syenite associated disseminated gold deposits in the abitibi greenstone belt canada a distinct group of gold deposits in the abitibi greenstone belt is spatially associated with quartz monzonite to syenite stocks and dikes the deposits occur mainly along major fault zones in association with preserved slivers of alluvial fluvial timiskaming type sedimentary rocks the deposits consist of disseminated sulfide replacement zones with variably developed stockworks of quartz carbonate k feldspar veinlets within zones of carbonate albite k feldspar and sericite alteration the syenitic intrusions are broadly contemporaneous with deposition of timiskaming sedimentary rocks and together with disseminated gold mineralization they have been overprinted by subsequent regional folding and related penetrative cleavage disseminated gold orebodies occur within composite syenitic stocks or along their margins along satellite dikes and sills and along faults and lithologic contacts away from intrusions orebodies in these different positions are interpreted to represent proximal to distal components of large magmatic hydrothermal systems centered on and possibly genetically related to composite syenitic stocks

kinematics of large syn orogenic intrusions example of the lower proterozoic saraya batholith eastern senegal the large saraya batholith intruded the lower proterozoic sequences of eastern senegal structural analysis indicates that this large granitic batholith corresponds to a composite body consisting of several coalescent plutons and interfering diapirs interpretation of the structural fabric patterns in these plutons and in the host rocks constitutes the basis of a hypothetical model of initiation ascent and emplacement as follows 1 development of a deep n s granitic ridge controled by a major crustal fracture generated by an earlier extensional phase re activated by a strike slip fault which probably would facilitate segregation collection and ascent of magmas 2 intrusion and splitting up of the ridge into several diapirs which ascended simultaneously and separately in a setting of a n s sinistral transcurrent deformation inducing a transverse nw se shortening and forceful emplacement of plutons and 3 final emplacement characterized by interference between regional deformation and pluton ballooning the latter leading to the coalescence of intrusions and to the formation of a larger apparently homogeneous body

systematics of chlorite alteration at the phelps dodge massive sulfide deposit matagami quebec hydrothermal alteration at the phelps dodge archcan volcanogenic massive sulfide deposit has converted rhyodacite and rhyolite footwall rocks into the following mineral assemblages quartz chlorite albite epidote or sericite quartz chlorite or sericite chlorite or sericite and chlorite talc or stilpnomelane alteration at constant al was initiated by hydration addition of fe mg silicification and leaching of ca na which produced al saturated chlorites with a wide range of fe fe mg ratios within the assemblage quartz chlorite albite epidote or sericite with continued addition of fe and less mg combined with extensive leaching of quartz the chlorite became al undersaturated quartz chlorite talc assemblage and ultimately formed a rock composed essentially of chlorite the chlorite talc or stilpnomelane assemblage is stable in late crosscutting veinlets and in the ore zone chlorites in the si al mg fe system form a rectangular solid solution plane with limited substitution of al for si and extensive substitution of fe for mg in si al space the solid solution plane is outlined by al saturation presence of other al rich mineral and al undersaturation presence of al poor or al free mineral boundaries al iv substitution for si in the chlorites increases from 2 3 to 3 01 formula units based on 8 si al iv with fe fe mg whereas al iv occupancy decreases from 2 88 to 2 65 the valence charge balance is compensated by octahedral site vacancies or by the conversion of fe super 2 to fe super 3 the al iv content of chlorite also increases with temperature chlorites on the al saturated boundary of the solid solution field form a potentially useful geothermometer t degrees c 106 al iv 18 al iv is corrected for changes with the fe fe mg ratio as al iv corrected al iv 0 7 fe fe mg the fe fe mg ratios of bulk rock and contained chlorite correlate well birefringence colors of the chlorite change from green to brown violet and berlin blue as fe fe mg changes from 0 18 to 0 64 the fe fe mg variation apparently depends largely on the proportions of mg rich seawater and fe rich hydrothermal fluid in the altering brine

the early proterozoic birimian supergroup of ghana and some aspects of its associated gold mineralization abstract rocks of the birimian supergroup of early proterozoic age sm nd isochron age 2166 66 ma form part of the west african craton ghana contains the eastern portion of the birimian terrane five parallel evenly spaced several hundred kilometre long volcanic belts consisting mainly of low grade metamorphic tholeiitic lavas morb chemistry characterize the birimian supergroup of ghana the belts are separated by basins containing isoclinally folded dacitic volcaniclastics wackes and argillitic sediments as well as granitoids lavas and sedimentary rocks were deposited contemporaneously as lateral facies equivalents within the sedimentary basins various lithofacies are distinguished in the transition zone between belt volcanics and basin sediments a chemical facies occurs this is defined by cherts manganiferous and carbon rich sediments fe ca mg carbonates and sulphide mineral disseminations after their emplacement birimian lavas and sediments were folded in the course of the eburnean tectono thermal event intruded by various types of granitoids uplifted and eroded the erosion products were deposited as sediments of the tarkwaian group in long narrow intramontane grabens which formed due to rifting preferentially in the central portions of all five birimian volcanic belts tarkwaian sediments were deformed by gravity tectonics two major types of eburnean granitoids in ghana are characterized as follows basin granitoids cape coast type occur as synorogenic foliated batholiths chiefly in the central portions of birimian sedimentary basins they are peraluminous and generally granodioritic in composition belt granitoids dixcove type mostly form late orogenic unfoliated intrusions in volcanic belts they possess a metaluminous character and commonly are of tonalitic composition bongo granitoids are of post tarkwaian age and k rich in contrast to these granitoid types which have strong mantle affinities the winneba type had an archean sialic precursor a model involving small scale equidimensional parallel and contemporaneously operating convection cells in the upper mantle which caused rifting of a highly attenuated protocrust as well as linear eruptions of tholeiitic magmas is proposed recent sm nd pb pb and rb sr data of volcanic and plutonic rocks are incorporated most ghanaian birimian gold occurrences and mines are concentrated in narrow corridors of 10 15 km width in the transition zone between volcanic belts and sedimentary basins as are the chemical facies and regionally extensive shear zones at the volcanics sediment interface birimian gold in ghana is present as two major types 1 the disseminated sulphide type which is generally lithofacies controlled i e controlled by chemical sediments and to a lesser extent by selvages of gold quartz veins and 2 the quartz vein type which is exclusively structure controlled data on geochemical underground traverses across both types of gold mineralization are briefly summarized

quantitative mineralogy from infrared spectroscopic data ii three dimensional mineralogical characterization of the rocklea channel iron deposit western australia this study demonstrates how mineralogy generated from spectroscopic visible near and shortwave infrared reflectance data collected using rapid drill core logging systems can be used to build one two and three dimensional models of the architecture of ore systems with applications toward exploration and mining in particular this study examines both spectroscopic and traditional mining data laboratory geochemistry and field geologist logging from 180 drill holes through the rocklea channel iron deposit in the hamersley region of western australia valuable infrared reflectance spectroscopy based mineralogy includes the types and abundances of iron oxyhydr oxides hematite vitreous goethite and ochreous goethite clays well and poorly ordered kaolin al and fe smectite and carbonates calcite and dolomite algorithms to determine the abundance and composition of these minerals from infrared reflectance spectra were validated in an associated study by haest et al 2012 and were found to be accurate e g root mean square error rmse for fe oxyhydr oxide abundance prediction of 9 1 wt fe petrographic analysis and validated infrared reflectance spectroscopy based mineralogy constrained the stratigraphy in the paleochannel and the paragenetic history of each horizon the rocklea channel iron deposit developed over weathered basalt and metasedimentary rocks with an internal channel stratigraphy dominated from bottom to top by the following 1 well ordered in situ kaolinite and a partially denaturated channel iron deposit 2 poorly ordered transported kaolinite 3 ochreous goethite with scarce ooidal textures no clays and late stage vitreous goethite silica replacement 4 mostly poorly ordered kaolinite and 5 calcrete with associated fe smectite the mineral paragenesis of the rocklea deposit was compared against current models for channel iron deposit formation a key finding is the relationship between parent rock composition superimposed regolith cover and channel iron deposit ore quality which are linked through ground water interactions and the physicochemistries of both the parent rock and the deposit this has implications for both exploration and ore deposit characterization 1 ore grade channel iron deposits can develop several kilometers downstream 50 km in this case from the proposed source rock areas above mafic volcanics and metasediments 2 vitreous goethite calcrete al and fe smectite cover the rocklea deposit and are proposed as potential vectors to channel iron deposit mineralization at depth 3 improved mineralogy from hyperspectral data in terms of iron oxyhydr oxide and clay content composition can improve iron ore resource delineation iron ore processing separation of vitreous and ochreous goethite and pit design differentiating swelling smectite requiring pit walls with lower slopes from kaolin or white mica

a fluid inclusion and stable isotope study at the loulo mining district mali west africa implications for multifluid sources in the generation of orogenic gold deposits the world class loulo mining district 15 5 moz resource in the birimian terrane of western mali contains a range of mineralogically diverse styles of orogenic gold mineralization the district is distinguished by as rich orebodies as at yalea as well as tourmaline bearing fe rich orebodies as at gara new fluid inclusion and stable isotope data presented here constrain the nature of the hydrothermal fluids responsible for these different types of mineralization and point towards the role of multifluid sources metamorphic and magmatic in the formation of orogenic gold deposits microthermometric and laser raman studies from yalea main and two other similar orebodies loulo 3 and baboto reveal the dominance of coexisting co 2 n 2 ch 4 type 1 and h 2 o nacl type 2 fluid inclusions these inclusions show evidence of fluid unmixing from reduced quartz faylite magnetite qfm buffered low salinity 10 wt nacl equiv co 2 rich h 2 o nacl n 2 ch 4 primary ore fluids the combination of microthermometric data and geothermometry based on ore and alteration assemblages indicate trapping temperatures and pressures of 270 to 340 c at 1 4 to 1 8 kbar the p t x state of the yalea style mineralizing fluids along with the 18 o fluid compositions of 8 8 to 10 7 is consistent with the derivation of auriferous fluids during greenschist facies regional metamorphism of the host terrane similar fluid compositions were previously reported elsewhere in the birimian crust of west africa and in other orogenic gold districts worldwide the precipitation of gold from the h 2 s rich metamorphic fluid is primarily linked to phase separation of the ore fluid which is controlled by p t fluctuations and or fluid rock interaction with carbonaceous host sedimentary rocks confirmed by low 13 c values of 21 7 to 15 8 fluid inclusion investigations from gara and a similar style orebody yalea north indicate the presence of coexisting co 2 n 2 ch 4 type 1 and mixed salinity 5 21 wt nacl equiv co 2 rich h 2 o nacl inclusions type 3 inclusion assemblages also contain common oxidized hematite magnetite hm buffered high temperature 400 c hypersaline 35 50 wt total dissolved solids metalliferous na fe ca cu ni w pb zn multiphase h 2 o rich co 2 nacl fecl 2 inclusions type 4 this inclusion type has not been previously reported in other birimian terranes the composition of the brines along with carbon isotope data 13 c of 14 4 to 4 5 suggests a magmatic input to the gara style hydrothermal system the coexistence in the fluid inclusion assemblages of the magmatic brines with yalea style co 2 rich metamorphic fluids and the positive correlation between salinity and homogenization temperatures suggest mineralization was locally controlled by fluid mixing the interaction of these two chemically contrasting fluids explains the distinctive petrographic characteristics of the gara style orebodies this includes the growth of widespread multi stage fe 3 rich tourmaline b rich granite source and sodic alteration and ore assemblages consisting of abundant nickeloan pyrite ree phosphates ni co pb zn minor trace sulfides and scheelite gold deposition in the gara style hydrothermal system is related to physical and chemical changes of the two fluids during mixing e g decreases in f o2 and t in the brines and retrograde boiling of the co 2 component in the metamorphic fluids a salting out effect

origin and significance of tourmaline rich rocks in the broken hill district australia tourmaline rich rocks are widespread minor lithologies within the early proterozoic willyama supergroup in the broken hill district australia tourmaline concentrations occur in strata bound and local stratiform tourmalinites clastic metasedimentary rocks quartz gahnite lode rocks stratiform pb zn ag sulfide ores garnet quartzites strata bound scheelite deposits quartz tourmaline nodules discordant quartz veins and granitic pegmatites most of the tourmaline rich rocks are within the broken hill group that hosts the main pb zn ag ores at the globe mine along the northeast end of the main lodes tourmalinites are closely associated with pb zn ag mineralization and in places are interbedded with mn rich garnet quartzites galena and other ore minerals occur locally in the cores of recrystallized tourmaline grains in these tourmalinites indicating that tourmaline and sulfides were present together prior to deformation and high grade metamorphism electron microprobe analyses of tourmalines intergrown with fe sulfides at the globe mine show mg rich compositions relative to tourmalines in sulfide free assemblages from the same area suggesting early premetamorphic introduction of boron and mg enrichment of tourmaline by sulfide silicate reactions during metamorphism combined field and geochemical data indicate that the district tourmalinites represent normal clastic sediments that were metasomatically altered by boron rich hydrothermal fluids at or below the sediment water interface whole rock chemical analyses of 33 tourmaline rich rocks show linear trends of data for major and trace elements that closely resemble the trends observed for unmineralized elastic metasedimentary rocks of the district average fe al mg al na al and ti al molar ratios of the tourmaline rich rocks and clastic metasediments are very similar the average k al molar ratio of the tourmaline rich rocks is significantly lower than that of the clastic metasediments reflecting the loss of k during tourmalinite formation chondrite normalized patterns of rare earth elements ree in the quartz rich tourmalinites are generally similar to those of the clastic metasediments except for minor depletions of light ree local positive and negative ce anomalies suggest tourmalinite formation in the presence of seawater or a seawater derived pore fluid the geochemical data imply relative immobility of al ti cr and heavy ree during hydrothermal alteration and later metamorphism boron isotope analyses of 52 tourmaline separates show a total range of delta 11 b values from 26 8 to 17 0 per mil fine grained euhedral nonpoikilitic tourmalines from tourmalinites in the andalusite muscovite zone in the northern part of the district e g black prince mine have delta 11 b values from 21 to 17 per mil whereas coarse granoblastic and poikilo blastic tourmalines from the sillimanite and two pyroxene granulite zones in the southern part of the district e g globe mine have delta 11 b values of 24 to 19 per mil tourmalines in strongly retrogressed tourmalinites have delta 11 b values from about 27 to 20 per mil the observed variations in delta 11 b are consistent with prograde and retrograde metamorphic fractionation of boron isotopes in which the fluid phase is preferentially enriched in the heavier isotope 11 b premetamorphic hydrothermal fluids that deposited the black prince tourmalinites had delta 11 b values of 8 to 5 per mil at 200 degrees to 300 degrees c suggesting a boron source from nonmarine evaporite borates tourmalinites in the broken hill district apparently formed by the same submarine hydrothermal processes as the main pb zn ag lodes and the siliceous ferromanganese protoliths of the garnet quartzites in our model the hydrothermal system s acquired abundant boron by leaching evaporitic borates within the thackaringa group the stratigraphic sequence that underlies the broken hill group and most of the tourmaline concentrations we suggest that evaporites of the thackaringa group provided a source of readily extractable boron for formation of the tourmalinites and also the source of the fluoride sulfur and perhaps the carbonate in the main lodes such evaporites may have been critical for increased metal chloride complexing and transport necessary for deposition of the high grade pb zn ag ores the broken hill deposit may have formed contemporaneously with the mount isa and mcarthur river pb zn ag deposits in similar evaporite bearing sequences during widespread early proterozoic continental rifting

fractal nature of the planetary boundary layer depth in the trade wind cumulus regime the fractal nature of the planetary boundary layer depth in the trade wind cumulus regime was explored the boundary layer depths were derived from discontinuities in airborne lidar backscatter profiles obtained during the frontal air sea interaction experiment fasinex in february 1986 scale invariant behaviour was observed over horizontal distances ranging from 50 m to 50 km the degree of intermittency and non stationarity of the signals were calculated using standard multifractal statistical methods

metamorphic origin of ore forming fluids for orogenic gold bearingquartz vein systems in the north american cordillera constraints froma reconnaissance study of δ15n δd and δ18o the western north american cordillera hosts a large number of gold bearing quartz vein systems from the mother lode of southern california through counterparts in british columbia and southeastern alaska to the klondike district in central yukon these vein systems are structurally controlled by major fault zones which are often reactivated terrane bounding sutures that formed in orogens built during accretion and subduction of terranes along the continental margin of north america mineralization ages span mid jurassic to early tertiary and encompass much of the evolution of the cordilleran orogen nitrogen contents and 15n values of hydrothermal micas from veins are between 130 and 3 500 ppm and 1 7 to 5 5 per mil respectively these values are consistent with fluids derived from metamorphic dehydration reactions within the phanerozoic accretion subduction complexes which have 15n values of 1 to 6 per mil the 18o values of gold bearing vein quartz from different locations in the cordillera are between 14 6 and 22 2 per mil but are uniform for individual vein systems the d values of hydrothermal micas are between 110 and 60 per mil ore fluids have calculated 18o values of 8 to 16 per mil and d values of 65 to 10 per mil at an estimated temperature of 300 c d values of ore fluids do not show any latitudinal control these results indicate a deep crustal source for the ore forming fluids most likely of metamorphic origin low dh2o values of 120 to 130 per mil for a hydrous muscovite from the sheba vein in the klondike district reflect secondary exchange between recrystallizing mica and meteoric waters collectively the n h and o isotope compositions of ore related hydrothermal minerals indicate that the formation of these gold bearing veins involved dilute aqueous carbonic and nitrogen bearing fluids that were generated from metamorphic dehydration reactions at deep crustal levels these data are not consistent with either mantle derived fluids or granitoid related magmatic fluids nor do they support a model involving deeply circulated meteoric water

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

dislocation formation and albitization in alkali feldspars from the shap granite orthoclase rich alkali feldspars in the lower devonian shap granite northwest england contain two generations of albite rich feldspar these have partially replaced earlier exsolution microtextures consisting of albite lamellae coarse semicoherent albite films and fine coherent albite platelets in tweed orthoclase the earlier generation of replacive albiterich feldspar ab90an9ori occurs together with orthoclase rich feldspar ab10or90 i n veins that crosscut exsolution microtextures throughout grain interiors this episode of recrystallization was mediated by magmatic fluids at 410 8c estimated from two feldspar geothermometry and was driven by stored elastic strain energy which was relatively homogeneously distributed throughout the microtextures the later generation of replacive albite rich feldspar which is restricted to grain margins and is compositionally pure ab 99 was produced by magmatic hydrothermal fluids at 370 8c this generation of albite rich feldspar does not crosscut exsolution microtextures and has selectively replaced volumes of highly elastically strained feldspar surrounding edge dislocations along semicoherent albite films marked differences in controls of the localization of the two generations of replacive albite rich feldspar by pre existing exsolution microtextures indicate that significant numbers of edge dislocations developed along albite films after the first phase of fluid feldspar interaction and associated albitization but before the second phase this relation indicates that edge dislocations formed between 410 and 370 8c these observations have important implications for understanding the factors that control the interaction of alkali feldspars with fluids both in cooling igneous rocks and in clastic sedimentary rocks during diagenesis

transcurrent tectonics and polycyclic evolution in the lower proterozoic of senegal mali abstract outcrops of the early proterozoic birimian series within the kedougou inlier straddling the mali senegal border have traditionally been described as being composed of a basal volcanic mako series overlain by a detrital carbonate dialle dalema series this geosynclinal context was later reinterpreted in terms of oceanization and subduction as the mako series contains successively basaltic pillow lavas of tholeiitic affinity and a calc alkaline complex however the sedimentary dialle dalema series lie in the core of a major anticlinorium and the rocks show overprinting of an earlier by a later tectonic phase the volcanic and sedimentary rocks of the mako series were affected only by the later tectonic phase in addition a fan delta deposit intercalated between the tholeiitic and calc alkaline rocks contains pebbles of schistose and folded sedimentary rock that clearly belong to the dialle dalema series these observations necessitate a complete revision of the local stratigraphy thus the clastic deposits of the dialle dalema are attributed to the lower birimian and are interpreted as having been deposited in intra cratonic basins the first tectonic phase took place before emplacement of the volcanic sereis which shows evidence of an extensional setting with major crustal fracturing the second deformation affected both the sedimentary and volcanic series structural analysis furthermore shows that the formation of the second cleavage s2 was contemporaneous with the emplacement of major granite bodies as well as with major transcurrent sinistral shear zones finally this evolution is shown to be comparable with that in other lower proterozoic terranes of the west african craton

metamorphism in archaean greenstone belts calculated fluid compositions and implications for gold mineralization an inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions for mafic and ultramafic sequences the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits xco2 0 2 0 3 this result follows from the calculation of mineral equilibria in the model system cao mgo feo al2o3 sio2 h2o co2 using a new expanded internally consistent dataset greenstone metamorphism cannot have involved much crustal over thickening because very shallow levels of greenstone belts are preserved such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere in this case the observed metamorphism which was contemporaneous with deformation is of the low p high t type for this type of metamorphism the metamorphic peak should have occurred earlier at deeper levels in the crust i e the piezothermal array should be of the deeper earlier type however at shallow crustal levels the piezothermal array is likely to have been of deeper later type as a consequence of erosion thus while the lower crust reached maximum temperatures and partially melted to produce the observed granites mid crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde we propose that these fluids are responsible for the gold mineralization thus the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution and does not mean that the mineralization has to be a consequence of igneous processes upward migration of metamorphic fluid via appropriate structurally controlled pathways will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower xco2 these assemblages are therefore grossly out of equilibrium with the fluid in the case of infiltrated metabasic rocks intense carbonation and sulphidation is predicted if as seems reasonable gold is mobilized by the fluid generated by devolatilization then the combination of processes proposed most of which are an inevitable consequence of the metamorphism leads to the formation of greenstone gold deposits predominantly from metamorphic fluids

early proterozoic ore deposits and tectonics of the birimian orogenic belt west africa abstract the early proterozoic of the west african craton comprises a series of volcanic troughs and sedimentary basins the birimian with granitic terranes accreted on archean nuclei in the man and reguibat shields studies of the birimian of the man shield indicate a model of polycyclic evolution with a major collision event d 1 2 1 ga thrusting part of the proterozoic terrane over the archean before individualization of numerous volcanic troughs and clastic infill basins the proposed evolution for the birimian orogenic belt comprises 1 deposition of the sedimentary lower birimian b1 with minor tholeiitic volcano sedimentary intercalations containing chert and or mn formations and with most of the detritus being derived from early proterozoic sources apart from contamination near the proterozoic archean contact 2 pre b2 crustal thickening related to d 1 thrusting 3 formation over about 40 ma of the upper birimian b2 with numerous volcanic troughs of different composition tholeiitic and rare komatiitic bimodal tholeiitic to calc alkaline volcano plutonic and tarkwaian clastic infill basins and 4 major transcurrent tectonics with sinistral d 2 and dextral d 3 strike slip faults such a tectonic evolution from a collision d 1 phase to a transcurrent d 2 d 3 phase is typical of collision belts and in the present case the evolution of the birimian orogenic belt can be extended into guyana the metallogenic history of the birimian shows a three phase evolution coinciding with the orogenic evolution and extends over almost 150 ma from the perkoa massive zn ag sulfides 2 12 ga with a clear mantle affinity to the latemesothermal au quartz veins 2 ga with according to lead isotopes a high crustal participation the economic mineralization of belt thus consists of 1 1 pre orogenic pre d 1 deposits related to early extension zones this was diverse with stratiform au tourmalinite type 1 au loulo in mali dorlin in guyana stratiform fe cu faleme in senegal and mn nsuta in ghana tambao in burkina faso and a single massive zn ag sulfide deposit perkoa in burkina faso associated with regional volcanosedimentary variably tholeiitic stratigraphic marker beds 2 2 syn orogenic post d 1 to syn d 2 d 3 deposits with disseminated au sulfides type 2 au yaoure in the ivory coast in extensional zones of the b2 followed by auriferous paleoplacers type 3 au in b2 extensional zones tarkwaian banket conglomerate or syn d 2 transtensional zones debris flow of orapu in guyana 3 3 late orogenic post peak d 2 d 3 deposits with mesothermal au mineralization evolving from a disseminated gold bearing arsenopyrite and au quartz lode type type 4 au ashanti in ghana to a quartz vein type with free gold and cu pb zn ag bi paragenesis most of the gold in west africa comes from this phase finally the metallogeny of the birimian appears as rich in au and mn formations and poor in volcanogenic mineralization and bif it differs from the archean metallogeny of west africa and other regions through the presence of certain deposit types such as au stratiform tourmalinite gold bearing conglomerate with au and fe ti oxides but no uranium or sulfides and au arsenopyrite rich shear zone deposits that are very common in but not exclusive to the proterozoic and paleozoic

the boron isotopic composition of tourmaline as a guide to fluid processes in the southwestern england orefield an ion microprobe study the boron isotopic composition 11b and chemistry of tourmalines from a series of sites in the western southwest england orefield has been determined using ion microprobe and electron microprobe techniques respectively the range of 11b values determined is relatively wide 0 6 to 12 7 and falls at the lighter end of the range for typical continental crust within this range granite tourmalines show a depletion in 11b from coarse grained early magmatic examples to fine grained late magmatic tourmaline we attribute this to fractionation of boron isotopes between aqueous and melt species during the exsolution of an aqueous fluid phase from the magma the aqueous phase formed by degassing of the magma is considered to have crystallised to form the tourmalinites and quartz topaztourmaline rocks common throughout the area decreases in the 11b values occur within certain zones of individual tourmaline crystals from these rocks this is ascribed to loss of 11b in the vapour phase during boiling caused by decompression of the aqueous fluid on fracturing the calculated boron isotope composition of fluids in equilibrium with tourmalines from the cligga head greisen bordered vein system agrees closely with that calculated for the tourmalinite fluid and suggests a constant granite derived source of boron during the early hydrothermal evolution of the stock the lack of isotopic zoning within the tourmalines from the cligga head hydrothermal veins suggests that phase separation did not occur or was very limited during hydrothermal circulation in the vein system

round hill shear zone hosted gold deposit macraes flat otago new zealand evidence of a magmatic ore fluid the 0 8 moz round hill shear zone hosted gold deposit is one of several located on or near the hyde macraes shear zone macraes flat new zealand field relationships show stockwork veins are the oldest part of the hydrothermal system followed by flat and ramp veins hanging wall shear veins formed early but have been modified by late deformation fluid inclusion microthermometric results for dominant type i inclusions show that flat and ramp veins have the same average wt percent nacl equiv values 2 2 0 6 and were precipitated from the same fluid salinity data for these veins overlap with those from hanging wall veins average 1 3 0 3 and stockwork veins 1 6 0 6 for vein margins 0 9 0 3 for vein centers although both of the latter trend toward lower overall wt percent nacl equiv values suggesting mixing between relatively dilute and more saline end members all of the above veins have similar temperatures of homogenization t h ranging between 110 and 188 c fluid inclusion trapping temperatures are considered to be 300 c equating to a 10 km depth of formation carbon dioxide dissolved in the hydrothermal fluid is responsible for up to one third of fluid inclusion apparent salinities stockwork and hanging wall veins thus have true salinities of 1 wt percent nacl equiv 0 17 molal and flat and ramp veins have true salinities of fluid inclusion volatiles from round hill and other macraes deposits are dominated by h 2 o 99 02 99 69 mol with lesser co 2 0 14 0 76 mol n 2 0 03 0 32 mol and ch 4 0 06 0 17 mol noticeable concentrations of hydrocarbons occur in many of the samples fluid inclusion cation and anion data show few components of relatively low concentration are dissolved in the hydrothermal fluid consistent with co 2 corrected microthermometric measurements a range in fluid inclusion d h 2 o values obtained for the round hill deposit is coincident with the different vein types and also indicates mixing between two fluids one fluid is considered to be o isotope shifted meteoric water as trapped by the early stockwork veins and has the lowest d h 2 o values of 80 per mil the other fluid is represented by flat and or ramp and hanging wall veins that have d h 2 o values up to 40 per mil the macraes d h 2 o data show a trend toward the magmatic water end member for geothermal systems associated with convergent plate boundaries and are consistent with a 25 to 50 percent magmatic component in the macraes hydrothermal system

the geology and mineralogy of the loulo mining district mali west africa evidence for two distinct styles of orogenic gold mineralization loulo is a world class orogenic gold mining district in the birimian terrane of western mali orebodies are located along second or higher order shears associated with the senegal mali shear zone with gold mineralization largely linked to a transtensional event the mine camp is divided into two distinct styles of gold deposit on the basis of differing geologic characteristics one group is typified by the gara deposit whereas the other by the yalea deposit both deposit styles are hosted by similar rock types calcareous graywackes and calciticdolomitic marbles gara style orebodies occur as sulfide disseminations or ankerite rich shear vein stockworks hosted in folded tourmalinized sandstones and breccias mainly within 2 km of the senegal mali shear zone these deposits are characterized by intense multi stage albitization and tourmalinization pre syn and postmineralization gold lodes are fe rich dominated by nickeloan pyrite contain cu ni co minor and trace sulfides e g chalcopyrite gersdorffite pentlandite cobaltite millerite and show consistently high levels of p ree w bearing phases apatite monazite xenotime and scheelite base metal concentrations show a marked increase in marble host rocks with the formation of nickeloan pyrite cobaltite clausthalite ores in contrast yalea style deposits are associated with quartz ankerite vein lodes and disseminated sulfide stringer zones mineralization occurs along highly altered tourmaline absent brittle ductile shears up to 8 km away from the senegal mali shear zone wall rock alteration is characterized by addition of k 2 o cao co 2 h 2 o and sio 2 with mineral assemblages consisting of chlorite sericite carbonate quartz albite ore paragenesis is enriched in as mainly as multistage growth of arsenopyrite and arsenian pyrite base metal sulfides scheelite and ree phosphates are extremely rare the diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region the as rich yalea style deposits have characteristics typical of birimian gold mineralization in ghana and auriferous fluids are likely derived from the dewatering of sedimentary rocks during regional metamorphism on the other hand the polymetallic fe b rich gara style orebodies show atypical features for birimian gold mineralization instead field relations and the mineralogy and geochemistry of the gara style lodes indicate a strong hydrothermal influence from surrounding intrusive stocks with possible links between gold mineralization and iron oxide skarn development in the region the data collected at loulo highlight the diverse nature of orogenic gold deposits especially in west africa this style of ore deposit can form from a variety of fluid sources both metamorphic and magmatic

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

formation of the auriferous and barren north dipper veins in the sigma mine val d or canada constraints from structural mineralogical fluid inclusion and isotopic data the sigma lamaque mine 100 metric tons t of gold is an example of an archean greenstone belt gold deposit formed by processes related to multistage hydrothermal fluid circulation the earliest recognized vein systems are steeply to moderately dipping fault fill veins within shear zones and contemporaneous subhorizontal veins which were overprinted by the late north dipper vein system the latter strikes east west dips moderately to the north and exhibits complex internal geometry with features similar to fault fill and extensional veins and therefore is interpreted as extensional shear veins this late vein system was considered one of the major sources of gold in the shallow underground and open pit ore reserve estimations these late north dipper veins were studied in detail in the sigma pit where they contain ore grade gold and are hosted by the calc alkaline tuffs of the val d or formation and in the north zone where they are barren and hosted by the tholeiitic volcanic rocks of the jacola formation in the sigma pit these veins have apparent thicknesses of 0 8 to 3 m and very irregular contacts with the vein walls they comprise irregular zones of milky quartz filling disrupted blocks of layered quartz and tourmaline and slabs of foliated pyrite rich wall rock layers with tourmaline and quartz crystals oriented perpendicular to the vein walls are observed close to the vein walls the veins contain minor amounts of calcite pyrite rutile and traces of chlorite muscovite pyrrhotite chalcopyrite galena bismuth tellurides and gold locally the sulfide content can attain 10 modal percent of the vein mineralogy in the veins native gold with 6 wt ag occurs rarely associated with the main vein filling minerals commonly as inclusions in pyrite main vein filling stage and most commonly filling fractures in pyrite tourmaline and quartz where it is alloyed with 8 to 17 wt percent ag and is associated with calcite and bi tellurides late auriferous stage the proximal wall rock alteration related to the auriferous north dipper veins overprints the metamorphic assemblage and is characterized by abundant fine grained plagioclase calcite quartz and minor chlorite tourmaline and pyrite in the tuffs and more abundant muscovite in the intrusive feldspar porphyry in the wall rock gold with 6 to 28 wt percent ag is found locally filling fractures in pyrite where it is associated with a late generation of calcite the barren north dipper vein also comprises mainly quartz and tourmaline but lacks calcite and gold has a very low content of sulfides aqueous carbonic fluid inclusions with salinities less than 8 wt percent nacl equiv and homogenization temperatures between 228 and 440 c and carbonic fluid inclusions with less than 0 2 mol percent ch 4 coexisting in the same healed fractures are common in the auriferous north dipper veins but very rare in the barren vein where aqueous fluid inclusions have lower temperatures dominantly 125 225 c crush leaching and laser ablation time of flight inductively coupled mass spectrometry la tof icpms of fluids in the inclusions indicate elevated concentrations of ni cu sb pb and ag in the fluids in the auriferous veins compared to the aqueous fluids in the barren veins this suggests different sources for the metals in the fluids associated with the gold possibly including both igneous and sedimentary rocks calculated oxygen isotope equilibration temperatures for quartz tourmaline pairs from the auriferous main vein filling stage are 323 to 370 c this is within the range documented in fluid inclusions related to the late auriferous stage indicating that the temperature of the auriferous hydrothermal fluids did not change appreciably during the evolution of the system calculated 18 o h 2 o values for the mineralizing fluids are 9 5 per mil consistent with either low water rock ratio systems associated with high grade metamorphism or a magmatic origin for the fluids gold included in pyrite may have precipitated during the main vein filling stage due to the decrease of h 2 sconcentration in the hydrothermal fluid following the deposition of sulfides however gold in fractures in the main vein filling minerals precipitated later due probably to dilution of the auriferous aqueous carbonic fluids by mixing with aqueous fluids the absence of ore grade gold in the barren north zone vein is probably due to the lack of circulation of auriferous aqueous carbonic fluids in the north zone located distal to the first order larder lake cadillac fault which may have served as the main regional conduit for the auriferous aqueous carbonic fluids

an epigenetic origin for the passagem de mariana gold deposit quadrilatero ferrifero minas gerais brazil abstract the gold tourmaline quartz vein deposit of passagem de mariana in the southeastern part of the quadrilatero ferrifero produced more than 60 tonne of gold from the end of the 17th century until 1954 the mine has not operated since 1985 orebodies are veins composed of white quartz 60 vol carbonate ankerite tourmaline sericite and sulfides tourmaline dravite up to 10 vol of the vein occurs as subhedral coarse commonly zoned crystals and is concentrated along vein boundaries and on the edges of host rock inclusions in the veins tourmaline is present in all rock types in the mine but the chemical composition of the host rocks determinates the intensity of tourmalinization with the alteration being greater in sericitic phyllites graphite sericite phyllites and calcareous rocks the most abundant sulfide is arsenopyrite which is normally associated with pyrite and pyrrhotite minor amounts of chalcopyrite galena lollingite berthierite and maldonite are present throughout the deposit sulfides are concentrated at veins boundary or are dispersed in the veins arsenopyrite is associated most commonly with calcareous rocks and graphite sericite phyllite pyrrhotite is usually found at the base of itabirites gold abundance is directly proportional to sulfide concentration hydrothermal alteration associated with the veins includes silicification tourmalinization and sulfidation the mineralized zone is a shear zone associated with a bedding parallel thrust fault that juxtaposes the itabirite lower proterozoic minas supergroup over other units this shear zone thrust fault extends for tens of km beyond the passagem mine and hosts numerous gold deposits the richest orebodies are along the itabirite footwall contact and within the graphite sericite phyllite main orebody although many lithologic units were mineralized the graphite sericite phyllite appears to have been most favorable for gold deposition the area underwent three phases of deformation d1 d2 and d3 mineral assemblages indicate upper greenschist to lower amphibolite conditions of regional metamorphism retrograde metamorphism characterized by chloritization of biotite and chloritization and biotitization of garnet developed locally the gold bearing veins crosscut the main foliation and lithologic contacts at low angle and occur within or are in contact with all lithotypes field and laboratory data indicate that gold mineralization at passagem de mariana is epigenetic gold deposition occurred after the peak of metamorphism within the late to post d2 period of deformation which is correlated with second set of structures of trasamazonian age of alkmim and marshak alkmim f f marshak s 1998 transamazonian orogeny in the southern sao francisco craton region minas gerais brazil evidence for paleoproterozoic collision and collapse in the quadrilatero ferrifero precambrian research 90 29 58 indicating that the gold mineralization occurred between 2 124 and 2 04 ga we choose to regard passagem de marina as an orogenic gold deposit as defined by groves et al groves d i goldfarb r j gebre mariam m hagemann s g robert f 1998 orogenic gold deposits a proposed classification in the context of their crustal distribution and relationship to other gold deposit types ore geology reviews 13 7 27 i e an epigenetic structurally hosted lode gold vein system in a deformed metamorphic terrane

fluid evolution and origins of iron oxide cu au prospects in the olympic dam district gawler craton south australia geologic observations suggest two stages of hydrothermal activity at a number of presently subeconomic iron oxide copper gold systems in the olympic dam district eastern gawler craton they contain high and moderate to low temperature fe oxide rich hydrothermal alteration the mineral assemblages include magnetite calc silicate alkali feldspar fe cu sulfides and hematite sericite chlorite carbonate fe cu sulfides u ree minerals in all documented prospects the minerals of the hematitic assemblages replace the minerals of the magnetite rich assemblages the bulk of the subeconomic cu au mineralization is associated with the hematitic alteration assemblages microanalysis by proton ion probe pixe of hypersaline fluid inclusions in magnetite rich assemblages however demonstrates that significant amounts of copper 500 ppm were transported by the early stage high temperature 400 c fluids responsible for the magnetite rich alteration these brine inclusions contain multiple solid phases liquid vapor multiple solids including chalcopyrite in some cases in comparison inclusions of the hematitic stage are relatively simple liquid vapor types with homogenization temperatures of 200 to 300 c and containing 1 to 8 wt percent nacl equiv the br cl ratios of the magnetite forming fluids measured by pixe lie beyond the range of typical magmatic and or mantle values allowing for the possibility that the fluids originated as brines from a sedimentary basin or the crystalline basement sulfur isotope compositions of chalcopyrite and pyrite demonstrate that sulfur in both alteration assemblages was derived either from cooling magmas and or crystalline igneous rocks carried by relatively oxidized fluids so42 h2s 34ssulfides from 5 to 2 or from crustal sedimentary rocks 34ssulfides from 5 to 10 oxygen and hydrogen isotope compositions of waters calculated for minerals of the magnetite rich assemblage have 18o values of 7 7 to 12 8 per mil and d values of 15 to 21 per mil the only available 18o and dfluid values for the hematitic assemblage are 4 7 and 9 per mil respectively the isotopic compositions of both fluids coupled with the available literature data can be explained in terms of fluid reequilibration with felsic gawler range volcanics or other felsic igneous rocks in the region and with metasedimentary rocks of the wallaroo group at low water to rock ratios prior to their arrival at the mineralization sites the lack of significant copper mineralization associated with magnetite forming fluids that carried copper suggests that there was no effective mechanism of saturation of copper minerals or the quantity of these fluids was not sufficient to produce appreciable copper mineralization association of the copper gold mineralization with the hematitic alteration in the subeconomic prospects can be explained by a two stage model in which preexisting hydrothermal magnetite with minor associated copper gold mineralization was flushed by late stage oxidized brines that had extensively reacted with sedimentary or metamorphic rocks the reduction of these brines driven by conversion of magnetite to hematite resulted in precipitation of copper and gold the oxidized brines may have contributed additional copper and gold to the system in addition to upgrading preexisting subeconomic cu au mineralization when compared to published models for the olympic dam deposit the new data for fluids in subeconomic fe oxide cu au prospects of the olympic dam district indicate the diversity of origins of iron oxide copper gold systems even within the same geologic region

the paleoproterozoic dorlin gold deposit french guiana genetic constraints of the stable isotope geochemistry abstract this paper reports on oxygen hydrogen carbon and sulphur isotopic compositions of silicates carbonates and sulphides from the dorlin gold deposit in the paleoproterozoic greenstone belt of central french guiana the dorlin orebodies result from the polydeformation of a large tourmalinite bearing stratabound sulphide deposit hosted by volcanic and sedimentary rocks of the paramaca series metamorphosed in the greenschist facies the mineralizing envelope would initially have consisted of an inner zone of tourmaline sulphide bearing ores m2 surrounded by a zone of chlorite sulphide bearing mineralization m1 the stratabound sulphide deposit was locally cut by a muscovite sulphide quartz stockwork m3 the 18 o values of quartz and chlorite from the chlorite sulphide mineralization are 12 2 and about 5 5 smow respectively those of quartz and tourmaline from the tourmaline sulphide mineralization are about 14 and 8 5 smow and those of quartz and muscovite from the discordant stockwork are 12 4 and 6 9 smow d values of the chlorite tourmaline and muscovite are about 45 28 and 38 smow respectively carbonates calcite dolomite ankerite and siderite in the mineralizations have 18 o values between 10 8 and 15 7 relative to smow and 13 c values of 5 2 to 0 8 relative to pdb 34 s values of samples from the three mineralizations range between 0 5 to 2 6 cdt with two exceptions at 2 8 and 2 9 the data support the existence of a zoned hydrothermal system largely controlled by a network of paleofractures and slightly reworked by a late muscovite sulphide quartz stockwork the isotopic geothermometers record temperatures of ore deposition in the range of 200 300 c the d and 18 o values of the chlorites tourmalines and muscovites would indicate a large input of seawater in the dorlin hydrothermal fluids whereas the 18 o and 13 c of the carbonates and 34 s of pyrite provide evidence of a magmatic component resulting probably for a large part from leaching of the volcanic host rocks

a carbon oxygen hydrogen and sulfur isotopic study of the gold mineralization at loulo mali abstract the isotopic compositions of oxygen hydrogen carbon and sulfur have been determined on silicate carbonate and sulfide samples from auriferous tourmalinized sandstone its footwall and hanging wall of birimian age 2 1 ga at loulo in mali the sandstones have undergone two phases of deformation the samples were taken from three different orebodies loulo 0 loulo 3 and p64 for loulo 0 18o values for vein quartz are consistently between 16 1 and 16 6 smow for oxygen with one exception at 11 6 the 18o values for detrital quartz grains from the tourmalinized sandstone are also 16 1 tourmaline has a high 18 o of 13 9 the 18o values for total rocks are also relatively homogeneous at 13 2 to 15 7 the lowest values 13 2 and 14 being those for the footwall samples dolomites from loulo 0 and p64 have 18o values between 15 1 and 19 5 smow for oxygen and 13c values of 13 5 to 2 0 pdb with the exception of one sample from the hanging wall which is of sedimentary origin 13 c 0 5 18 o 22 2 the sampled dolomites are in veinlets or veins or disseminated in rocks and were deposited by hydrothermal fluids the 34s values of samples from the three orebodies range between 6 0 and 12 9 cdt they fall into different groups according to location but pre and post schistosity sulfides within each group are indistinguishable the results are interpreted as indicating the existence of two hydrothermal systems enriched in 18o at different times one of these gave rise to tourmalinization silicification and dolomitization of the sandstone and the deposition of sulfides the high d values of the tourmalines in agreement with petrological data indicate that the fluid of this system was of marine origin the two phases of folding intervened between the activity of this hydrothermal system and the initiation of the second system which gave rise to the stockwork this fluid rich in 18o was possibly of metamorphic origin with a low water rock ratio it remobilized the pre schistosity sulfides concentrating them into post schistosity veins the partly remobilized silica and carbonates were deposited under temperature conditions such that their isotopic compositions were similar to those observed for the tourmalinized sandstone the vertical extent of this hydrothermal system is as yet poorly known it is very extensive horizontally and strongly pervasive in the tourmalinized sandstone but in the vertical sense appears to be confined to fractures in the footwall

boron isotope composition of tourmalinite and vein tourmalines associated with gold mineralization serra do itaberaba group central ribeira belt se brazil abstract important concentrations of tourmaline occur as gold bearing stratiform tourmalinites and in mineralized quartz tourmaline veins at the tapera grande and quartzito gold prospects in the mesoproterozoic serra do itaberaba group central ribeira belt sao paulo state se brazil the main rock types in both prospects constitute the volcanic sedimentary morro da pedra preta formation which formed in a submarine back arc setting at tapera grande the volcanic sedimentary sequence is composed of metabasic and metavolcaniclastic rocks graphitic and sulfur rich metapelites banded iron formation metandesite metarhyolite calc silicates tourmalinites and metahydrothermalites derived from mafic and felsic rocks the mesoproterozoic rocks at quartzito prospect are lithologically similar but they have been affected by neoproterozoic faulting and shearing and by the emplacement of granitic rocks resulting in the formation of tourmaline rich quartz carbonate veins with gold and base metal mineralization we conducted a chemical and b isotope study of tourmalines in order to better understand the origin of the stratiform tourmalinites in the morro da pedra preta formation and their relationship with gold mineralization the overall range of 11b values obtained for the tourmalinite and vein tourmalines is between 15 and 5 with the tourmalinites falling at the low end of this range 15 to 8 such values are typical for continental crust and inconsistent with a primary marine boron signature as expected from the submarine exhalative model for the gold prospects we conclude from this that tourmaline formed or recrystallized from crustal fluids related to the amphibolite grade metamorphism which affected the serra do itaberaba group and that gold deposition occurred syn to post peak metamorphism by phase immiscibility as attested by fluid inclusions in tapera grande tourmalinite tourmaline and quartz the vein hosted tourmalines at quartzito have isotopically variable boron signatures with heavier 11b values of 5 to 8 for acicular green tourmalines and lighter values 15 to 7 for light blue ti free tourmaline from quartz carbonate veins we attribute the heavier boron to fluids derived from the volcano sedimentary rocks of marine affinity whereas the lighter boron was contributed by crustal fluids related to the granitoids or metasediments in the continental crust

mineralogical fluid inclusion and stable isotope constraints on mechanisms of ore deposition at the samgwang mine republic of korea a mesothermal vein hosted gold silver deposit the samgwang mine is located in the cheongyang gold district cheonan metallogenic province of the republic of korea it consists of eight massive gold bearing quartz veins that filled ne and nw striking fractures along fault zones in precambrian granitic gneiss of the gyeonggi massif their mineralogy and paragenesis allow two separate vein forming episodes to be recognized temporally separated by a major faulting event the ore minerals occur in quartz and calcite of stage i associated with fracturing and healing of veins hydrothermal wall rock alteration minerals of stage i include fe rich chlorite fe fe mg ratios 0 74 0 81 muscovite illite k feldspar and minor arsenopyrite pyrite and carbonates sulfide minerals deposited along with electrum during this stage include arsenopyrite pyrite pyrrhotite sphalerite marcasite chalcopyrite galena argentite pyrargyrite and argentian tetrahedrite only calcite was deposited during stage ii fluid inclusions in quartz contain three main types of c o h fluids co2 rich co2 h2o and aqueous inclusions quartz veins related to early sulfides in stage i were deposited from h2o nacl co2 fluids 1 500 5 000 bar average 3 200 with thtotal values of 200 c to 383 c and salinities less than about 7 wt nacl equiv late sulfide deposition was related to h2o nacl fluids 140 1 300 bar average 700 with thtotal values of 110 c to 385 c and salinities less than about 11 wt nacl equiv these fluids either evolved through immiscibility of h2o nacl co2 fluids as a result of a decrease in fluid pressure or through mixing with deeply circulated meteoric waters as a result of uplift or unloading during mineralization or both measured and calculated sulfur isotope compositions 34sh2s 1 5 to 4 8 of hydrothermal fluids from the stage i quartz veins indicate that ore sulfur was derived mainly from a magmatic source the calculated and measured oxygen and hydrogen isotope compositions 18oh2o 5 9 to 10 9 d 102 to 87 of the ore forming fluids indicate that the fluids were derived from magmatic sources and evolved by mixing with local meteoric water by limited water rock exchange and by partly degassing in uplift zones during mineralization while most features of the samgwang mine are consistent with classification as an orogenic gold deposit isotopic and fluid chemistry indicate that the veins were genetically related to intrusions emplaced during the jurassic to cretaceous daebo orogeny

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

timing and structural controls on gold mineralization at the bogoso gold mine ghana west africa the bogoso gold deposit comprises 20 open pits scattered along 18 km of a major fault zone which separates birimian supergroup turbiditic sedimentary rocks to the northwest from tarkwaian group rocks to the southeast individual pits are centered on 200 to 400 m long ore shoots which are located in three broad structural settings steeply pitching relatively high grade orebodies are commonly localized in left hand dilational jogs in the strike of the controlling faults these jogs often coincide with intersecting minor faults gold is largely confined to the graphitic mylonite which defines the fault and imbricated slices of carbonate altered rock gently dipping mineralized quartz vein stockworks and disseminated auriferous pyrite and arsenopyrite form broader zones between more steeply dipping bounding faults in the vicinity of right hand bends and splays in these faults smaller zones of disseminated sulfide mineralization coincide with areas where bedding and or foliation in the adjacent birimian sedimentary rocks is strongly discordant with the adjacent mineralized fault generally where the faults cut the short limbs of tight upright east trending folds approximately 75 percent of the gold extracted from the bogoso district has been mined from a cluster of pits that coincides with the densest and most highly connected concentration of larger faults in the bogoso concession the majority of ore shoots are located along the largest of these faults the narrower faults generally host smaller and less frequent ore shoots the fault network originated during an early episode of regional scale shortening involving thrusting and isoclinal folding the structural setting and geometry of the ore shoots implies that mineralization occurred during a later minor episode of sinistral strike slip movement on the controlling faults after development of the fault network the undeformed nature of the auriferous sulfides implies that this was the last phase of ductile brittle deformation on the host faults this conclusion is consistent with earlier work indicating that the giant ashanti deposit 150 km along strike from bogoso also developed during a period of small displacement sinistral offset late in the history of the host fault network

late kinematic timing of orogenic gold deposits and significance for computer based exploration techniques with emphasis on the yilgarn block western australia abstract orogenic gold deposits are a widespread coherent group of epigenetic ore deposits that are sited in accretionary or collisional orogens they formed over a large crustal depth range from deep seated low salinity h2o co2 ch4 n2 ore fluids and with au transported as thio complexes regional structures provide the main control on deposit distribution in many terranes first order faults or shear zones appear to have controlled regional fluid flow with greatest ore fluid fluxes in and adjacent to lower order faults shear zones and or large folds highly competent and or chemically reactive rocks are the most common hosts to the larger deposits focusing of supralithostatic ore fluids into dilatant zones appears to occur late during the evolutionary history of the host terranes normally within d3 or d4 in a d1 d4 deformation sequence reactivation of suitably oriented pre existing structures during a change in far field stress orientation is a factor common to many deposits and repeated reactivation may account for multiple mineralization episodes in some larger deposits absolute robust ages of mineralization support their late kinematic timing and in general suggest that deposits formed diachronously towards the end of the 100 to 200 m y long evolutionary history of hosting orogens for example in the yilgarn block a region specifically emphasised in this study orogenic gold deposits formed in the time interval between 40 and 90 m y with most about 60 to 70 m y after the youngest widespread basic ultrabasic volcanism and towards the end of felsic magmatism the late timing of orogenic gold deposits is pivotal to geologically based exploration methodologies this is because the present structural geometries of i the deposits ii the hosting goldfields and iii the enclosing terranes are all essentially similar to those during gold mineralization at least in their relative position to each other thus interpretation of geological maps and cross sections and three dimensional models can be used to accurately simulate the physical conditions that existed at the time of ore deposition it is particularly significant that the deposits are commonly related to repetitive and predictable geometries such as structural heterogeneities within or adjacent to first order structures around rigid granitoid bodies or in specific locked up fold thrust structures importantly the two giant greenstone hosted goldfields kalgoorlie and timmins show a remarkably similar geometry at the regional scale computer based stress mapping and gis based prospectivity mapping are two computer based quantitative methodologies that can utilize and take advantage of the late timing aspect of this deposit type to provide important geological aids in exploration both in broad regions and more localized goldfields both require an accurate and consistent solid geology map stress mapping requires knowledge of the far field stresses during mineralization and the empirical prospectivity mapping requires data from a significant number of known deposits in the terrane the kalgoorlie terrane in the yilgarn block meets these criteria and illustrates the potential of these methodologies in the exploration for orogenic gold deposits low minimum stress anomalies interpreted to represent dilational zones during gold related deformation coincide well with the positions of known goldfields rather than individual gold deposits in the terrane and there are additional as yet unexplained anomalies the prospectivity analysis confirms that predictable and repetitive factors controlling the siting of deposits are i proximity to and orientation and curvature of granitoid greenstone contacts ii proximity to segments of crustal faults which strike in a preferred direction iii proximity to specific lithological contacts which have similar preferred strike iv proximity to anticlinal structures and v the presence of preferred reactive host rocks e g dolerite the prospectivity map defines a series of anomalous areas which broadly conform to those of the stress map 78 correspondence the most prospective category on this map covers less than 0 3 of the greenstone belts and yet hosts 16 of the known deposits which have produced 80 of known gold thus it discriminates in favour of the larger economically more attractive deposits in the terrane the successful application of stress mapping and prospectivity mapping to geology based exploration for orogenic gold deposits indicates that more quantitative analysis of geological map data is a profitable line of research the computer based nature of these methodologies is ideal for the production of an ultimate integrated deposit target map which can be compared to other more conventional targeting parameters such as geophysical and geochemical anomalies such an integrated strategy appears the way forward in the increasingly difficult task of cost effective global exploration for orogenic gold deposits in poorly exposed terranes

archean gold mineralization and oxidized hydrothermal fluids sulfate minerals hematite and isotopically fractionated sulfides are relatively rare in rocks of archean age several archean au deposits however contain one or more of these features indicative of oxidized hydrothermal fluids they include hemlo mcintyre hollinger macassa lake shore and ross which are all in ontario canada and kalgoorlie in australia examples of lesser deposits are in hislop township and in the matachewan area ontario canada with the exception of hemlo these are quartz carbonate vein deposits spatially and temporally associated with felsic plutonism because metamorphic fluids and archean surface waters were mostly reducing the most feasible general source for the fluids was felsic magmas parent magmas were relatively oxidized hydrous and co 2 rich the oxidized character may have favored the incorporation of au into the partial melt and its partitioning into magmatic aqueous fluids high contents of co 2 are conducive to the separation of an aqueous fluid from magma at greater depth than otherwise fluid inclusion data support the formation of major archean au vein deposits at deeper crustal levels than more recent magmatic hydrothermal mineralization such as porphyry cu deposits co 2 rich fluids can unmix into two phases at relatively high temperatures and pressures a saline aqueous liquid and a low salinity h 2 o co 2 liquid this can cause partitioning of cl complexed base metals from other metals including au sb and as the extensive carbonatization around most deposits has consistent delta 13 c values in the range of juvenile magmatic co 2