30 to 600 c and in a wide variety of geological environments the pyrite au as data form a wedge shaped zone in compositional space and the fact that most data points plot below the solid solubility limit defined by reich et al 2005 indicate that au1 is the dominant form of au in arsenian pyrite and that au bearing ore fluids that deposit this sulfide are mostly undersaturated with respect to native au the analytical data also show that the solid solubility limit of au in arsenian pyrite defined by an au as ratio of 0 02 is independent of the geochemical environment of pyrite formation and rather depends on the crystal chemical properties of pyrite and post depositional alteration compilation of au as concentrations and formation temperatures for pyrite indicates that au and as solubility in pyrite is retrograde au and as contents decrease as a function of increasing temperature from 200 to 500 c based on these results two major au as trends for au bearing arsenian pyrite from ore deposits are defined one trend is formed by pyrites from carlin type and orogenic au deposits where compositions are largely controlled by fluid rock interactions and or can be highly perturbed by changes in temperature and alteration by hydrothermal fluids the second trend consists of pyrites from porphyry cu and epithermal au deposits which are characterised by compositions that preserve the au as signature of mineralizing magmatic hydrothermal fluids confirming the role of this sulfide in controlling metal ratios in ore systems

invisible gold in ore and mineral concentrates from the hillgrove gold antimony deposits nsw australia vein hosted mesothermal stibnite gold mineralisation at the hillgrove au sb mine in northeastern new south wales has a halo of veinlet and disseminated auriferous arsenopyrite and arsenian pyrite in metasedimentary and granitic host rocks about 50 55 of the gold produced at hillgrove occurs invisibly in arsenopyrite and pyrite gold losses of 20 into tailings are due to this mineral chemical factor from pixe probe analyses it has been found that arsenopyrite contains 255 1500 ppm au and pyrite 24 223 ppm au with au contents of each mineral correlating moderately with as content arsenopyrite and pyrite also contain anomalous values of cu ag and sb whereas paragenetically later stibnite contains little invisible gold but minor fe as ag cu and pb the precipitation of invisible gold in arsenopyrite and pyrite by a possible fe au 3 as s 3 substitution mechanism may have been facilitated by rapid non equilibrium conditions involving pressure decreases and wall rock reaction sulphidation carbonatisation as a prelude to the main stage of stibnite and gold deposition

pyrite physical and chemical textures pyrite may crystallize initially in forms as diverse as framboids or cubes depending upon the temperatures and pressures fluid rich diagenesis or low grade metamorphism clearly results in thorough recrystallization and the common formation of cubes once these have formed the pyrite becomes much more refractory and retains many characteristics even in deposits which have undergone penetrative deformation this is in strong contrast to the behavior of most of the accompanying sulfides which often undergo ductile deformation solid state or chemical remobilization and annealing pyrite deforms sparingly until there is brittle failure however there may be significant pyrite corrosion and regrowth during metamorphism as the result of sulfur exchange with other minerals especially pyrrhotite pyrite fabrics may also be significantly modified by pressure solution or coble creep optical microscopic examination and electron microprobe chemical mapping of pyrites from a variety of mineral deposits including several high grade metamorphic ones reveals that the pyrites frequently contain both physical and chemical textures that may be interpreted in terms of the depositional and the post depositional history of the deposits inclusions of sulfides or other minerals reveal information on the timing of the crystallization or recrystallization of the pyrite chemical mapping of elements such as ni co and as reveals information on the relative time of transport of these elements in the ore fluids

biogenic overgrowth on detrital pyrite in ca 3 2 ga archean paleosols regionally traceable paleosols in the lower moodies group of the barberton greenstone belt ca 3 22 ga northeastern south africa and swaziland contain locally abundant silicified nodules originally composed of pedogenic carbonates and sulfates interbedded with heavy mineral laminae dominated by pyrite pyrite grains show rounded detrital cores and secondary idiomorphic rims with trace element concentrations and 34 s ratios clearly different from those of the cores while cores have low co and ni concentrations and high co ni ratios rims show as much as 5 5 wt of these elements and low co ni ratios reflecting the weathering of nearby ultramafic rocks in situ sulfur isotope analyses of pyrite cores show 34 s vcdt vienna canyon diablo troilite values between 5 and 5 while the rims show 34 s vcdt values between 20 and 24 5 suggesting biogenic fractionation of sulfur the close spatial association and microtextural evidence for nearly contemporaneous formation of the pedogenic sulfate nodules and the secondary pyrite rims suggests microbial processing of sulfur in the paleosols which provided reduced and 34 s depleted sulfur for the growth of authigenic pyrite this indicates that vadose zone soil forming processes in the archean involved not only physical and chemical modification of moist unconsolidated sediment in a terrestrial environment but also already included its microbiological modification

antimony in rutile as a pathfinder for orogenic gold deposits abstract in our study we explore the applicability of rutile as a pathfinder for orogenic gold deposits which are an important source of this metal worldwide we analysed rutile associated with orogenic au deposits from three different precambrian terranes the capricorn orogen the barberton greenstone belt and the ashanti belt all of which formed under greenschist conditions and share similarities in the style of mineralisation microtextural evidence from scanning electron microscopy and electron back scatter diffraction indicates that rutile formed during the main deformation and alteration stage in these rocks and is therefore related to mineralisation we used electron microprobe and laser ablation icp ms to investigate the trace element compositions of rutile and we compared our results to other gold deposits we find that hydrothermal rutile from gold deposits contains certain trace element characteristics in particular high sb concentrations up to 1500 ppm in au deposits of the capricorn orogen that are distinct from rutile from non mineralised rocks of various petrogenetic origin other elements such as w and sn are found to be more enriched in rutile from other rock types namely felsic magmatic rocks and hydrothermal veins and are therefore not diagnostic of au mineralisation in this type of deposits we also find that the presence of sub m scale inclusions in particular zr si th bearing phases sulfide minerals and native au can severely affect analyses of this type of rutile and compromise the applicability of zr in rutile geothermometry

illumination of vein quartz textures in a porphyry copper ore deposit using scanned cathodoluminescence grasberg igneous complex irian jaya indonesia vein quartz from the grasberg igneous complex in irian jaya indonesia displays a wide range of features when observed using scanned cathodoluminescence imaging on the electron microscope examination of seventeen samples revealed concentric growth zoning quartz filled microfractures dark luminescence near sulfides changes in crystal size and orientation from vein edges to vein centers turbid growth zoning and the truncation of concentric growth zoning individual features observed along two large photomosaic traverses from vein margins to vein centers coupled with trace element analyses from the electron microprobe allow determination of a detailed history of fracture infilling the two traverses indicate a simple two stage history of crystal growth into an open fracture followed by mechanical closing of the fracture with resulting microfractures electron microprobe trace element analyses show that the fe content of quartz reaches values of up to 4900 ppm in dark luminescent regions near sulfide crystals and decreases smoothly to 200 ppm along traverses away from the sulfides the solution to the diffusion equation coupled with existing experimental data about fe diffusion in sio2 indicate the duration of the diffusion was perhaps on the order of thousands of years

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

trace element zoning of sulfides and quartz at sheba and fairview gold mines clues to mesoarchean mineralisation in the barberton greenstone belt south africa abstract the fairview and sheba mines are two of the major gold mines in the paleoarchean barberton greenstone belt of southern africa at these mines gold is associated with quartz carbonate rutile veins and occurs both as invisible gold finely dispersed in sulfides primarily pyrite and arsenopyrite and as visible electrum grains hosted in pyrite up to approximately 1000 ppm au are contained in pyrite and up to approximately 1700 ppm in arsenopyrite mapping of trace element distribution in sulfide minerals using electron microprobe and proton probe techniques revealed multiple events of ore formation and au mineralisation at fairview mine three stages of pyrite formation were identified the last of which is associated with arsenopyrite electrum and other sulfide minerals sphalerite chalcopyrite galena gersdorffite and sb sulfides at sheba mine pyrite was deposited in two stages and electrum is associated with the second stage at both mines the last stage of sulfide formation is the main stage of au deposition and is associated with mobilisation of au as sb cu zn and ni the host rock composition seems to have affected the composition of pyrite since higher ni and co concentrations up to 1 4 and 1 6 wt respectively have been measured in meta ultra mafic host rocks in comparison with chert and metagreywacke arsenopyrite is chemically zoned and has sb and s rich cores and as and ni rich rims this zoning indicates variations in fluid compositions decreasing sb and increasing ni and crystallisation conditions increasing as content for increasing temperature geothermometric estimates based on the as content of arsenopyrite as 32 at indicate temperatures up to 420 c for the crystal rims petrographic and cathodoluminescence observations of quartz associated with gold mineralisation show only local brittle deformation and no plastic deformation this supports the notion that the ore transporting veins were emplaced late in the deformation history variations of cathodoluminescence of quartz are correlated with changing al contents al 0 16 wt and can be related to fluctuations in the ph of the mineralising fluids

genetic significance of the trace element content in metamorphic and hydrothermal quartz a reconnaissance study abstract a reconnaissance study on trace elements in metamorphic and hydrothermal quartz was carried out using quartz samples from the tin district erzgebirge germany the gold mineralization at kasperske hory czech republic and the gold quartz vein deposits muruntau and myutenbai uzbekistan a new method of sample preparation has been developed to prepare pure quartz samples by combining conventional hand picking with microscopic and spectroscopic studies as well as acid wash etch procedures preparation of monomineralic samples was followed by sample dissolution and measurement by icp ms the metamorphic quartz has very low concentrations of li 0 4 ppm al 30 ppm k 35 ppm rb 50 ppb sr 0 3 ppm and y 15 ppb moreover it is characterized by light rare earth element enriched lanthanide distribution patterns lacking eu anomalies the low element concentrations in metamorphic quartz are interpreted to result from recrystallization metamorphic quartz from alteration halos enveloping tin and gold deposits has distinctly different trace element signatures these differences are related to the hydrothermal overprint of the pre existing metamorphic quartz by the mineralizing fluids hydrothermally altered metamorphic quartz from tin deposits has elevated concentrations of li 0 9 ppm al 50 ppm k 45 ppm rb 250 ppb and y 40 ppb whereas altered metamorphic quartz from gold deposits is characterized by elevated concentrations of sr 0 5 ppm the rare earth element distribution patterns of altered metamorphic quartz show variable enrichments of the heavy rare earth elements and frequently display positive eu anomalies hydrothermal vein quartz from the gold deposits usually has elevated al 50 ppm and sr 0 6 ppm contents the lanthanide distribution patterns exhibit variable enrichments of the heavy rare earth elements and commonly show positive eu anomalies the elevated sr concentrations in the quartz from gold deposits are best explained by sr release during feldspar alteration in the wall rocks this mechanism may also account for the relative enrichment of eu in the mineralizing fluids although other processes may not be unambiguously ruled out

multi stage alteration rheological switches and high grade gold mineralization at sheba mine barberton greenstone belt south africa abstract structural petrographic and geochemical data from sheba gold mine in the barberton greenstone belt of south africa indicate that gold mineralization formed at the end and as the result of a multi stage structural fluid flow and alteration history this multi stage evolution is a prerequisite for the formation of high grade 40 g t au ore bodies the sheba mine is situated in well bedded metaturbidites of the mesoarchaean fig tree group and structurally infolded ultramafic lavas in the first order refolded ulundi syncline in the northern parts of the belt original komatiitic lava flows are pervasively altered to talc carbonate greyschist this regionally widespread alteration relates to an early phase of deformation and associated fluid flow the reaction softening associated with this alteration accentuates rheological anisotropies within the heterogeneous wall rock sequence this results in the localization of strain along the contact between talc carbonate greyschist and overlying massive greywacke and chert units during later stage refolding and associated flexural slip pervasive quartz veining silicification k enrichment and the progressive replacement of the earlier talc carbonate assemblages by a high variance quartz fuchsite green schist alteration testify to the localized fluid flow along this contact the newly formed finely intergrown green schist assemblage leads in turn to a reaction hardening and embrittlement of the former greyschist chert contact high grade gold mineralization is hosted by brittle quartz carbonate vein networks and breccias developed along brittle ductile structures that cross cut the competent green schist at high angles the siting of gold and predominance of free gold suggest that gold precipitation is related to extreme fluid pressure cycling and phase separation of the mineralizing fluid during the formation of the brittle fracture networks the thrust kinematics of the late stage controlling structure studied here indicate that the mineralized structure accommodated regional nw se subhorizontal shortening when further tightening of the ulundi syncline passed the lock up stage could no longer be accommodated by flexural slip the results highlight that the formation of favourable conditions for high grade mineralization such as those at sheba mine may be the result of the interaction of temporally distinct fluid flow and alteration events and lithological heterogeneities during the progressive structural evolution of a region

ore shoot formation in the main reef complex of the fairview mine multiphase gold mineralization during regional folding barberton greenstone belt south africa gold production in fairview mine in the barberton greenstone belt south africa is focused on high grade steeply plunging ore shoots contained within a gently undulating system of spaced low displacement broadly bedding parallel faults and shear zones collectively referred to as the main reef complex this shear zone system is developed in metaturbiditic sedimentary rocks of the mesoarchean fig tree group on the steep western limb of the regional d3 refolded and distinctly arcuate ulundi syncline shear zones are preferentially localized along lithological contacts with pronounced rheological contrasts particularly pronounced along the limbs of tightly infolded ultramafic schist against fig tree group metasedimentary rocks the main ore shoot of the mrc at fairview shows a steep southeasterly plunge that can be traced for 2000 m down plunge parallel to the fold hinge f3b that refolds the ulundi syncline fabrics and structures in the ore shoot record combined top to the nw thrust sense and dextral strike slip kinematics and strongly constrictional strains interpreted to indicate dextral transpression and associated steep extrusion of the rocks during progressive nw se d3 shortening a combination of structural and lithological controls govern ore shoot formation and gold mineralization the main ore shoot corresponds to dilational jog geometry that developed during dextral transpression bedding parallel shearing consistent with the flexural slip refolding f3b of the ulundi syncline high grade pockets correlate with sheared commonly dismembered graphite rich and sulfide mineralized shale units testifying to the preferential fluid focusing and strain localization into incompetent shale units during deformation cross cutting relationships with other auriferous reefs from the fairview mine complex indicate a late timing of the mrc type mineralization underlining the complex fluid focusing and utilization of differently orientated structures with different kinematics during progressive deformation and either episodic or protracted fluid flow events

an atmospheric source of s in mesoarchaean structurally controlled gold mineralisation of the barberton greenstone belt abstract the barberton greenstone belt of southern africa hosts several mesoarchaean gold deposits the ores were mostly formed in greenschist facies conditions and occur as hydrothermal alteration zones around extensional faults that truncate and post date the main compressional structures of the greenstone belt ore deposition was accompanied by the intrusion of porphyries which has led to the hypothesis that gold may have been sourced from magmas because the transport of au in the hydrothermal fluids is widely believed to have involved s complexes tracing the origin of s may place strong constraints on the origin of au we measured multiple s isotopes in sulfide ore from sheba and fairview mines of the barberton greenstone belt to distinguish deep s sources e g magmas from surface s sources i e rocks of the volcano sedimentary succession that contain s processed in the atmosphere preserved as sulfide and sulfate minerals ion probe sims analyses of pyrite from ore zones indicate mass independent fractionation of s isotopes 33s 0 6 to 1 0 and the distribution of the analyses in the 33s 34s space matches the distribution peak of previously published analyses of pyrite from the entire volcano sedimentary succession notwithstanding that the h2o co2 components of the fluids may have been introduced from a deep source external to the greenstone belt rocks the fact that s bears an atmospheric signature suggests the hypothesis that the source of au should also be identified in the supracrustal succession of the greenstone belt our findings differ from conclusions of previous studies of other archaean shear hosted au deposits based on mineralogical and isotopic evidence which suggested a magmatic or mantle source for au and imply that there is no single model that can be applied to this type of mineralisation in the archaean