the nature of the witwatersrand hinterland conjectures on the source area problem although the au and u deposits of the witwatersrand basin are generally regarded as modified placer in origin the nature of the source area and the ultimate origin of au and u still remain a major enigma a number of domal outcrops of the archean basement are exposed in the witwatersrand hinterland revealing the presence of a diverse suite of granitoids and a minor 20 greenstone belt component in addition samples from over 130 boreholes drilled adjacent to the basin have revealed the character of subsurface archean basement beneath early proterozoic sedimentary and volcanic cover the granitic subsurface basement is commonly characterized by pervasive hydrothermal alteration evident as propylitization sericitization carbonation and greisenization as well as a variety of vein related alterations comprising quartz or k feldspar or chlorite or co 3 or sulfide or fluorite parageneses these hydrothermally altered granites appear to have been preserved beneath early proterozoic cover but have largely been eroded from surface outcrops the hydrothermally altered granites which are believed to be or 3 050 ma are significantly enriched in au and u with respect to both surface granitoids from the witwatersrand hinterland and the entire range of granites from the barberton mountain land the presence of enriched hydrothermally altered granites in the witwatersrand hinterland lends support to the view that mineralization processes in the basin were dominated in the first instance by placer processes the remarkable concentrations of au and u that are unique to the central rand group of the witwatersrand basin indicate that the sediments in the upper portions of the sequence tapped a fertile source not present in the hinterland during earlier cycles of sedimentation the erosional remnants of this source area are represented by the hydrothermally altered granites which are considered to have been a suite of high level granitoids the upper portions and roof zones of which were mineralized in au and u and easily accessible to erosion the emplacement of these granites and the development of extensive zones of attendant high level hydrothermal alteration was synchronous with witwatersrand sedimentation the hydrothermally altered granites not only contributed to the au and u rich detritus in the witwatersrand but their emplacement may have been linked causally to the deposition of the upper part of the sedimentary sequence

archaean lode gold deposits in africa crustal setting metallogenesis and cratonization abstract archaean rocks in africa are preserved in seven important tectonic and physiographic terranes the zimbabwe craton the kaapvaal craton of south africa the tanzanian shield the north zaire massif which extends westwards into the central african republic the kasai angola massif of southern zaire the atlantic rise of cameroun gabon and congo extending southwards to the malanje strip of northern angola the guinea rise of guinea sierra leone and liberia and the reguibat rise of mauritania morocco and northwest algeria dilational vein and brittle to ductile shear zone mineralization is common in the zimbabwe craton distribution of the gold being controlled by the permeability characteristics of the hydrothermal conduits and by the competence and chemistry of the wallrocks within the kaapvaal craton the barberton greenstone belt has the highest productivity with a marked concentration of lode type mineralization at or near the tectonized contact between the basalt dominated onverwacht group and the metasediments of the fig tree group smaller lode gold deposits are also present in the greenstone belts of the central and northern kaapvaal craton the more important being the stibnite gold mineralization that accompanied late brittle deformation of carbonatized high mg basalts in the murchison greenstone belt the archaean basement and supracrustal sequences of the guinea rise the reguibat rise the atlantic rise and the kasai angola massif are dominated by medium and high grade metamorphic rocks and with the exception of small greenschist facies sequences in central sierra leone the guinea rise are mostly devoid of gold mineralization however vein type gold lodes are more numerous in the low metamorphic grade greenstone belts of the north zaire massif and the tanzanian shield the important gold producing terranes are characterized by a complex tectonic history of strike slip and reverse dip slip shearing the productive lodes commonly being hosted by second or higher order shear zones these shear zones demonstrate an important component of late reverse dip slip activation or reactivation which provided optimum conditions for crustal de watering during prograde metamorphism and emplacement of tonalitic magmas the stratigraphic sequences and tectonic evolution are generally compatible with but do not unequivocally demonstrate cordilleran type accretionary regimes a link between gold metallogeny and the tectonothermal events that preceded and ultimately culminated in cratonization is commonly evident however cratonization in africa in a broad sense was diachronous ranging from ca 3 0 ga in the barberton greenstone belt through ca 2 6 2 5 ga in the zimbabwe and tanzanian cratons to the ca 2 0 ga eburnean event of west africa the latter contributing to the important gold mineralization of southwest ghana this multiplicity of tectonothermal events and the evidence for higher and more persistent crustal heat fluxes in the early precambrian terranes of west and equatorial africa have important ramifications for crustal evolution metallogeny and gold exploration

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

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

gold mineralization patterns in relation to the lithologic and tectonic evolution of the chibougamau mining district quebec canada the chibougamau mining camp a major gold producer has so far produced a total of 1 050 mt of gold at an average grade of 1 85 g tonne in contrast to a general tendency to group the deposits as vein type with varying compositions this paper demonstrates the strong influence of regional lithologic and tectonic evolution of the area on the gold mineralization patterns a syn volcanic period of mineralization comprises both the volcanogenic massive sulphide and disseminated mineralization and the subsequent epithermal mineralization the latter is also associated with evolving volcanic landforms and syn volcanic intrusions the emplacement of the typical archean lode gold deposits and later cu au deposits of uncertain origin coincides with the syn deformational kenoran period the spatial relationship between the east west trending shear zones and a north east trending fault system is shown to be a possible mechanism for generating added dilatancy during the gold mineralizing phase a late to post kenoran shear system controlled the emplacement of the last major phase of gold mineralization within a stratiform intrusion

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

age constraints on tarkwaian palaeoplacer and lode gold formation in the tarkwa damang district sw ghana two major epigenetic gold forming events are recorded in the world class gold province of southwest ghana a pre tarkwaian event was the source of the world class tarkwa palaeoplacers whereas post birimian and tarkwaian deformation which was related to the eburnean orogeny gave rise to the world class e g prestea to giant e g obuasi orogenic gold deposits which have made the region famous for more than 2 500 years a maximum age of 2133 4 ma for tarkwaian sedimentation is provided by 71 of 111 concordant shrimp ii u pb dates from detrital zircons in tarkwaian clastic rocks from damang and bippo bin northeast of tarkwa the overall data distribution broadly overlaps the relatively poorly constrained ages of birimian volcanism and associated dixcove type granitoid emplacement indicating syntectonic development of the tarkwaian sedimentary basin these zircon ages argue against derivation of the palaeoplacer gold from an orogenic gold source related to the compressional phase of an orogeny significantly older than the eburnean orogeny instead they suggest that the gold source was either orogenic gold lodes related to an earlier compressional phase of a diachronous eburnean orogeny or ca 2200 2100 ma intrusion related gold lode the co2 rich fluid inclusions in associated vein quartz pebbles are permissive of either source at the damang deposit an epigenetic orogenic lode gold system clearly overprinted and sulphidised low grade palaeoplacer hematite magnetite gold occurrences in the banket series conglomerate within the tarkwaian sedimentary sequence gold mineralisation is demonstrably post peak metamorphism as gold related alteration assemblages overprint metamorphic assemblages in host rocks in alteration zones surrounding the dominant subhorizontal auriferous quartz veins there are rare occurrences of hydrothermal xenotime which give a shrimp u pb age of 2063 9 ma for gold mineralisation the similar structural timing of epigenetic gold mineralisation in tarkwaian host rocks at damang to that in mainly birimian host rocks elsewhere in southwest ghana particularly at obuasi suggests that 2063 9 ma is the best available age estimate for widespread orogenic gold mineralisation in the region argon argon ages of 2029 4 and 2034 4 ma for hydrothermal biotite from auriferous quartz veins appear to represent uplift and cooling of the region below about 300 c as estimates of the temperature of gold mineralisation are higher at around 400 c if peak metamorphism with temperatures of about 550 c is assumed to have occurred at about 2100 ma the biotite ages in combination with the xenotime age suggest a broadly constant uplift rate for the region of about 1 km per 10 million years from about 2100 to 2025 ma

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

lithostratigraphy geochronology and gold metallogeny in the northern guiana shield south america a review abstract with a surface area of nearly 900 000 km 2 the guiana shield represents the northern segment of the amazonian craton in south america lying for the most part between the amazon and orinoco river basins most of the guiana shield formed during protracted periods of intense magmatism metamorphism and deformation culminating with the trans amazonian tectono thermal event bracketed between 2 1 and 1 9 ga the guiana shield is among the least known precambrian terranes because it is relatively inaccessible lacks bedrock exposure due to intense weathering and is poorly documented in the international geological literature this situation has significantly improved during the last 20 years when shallow in situ gold occurrences attracted exploration and mining companies to initiate geological programs aimed at better understanding the geology and the mineral deposits of the shield the only archean terrane ca 3400 ma known to date in the guiana shield is the imataca complex in venezuela the paleoproterozoic low grade volcano sedimentary greenstone sequences and associated granitoid intrusions have yielded ages between 2 25 and 2 08 ga recent u pb age determinations of the granitoid greenstone belts suggest protracted magmatic cycles from pre to post peak regional metamorphism the younger terranes comprise anorogenic sedimentary sequences of the roraima formation as well as felsic volcanic rocks and associated intrusions of the uatuma formation mafic dikes of the avanavero suite and rapakivi type and alkali intrusions several large scale ductile shear zones have been documented in the guiana shield in northcentral venezuela the most outstanding structure documented to date the ne sw trending guri fault juxtaposes the archean imataca complex against paleoproterozoic terranes the central guiana shear zone cgsz extends from french guiana westerly towards central suriname and further west towards northcentral guyana where it matches with the makapa kuribrong shear zone mksz in french guiana the north guiana trough ngt is interpreted as a sinistral strike slip formed during the trans amazonian orogeny most gold deposits and occurrences discovered to date in the guiana shield are sited in close proximity to major structures in addition they are linked with low to medium metamorphic grade granitoid greenstone belts similar to other better explored precambrian terranes at a local scale the gold deposits are hosted within or in close proximity to quartz veins that are syn to late tectonic and to a lesser extent in stockworks breccias and lenses they are commonly located in units that behaved in a more brittle manner than the country rocks available information suggests that gold deposits are mainly epigenetic although some are associated with specific lithostratigraphic units pyrite pyrrhotite chalcopyrite galena sphalerite scheelite molybdenite and tellurides are the main metallic minerals associated with gold non metallic minerals are mainly quartz and carbonates ankerite calcite siderite associated with minor chlorite epidote albite muscovite and fuchsite silica carbonate propylitic and potassic alteration is common high erosion rates expected after the creation of an orogenic belt did not occur in the northern guiana shield shallow level deposits preserved in many settings suggest that the granitoid greenstone belts represent first order exploration targets for large tonnage low grade gold deposits

the 2 7 2 0 ga volcano sedimentary record of africa india and australia evidence for global and local changes in sea level and continental freeboard abstract the 2 7 2 0 ga volcano sedimentary records of the african indian and australian cratons indicate two broadly defined periods of extensive drowning of the emergent continental areas concomitant with lowered freeboard carbonate banded iron formation bif platforms characterised the first such event at ca 2 6 2 4 ga africa and australia to 2 7 ga india these earlier globally enhanced sea levels are ascribed to increased mid ocean ridge activity possibly related to breakup of a postulated late archaean southern supercontinent alternatively a transition from global scale catastrophic mantle overturn events to the onset of plate tectonics may have occurred in the late archaean nelson 1998 earth planet sci lett 158 109 119 both explanations of increased mid ocean ridge activity are compatible with significant early to middle archaean crustal growth armstrong 1981 phil trans r soc london a 301 443 472 with the emergent high freeboard cratons being subjected to aggressive weathering and erosion enhanced continental crustal growth near the archaean proterozoic boundary mclennan and taylor 1982 j geol 90 347 361 related to the development of significant island arc complexes would have resulted in common lowered freeboard enhanced sea level conditions at the passive margins of the southern cratons the diachronous nature of these earlier transgressions in the various cratons may reflect the effect of local tectonic movements and or the thermal state of the cratons from ca 2 4 2 2 ga cratons that make up the present day continents of india africa and australia had relatively high continental freeboard and lowered sea levels glacigenic deposits are preserved on the kaapvaal africa singhbhum india and pilbara australia cratons the second broadly defined drowning event at ca 2 15 ga was probably due to post glacial climatic amelioration freeboard was reduced by the combination of eustatic rise and the reestablishment of aggressive weathering as warmer palaeoclimates returned in india carbonates were more prominent than the siliciclastic sediments including prominent black shales seen in africa and australia

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

constraints on the timing of late eburnean metamorphism gold mineralisation and regional exhumation at damang mine ghana the damang gold deposit in southwest ghana is unique among known deposits in ghana comprising gold mineralisation in two distinct styles early gold hosted in a stratigraphically controlled auriferous quartz pebble conglomerate horizon is overprinted by later gold mineralisation contained in a sub horizontal fault fracture quartz vein array a multi system geochronological study is used to constrain the timing of igneous activity regional metamorphism gold mineralisation and the thermal history at damang u pb analysis of zircons from birimian volcaniclastic and intrusive rocks constrain volcanism and associated intrusive activity at 2178 0 9 3 ma and 2164 6 8 0 ma respectively which is consistent with previous studies the age of formation of staurolite grade amphibolite facies peak metamorphic mineral assemblages at 2005 26 ma is provided by u th total pb epma analysis of metamorphic monazite grains in the tarkwa phyllite measured 40ar 39ar biotite ages range between 1980 9 ma and 1898 11 ma argon diffusion modelling with the programme diffarg suggests that this age range could be achieved by a period of rapid cooling at a rate of approximately 17 c ma followed by a prolonged period of much slower cooling at a rate of 0 15 c ma the period of rapid cooling is interpreted to represent localised exhumation of the damang host rocks during the latest stage of the eburnean orogeny at the time of hydrothermal gold mineralisation given these age constraints hydrothermal gold mineralisation is inferred to have occurred between approximately 2030 ma and 1980 ma these ages constrain metamorphism fluid flow and gold mineralisation at damang and are the youngest currently recognised in the birimian of sw ghana

gold mineralisation in the essakane goldfield in burkina faso west african craton abstract the west african craton is known for its structurally hosted au deposits in ghana burkina faso cote d ivoire mali and niger the essakane goldfield in northeast burkina faso has produced 1 606 000 oz of gold since 2010 from the essakane main zone the essakane goldfield is made up of several exploration and artisanal sites that include essakane main zone gossey falagountou sokadie tin zoubratan essakane north and south korizena bom kodjele tin taradat tassiri gaigou and takabangou gold mineralisation in sheeted and stockworks quartz carbonate and tourmaline veins occurs with pyrite arsenopyrite and traces of pyrrhotite galena and hematite it is hosted in sheared folded and contact metamorphosed volcanic volcanoclastic and sedimentary birimian supergroup sequences the maximum age of gold mineralisation in the essakane goldfield is syn deformational and formed during the eburnean orogeny d2 at 2130 1980 ma

the witwatersrand controversy revisited recent revisionist criticism of the modified placer theory of the origin of the witwatersrand ores is singularly unconvincing in response to that criticism it is shown that the so called enigma of the sulfidation of black sands is a myth sulfide assemblages viewed as unusual are quite plausibly attributed to a source effect involving detrital transport prolonged weathering and recycling under oxygen deficient atmospheric conditions have favored preservation of detrital sulfides native elements and uraninite although many of these minerals have been reconstituted to varying degrees as a result of diagenesis and metamorphism a supposed example of structural control of mineralization is invalid evolution of the mineralogy of the witwatersrand ores is intimately linked to extensive reworking and recycling of constituents derived from deep weathering of source rocks these rocks are composed of about 60 percent arenaceous sequences 15 to 20 percent greenstone belt rocks and 20 to 25 percent metamorphic and felsic igneous rocks of the sialic crust revisionist arguments for a widespread hydrothermally induced metasomatic origin of reported precambrian paleosols associated with the ore bearing conglomerates are shown to be ill founded and wrong in summary revisionist criticism of the modified placer hypothesis proves to be at best premature the placer hypothesis as presently modified is alive and well enjoying a formidable predictive capacity as the favored working hypothesis

nd pb and sr isotope systematics of fluorite at the amba dongar carbonatite complex india evidence for hydrothermal and crustal fluid mixing the cretaceous amba dongar complex contains one of the largest fluorite deposits 11 6 million tons at 30 wt caf 2 associated with any carbonatite fluorite mineralization postdates carbonatite emplacement and takes the form of replacement hydrothermal veins concentrated at the carbonatite country rock contact sr isotope ratios from the amba dongar fluorites reveal large isotopic variations that can only be attributed to reaction between a carbonatite derived f rich fluid and continental crust or crustal derived fluid the extremely variable initial 87 sr 86 sr ratios 0 70910 0 71729 are intermediate between those of the carbonatites 0 70549 0 70628 and the host bagh sandstones 0 75359 0 78274 they correlate negatively with sr abundances dances sr isotope ratios from intrasample fluorites show large isotopic variations 0 70910 0 71425 that cover almost the entire range of values found for all of the fluorites measured from amba dongar the pb isotope ratios from the fluorites are also variable and differ from those of the country rocks compared to the carbonatites most contain similar 207 pb 204 pb and 208 pb 204 pb ratios but more radiogenic 206 pb 204 pb values galena and pyrite coeval with fluorite mineralization have pb isotope ratios which overlap those from most of the fluorites but which are isotopically distinct from one another in contrast initial 143 nd 144 nd isotope ratios from most of the fluorites are relatively uniform 0 51240 051247 quite different to those from the surrounding bagh sandstones 0 51122 0 51149 but similar to those from the carbonatites 0 51248 0 51253 the new isotopic results are partly consistent with a model for fluorite deposition involving interaction between ground waters within the surrounding country rocks and an f rich carbonatite derived fluid deans and powell 1968 the variation in the sr isotope composition for the fluorites suggests that certain physio chemical parameters of the mineralizing fluids were changing with time of deposition

rb sr and sm nd geochronology of lower proterozoic granite greenstone terrains in french guiana south america abstract nine samples of metavolcanic rock from the lower parts of greenstone belts in central french guiana the paramaca series and 14 granitic samples from the intrusive gneisses the degrad roche and arawa gneisses were selected for sm nd and rb sr analysis the sm nd results from the metavolcanic series including two tholeiites five peridotitic komatiites and two andesites yield an isochron age of 2 11 0 09 2 ga with an initial 143 nd 144 nd ratio ind of 0 51002 9 2 corresponding to nd t 2 1 1 8 this isochron is interpreted as representing the age of initial volcanism of the paramaca series acid intrusives were dated by the rb sr method a whole rock rb sr isochron including data points from both the degrad roche and arawa gneisses yields an age of 2 00 0 07 2 ga with initial 87 sr 86 sr ratio isr value of 0 7019 4 2 this result is considered to be the time of emplacement of the orthogneiss protoliths the positive end value 2 1 1 8 obtained from the metavolcanic rocks of french guiana suggests that their mantle sources have evolved in reservoirs slightly depleted in light rare earth elements lree this result confirms the possible existence of ancient lree depleted reservoirs within the lower proterozoic mantle moreover the high end t value for these rocks excludes any significant crustal contamination during magma genesis the french guianese orthogneisses yield a low isr value 0 7019 4 2 which together with geochemical considerations suggests that their granitic protoliths could have originated by partial melting of short lived crustal precursors of basaltic to granodioritic composition the present geochronological and isotopic study suggests that the guiana shield may represent a major continental accretion event during the lower proterozoic

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

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

palaeoproterozoic gold mineralization of the eteke archaean greenstone belt gabon its relation to the eburnean orogeny abstract the gold deposits of the eteke district gabon are hosted by metasediments and intermediate to ultramafic metavolcanites of the eteke archaean unit 2940 6 ma a greenstone belt within the folded and metamorphosed ogooue domain that was formed by the successive transport and stacking of nappes during palaeoproterozoic eburnean d1 d2 collisional tectonism this lithotectonic pile in the east of the eteke district rests on the chaillu archaean basement and palaeoproterozoic francevillian foreland basin deposits the gold is contained in quartz carbonate stockworks hydrothermalite lenses and mineralized quartz veins that indicate mineralization during a single d2 hydrothermal event extending from an early d2 stage stockwork to a late d2 stage veins the mineralization began during the early d2 stage following a major phase of tangential tectonism d1 when the eteke unit still in a shallow position brittle deformation in the outer foreland zone of the orogenic belt would have received the repercussions of crustal thickening taking place in the inner zone of the belt ogooue series s s these repercussions are reflected by the development of stockworks and hydrothermalite lenses related to reverse faults that favoured the emplacement of tonalitic dykes and sills and channelled the first hydrothermal fluids at the peak d2 stage the eteke unit which by this time formed part of the outer zone of the orogenic belt was caught up in nappe tectonism that imparted a major s2 foliation with microfolding and foliation of the early stockworks the early d2 reverse faults evolved to synmetamorphic thrusts epi to mesozonal conditions along which syntectonic granites and rare mineralized veinlets were emplaced reflecting a dormancy of the hydrothermal event the nappe tectonism ended with transport of the peak d2 lithotectonic pile onto the chaillu archaean terrain and nearby francevillian foreland basins accompanied by a late d2 emplacement of gold bearing quartz veins cutting the s2 foliation and marking the end of the mineralizing hydrothermal event

infiltration of a liquid front in an unsaturated fractured porous medium we consider liquid infiltrating by gravity flow into a system of parallel regularly spaced fractures in an unsaturated porous medium the position of the fracture liquid front as a function of time under some simplifying assumptions is shown to obey a nonlinear integrodifferential equation approximate analytic solutions are developed showing that the movement of the liquid front exhibits three major flow periods 1 at early time the frontal position is determined by the fracture inlet boundary condition and the gravity driven flow behavior of the fracture with negligible influence by the matrix 2 at intermediate time matrix imbibition retards the frontal advance against the pull of gravity 3 at late time the matrix approaches saturation and the frontal velocity approaches a limiting value a two dimensional numerical model is used to confirm the approximate solutions implications of the model for nuclear waste storage are discussed the analysis is applicable not only to fractured rock but also to lateral infiltration into coarse grained sediments lying between layers of fine grained soil

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

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