discriminating fluid source regions in orogenic gold deposits using b isotopes abstract the genesis of orogenic gold deposits is commonly linked to hydrothermal ore fluids derived from metamorphic devolatilization reactions however there is considerable debate as to the ultimate source of these fluids and the metals they transport tourmaline is a common gangue mineral in orogenic gold deposits it is stable over a very wide p t range demonstrates limited volume diffusion of major and trace elements and is the main host of b in most rock types we have used texturally resolved b isotope analysis by secondary ion mass spectrometry sims to identify multiple fluid sources within a single orogenic gold ore district the loulo mining district in mali west africa hosts several large orogenic gold ore bodies with complex fluid chemistry associated with widespread pre ore na and multi stage b metasomatism the gara deposit as well as several smaller satellites formed through partial mixing between a dilute aqueous carbonic fluid and a hypersaline brine hydrothermal tourmaline occurs as a pre ore phase in the matrix of tourmalinite units which host mineralization in several ore bodies clasts of these tourmalinites occur in mineralized breccias disseminated hydrothermal and vein hosted tourmaline occur in textural sites which suggest growth during and after ore formation tourmalines show a large range in 11 b values from 3 5 to 19 8 which record a change in fluid source between paragenetic stages of tourmaline growth pre mineralization tourmaline crystals show heavy 11 b values 8 19 8 and high x site occupancy na ca 0 69 1 apfu suggesting a marine evaporite source for hydrothermal fluids syn mineralization and replacement phases show lighter 11 b values 3 5 to 15 1 and lower x site occupancy 0 62 0 88 apfu suggesting a subsequent influx of more dilute fluids derived from devolatilization of marine carbonates and clastic metasediments the large overlapping range in isotopic compositions and a skew toward the opposing population in the 11 b data for both tourmaline groups reflects continual tourmaline growth throughout mineralization which records the process of fluid mixing a peak in 11 b values at 8 largely controlled by tourmalines of syn to post ore timing represents a mixture of the two isotopically distinct fluids this paper demonstrates that b isotopes in tourmaline can be instrumental in interpreting complex and dynamic hydrothermal systems the importance of b as an integral constituent of orogenic ore forming fluids and as a gangue phase in orogenic gold deposits makes b isotope analysis a powerful tool for testing the level of source region variability in these fluids and by extension that of metal sources

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

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

fluid inclusions in quartz pebbles of the gold bearing tarkwaian conglomerates of ghana as guides to their provenance area quartz pebbles of the early proterozoic au bearing tarkwaian conglomerates in ghana reveal several original inherited pre sedimentary fluid inclusions these inclusions are co2 n2 rich and display a distinct high density up to 1 15 g cm3 the unusual high density and composition compare well with co2 n2 rich inclusions in quartz vein type gold deposits of the birimian supergroup in ghana and burkina faso this type of fluid inclusions has not been reported from any other lode gold deposit of greenstone affiliation and is thus a specific characteristic for birimian hosted gold deposits therefore it can be used as an unequivocal pathfinder for epigenetic as well as for syn sedimentary gold mineralization of the early proterozoic of west africa the inherited fluid inclusions with the unique physicochemical characteristics suggest that the tarkwaian quartz pebbles and possibly some gold were derived from au quartz vein deposits comparable in mineralogy petrography and genesis to those along the nw margin of the ashanti belt e g ashanti mine prestea mine

geochronological constrains on the timing of magmatism deformation and mineralization at the karouni orogenic gold deposit guyana south america abstract the karouni deposit is located in central guyana 35 km to the w of the former producing 5 moz omai mine and is interpreted to be an orogenic gold deposit it is hosted in rhyacian aged 2 2 to 2 1 ga volcano sedimentary and intrusive rocks metamorphosed to greenschist facies detailed structural and crosscutting relationships coupled with robust new u pb geochronology on intermediate to felsic intrusive rocks and hydrothermal minerals from the karouni camp have constrained the structural evolution of the deposit and provided a new age for gold mineralization with important implications for the structural framework and timing of gold mineralization throughout the region magmatism within the 200 km2 of the karouni camp progressed from early synvolcanic intermediate composition dikes at 2147 ma to a major voluminous pulse of felsic magmatism coeval with regional deformation during the trans amazonian orogeny between 2120 and 2088 ma a late pulse of granitic magmatism at 1950 ma may be related to a post orogenic event inherited zircons within the felsic intrusive rocks indicate assimilation of crust as old as 2223 32 ma gold mineralization associated with camp scale d2 deformation has been dated to 2084 15 ma on the basis of u pb geochronology of hydrothermal titanite these results show gold mineralization is coeval with felsic magmatism and contemporaneous with a major pulse of orogenic gold mineralization in the birimian province of west africa

paleoproterozoic volcano sedimentary series in the ca 2 27 1 96 ga birimian orogen of the southeastern west african craton abstract the ca 2270 1960 ma accretionary collisional birimian orogen in the southern west african craton swac contain a number of volcano sedimentary series which were deposited in basins that developed during the regional eburnean collisional orogeny ca 2160 1960 ma understanding their spatiotemporal distribution is thus crucial for constraining the tectonic evolution of the orogenic system this study presents new la icpms data on detrital zircon from volcano sedimentary series in southeastern swac which indicate maximum depositional ages between ca 2160 2130 ma the detrital zircon population is predominantly composed of grains with ages compatible with derivation from local birimian crust but older paleoproterozoic and archean components are also present these may have been derived from crustal domains now present in south american cratons and terranes and which would have been located to the present east or southeast of the swac the deposition of the volcano sedimentary series also coincided with the onset of the eburnean collisional orogeny in eastern swac which suggest a dynamic coupling between compression on one hand and extension on the other where the latter controlled the establishment of the depocentres in which the series were deposited the data in this study support previous observations that sedimentation across swac was diachronous during the course of the eburnean orogeny and that an early pulse occurred primarily in the east between ca 2165 2125 ma while a later pulse occurred in the west around ca 2115 2080 ma followed by limited volcanism and sedimentation at ca 2065 ma this diachroneity is also reflected in the lithostratigraphy with areally extensive shallow water deposits being more frequent in the east whereas carbonates are only present in the younger successions to the west this may reflect a hinterland foreland style relationship between eastern and western swac respectively where sedimentation occurred progressively during convergence related to the regional and diachronous eburnean orogeny

crustal scale transcurrent shearing in the paleoproterozoic sefwi sunyani comoe region west africa abstract the paleoproterozoic sefwi sunyani comoe region that straddles the ghana ivory coast border in west africa has been characterised as resulting from a combination of compression and simple shear with leucogranite intrusion either being prior to the compression or synchronous with late shearing the analysis of regional magnetic datasets combined with field observations allows us to better define the geometry of the major lithostratigraphic packages and their structural contacts in this region this analysis reveals a series of elongate rounded leucogranite intrusions enveloped by deformed metasediments recent finite element modelling of two phase aggregates has shown that we can analyse the geometry of these systems both in terms of their finite deformation and their mechanical contrast we interpret the geometries we see in the sefwi sunyani comoe region as reflecting the activity of a major crustal deformation zone which was dominated by simple shear the comparison with our modelling suggests a finite shear strain of approximately 5 gamma which in turn implies a lateral displacement of 400 km parallel the sefwi greenstone belt which places southern ghana eoghana near eastern burkina faso prior to deformation our analysis also suggests that the leucogranites were already acting as more rigid bodies during the shearing suggesting that their emplacement was predominantly pre kinematic

gold remobilisation and formation of high grade ore shoots driven by dissolution reprecipitation replacement and ni substitution into auriferous arsenopyrite abstract both gold rich sulphides and ultra high grade native gold oreshoots are common but poorly understood phenomenon in orogenic type mineral systems partly because fluids in these systems are considered to have relatively low gold solubilities and are unlikely to generate high gold concentrations the world class obuasi gold deposit ghana has gold rich arsenopyrite spatially associated with quartz veins which have extremely high localised concentrations of native gold contained in microcrack networks within the quartz veins where they are folded here we examine selected samples from obuasi using a novel combination of quantitative electron backscatter diffraction analysis ion microprobe imaging synchrotron xfm mapping and geochemical modelling to investigate the origin of the unusually high gold concentrations the auriferous arsenopyrites are shown to have undergone partial replacement 15 by au poor nickeliferous arsenopyrite during localised crystal plastic deformation intragranular microfracture and metamorphism 340 460 c 2 kbars our results show the dominant replacement mechanism was pseudomorphic dissolution reprecipitation driven by small volumes of an infiltrating fluid that had relatively low s 2 and carried aqueous nicl 2 we find that arsenopyrite replacement produced strong chemical gradients at crystal fluid interfaces due to an increase in s 2 during reaction which enabled efficient removal of gold to the fluid phase and development of anomalously gold rich fluid potentially 10 ppm or more depending on sulphur concentration this process was facilitated by precipitation of ankerite which removed co 2 from the fluid increasing the relative proportion of sulphur for gold complexation and inhibited additional quartz precipitation gold re precipitation occurred over distances of 10 m to several tens of metres and was likely a result of sulphur activity reduction through precipitation of pyrite and other sulphides we suggest this late remobilisation process may be relatively common in orogenic belts containing abundant mafic ultramafic rocks which act as a source of ni and co scavenged by chloride bearing fluids both the preference of the arsenopyrite crystal structure for ni and co rather than gold and the release of sulphur during reaction can drive gold remobilisation in many deposits across broad regions

paleoproterozoic gold deposits hosted by albite and carbonate altered tonalite in the chirano district ghana west africa paleoproterozoic birimian supergroup rocks of southwest ghana host numerous mesothermal lode gold deposits including the 38 million ounce moz ashanti deposit at obuasi and other multimillion ounce deposits at bogoso konongo prestea bibiani and in the yamfo sefwi region gold deposits of the chirano district are localized along the chirano lode horizon a 9 km segment of the chirano and related shear zones on the southeast margin of the sefwi bibiani belt economically significant mineralization at chirano is largely hosted by altered tonalite intrusions weaker alteration and subparts per million gold grades extend into adjacent birimian mafic igneous rocks and tarkwaian group sedimentary rocks pink albite alteration albite quartz pyrite hematite rutile leucoxene chalcopyrite gold and later pale carbonate alteration ankerite sericite quartz pyrite hematite gold fineness 910 940 ag au telluride characterize the gold deposits of the chirano district alteration assemblages and the bi te and mo contents of the mineralized rocks at chirano are more typical of lode gold deposits than orthomagmatic gold deposits alteration assemblages at chirano contrast with the graphite carbonate pyrite arsenopyrite pyrrhotite assemblages that dominate the lode gold deposits in southwest ghana which are hosted by graphitic shear zones and birimian sedimentary rocks differences between alteration assemblages at chirano and those in graphitic shear zones may reflect the different wall rock assemblages with which the hydrothermal fluid equilibrated rather than different fluid sources mineralization in the chirano district formed during a relatively late episode of small displacement brittle cataclasis on the chirano lode horizon after reverse ductile shearing associated with regional scale shortening this structural history is similar to that documented at the larger ashanti and bogoso deposits on the opposite side of the kumasi basin timing relationships are compatible with all three deposits forming during a single mineralizing event late in the eburnean 2 orogeny at chirano limited fault vein geometric relationships imply a component of reverse movement during mineralization whereas sinistral strike slip movement accompanied mineralization at ashanti and bogoso no consistent correlation was observed between the asymmetry of bends and splays in the strike of the chirano lode horizon and the location or style of individual gold deposits unlike at ashanti and bogoso the structural settings of deposits in the chirano district indicate that an exploration strategy which assumes mineralization occurred during sinistral offset in the host structures similar to relationships at ashanti and bogoso may result in significant targets being ignored

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

vein mineralization at the damang gold mine ghana controls on mineralization abstract two distinct styles of au mineralization occur at the damang gold mine palaeoproterozoic sediments of the tarkwaian group host both one style of mineralization is stratabound within quartz lithic conglomerates of the banket series the second style of mineralization is associated with an extensive low displacement fault fracture mesh that formed in a compressional stress regime late in the deformational history and after the peak metamorphism regional deformation within the tarkwaian involved initial nw se directed shortening d 1 a major nne trending f 1 anticline hosts the damang orebody broadly n s shortening during d 2 resulted in the formation of e w trending thrusts with small displacements the d 3 shortening direction was similar to that of d 1 steep d 1 faults were reactivated and a new set of low angle thrusts and associated flat lying extension veins were formed the bulk of the mineralization observed at damang is associated with the low displacement d 3 fault fracture mesh the presence of flat lying extensional veins and the reactivation of some misoriented d 1 structures is indicative of periodic episodes of supralithostatic fluid pressures low differential stress and fault valve behavior towards the end of the deformation history

geochemistry of impactites and basement lithologies from icdp borehole lb 07a bosumtwi impact structure ghana in 2004 a drilling project by the international continental scientific drilling program icdp at the bosumtwi impact crater ghana 1 07 myr old and 10 5 km in diameter obtained drill core lb 07a which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure the lb 07a core consists of three sequences 82 29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54 88 m of so called lower impactite of monomict impact breccia with several suevite intercalations and 74 53 m of meta graywacke and altered shale of the basement also containing a number of suevite intercalations major and trace element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein compositions of polymict impact breccias of the crater fill revealed by core lb 07a are compared with the compositions of the ivory coast tektites and the fallout suevites the impactites of the lb 07a borehole appear well homogenized with respect to the silicate component and little change in the ranges of many major and trace element differences is seen along the length of the borehole except for fe2o3 mgo and cao contents much scatter is observed for a number of elements and in many cases this increases with depth it is proposed that any variability in composition is likely the function of clast population differences i e also of relatively small sample sizes no systematic compositional difference between polymict lithic and suevitic impact breccias is evident an indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies the impactites of the borehole generally show intermediate compositions to previously defined target rocks the fallout suevites have comparable major element abundances except for relatively lower mgo contents the ivory coast tektites are generally similar in composition to the lb 07a suevites but broader ranges in mgo and cao contents are observed for the lb 07a suevites

high co2 content of fluid inclusions in gold mineralisations in the ashanti belt ghana a new category of ore forming fluids a reply fluid inclusions were studied in samples from the ashanti konongo southern cross prestea abosso damang and ayanfuri gold deposits in the ashanti belt ghana primary fluid inclusions in quartz from mineralised veins of the ashanti prestea konongo southern cross and abosso damang deposits contain almost exclusively volatile species the primary setting of the gaseous i e the fluid components co2 ch4 and n2 fluid inclusions in clusters and intragranular trails suggests that they represent the mineralising fluids microthermometric and raman spectroscopic analyses of the inclusions revealed a co2 dominated fluid with variable contents of n2 and traces of ch4 water content of most inclusions is below the detection limits of the respective methods used aqueous inclusions are rare in all samples with the exception of those from the granite hosted ayanfuri mineralisation here inclusions associated with the gold mineralisation contain a low salinity co2 and low salinity 6 eq wt nacl however fluid inclusions in quartz from the gold mineralisations in the ashanti belt point to distinctly different fluid compositions specifically the predominance of co2 and co2 h2o have to be emphasized fluid systems with this unique bulk composition were apparently active over more than 200 km along strike of the ashanti belt fluids rich in co2 may present a hitherto unrecognised new category of ore forming fluids

intrusion related affinity and orogenic gold overprint at the paleoproterozoic bonikro au mo deposit cote d ivoire west african craton the bonikro gold deposit and satellite hire open pits are located at the southern tip of the oume fettekro granite greenstone belt in cote d ivoire country rocks in the region have undergone polyphase deformation and prolonged arc magmatism during the paleoproterozoic eburnean orogeny intrusive host rocks at hire and bonikro have been dated at 2180 6 and 2086 4 ma u pb on zircon respectively these plutonic bodies acted as favourable sites for fluid flow due to their brittle rheological characteristics gold mineralisation at bonikro is hosted by i a sheeted quartz vein array characterised by an au w bi te ag metal association in the cupola of a porphyritic granodiorite and ii an overprinting swarm of fault fill smoky quartz molybdenite veins gold mineralisation at hire is shear hosted and shares a similar relative timing with the latter set of auriferous veins this second phase of gold mineralisation occurred during a period of transcurrent tectonics late in the eburnean tectono magmatic history soon after the cessation of compressional deformation it is responsible for the bulk of gold resources in the region e g bonikro hire agbahou and has been dated at 2074 16 ma re os on molybdenite the integration of structural paragenetic relationships with high precision dating of magmatic and hydrothermal events highlights the superposition of late eburnean orogenic gold mineralisation on an earlier intrusion related gold system linked to the local emplacement of a porphyritic granodiorite at bonikro and illustrates the genetic diversity of paleoproterozoic granitoid hosted gold mineralisation in the west african craton it also further supports that gold mineralisations in the west african craton occurred diachronously throughout the eburnean orogeny through a variety of deposit types including au skarn intrusion related au orogenic au and porphyry cu au

zircon hf and o isotope constraints on the evolution of the paleoproterozoic baoule mossi domain of the southern west african craton abstract the use of zircon isotopic systems such as lu hf and o has proven to be an invaluable tool for unraveling the evolution of the early earth in order to achieve a comprehensive understanding of the evolution of the southern west african craton and in particular the paleoproterozoic baoule mossi domain zircon grains from 75 felsic intrusions were analyzed for hf isotopes 22 of those samples were analyzed for o isotopes covering an area that covers more than 1000 km2 from southwest burkina faso across southern mali and into eastern guinea the results of the study reveal that the baoule mossi domain comprises two magmatic regions that display broadly juvenile isotopic character but with significant localized evidence of crustal contamination the hf model age data show that the crustal contaminant is potentially as old as ca 2800 ma and indicate that the two domains may have evolved slightly differently in fact the westernmost region presents a slightly older or less radiogenic hf isotope signature when compared to the easternmost region furthermore the o isotopic data support the notion that the felsic magmas were subject to supra crustal contamination as 18o values range between 6 and 12 the data presented in this study provide robust evidence that the baoule mossi domain of the southern west african craton evolved by accretionary processes between ca 2250 2050 ma

the origin and hydrothermal mobilization of carbonaceous matter associated with paleoproterozoic orogenic type gold deposits of west africa abstract the chemical and physical properties of carbonaceous matter cm were studied in paleoproterozoic metasediment hosted orogenic type gold deposits in burkina faso the inata deposit in mali the syama deposit and in ghana the obuashi and bogoso deposits two types of cm occur in all the studied deposits metamorphosed and hydrothermal metamorphosed cm prevails in all the deposits hydrothermal cm occurs in small veinlets in hydrothermally altered rocks and in quartz veins or forms irregular accumulations parallel or sub parallel to c type cleavage within the shear zones the origin of hydrothermal cm which occurs in paragenesis with au bearing arsenopyrite or pyrite seems to have been due to supersaturation of hydrothermal fluids with carbon at the deeper or middle crustal levels the isotopic composition of carbon in bulk cm 33 1 to 26 2 vpdb indicates its biogenic origin the isotopic composition of carbon in hydrothermal carbonates ranges from 14 5 to 4 4 vpdb which suggest mixing of carbon derived from a deep seated source with carbon derived from an organic source the interaction of hydrothermal fluids with metamorphosed cm could be one of the causes of the reduction of hydrothermal fluids and formation of the respective mineralization the optical properties and raman spectra of the metamorphosed cm particles in the individual studied mineral deposits differ considerably the temperatures calculated on the basis of the raman spectra of metamorphosed cm vary between 280 and 440 c depending on the thermometer used and correspond to temperatures of metamorphism of upper sub grenschist and greenschist facies the temperatures calculated for hydrothermal cm at the individual deposits are only slightly lower compared to the metamorphosed cm at the same deposits which indicates approximately the same temperature of the metamorphic and hydrothermal processes

the affinity of archean crust on the yilgarn albany fraser orogen boundary implications for gold mineralisation in the tropicana zone abstract craton margins can be subject to a wide array of gold genesis and redistribution processes although high grade terrains on craton margins are frequently viewed as less prospective than lower grade counterparts in contrast to this the high grade tropicana zone a newly defined archean crustal component on the eastern margin of the yilgarn craton within the albany fraser orogen afo contains a significant proterozoic gold deposit this deposit and zone comprise mid amphibolite to granulite facies gneissic rocks with evidence of partial melting and granite injection the tropicana zone contains significant low si lile enriched granites classed as sanukitoids along with the distinctive compositions the rarity of these rocks within any archean craton suggests that the granitoid protoliths represent a single suite emplaced during one event due to the intense granulite facies overprinting of the tropicana zone rocks determination of the magmatic protolith age for these sanukitoids is challenging nonetheless the best age estimate for magmatism is 2692 16 ma based on the youngest zircons preserving textural evidence of growth within a viscous silicate melt this age is older than compositionally similar magmatism found within the yilgarn craton although a sanukitoid in the northern foreland of the afo has a similar age furthermore the granulite facies metamorphic zircon growth in the tropicana zone at 2718 2554 ma was prolonged compared to that in the yilgarn craton nonetheless the hf isotopic signature of the tropicana zone zircon shares strong similarity to that from the eastern goldfields superterrane of the yilgarn craton this implies that the tropicana zone reflects a deeper crustal level of the yilgarn craton exhumed and thrust nw an unknown distance over the craton edge in addition we observe that the granulite facies zircons have a less radiogenic hf isotope signature than the preserved pre metamorphic zircon cores based on correlations with alpha dose u and th content and 176hf 177hf we suggest this reflects the preferential destruction and release of unradiogenic hf from inherited zircon whereas the protolith sanukitoid zircon with lower u and th content was more resistant to mobilisation during high grade metamorphism we note this situation may be a more general response of the hf isotopic system in which zircon grown in a more mafic melt is less likely to contribute to the metamorphic hf reservoir than its felsic counterpart juvenile granitic veins dated at c 1780 ma intruded into the tropicana zone indicate that the tropicana zone was structurally emplaced at or before c 1780 ma given similar proterozoic magmatic events are well documented from para autochthonous adjacent units re os dating of pyrite coeval with one generation of gold in these rocks indicates model ages of c 2100 ma supportive of a palaeoproterozoic age of mineralisation this mineralisation event is distinct from major proterozoic tectonothermal events elsewhere in the afo sanukitoid magmas are well known for gold fertility and were likely the original source of gold in the tropicana zone which was subsequently concentrated into brittle structures during several episodes gold mineralisation post dated peak metamorphic conditions and is significantly younger than gold mineralisation within other parts of the adjoining yilgarn craton

fluid pressure dominated orogenic gold mineralization under low differential stress case of the yaoure gold camp cote d ivoire west africa the 1 5 moz yaoure gold deposit which is located within the birimian greenstone belts of central cote d ivoire west africa is associated with two stages of economic gold mineralization both ore forming stages occurred during protracted e w directed shortening associated with the eburnean orogeny the first mineralization stage is hosted by fault fill veins along conjugate ene striking dextral and wnw striking sinistral strike slip fault arrays this ore stage is associated with a polymetallic assemblage including mo bi pb w te ag and au and cryptic alteration in wall rocks the second and main gold mineralization stage is associated with a low displacement fault fracture vein mesh formed during a reverse faulting episode this second gold phase is associated with a polymetallic assemblage including w and au pb and is characterized by pronounced albite ankerite pyrite hydrothermal alteration in wall rocks shear veins associated with this second mineralization episode formed during episodic fluid pressure variations under low differential stress within pre existing low angle reverse fault planes permeability destruction by albite ankerite pyrite hydrothermal sealing of these low angle reverse faults resulted in fluid pressure build up and ultimately in the development of extension veins under low differential stress in the late stages of the structural evolution of the deposit this study presents the structural evolution of a polyphase orogenic gold deposit and highlights that the interplay between fault zone activity hydrothermal alteration and fluid pressure fluctuations at low differential and deviatoric stresses were key in controlling fluid flow and the formation of ore within optimally oriented faults

stable c o and s isotope record of magmatic hydrothermal interactions between the faleme fe skarn and the loulo au systems in western mali the gara yalea and gounkoto au deposits of the 17 moz loulo mining district largely hosted by the kofi series metasediments are located several kilometers to the east of the 650 mt fe skarn deposits in the adjacent faleme batholith the au deposits are interpreted to have formed through phase separation of an aqueous carbonic fluid which locally mixed with a hypersaline brine of metaevaporite origin recognition of an intrusive relationship between the faleme batholith and kofi series opens the possibility that the fe skarns and au deposits are part of the same mineral system in this paper we combine new 13c 18o and 34s data from the karakaene ndi skarn au occurrences along the western margin of the kofi series and zircons within plutonic rocks of the faleme batholith we combine these with existing data from the loulo au deposits to model the contribution of magmatic volatiles to au mineralization c and o isotope compositions of auriferous carbonate quartz sulfide veins from the loulo au deposits have wide ranges 13c 21 7 to 4 5 and 18o 11 8 to 23 2 whereas values from carbonate veins in kofi series au prospects close to the faleme batholith and the karakaene ndi fe skarn deposit have more restricted ranges 13c 16 8 to 3 7 18o 11 4 to 17 2 and 13c 3 0 1 18o 12 6 1 respectively kofi series dolostones have generally higher isotopic values 13c 3 1 to 1 3 and 18o 19 1 to 23 3 pyrite from kofi series au prospects adjacent to the faleme batholith have a wide range of 34s values 4 6 to 14 2 similar to pyrite from the karakaene ndi skarn 2 8 to 11 9 whereas 34s values of pyrite and arsenopyrite from the loulo deposits are consistently 6 comparison of the c and o isotope data with water rock reaction models indicates the loulo au deposits formed primarily through unmixing of an aqueous carbonic fluid derived from the devolatilization of sedimentary rocks with an organic carbon component isotopic data are permissive of the hypersaline brine that enhanced this phase separation including components derived from both kofi series evaporite horizons interlayered with the dolostones and a magmatic hydrothermal brine this magmatic hydrothermal component is particularly apparent in o c and s isotope data from the gara deposit and au prospects immediately adjacent to the faleme batholith

gold mineralization in the ashanti belt of ghana genetic constraints of the stable isotope geochemistry the ashanti belt of ghana is the key district of gold mineralization in the paleoproterozoic terrane of west africa the area considered in southwest ghana is covered by lithologies of the volcanic sedimentary birimian supergroup and the overlying elastic sedimentary tarkwaian group which were jointly folded and metamor phosed under greenschist facies conditions during the eburnean teetonothermal event at about 9 1 ga regional fotiation md subparallel shear zones hosting mesothermal gold mineralization developed during deformation coeval with metamorphism four major tsoes of primary gold mineralization are present in the ashanti belt 1 mesothermal generally steeply dipping quartz veins in shear zones mainly in birimian sedimentary rocks 9 sulfide ores with auriferous arsenopyrite and pyrite spatialty dosely associated with the quartz veins 3 sulfide disseminations and stockworks in granitoids and 4 paleoplaeers of the tarkwaian group this study concentrates on types 1 and 9 of the hydrotherlnal gold mineralization stable isotope analyses of host rock and ore components were performed with the ailn of obtaining parameters relevant to the origin and evolution of the fluids that produced gold mineralization carbonaeeous matter in the birimian metasediments displays 93c values ranging from 11 4 to 9 8 3 per mil relative to pdb indicating an organogenie origin carbonates display a unimodal distribution of 9ac values ranging from 9 9 to 17 0 per rail relative to pdb coz extracted from fluid inclusions in the auriferous quartz veins has 93c values ranging from 9 5 to 15 7 per mil relative to pdb it is proposed that these carbon isotope compositions of carbonates and coz reflect extensive interaction of the co 2 rieh hydrothermal fluids with reduced carbon in birimian sediments in the deeper parts of the hydrothermal systems carbonates and auriferous vein quartz have 6 so values ranging from 19 9 to 9 9 9 and 19 8 to 15 6 per mil relative to smow respectively carbonates and quartz were deposited in near isotopic equilibrium with respect to 6 so indicating fluid dominated conditions during ore formation from fluids of metamorphic or magmatie origin such an origin is corroborated bysd values of water extracted from fluid inclusions in vein quartz 37 to 53 relative to smoxv pyrite of synsedimentary diagenetie origin in birimian schists displays sulfur isotope compositions ranging from 7 3 to 9 0 9 per rail median ca 10 relative to cdt similar compositions and wide ranges are usually attributed to sulfide generation by bacterial sulfate reduction from seawater arsenopyrite and eogenetie pyrite from the sulfide ores generally have 534s values in the range 5 3 to 10 9 per rail relative to cdt the tight unimodal distribution of 534s values indicates a large homogeneous fluid reservoir the low 534s values are interpreted as source inherited not related to unusual ph eh temperature or depositional conditions sulfides in birimian sediments represent the most likely sulfur reservoir tapped by the fluid systems the c o h and s isotope compositions of ore related hydrothermal minerals and fluid inclusion compo nents indicate that the mineralizing fluids interacted extensively with the paleoproterozoie rocks especially birimian sediments at deeper crustal levels and at high temperatures the isotopic compositions are most compatible with the formation of fluids from devolatilization reactions invomng birimian strata during pro grade metamorphism at depth metamorphic fluids

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

reassessment of proterozoic granitoid ages in ghana on the basis of u pb zircon and monazite dating abstract the west african craton in ghana is of early proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the birimian supergroup as well as younger conglomerates and quartzites of the tarkwaian group and extensive granitoid plutons no consensus exists on absolute and relative ages of plutons within belts and basins u pb geochronology of four different granitoid complexes indicates that contrary to long held views belt granitoid plutons ashanti belt 2172 2 ma sefwi belt 2179 2 ma are 60 to 90 ma older than basin plutons kumasi basin 2116 1 ma belt granitoid are approximately coeval with volcanic rocks of the birimian supergroup while basin plutons are late kinematic and postdate tarkwaian sedimentation the first high precision geochronological data for rocks from ghana i e the eastern part of the birimian domain on the west african craton presented here show that magmatism within this region occurred over a time span of at least 90 ma and provide further evidence that the period between 2 1 and 2 2 ga was one of important magmatism in west africa

differential response of u pb systems in coexisting accessory minerals winnipeg river subprovince canadian shield implications for archean crustal growth and stabilization the u pb isotopic systems of zircon monazite titanite and some apatite and the pb isotopic composition of k feldspar have been investigated in three areas of the winnipeg river subprovince wrs of the superior province canada in order to define the timing of magmatic and metamorphic processes in this archean gneissic granitoid terrain

pre mineralization thermal evolution of the palaeoproterozoic gold rich ashanti belt ghana the region of the gold rich ashanti belt in southern ghana was chosen as the subject for a detailed regional thermal modelling study geological studies in addition to laboratory measurements of thermal properties and heat production rates allow us to constrain a finite element thermal modelling scenarios integrating variations of the structure of the crust and various chronological settings were examined we calculated the thermal regime before and after the thrust tectonism that affected the region during the eburnean orogeny 2130 to 2095 ma just before ore deposit formation this gives a new insight into the regional thermal state of the crust before the mineralizing events to satisfy the thermobarometric observations the most probable mantle heat flow must be 60 mw m 2 which is at least three times greater than the present day value at shallow depths our results also indicate anomalies of lateral heat flow reaching 25 mw m 2 focused on the margins of each lithological unit including the ashanti belt these anomalies are related to the distortion of the isotherms in the first few kilometres that can be mostly explained by lateral contrasts in thermal conductivity such anomalies could be of importance for the mineralizing events as they would favour fluid circulation locally

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

zircon u pb ages from the wabigoon manitou lakes region wabigoon subprovince northwest ontario three distinct stratigraphic units have been recognized in the crow lake savant lake belt these are a basal unit consisting of high magnesium tholeiitic flows a middle unit consisting mostly of tholeiitic to calc alkaline pyroclastic rocks and flows and an upper unit of high iron tholeiitic flows a zircon u pb study has been carried out on seven rocks from the wabigoon manitou lakes region of this belt in the manitou lakes area a minimum age of 2755 ma on a porphyry pluton intruded into the wapageisi volcanics gives a minimum age for the high magnesium tholeiitic sequence a trondhjemite phase of the atikwa batholith near wabigoon lake that is dated at 2732 2 2 9 ma has the same age within experimental error as a rhyolite flow from the lower wabigoon volcanics in the same area that is dated at however a trondhjemite phase of the atikwa batholith at eagle lake that is dated at is distinctly younger than a dacite flow collected nearby from the lower wabigoon volcanics that is dated at both thes

40ar 39ar thermochronology of paleoproterozoic granitoids of northeast burkina faso west african craton implications for regional tectonics abstract the greenstone belts of northeast burkina faso in the west african craton wac comprise birimian meta sedimentary and mafic ultramafic complexes that are intruded by granitoids and late dolerite dykes the granitoid greenstone belt assemblages underwent at least two phases of deformation that have resulted in their principal nw and ne trends we performed a detailed 40ar 39ar dating together with zircon u pb ages in order to unravel the thermo chronological history of the studied granitoids our previous zircon u pb data indicated that the granitoids were emplaced between 2138 7 and 2181 7 ma this period is well known for crustal growth in the wac the emplacement of granitoids was concomitant with d1 or the tangaean event 2130 2170 ma amphibole and micas yield two 40ar 39ar age groups the first group 2046 12 2132 13 ma constrained the d2 deformation event and the second group 1918 24 to 1984 12 ma constrained the d3 or wabo tempelse event considering closure temperatures of the isotopic systems the u pb ages and the present 40ar 39ar dating results suggest that after emplacement the granitoids cooled to 315 c at ca 2020 ma with a mean cooling rate of 6 5 c ma corresponding to a mean uplift rate of 0 65 km ma the additional k feldspar 40ar 39ar data indicate a later slow cooling stage with a rate of 0 54 c ma corresponding to a mean uplift rate of 0 054 km ma subsequent to the end of the major crustal growth in the wac at 2000 ma

basin analysis of the jurassic lower cretaceous southwest tarim basin northwest china the jurassic through cretaceous southwest tarim basin northwest china contains more than 6 km of fluvial and lacustrine strata deposited in a foreland setting during the successive collisions with eurasia of the changtang block during late triassic early jurassic time and with the mega lhasa block during late jurassic early cretaceous time this tectonism is chronologically linked with the creation of a narrow lower middle jurassic transtensional basin with thick sedimentary infill succeeded by a broader upper jurassic lower cretaceous compressional basin with thinner sedimentary infill the older basin formed between a north northwest striking dextral fault on the eastern side of the southwest tarim basin in the tian shan and a postulated strike slip or normal fault on the western margin of the basin along the kunlun shan the former fault is now the talas ferghana fault the latter may be a predecessor to the main pamir thrust subsidence analysis of the thickest sedimentary section suggests thermal subsidence interpreted as the result of transtension between the two basin bounding faults the younger basin extends farther east and west and does not preserve evidence of activity along the talas ferghana fault the change in basin style between these two episodes of basin development likely reflects either a small counterclockwise rotation of basin bounding structures during the first episode or a small clockwise rotation of the maximum compressive stress between the two episodes

hydrochemical characteristics of groundwater and surface water for domestic and irrigation purposes in vea catchment northern ghana the vea catchment mainly underlain by crystalline basement rocks is located in northern ghana hydrogeochemical studies were carried out in this area with the objective of identifying the geochemical processes influencing water quality and suitability of surface and groundwater for agricultural and domestic uses sixty one groundwater and four surface water samples were collected from boreholes dams and rivers and analysed for ca2 mg2 na k hco3 cl and so4 2 fetot po4 3 mntot nh4 no3 no2 in addition ph total dissolved solids electrical conductivity total hardness turbidity colour salinity and dissolved oxygen were analysed chloro alkaline indices 1 and 2 and characterization of weathering processes suggest that the chemistry of groundwater is dominated by the interaction between water and rocks cation exchange and silicate weathering are the dominant processes controlling the chemical composition of the groundwater in the area studied mineral saturation indices indicate the presence of at least three groups of groundwater in the vea catchment with respect to residence time the meteoric genesis index suggests that 86 of the water samples belong to the shallow meteoric water percolation type the findings further suggest that the groundwater and surface water in the basin studied are mainly ca mg hco3 water type regardless of the geology compared to the water quality guidelines of who the study results on sodium absorption ratio sodium percentage magnesium hazard permeability index and residual sodium carbonate indicate that groundwater and surface water in the vea catchment are generally suitable for drinking and irrigation purposes

episodic collisional orogenesis and lower crust exhumation during the palaeoproterozoic eburnean orogeny evidence from the sefwi greenstone belt west african craton abstract new constraints on the architecture deformation sequence and tectonic evolution of the sparsely exposed palaeoproterozoic sefwi greenstone belt ca 2200 2070 ma in sw ghana are derived from the integration of field observations structural mapping and interpretation of regional airborne geophysical data our findings provide new insights into the tectonic evolution of the west african craton during the palaeoproterozoic eburnean orogeny ca 2150 2070 ma domains of contrasting metamorphic grade comprising high grade paragneisses and meta volcanic rocks are tectonically juxtaposed against low grade volcano sedimentary packages the earliest deformation event is characterised by a bedding parallel high grade metamorphic foliation s1 that is axial planar to isoclinal folds formed during nnw sse shortening and late d1 partial melting this crustal thickening event generated biotite muscovite granites emplaced parallel to the nw margin of the sefwi belt between ca 2092 and 2081 ma providing a minimum age for d1 d2 deformation is characterised by a moderate to steeply dipping ene wsw to ne sw striking penetrative s2 mineral foliation within the sefwi belt coaxial f2 fold axes and l2 stretching lineations pitch shallowly ene wsw to ne sw oblique to the major shear zones indicative of ene wsw transtension and associated constrictional deformation differential exhumation of middle and lower crustal segments along nne striking d2 extensional detachments was coincident with sinistral oblique reactivation of ne sw regional scale shear zones the timing of exhumation is constrained by metamorphic monazite u pb ages at ca 2073 ma subsequent e w shortening is associated with dextral reactivation of regional ne sw striking shear zones and refolding of earlier structures away from the major shear zones we propose that the nw margin of the sefwi belt represents the collision d1 of southern ghana and central and nw ghana ivory coast which record different tectonic histories furthermore we suggest the amalgamation of the west african craton is the product of episodic collisional orogenesis providing new insights into the evolution of orogenic processes during the palaeoproterozoic

compilation of west african mineral deposits spatial distribution and mineral endowment abstract the west african craton is highly endowed in minerals and their spatial and temporal distribution varies from single to multi phase mineralization events they are broadly related to three major tectono metallogenic elements and formed during distinct mineral epochs 1 in both archean shields kenema man and reguibat and paleoproterozoic domains baoule mossi eglab these are characterized by giant iron ore deposits that formed between ca 2 5 2 3 ga nearly all gold porphyry copper lead zinc and sedimentary manganese ore that developed between 2 2 and 2 1 ga and primary diamonds that formed between two intervals at ca 2 2 2 0 ga and in the mesozoic 2 across pan african and variscan belts these are distinguished by major precambrian iocg s copper gold that formed at ca 2 1 ga and approximately 680 ma and neoproterozoic sedimentary iron ore and phosphate deposits 3 within intracratonic and coastal basins these include the development of cenozoic lateritic bauxites over mesozoic dolerites tertiary quaternary mineral sands deposits oolitic iron ore and sedimentary phosphate deposits geological spatial and temporal correlations using the multi commodity west african mineral deposit database highlight that gold and non gold commodities formed in multiple phases this commenced in the liberian orogeny 2 9 2 8 ga with the enrichment of iron ore nickel sulphides diamonds and gold in the earth s crust the pre eburnean or tangaean eoeburnean eburnean i event yielded gold and the major eburnean orogeny yielded gold iron ore manganese diamonds magmatic nickel sulphides copper gold lead zinc and ree minerals throughout the pan african event sedimentary manganese deposits lead zinc ree minerals sedimentary phosphates and again gold were formed primary diamonds and magmatic nickel sulphides are related to the break up of gondwana followed by an intense lateritic weathering period that formed bauxite deposits along the craton margin

geology and fluid characteristics of the mina velha and mandiocal orebodies and implications for the genesis of the orogenic chega tudo gold deposit gurupi belt brazil the chega tudo gold deposit is located in the gurupi belt of northern brazil gold mineralization in the mina velha upper level and mandiocal intermediate and lower levels orebodies is hosted in highly strained paleoproterozoic 2148 2160 ma felsic to intermediate metavolcanic rocks and schists that record greenschist facies metamorphism the orebodies are broadly concordant with the regional northwest oriented foliation and mineralization comprises thick quartz vein sets and small quartz carbonate sulfide veinlets and enclosing hydrothermally altered host rocks the hydrothermal mineral assemblage is postpeak metamorphism syn to late tectonic and includes quartz calcite chlorite white mica pyrite subordinate chalcopyrite and traces of sphalerite and galena free milling gold occurs in sulfide poor veins in the upper and lower zones of the deposit whereas refractory gold was deposited in fractures of pyrite and quartz in the more gold enriched intermediate lower level fluid inclusion and stable isotope o h c and s data indicate that the two styles of gold deposition were produced by a combination of fluid immiscibility fluid rock reactions sulfidation carbonatization and probably mixing occurring in two discrete stages within a single mineralizing event associated with fluid flow within and around an active shear zone under fluctuating pressure conditions the refractory gold was deposited at 340 to 370 c from a co2 ch4 h2o nacl fluid having salinity of 5 8 2 7 wt percent nacl equiv variable co2 contents typically 12 22 mol and up to 6 mol percent ch4 the 18o and d values of this fluid are 7 9 to 9 4 per mil and 29 to 37 per mil respectively indicating a metamorphic origin carbon isotopes show contrasting values in fluid inclusion co2 13c 24 1 and calcite 13c of 3 8 the strongly negative value of fluid inclusion co2 is interpreted to be an organic signature acquired at the site of deposition by reaction of the deeply sourced ore forming fluid with carbonaceous schists present in the deepest part of the deposit the higher value for calcite probably records a crustal source oxygen fugacities calculated for the whole range of t p xco2 conditions yielded log fo 2 between 28 7 and 30 5 indicating reduced conditions for the fluid which is in agreement with the mineral paragenesis fluid inclusion compositions and sulfur isotope values the second fluid responsible for the deposition of the free milling gold in veins and probably part of the gold of the lower zone of the deposit had a lower temperature 330 340 c this co2 h2o nacl fluid had a salinity of 1 6 to 2 5 wt percent nacl equiv and contained 11 to 13 mol percent co2 the 18o and d values of this fluid are 5 3 to 7 2 per mil and 12 to 30 per mil respectively also indicating a metamorphic source the 13c value of fluid inclusion co2 is 6 9 per mil likely representing a mantle derived carbon source log fo 2 values between 30 5 and 31 5 also indicate slightly more reduced conditions for this fluid both fluids had near neutral ph between 5 and 6 2 and 34s values of h2s between 0 2 and 1 0 per mil interpreted to reflect derivation of sulfur from magmatic sulfides

mapping of prospectivity and estimation of number of undiscovered prospects for lode gold southwestern ashanti belt ghana in the southwestern part of the ashanti belt the results of fractal and fry analyses of the spatial pattern of 51 known mines prospects of mostly lode gold deposits and the results of analysis of their spatial associations with faults and fault intersections suggest different predominant structural controls on lode gold mineralisation at local and district scales intersections of nne and nw trending faults were likely predominantly involved in local scale structural controls on lode gold mineralisation whilst nne trending faults were likely predominantly involved in district scale structural controls on lode gold mineralisation the results of the spatial analyses facilitate the conceptualisation and selection of spatial evidence layers for lode gold prospectivity mapping in the study area the applications of the derived map of lode gold prospectivity and a map of radial density of spatially coherent lode gold mines prospects results in a one level prediction of 37 undiscovered lode gold prospects the applications of quantified radial density fractal dimensions of the spatial pattern of spatially coherent lode gold mines prospects result in an estimate of 40 undiscovered lode gold prospects the study concludes finally that analysis of the spatial pattern of discovered mineral deposits is the key to a strong link between mineral prospectivity mapping and assessment of undiscovered mineral deposits

age constraints on gold mineralization and paleoproterozoic crustal evolution in the ashanti belt of southern ghana abstract u pb isotopic dating in the region of the ashanti belt in ghana has been carried out to improve understanding of crustal evolution and gold mineralization in this paleoproterozoic terrane a u pb zircon age of ca 2174 2 ma for a belt type granitoid from sekondi accords with previous ages on belt type plutons in ghana but metamorphic titanite within this granitoid gives an age of 2092 3 ma coeval with intrusion of the late basin type granitoids the within belt granitoid at banso village has an age close to 2097 3 ma which falls into the age range typical for basin type granitoids 2116 2088 ma these results indicate that widespread plutonism in the birimian sedimentary basins also produced magmatic and metamorphic effects within the volcanic belts most dated detrital zircons from birimian metasediment samples at the ashanti gold mine agree in age at 2155 2 ma with others that are slightly older this gives a maximum estimate for the depositional age of the sediments and for gold mineralization at the ashanti gold mine hydrothermally altered and auriferous granitoid plutons along the western flank of the ashanti belt at anyankyerim nhyiaso yaomensakrom and esuajah ayanfuri all have intrusion ages within error of 2105 2 ma this is also the age of metamorphic titanite in an unmineralized basin type granitoid at kwapia rutiles from the hydrothermally altered yaomensakrom and the mineralized esuajah ayanfuri granitoid were dated at 2098 7 and 2086 4 ma respectively and probably provide minimum ages on gold mineralization these data indicate a close temporal association among auriferous granitoid plutons near the western margin of the ashanti belt at 2105 ma gold mineralization may be only slightly younger than magmatic emplacement of the plutons granitic plutonism in the sedimentary basins metamorphism and hydrothermal gold mineralization in southern ghana are all part of a major crust formation event the eburnean orogeny that can be constrained to the time range 2120 2080 ma

a new metamorphic constraint for the eburnean orogeny from paleoproterozoic formations of the man shield aribinda and tampelga countries burkina faso abstract progressive metamorphic zones developed in pelites around the aribinda granite burkina faso indicate contact metamorphism at 5 6 kbar biotite chloritoid staurolite kyanite zones are distributed concentrically around the aribinda pluton on its northern margin but are displaced by later thrust faults in the south of the aribinda area and in the nearly tampelga area these metamorphic and structural characteristics provide new constraints for the geodynamical evolution of the eburnean orogeny in the north part of the baoule mossi domain they show the leading part of the granitoid emplacement in the thermic structuration of the greenstone belts and the post metamorphic character of the tangential tectonic in burkina faso the eburnean orogeny underwent 1 initial greenschist facies metamorphism 2 emplacement of granitoids at mid deep crustal levels leading to unusual medium pressure metamorphism 3 later thrusting

paleoproterozoic turbidite hosted gold deposits of the ashanti gold belt ghana west africa comparative analysis of turbidite hosted gold deposits and an updated genetic model abstract the paleoproterozoic gold deposits of the ashanti gold belt ghana occur mainly as auriferous quartz veins in graphitic shears proximal to contacts between deformed and metamorphosed turbidites and tholeiitic volcanics alternately block faulted tarkwaian sediments they are suggested in this article to belong to that class of gold deposits recognized as turbidite hosted hence evidence from previous investigations of turbidite hosted and turbidite associated gold deposits occurring in circumstances roughly similar to the ashanti deposits are used where relevant to produce an updated genetic model for the latter given the relatively significant amounts of graphite associated with the major ashanti deposits the role of non carbonate carbon is assessed in the light of conclusions reached by work on other turbidite hosted and turbidite associated gold belts conclusions as to the origins of mineralizing fluids and temporary repositories of gold auriferous interflow sediments reached by work on these other belts are also analysed a concept of the tectonic development of the ashanti belt kumasi basin area introduced in this article is deemed to be critical to the origin of the ashanti gold deposits it encompasses the role of non carbonate carbon turbidites and the primary sources and secondary repositories of au and as in the genesis of the major ashanti gold deposits it examines the significance of the intimate relationship between calc alkaline volcanism and the basin sediments the occurrences of felsic metasandstones in transition zone sediments and ttg granitic material and fragments of chemical sediments in turbidites all of which are incompatible with one or the other of existing concepts of tectonic development in the area

chemical and sr nd isotopic compositions and zircon u pb ages of the birimian granitoids from ne burkina faso west african craton implications on the geodynamic setting and crustal evolution abstract the west african craton was the site of massive juvenile crustal addition in the early proterozoic during the eburnean orogeny 2 1 ga in this study we determined chemical and isotopic compositions as well as zircon u pb ages of granitoid samples from ne burkina faso these data were then used to constrain the chemical characteristics emplacement age and tectonic environment of these rocks in the context of the west african craton evolution birimian paleoproterozoic granitoids of ne burkina faso are associated with volcanic and volcano sedimentary sequences of the greenstone belts analyses indicate that the na rich birimian granitoids of tonalite trondhjemite granodiorite ttg composition were generated by partial melting of a garnet bearing amphibolite crust granite intrusions derived from partial re melting of the deep basement of felsic calc alkaline nature with contribution of metasediments zircon u pb isotopic analyses yielded ages between 2122 15 ma and 2181 7 ma for the birimian na rich granitoids and 2151 10 ma for a biotite granite ij10 sr nd isotopic data show very low initial sr composition i sr t 0 7007 positive epsilon neodymium values nd t 0 7 to 2 5 and sm nd model ages t dm of 2363 2558 ma these data are similar to those of mafic rocks of the pissila greenstone belt i sr t 0 7003 0 7015 nd t 3 2 to 4 1 t dm 2336 2552 ma and further suggest that the na rich granitoids are of juvenile nature the present study together with published data on the birimian volcanics and meta sedimentary rocks of the west african craton preclude contamination of the juvenile birimian rocks of ne burkina faso by reworked archean materials the birimian granitoids of ne burkina faso were mostly emplaced during the early stage 2 19 2 15 ga of a large scale crustal growth event 2 2 2 0 ga where large amount of juvenile materials were added to the crust from the depleted mantle in a tectonic environment comparable to modern volcanic arcs

lead isotope signature of early proterozoic ore deposits in western africa comparison with gold deposits in french guiana lead isotope analyses of samples from ore deposits of the west african shield trace the metallogenic evolution of the early proterozoic or birimian period the selected deposits which include the loulo poura and banora gold deposits the faleme iron deposit and the perkoa gan hanhui and pilimpikou base metal deposits are representative of each stage of the so called eburnean evolution the study was completed by an analysis of gold deposits in the french guianan shield which is a rifted remnant correlative of the west african craton the isotopic compositions of most of the galenas cluster in a narrow field 206 pb 204 pb 14 60 15 15 207 pb 204 pb 14 96 15 23 208 pb 204 pb 34 34 34 71 corresponding to the initial lead isotopic composition of birimian mineralization secondary isochrons obtained on cogenetic lead depleted sulfides all converge toward the same narrow field it is likely that they reflect an in situ addition of radiogenic lead to older lead from the eburnean event the calculated ages 2236 2741 ma have no clear geologic significance the elongated field of initial lead compositions requires a mixing between juvenile mantle derived lead as expressed in the syngenetic deposits perkoa mu 9 11 and a more radiogenic component such as old 2 150 ma crustal lead as expressed in the late orogenic deposits dorlin thr stockwork mu 10 06 this increase in mu values reflects a gradual increase in the older recycled crustal lead component favored by the tectonic change from d 1 thrusting to d 2 gold bearing vertical strike slip faulting the age of the old recycled crust would almost certainly be earliest proterozoic 2300 2145 ma dabakalian no perceptible archcan lead influx is required to explain the genesis of these deposits geochronological data from other sources bracket most of the eburnean metallogenesis between 2153 or 13 ma 2sigma age of the dabakalian metamorphism and 1949 or 55 ma 2sigma age of the last post tectonic granites pb pb model ages respectively 2113 ma for the early so called syngenetic ore deposits and 2014 ma for the late orogenic mesothermal gold deposits are consistent with these independent geochronologieal data and probably reflect fairly well the approximate geologic ages of these deposits

the gounkoto au deposit west africa constraints on ore genesis and volatile sources from petrological fluid inclusion and stable isotope data the loulo gounkoto complex in the kedougou kenieba inlier hosts three multi million ounce orogenic gold deposits situated along the senegal mali shear zone this west malian gold belt represents the largest west african orogenic gold district outside ghana the gounkoto deposit is hosted to the south of the gara and yalea gold mines in the kofi series metasedimentary rocks the ore body is structurally controlled and is characterised by sodic and phyllic alteration as and fe rich ore assemblages with abundant magnetite and overall enrichment in fe as cu au ag w ni co ree minor te pb se cd fluid inclusion analysis indicates that the deposit formed at p t conditions of approximately 1 4 kbar and 340 c and that two end member fluids were involved in mineralisation 1 a moderate temperature 315 340 c low salinity 10 wt nacl equiv low density 1 g cm 3 h2o co2 nacl h2s n2 ch4 fluid 2 a high temperature up to 445 c hypersaline 40 wt nacl equiv high density 1 3 g cm 3 h2o co2 nacl fecl2 fluid partial mixing of these fluids within the jog zone at gounkoto enhanced phase separation in the aqueo carbonic fluid and acted as a precipitation mechanism for au these findings demonstrate the widespread if heterogeneously distributed nature of fluid mixing as an ore forming process in the loulo gounkoto complex operating over at least a 30 km strike length of the shear zone stable isotope analyses of ore components at gounkoto indicate a dominant metamorphic source for h2o h2s and co2 and by extension au it thus can be reasoned that both the aqueo carbonic and the hypersaline fluid at gounkoto are of metamorphic origin and that the high levels of salinity in the brine are likely derived from evaporite dissolution

spatial sampling of the thermospheric vertical wind field at auroral latitudes 1 results are presented from two nights of bistatic doppler measurements of neutral thermospheric winds using fabry perot spectrometers at mawson and davis stations in antarctica a scanning doppler imager sdi at mawson and a narrow field fabry perot spectrometer fps at davis have been used to estimate the vertical wind at three locations along the great circle joining the two stations in addition to the vertical wind routinely observed above each station these data were obtained from observations of the 630 0 nm airglow line of atomic oxygen at a nominal altitude of 240 km low resolution all sky images produced by the mawson sdi have been used to relate disturbances in the measured vertical wind field to auroral activity and divergence in the horizontal wind field correlated vertical wind responses were observed on a range of horizontal scales from 150 to 480 km in general the behavior of the vertical wind was in agreement with earlier studies with strong upward winds observed poleward of the optical aurora and sustained though weak downward winds observed early in the night the relation between vertical wind and horizontal divergence was seen to follow the general trend predicted by burnside et al 1981 whereby upward vertical winds were associated with positive divergence and vice versa however a scale height approximately 3 4 times greater than that modeled by nrlmsise 00 was required to best fit the data using this relation

lithostratigraphic evolution of the bandamian volcanic cycle in central cote d ivoire insights into the late eburnean magmatic resurgence and its geodynamic implications late to post collisional volcanism in the southern portion of the west african craton swac has been described along the paleo craton margin between the paleoproterozoic baoule mossi domain and archean kenema man nucleus new lithostratigraphic and isotopic data for central cote d ivoire as well as revised regional stratigraphic correlations across the western swac indicate that this volcanic resurgence is not only restricted to the paleo craton margin but extends to most greenstone belts of the western half of the swac this late volcanic event historically referred to as the ca 2117 2080 ma bandamian volcanic cycle has received little attention since its identification in 1996 detailed geological investigation of the bouafle granite greenstone belt in cote d ivoire located 150 km east of paleo craton margin along with newly acquired u pb geochronological data indicate a peak in igneous activity showing evidence for bimodal magmatism affiliated to the bandamian volcanic cycle in central cote d ivoire between 2108 5 ma and 2105 4 ma whole rock geochemistry and lu hf isotopic measurements on magmatic zircons from the bouafle region show that these bandamian rocks are marked by a juvenile signature with little degree of crustal contamination collectively the data collected in this study show that the bandamian volcanic cycle represents an important thermal event that affected the swac during the late stages of the eburnean orogeny spanning the incremental change from folding and thrusting to dominantly transcurrent tectonics along major faults and shear zones this paper discusses the possible geodynamic settings that may account for such a peak in thermal activity during the late stages of the eburnean orogeny

re os geochronological evidence for multiple paleoproterozoic gold events at the scale of the west african craton the archean proterozoic craton of west africa hosts numerous gold deposits which are spatially and temporally related to the eburnean orogeny that took place between 2250 and 1980 ma and included multiple deformation events the majority of these gold deposits are located along shear zones the structural history is relatively well established for most gold deposits but absolute timing of the mineralization is commonly lacking five deposits hosted in the baoule mossi domain located in the southwestern part of the west african craton were studied to better constrain timing of gold events nassara and kiaka in burkina faso and wassa damang and obuasi in southern ghana gold mineralization was structurally characterized at each deposit and dated by the re os method on pyrite arsenopyrite and pyrrhotite grains that were coeval with the gold deposition combined structural and re os geochronological constraints allow two groups of gold deposits to be distinguished early orogenic gold formed during the eoeburnean orogeny i e between 2190 and 2125 ma as represented by kiaka1 at 2157 24 ma and wassa1 at 2164 14 ma these early gold occurrences are expressed as early disseminated gold enrichment 1 3 g t au at kiaka and gold hosted in folded and boudinaged quartz veins that are transposed along the primary s foliation at wassa these re os ages on the early sulfides at wassa and kiaka are significant because they definitively provide the first direct age constraints on gold only eoeburnean mineralization in the birimian of west africa late orogenic gold hosted by brittle structures and with higher gold grades up to 60g t at kiaka formed during late eburnean deformation d d events between 2120 and 2000 ma as represented by the re os dating of these ores at wassa at 2055 18 ma and obuasi at 2045 40 ma similar style ores dated at nassara at 2131 99 ma and damang at 2080 135 ma but both with large uncertainties and an undated younger event at kiaka are also suggested to be part of this later mineralizing episode the high uncertainties on the nassara and damang ages may be directly linked to the low rhenium and osmium contents of the studied samples these results highlight the polyphase character of the widespread gold mineralization in the west african craton although the late stages of the eburnean orogeny constitute a prolific period for the formation of high grade gold mineralization identification of less well studied early stage gold deposits which can also contain large quantities of gold is critical for mineral exploration in the west african craton

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

geology geodynamics and orogenic gold prospectivity modelling of the paleoproterozoic kumasi basin ghana west africa abstract this paper describes the geology and tectonics of the paleoproterozoic kumasi basin ghana west africa as applied to predictive mapping of prospectivity for orogenic gold mineral systems within the basin the main objective of the study was to identify the most prospective ground for orogenic gold deposits within the paleoproterozoic kumasi basin a knowledge driven two stage fuzzy inference system fis was used for prospectivity modelling the spatial proxies that served as input to the fis were derived based on a conceptual model of gold mineral systems in the kumasi basin as a first step key components of the mineral system were predictively modelled using a mamdani type fis the second step involved combining the individual fis outputs using a conjunction product operator to produce a continuous scale prospectivity map using a cumulative area fuzzy favourability caff curve approach this map was reclassified into a ternary prospectivity map divided into high prospectivity moderate prospectivity and low prospectivity areas respectively the spatial distribution of the known gold deposits within the study area relative to that of the prospective and non prospective areas served as a means for evaluating the capture efficiency of our model approximately 99 of the known gold deposits and occurrences fall within high and moderate prospectivity areas that occupy 31 of the total study area the high and moderate prospectivity areas illustrated by the prospectivity map are elongate features that are spatially coincident with areas of structural complexity along and reactivation during d4 of ne sw striking d2 thrust faults and subsidiary structures implying a strong structural control on gold mineralization in the kumasi basin in conclusion our fis approach to mapping gold prospectivity which was based entirely on the conceptual reasoning of expert geologists and ignored the spatial distribution of known gold deposits for prospectivity estimation effectively captured the main mineralized trends as such this study also demonstrates the effectiveness of fis in capturing the linguistic reasoning of expert knowledge by exploration geologists in spite of using a large number of variables the curse of dimensionality was precluded because no training data are required for parameter estimation

early proterozoic crustal evolution in the birimian of ghana constraints from geochronology and isotope geochemistry abstract representative suites of ghanaian granitoids metavolcanics and metasediments were selected from birimian rock units for rb sr pb pb and sm nd analyses isotopic data on the igneous rocks provide evidence for a major early proterozoic crust forming episode which took place over a maximum time interval from 2 3 to 2 0 ga by a differention from a depleted mantle source there is little evidence for the involvement of significantly older e g archaean crust in the genesis of the igneous rock units the only exception so far found is the winneba granitoid which appears to contain a magmatic contribution from presumably underlying archaean basement sm nd model ages indicates that birimian sediments were derived from adjacent penecontemporaneous volcanic belts the birimian of ghana forms part of a major proterozoic eburnean episode of juvenile crustal accretion which has been recognised in the surrounding areas of mauretania senegal ivory coast burkina faso mali and niger where it has been dated at 2 1 2 2 ga abouchami et al the birimian terranes bridge a major gap in mantle activity and associated crustal evolution for a period considered as quiescent on other continents

the alamoutala carbonate hosted gold deposit kedougou kenieba inlier west africa the alamoutala gold deposit is located in the kedougou kenieba inlier a window of paleoproterozoic rocks that crop out in eastern senegal and western mali the deposit is part of the 3 moz au yatela district and produced 308 400 oz au between 2002 and 2012 country rocks in the alamoutala open pit consist of carbonate rocks arenites wackes and siltstones the sedimentary rocks have been subjected to polycyclic deformation and regional greenschist facies metamorphism synkinematic calc alkaline intermediate quartz feldspar porphyry stocks intruded the metasedimentary rocks at 2083 7 ma u pb zircon amphibolite facies contact metamorphism of carbonate rocks resulted in the formation of a high temperature low pressure magnetite bearing skarn field relationships and microtextural data however indicate that economic gold mineralization is shear hosted and occurred during retrograde contact metamorphism geometric and kinematic analyses indicate that the ore hosting structures were undergoing sinistral reverse displacement at the time of mineralization ore minerals deposited synchronous with a potassic hydrothermal alteration assemblage defined as biotite calcite pyrite k feldspar actinolite tremolite quartz muscovite and tourmaline petrographic studies reveal that ore related sulfides largely consist of pyrite with lesser pyrrhotite and chalcopyrite the ore also contains accessory ag sb as hg bi te mo w zn pb ni co u bearing mineral species the alamoutala deposit represents two distinct hydrothermal systems in which a locally developed magnetite bearing skarn was overprinted by a region wide orogenic gold event in the late eburnean

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