mobilization of gold as a polymetallic melt during pelite anatexis at the challenger deposit south australia a metamorphosed archean gold deposit the challenger gold deposit in south australia is hosted by pelitic migmatites that underwent peak metamorphism at 7 5 1 5 kbars and at least 800 c nd model ages suggest a protolith age of 2900 ma zircon u pb and garnet sm nd dating indicates that peak metamorphism occurred at 2440 ma and that garnet was present during this event at this time in situ vapor absent melting affected the pelitic host rock indicating that peak metamorphism occurred without introduction of a foreign fluid or melt thus evidence indicating the presence of ore minerals during peak metamorphism implies a prepeak mineralization event this evidence includes the occurrence of invisible gold in lollingite but not in adjacent arsenopyrite and the presence of spherical gold sulfide inclusions in peak metamorphic garnet and other silicates textures developed between pyrrhotite lollingite arsenopyrite and gold the lack of silicate alteration minerals supports this conclusion experimental evidence is presented to support our interpretation that a gold rich polymetallic melt was mobilized into leucosomes synchronously with silicate melt during peak metamorphism visible gold is restricted to migmatitic leucosomes and to a lesser extent melanosomes large inclusions of gold with coexisting arsenopyrite pyrrhotite and bismuth are hosted within peak metamorphic silicate minerals and at grain boundaries trails of micrometric spherical inclusions of predominantly gold and bismuth propagate along annealed fractures from these larger inclusions we show that these gold sulfide inclusions represent the crystallized products of an immiscible polymetallic melt metamorphism of the challenger deposit resulted in partial melting of the pelitic host rock and formation of this gold rich melt these two melts were synchronously redistributed to accumulate as leucosomes during development of a stromatic migmatite formation of a polymetallic melt thereby enabled extensive mobilization of gold producing leucosomes enriched in gold this occurred because the immiscible metallic melt was physically entrained rather than chemically dissolved within the silicate melt gold rich shoots at challenger parallel the plunge of ptygmatically folded leucosomes that are shown to be parasitic to a larger scale fold geometry which appears to be structurally related to the ore shoots it is interpreted that concurrent migration of polymetallic and silicate melt allowed concentration of gold into a series of dilational structures which developed within the larger scale fold geometry challenger represents a new deposit type other deposits around the world such as renco and griffins find are hosted in granulite facies rocks but challenger is the first reported example of leucosome hosted gold in migmatites

exhumation driven devolatilization as a fluid source for orogenic gold mineralization at the damang deposit ghana gold mineralization at the damang deposit is unique among currently known orogenic gold deposits in ghana comprising gold hosted within metasediments of the tarkwaian system and contained in a subhorizontal extensional quartz vein array that formed during regional compression the damang region has an extended paragenesis involving numerous structural metamorphic igneous and metasomatic events orogenic gold mineralization occurred late in the geologic paragenesis at damang postdating regional metamorphism and an earlier episode of hydrothermal alteration locally termed pink hematite alteration associated with the intrusion of mafic sills and dikes this earlier pink hematite alteration event involves extensive silicification that changed the rheology of the altered rocks and promoted later fracturing following peak regional metamorphism at around 2005 ma the damang region underwent a short period of rapid exhumation as constrained through numerical thermal modeling of existing pressure temperature time data this exhumation triggered the generation of a subhorizontal fracture array that was fed by fluids released through decompression driven metamorphic devolatilization the interaction between these fluids and the host rock resulted in precipitation of gold in association with sulfide carbonate potassic alteration halos around quartz veins such postpeak metamorphic exhumation driven devolatilization is unlikely to be a singular occurrence and represents a potentially important source of fluid for orogenic gold deposits elsewhere in ghana west africa and globally

fluid inclusion studies of the abawso gold prospect near the ashanti belt ghana the major ghanaian lode gold deposits are preferentially aligned along the western and eastern contacts of the kumasi basin with the ashanti and sefwi belts respectively the investigated area of the abawso small scale concession covering the workings of the old ettadom mine is situated 3 km west of the lithological contact of the birimian metavolcanic rocks of the akropong belt in the east with the birimian metasedimentary rocks of the kumasi basin in the west the rocks of the abawso concession represent a steeply nw dipping limb of a se verging anticline with an axis plunging to the sw quartz veining occurs predominantly in the form of en echelon dilatational veins along nne ssw striking shear zones of a few metres width and shows evidence of brittle and ductile deformation also stockwork style quartz veining occurs in the vicinity of the main shaft of the old ettadom mine hydrothermal alteration includes sericitisation sulphidation and locally carbonatisation the auriferous quartz veins mainly follow the trend of brittle to ductile deformed quartz veins however some occur in stockwork fluid inclusion studies reveal a large number of h2o inclusions along intragranular trails in auriferous quartz vein samples as well as an overall dominance of h2o and h2o co2 inclusions over co2 inclusions textural observations and physico chemical fluid inclusion properties indicate post entrapment modifications for all quartz vein samples due to grain boundary migration recrystallisation this process is interpreted to be responsible for the generation of the co2 inclusions from a h2o co2 parent fluid in comparison with mineralisation at the ashanti and prestea deposits which are characterised by co2 n2 inclusions the observed inclusion assemblage may be due to a shallower crustal level of mineralisation or different degrees and styles of recrystallisation or a less pronounced development of laminated quartz veins due to comparably restricted pressure fluctuations furthermore the microthermometric observations allow the reconstruction of a possible retrograde p t path depicting near isothermal decompression in the p t range of the brittle ductile transition

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

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

fast model building using demigration and single step ray migration this work provides explorationists with simple procedures to perform depth conversion more accurately than can be achieved with simple vertical layer cake depth conversion the use of image rays which are inadequate in structurally complex areas is avoided migrated time interpretations are still used and are demigrated using the kirchhoff time migration equations this backs out the effect of the time migration prior to a ray depth migration and enables the lateral shifts between the time migrated image and a depth migrated image to be quantified these shifts can be separated into a mismigration component and a refraction component the relative size of the components define whether time or depth migration is required and may be used to justify a remigration of the seismic image furthermore the tedious layer by layer approach to ray depth migration may be avoided by using the velocity depth model from the vertical layer cake depth conversion of the time migrated data for ray depth migration of the

the metallogenic relationship between birimian and tarkwaian gold deposits in ghana the traditional concept of the early proterozoic gold deposits in ghana i e gold bearing shear zones overlain by tarkwaian paleoplacers containing reworked gold derived from the shear zones needs to be reconsidered in the light of recent research in ghana the ivory coast and french guiana this research has revealed a consistent pattern of geostructural and metallogenic evolution in which both the birimian and the tarkwaian rocks were deformed by a major eburnean compression d2 it has shown that the ne sw faults controlling the gold coast range shear zone mineralization ashanti prestea were formed during the eburnean d2 episode of thrusting that was followed by hydrothermal activity with the emplacement of auriferous arsenopyrite and then by the development of quartz veinlets and native gold thus the shear zone mineralization could only have appeared during the d2 late orogenic stage it has also shown evidence of post depositional d2 deformation in the gold sites examined in the tarkwa gold bearing conglomerate although the effects are limited and primary lithological controls have been preserved that reveal these deposits to be modified paleo placers thus the tarkwaian gold could not be derived from the gold bearing shear zones

the nature and dimensions of regional and local gold related hydrothermal alteration in tholeiitic metabasalts in the norseman goldfields the missing link in a crustal continuum of gold deposits the archaean lode gold deposits at norseman western australia consist of auriferous quartz veins in dextral reverse ductile brittle shear zones within tholeiitic metabasalts of upper greenschist to amphibolite facies metamorphic grade three types of deposits northern central southern are delineated on the basis of their spatial distribution veining style alteration mineraloty and metamorphic grade of host rocks northern deposits hosted in upper greenschist to lower amphibolite facies rocks comprise massive to laminated quartz veins with selvedges of quartz chlorite calcite biotite plagioclase assemblages central deposits hosted in lower amphibolite facies rocks consist of laminated to massive quartz veins with selvedges of quartz actinolite biotite plagioclasecalcite assemblages southern deposits hosted in middleamphibolite facies metabasalts consist of banded quartz diopside calcite microcline zoisite veins all deposits exhibit variable ductile deformation of veins and contiguous alteration haloes consistent with a syn deformational genesis at high temperatures from northern to southern deposits the alteration assemblages are indicative of higher temperatures of formation and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals these observations imply that a thermal variation of gold related hydrothermal alteration exists within the norseman terrane over a distance of 40 km with tnorthern tcentral tsouthern this thermal zonation is corroborated by t xco 2 phase relations between vein selvedge assemblages which signify formation temperatures of approximately 420 475 c 470 495 c and 500 c for northern central and southern deposits respectively the sum of structural petrographic and mineral chemistry data indicates that the alteration assemblages formed in high temperature open hydrothermal systems and have not been subsequently metamorphosed the thermal differences between the deposit groups may reflect 1 a temperature gradient at relatively constant p corresponding to the proximity of the deposits to regional granitoid complexes or 2 formation of the deposits at progressively deeper crustal levels from north to south in either case the deposits represent a continuum of gold deposition from upper greenschist to amphibolite facies now exposed in an oblique section through the archaean crust at norseman

high grade metamorphic processes which influence archaean gold deposits with particular reference to big bell australia mineralogical and geochemical evidence indicates that partial melting and desulphidation have occurred in the big bell gold deposit decarbonation may also have occurred to account for the lack of a carbonate alteration halo this is compatible with the present data but difficult to test the big bell deposit consists of auriferous sulphide bearing lode schists with muscovite and k feldspar and surrounding biotite schists all derived by intense premetamorphic alteration of rocks of mafic composition assemblages which include cordierite sillimanite k feldspar garnet biotite quartz suggest peak metamorphic conditions of 4 5 kbar and 650 700 c based on phase relations geobaro meters and garnet biotite fe mg exchange partitioning partial melting occurred at peak metamorphism particularly in the altered mafic rocks in and around the deposit and its occurrence may have been essential to the preservation of the deposit melting greatly limited the importance of devolatilization reactions resulting in negligible aqueous fluids and no means of removing appreciable gold minor gold loss may have accompanied desulphidation a diversity of complex metamorphic assemblages occurs around the mine compared to the assemblages developed regionally variable bulk rock composition influences this contrast but there is no evidence of higher metamorphic grades at the mine nor that this might have been the prime control on the different assemblages in this narrow belt it is suggested that the big bell and hemlo deposits are the higher metamorphic grade equivalents of the more abundant greenschist facies gold deposits within archaean greenstone belts this interpretation is favoured by the host rock setting and geochemical characteristics of big bell alternative models that suggest that this class of deposit is a new type must account for the absence of high grade equivalents of the greenschist facies deposits and also the lack of low grade equivalents of the big bell hemlo type archaean gold deposits in high grade metamorphic terrains have undergone a series of processes that are not recorded in the more typical gold deposits of the greenschist facies

tectonic constraints on the timing relationships of metamorphism fluid production and gold bearing quartz vein emplacement abstract there is an ongoing discussion about the genetic significance of the postmetamorphic emplacement of many gold mineralizations as a contribution to this discussion this paper reviews the timing relationships between metamorphic devolatilization and metamorphism at different crustal levels for different tectonic processes for example denudation accompanying or following thickening causes metamorphism to occur later at depth than at shallow levels thus many internally heated terrains e g barrovian terrains are of deep later type that is peak metamorphism and devolatilization at deeper levels occurs at later times metamorphic processes of the deep later type may produce and release metamorphic fluids during prograde metamorphism at depth and precipitate them in the form of postmetamorphic quartz veins at shallow levels magmatic heating from below on the other hand is a deep earlier process causing inverse timing relationships in many metamorphic terrains deep later and deep earlier processes may compete for example if denudation at the surface accompanies heating from below if denudation and heating from below are of similar duration deep later characteristics prevail and a strongly bi modal age distribution of peak metamorphism in different crustal levels may arise the upper crust will experience metamorphism early but largely contemporaneously and the lower crust will experience metamorphism much later this process is particularly favourable for substantial simultaneous devolatilization of large sections of the lower crust to postdate metamorphism at shallow levels thus such processes may result in late infiltration events in cooling rocks of low metamorphic grade the common observation of postmetamorphic fluid infiltration and mineralization may therefore indicate but does not require that the mineralising fluids are related to the same metamorphic event

mineralogical fluid inclusion and stable isotope characteristics of birimian orogenic gold mineralization at the morila mine mali west africa abstract gold mineralization at morila is hosted in flat lying meta graywacke associated with a birimian aged massigui schist belt in mali three major events of deformation metamorphism fluid flow d 1 d 2 and d 3 are recognized at morila with mineralization associated with various increments of deformation and fluid flow during d 2 three distinct vein sets v 1 v 3 associated with d 1 d 3 respectively have also been recognized v 1 vein sets are thin poorly defined quartz feldspar metamorphic segregations broadly parallel to the regional fabric v 2 veins are generally boudinaged foliation parallel quartz feldspar veins or occur as felsic melt segregations in the form of quartz k feldspar muscovite biotite blebs located essentially within the ore zone v 3 veins occur as sub parallel to high angle biotite arsenopyrite gold veins that appear to be extensional in nature the ore zone is characterized by compositionally distinct biotite higher mgo and lower feo and more calcic plagioclase than otherwise texturally identical minerals in the footwall and hanging wall of the mineralized zone suggesting that prograde metamorphic minerals were modified by post metamorphic mg and ca rich mineralizing fluids gold forms discrete grains in the v 2 veins or in the adjacent host rock and is closely associated with pyrrhotite and arsenopyrite gold also occurs with sb bi ore minerals in rare microfractures numerous thin biotite arsenopyrite gold veinlets cut d 1 and d 2 fabrics and often carry significant gold associated with kink bands in the d 2 fabric fluid inclusion and oxygen isotope study provide evidence that morila fluids are intermediate salinity 12 16 wt nacl equivalent nacl cacl 2 mgcl 2 h 2 o ch 4 n 2 fluids of magmatic origin 18 o water 10 9 0 3 the ore forming fluid subsequently unmixed to form a compositional range of vapor rich ch 4 n 2 dilute nacl ch 4 n 2 aqueous and liquid rich brine associated with the opening of a micro fracture network and vein development quartz biotite isotope geothermometry indicates that mineralization occurred between 350 to 450 c gold mineralization at morila was an integral part of coeval plutonism deformation and regional metamorphism while the modification of the ore fluid by unmixing was the most important factor associated with the concentration of economically viable gold grades at morila

pb isotope evidence for contaminant metal dispersal in an international river system the lower danube catchment eastern europe abstract lead isotope signatures 207pb 206pb 208pb 206pb 208pb 204pb 206pb 204pb determined by magnetic sector icp ms in river channel sediment metal ores and mine waste have been used as geochemical tracers to quantify the delivery and dispersal of sediment associated metals in the lower danube river catchment due to a diverse geology and range of ore body ages pb isotope signatures in ore bodies within the lower danube river catchment show considerable variation even within individual metallogenic zones it is also possible to discriminate between the pb isotopic signatures in mine waste and river sediment within river systems draining individual ore bodies lead isotopic data along with multi element data were used to establish the provenance of river sediments and quantify sedimentary contributions to mining affected tributaries and to the danube river data indicate that mining affected tributaries in serbia and bulgaria contribute up to 30 of the river channel sediment load of the lower danube river quantifying relative sediment contributions from mining affected tributaries enables spatial patterns in sediment associated metal and as concentrations to be interpreted in terms of key contaminant sources combining geochemical survey data with that regarding the provenance of contaminated sediments can therefore be used to identify foci for remediation and environmental management strategies

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

craton scale distribution of archean greenstone gold deposits predictive capacity of the metamorphic model genetic models should not only attempt to explain features of archean gold mineralization on the scale of mines or districts but should also explain its marked spatial and temporal heterogeneity in greenstone belts the application of genetic models developed with such a philosophy can have predictive capacity in gold exploration the metamorphic model one of a number of current genetic models for archean gold deposits involves generation of auriferous h 2 o co 2 fluids by devolatilization of the stratigraphically lowermost greenstones during metamorphism and broadly synchronous synkinematic granitoid intrusion crustal scale fault shear systems are required to induce channeled fluid flow and to focus ore fluids toward suitable host rocks and structural sites declining temperature below the amphibolite greenschist transition combined with sulfidation of fe rich host rocks is a particularly efficient au depositing mechanism the model suggests that the major controlling factors in declining order of importance are 1 high temperature low pressure generally low strain metamorphic regimes 2 crustal scale fault systems 3 unaltered fe rich rocks tholeiites banded iron formation at a suitable stratigraphic metamorphic level and 4 abundant au enriched komatiites and s rich sediments in source regions the low pressure metamorphic conditions combined with the occurrence of elongate high strain zones in most if not all generally low strain greenstone belts may explain the ubiquitous occurrence of gold deposits in archean terranes all four favorable parameters appear to act in concert to produce the largest gold deposits and most abundant gold mineralization in 2 7 or 0 1 b y old rift phase greenstone belts interpreted to have formed under conditions of high crustal extension and extensive crustal thinning i e with predicted high geothermal gradients high fault density limited synvolcanic alteration and abundant komatiites and sulfidic sediment horizons more widespread commonly older platform phase greenstone belts which formed under conditions of lesser crustal extension and thinning are less mineralized as predicted by the metamorphic model the ability of the model to explain the regional distribution of known archean gold deposits lends credence to its predictive capacity for example 1 greenstone belts in any one craton can be broadly ranked in order of likely relative gold potential 2 the likelihood of discovery of large deposits in poorly exposed or poorly explored belts of known type can be estimated 3 areas with the highest potential can be defined in rift phase greenstone belts by the coincidence of critical parameters such as specific host rock sequences and metamorphic facies 4 prospective structural sites for gold mineralization can be postulated in platform phase greenstone belts affected by diapiric tectonics and 5 the possible spacing of large deposits can be estimated further the potential use of the model in exploration emphasizes serious gaps in our knowledge of parameters crucial to understanding greenstone belt metallogeny including alteration patterns metamorphic p t paths and regional scale structural regimes

geochemistry of paleoproterozoic metavolcanic rocks from the southern ashanti volcanic belt ghana petrogenetic and tectonic setting implications abstract geochemical data are presented for paleoproterozoic metavolcanic rocks from the southern ashanti volcanic belt with the aim of inferring their petrogenesis and tectonic setting in which they were formed the metavolcanic rocks which are predominantly basalts basaltic andesites and andesites have high cr and ni contents indicating that they have not undergone significant fractional crystallization from mantle derived melts two types of basalts basaltic andesites type i and type ii were identified the type i basalts show flat to slightly lree depleted patterns with la sm n 0 69 1 03 la yb n 0 57 0 71 and minor negative and positive eu anomalies eu eu 0 92 1 24 the type ii basalts basaltic andesites show fractionated ree patterns with la sm n 1 34 2 31 la yb n 2 08 4 25 and minor positive eu anomalies eu eu 1 09 1 13 the andesites also show fractionated ree patterns la sm n 1 97 2 78 and la yb n 4 11 8 48 with minor positive to non existent eu anomalies eu eu 0 99 1 15 n morb normalized trace element patterns show that the type ii basalts basaltic andesites and the andesites have geochemical patterns characterized by enrichment in lile relative to hfse and in lree relative to hree the andesites and the type ii basalts basaltic andesites exhibit characteristics of subduction zone related magmas i e the former displays strong nb and ti anomalies and relatively high th nb ratios 0 69 95 whereas the latter displays strong negative nb anomalies but relatively smaller ti anomalies and lower th nb ratios 0 35 0 37 the type i basalts basaltic andesites are generally moderately enriched in liles and depleted in hfses and hrees relative to n morb like the andesites and the type ii basalts basaltic andesites the type i basalts basaltic andesites display subduction related trace element characteristics of positive ba and sr anomalies together with slightly negative nb ta anomalies the low th nb ratios of 0 06 0 11 coupled with their morb like ree patterns indicate no or negligible amount of sedimentary component in the type i basalts basaltic andesites crustal contamination may be ruled out as the cause of the negative th and hfse anomalies observed in the basaltic and andesitic rocks rather the anomalies are interpreted on the basis of th nb la ce to reflect a recycled slab derived lithosphere component the lree depleted tholeiitic type i basalts basaltic andesites exhibit back arc basin geochemical signatures the calc alkaline andesites and the type ii basalts basaltic andesites show intra oceanic island arc signatures the high mg basaltic andesitic rocks may however have evolved in a forearc setting we therefore infer an intra oceanic island arc forearc backarc setting for the paleoproterozoic birimian metavolcanic rocks from the southern ashanti volcanic belt ree modeling indicates that the magma from which the type i basalts were formed was generated by about 17 30 partial melting of a depleted mantle dmm composed of spinel lherzolite in an extensional marginal basin the parental magmas of the high mg calc alkaline basaltic andesitic rocks were produced in the forearc by about 20 30 partial melting of the metasomatized mantle and the type ii basalts basaltic andesites and the andesites are related by fractional crystallization

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

fluid characteristics of granitoid hosted gold deposits in the birimian terrane of ghana a fluid inclusion microthermometric and raman spectroscopic study fluid inclusions in vein quartz from 10 granitoid hosted gold deposits and prospects in the birimian terrane of ghana as well as from the sansu mine ashanti shear zone type and quartz veins in the nonmineralized princess town granodiorite were studied by microthermometry and raman microspectrometry fluid inclusions from the granitoid hosted gold deposits are dominated by aqueous h2o co2 nacl type 1 and liquid co2 n2 ch4 type 2 with minor 10 aqueous h2o nacl type 3 type 1 inclusions show large variations in co2 phase volume proportions 10 90 at 25 c and have salinities commonly between 0 and 6 wt percent nacl equiv their bulk densities fall in a major range from 0 62 to 1 11 g cm3 type 2 inclusions show no visible h2o phase at room temperature and have bulk densities between 0 30 and 0 92 g cm3 type 3 inclusions containing 10 to 20 vol percent h2o vapor have salinities of 1 to 8 wt percent nacl equiv and bulk densities of 0 77 to 1 03 g cm3 in most cases the three types of fluid inclusions coexist as groups in individual quartz grains of the samples studied these fluid inclusions are interpreted to be trapped during phase separation of an originally homogeneous h2o co2 fluid with low salinity 6 wt nacl equiv and moderate to high density 0 65 0 95 g cm3 the type 1 fluid inclusions are suggested to be heterogeneous mixtures of the two end members of type 2 and 3 inclusions fluid immiscibility is documented by petrographic characteristics and microthermometric results of the three types of inclusions trapping temperatures and pressures estimated from microthermometry of type 3 inclusions equation of state and the p t x nature of the h2o co2 nacl system are mainly between 200 and 350 cand 1 and 3 kbars for the gold deposits by contrast fluid inclusions in the sansu mine at ashanti mostly comprise the liquid co2 n2 ch4 type 2 the inclusions are considered to represent postentrapment modifications of trapped fluids in addition fluid inclusions in barren vein quartz from the princess town granodiorite comprise the low salinity commonly 6 wt nacl equiv h2o nacl type 3 raman microspectrometry shows that gaseous compositions of fluid inclusions from both granitoid and shear zone hosted gold deposits are mainly composed of co2 80 95 mol with significant amounts of n2 2 20 mol and ch4 0 10 mol the distinct low salinity h2o co2 rich fluids from the granitoid hosted gold deposits are comparable in composition to those from the major ashanti and tarkwaian types of gold deposits in the birimian terrane of ghana these fluids are most likely to be metamorphic in origin and associated with the waning stages of the regional birimian orogeny gold deposition within the birimian granitoids was related to fluid phase separation and sulfidization of host rocks during hydrothermal alteration and mineralization the present study together with previous publications suggests that fluid inclusions are characterized by h2o co2 nacl and or co2 n2 ch4 types in mineralized areas whereas h2o nacl fluid compositions dominate in barren areas fluid inclusion characteristics may therefore be a useful tool for regional gold exploration in the birimian terrane of ghana

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

conflicting evidence on the timing of mesothermal and paleoplacer gold mineralisation in early proterozoic rocks from southwest ghana west africa rocks of early proterozoic age ca 2100 ma host the major gold deposits in ghana the deposits are either located in mesothermal quartz vein systems or hosted in a quartz pebble conglomerate that represents a paleoplacer both types of mineralisation are largely confined to the ashanti belt one of four parallel northeast trending volcanic belts while the stratigraphy and structure of the belts are similar the ashanti belt is characterised by a more tectonised northwest margin where most of the epigenetic gold deposits are located in these deposits gold mineralisation is located in faults that parallel the regional trend of the belts and were active late in the deformation history of the terrane the auriferous quartz pebble conglomerate is part of a clastic sequence that is largely derived from the adjacent volcanic and plutonic rocks with the gold widely regarded as having originated from eroded vein deposits structural data however show that both the volcanic rocks and clastic sequence were deformed jointly prior to epigenetic gold mineralisation thus the quartz vein deposits could not have been the source of the paleoplacer mineralisation the paleoplacer gold could have originated from one of several possible sources but none has been unequivocally identified

on the timing relationship between fluid production and metamorphism in metamorphic piles some implications for the origin of post metamorphic gold mineralisation abstract the relative timing of metamorphism at different crustal levels is quantified for a range of metamorphic environments associated with convergent deformation we characterise different metamorphic environments by making simple assumptions about the nature of heat input and the nature of heat withdrawal from the terrane if internal heat production is the major heating agent during crustal thickening then deep crustal levels reach their metamorphic temperature peak later than shallow levels in terranes heated from below such relationships apply only if erosion commenced early in the thermal history or if erosion rates are fast quantitative knowledge of these relations can be used together with simple assumptions about prograde fluid release to predict the likely timing of fluid passage through rocks higher in the column in terranes with deep later characteristics the evidence for fluid penetration may be preserved in the form of mineralised quartz veins or late stage alteration within our simple parameterisation of crustal heat sources we can use the grade of a metamorphic rock together with field evidence on the timing of metamorphism igneous intrusion and deformation in order to constrain the time scale of the underlying thermal perturbation and erosion process for a given terrane we demonstrate that time intervals up to several tens of million years may separate metamorphism and fluid emplacement in model evolutions that are scaled to describe greenschist facies metamorphic terranes that formed at low pressure such timing relationships are in fact recorded in many greenschist facies metamorphic terranes that host late stage mesothermal gold deposits we suggest therefore that there is no conflict between the observation that the emplacement of many gold deposits occurred demonstrably after peak metamorphism of their host rocks and the interpretation that metamorphic fluids are responsible for the mineralisation

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

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

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

the gold carbonate association source of co2 and co2 fixation reactions in archaean lode deposits abstract abundant carbonate is a ubiquitous feature of archaean lode gold deposits both in domains of alteration enveloping veins and in rocks enclosing auriferous sediments detailed studies of chemical mass balance in metabasalts progressively altered towards gold bearing quartz carbonate veins disposed within shear zones at yellowknife reveal massive additions of co2 k si and fe accompanying mineralisation with concomitant depletions of na coherent behaviour of al sc zr also v and nb provides a reference for constraining the volume relations during hydrothermal alteration in the peripheral regions of alteration depletions of ca and mg result in overall volume reduction but these elements are added to veins and their immediate alteration envelopes where there is a large positive volume change precious metals together with cr ni cu zn pb rb cd and ba have been added to the veins and altered wall rocks whereas quartz noble metals and other trace elements have been precipitated from hydrothermal fluids solution of ca mg and some fe indigenous to peripheral alteration regions combining with co2 from the mineralising reservoir appears to be the process for forming the abundant ferri dolomite gangue observations of many archaean au deposits reveals that carbonate chemistry reflects the nature of wall rocks with wall rocks donating the bivalent metal cations and hydrothermal fluids the co2 alteration reactions at yellowknife involved hydrolysis of albite accompanied by fixation of kaqueous to produce muscovite with na loss and hydrolysis of chlorite epidote with co2 fixation to form ferri dolomite studies of oxygen isotopes fe 2 fe 2 fe 3 and structure reveal that au quartz and carbonate were precipitated in the presence of fluids of probable metamorphic origin 18 o 8 9 at low redox potential and at ambient temperatures of 400 450 c during episodes of hydrofracturing the abundant co2 and k required for extensive carbonate muscovite replacement alteration could be supplied by fluids released during prograde metamorphism under greenschist or greenschist amphibolite facies conditions where the relative proportion of co 2 h 2 o is in the order of 0 2 to 0 5 and k na 1 given high co2 co in the hydrothermal reservoir these molecules may act as complexing agents for transport of au and other rare elements e g w pd ni cr