the silicon isotope record of early silica diagenesis the heavy isotopes of silicon are strongly enriched in some of the youngest early diagenetically formed porcellanite layers from the southwest indian ridge pleistocene and the maud rise pliocene these porcellanite layers are composed of opal ct and were formed by the conversion of amorphous silica opal a from siliceous sediment via dissolution reprecipitation their bulk si30 values range between 1 7 and 2 3 detritus poor siliceous sediment surrounding these layers is significantly lower at 0 3 to 1 5 sequential chemical extractions of bulk siliceous sediment show i preferential dissolution of diatoms featuring higher si30 than radiolaria and al si components the detailed investigation of porcellanite layers by micro scale si isotope and al si analyses using uv femtosecond laser ablation icp mass spectrometry show that ii precipitation of authigenic aluminum silicates enriched in light si isotopes drives pore waters to even higher si30 we suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert we infer that past environmental conditions can be reconstructed with high fidelity from the stable si isotope composition of chert when initial seawater si concentrations were high such as in the precambrian exchange of si between layers during phase transformation from opal a to opal ct and from opal ct to quartz is impeded when variable amounts of detrital minerals are present because they control rates of silica phase transformation and hence the timing of dissolution reprecipitation during burial

laminated cyanobacterial mats in sediments of solar salt works some sedimentological implications formation of microlaminated sediments in solar salt works along the mediterranean coast in southern france only occurs within a restricted salinity range of 60 150 gl 1 these salinities are associated with development of a laminated cyanobacterial mat composed primarily of the filamentous cyanobacteria microcoleus chthonoplastes interbedded with detrital laminae transplants of the cyanobacterial mat to a less saline zone 36 60 gl 1 indicated that the cyanobacterial mats failed to colonize the less saline waters due to herbivorous snails and competition for light from floating algal masses of cladophora and enteromorpha neither the snails nor the cladophora and enteromorpha masses are tolerant of salinities above 60 gl 1 and therefore the microcoleus mats are restricted to those areas of the solar salt works with these higher salinities analyses of salinity conductivity dissolved oxygen and ph in shallow salt pans with salinities of 60 150 gl 1 established a relationship between the daily development of oxygen supersaturation and cyanobacterial photosynthesis sediments are unlaminated in those portions of the solar salt works where there are no cyanobacterial mats these mats are frequently drained of their overlying water and thus desiccation cracks divide them into polygonal plates the development and translocation of these plates is enhanced by gas bubbles which form under the surface of the mats no correlation between the microlaminae in sections from two cores located approximately 1 m apart was observed this was consistent with the hypothesis that the surface of the desiccation crack polygons can be removed by currents and redeposited on the top of other cyanobacterial mat polygons this process results in a patchwork quilt of young and old cyanobacterial mat polygons with an irregular microlamination pattern the presence of such an irregular pattern of laminae permits an important distinction to be made between sediments associated with stromatolite formation and those associated with the very fine and horizontal varved sediments of stratified meromictic water bodies the sedimentological significance of these observations is reviewed in relation to the processes of stromatolite genesis

the role of fungi in the diagenetic alteration of spar calcite fungi may play a far more important role in carbonate diagenesis than has been previously suspected this can be demonstrated by placing known species of fungi on crystals of iceland spar calcite such experiments show that fungi can 1 cause sparmicritization through dissolution 2 produce spiky calcite through dissolution 3 trap and bind loose particles of calcite and 4 promote the growth of new calcite crystals although not observed in this series of experiments fungi can bore into substrates and cause sparmicritization the activity of fungi involves a complex interplay of substrate destruction and construction without the intervention of vast quantities of fluid

evidence for two stages of mineralization in west africa s largest gold deposit obuasi ghana the supergiant obuasi gold deposit is the largest deposit in the paleoproterozoic birimian terranes of west africa with 62 moz of gold past production resources the deposit is hosted in the paleoproterozoic kumasi group sedimentary rocks composed of carbonaceous phyllites slates psammites and volcaniclastic rocks intruded by different generations of felsic dikes and granites a three stage deformation history is defined for the district the d1 ob stage is weakly recorded in the sedimentary rocks as a layer parallel fabric and indicates that bedding parallel shearing occurred during the early stage of deformation at obuasi the d2 ob is the main deformation stage affecting the obuasi district and corresponds to a nw se shortening tight to isoclinal folding as well as intense subhorizontal stretching occurred during d2 ob parallel with the plane of a pervasive ne striking subvertical foliation s2 ob finally a n s shortening event d3 ob refolded previously formed structures and formed a distinct ene striking variably dipping s3 ob cleavage that is domainal in nature throughout the deposit two economic styles of mineralization occur at obuasi and contribute equally to the gold budget these are 1 gold bearing sulfides dominantly arsenopyrite mainly disseminated in metasedimentary rocks and 2 native gold hosted in quartz veins that are as much as 25 m wide microstructural evidence such as strain shadows surrounding gold bearing arsenopyrite parallel with s2 ob but folded by s3 ob indicates that the sulfides were formed during d2 ob concentrations of as much as 700 ppm au are present in the epitaxial growth zones of the arsenopyrite grains although the large mineralized quartz veins are boudinaged and refolded indicating their formation during d2 ob field and microanalytical observations demonstrate that the gold in the veins is hosted in microcracks controlled by d3 ob where the s3 ob cleavage crosscuts the quartz veins in the main ore zones thus these observations constitute the first evidence for multiple stages of gold deposition at the obuasi deposit futhermore three dimensional modeling of stratigraphy structure and gold orebodies highlights three major controls on oreshoot location which are 1 contacts between volcaniclastic units and pre d 1 felsic dikes 2 fault intersections and 3 f3 ob fold hinges the maximum age for the older disseminated gold event is given by the age of the granites at 2105 2 ma which is within error of hydrothermal rutile in the granites of 2098 7 ma the absolute age of the younger gold event is not known

the paleoclimatic record provided by eolian deposition in the deep sea the geologic history of wind the mineral component of pelagic sediment is brought to the deep sea by transport in the wind extraction and analysis of this dust allows estimation of the past aridity of the eolian source region via flux determinations and of the intensity of the transporting winds from grain size data these two parameters the grain size and mass flux of dust vary independently there are three significant sources of modern dust eastern and central asia northwest africa and arabia all in the northern hemisphere as the rainfall associated with the intertropical convergence zone is an effective barrier to southerly transport of dust the northern hemisphere is an order of magnitude more dusty than is the southern an asymmetry that has characterized most of the cenozoic eolian flux data show that most of the northern hemisphere was more arid during glacial maxima with 3 to 5 times as much dust transported during glacial stages than during interglacials only northwestern south america varied in the opposite sense the periodicity of quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the milankovitch cycles of orbital variability wind intensities vary on a shorter time scale than the general 100 kyr cycles of glaciation and general aridity eolian grain sizes display forcing at precessional 19 and 23 kyr tilt 41 kyr and at approximately 30 kyr periodicities as a result the generalization that winds are uniformly stronger during glacial times is not valid whole cenozoic records show that the largest change in dust flux an order of magnitude increase occurred in the northern hemisphere and reflects continental drying associated with the late pliocene onset of northern hemisphere glaciation southern hemisphere eolian records show no sign of paleoclimatic changes in the late pliocene the most important change in cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the paleocene eocene boundary before then latest cretaceous and paleocene winds were essentially as strong as those of the late cenozoic this shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 ma

impressions of algal mats from the middle proterozoic belt supergroup northwestern montana u s a irregular low relief bedding plane markings which probably represent impressions of thin wrinkled algal mats have recently been detected in mudstones from the 1100 m y old snowslip formation belt supergroup glacier national park montana algal mat impressions are rarely reported from ancient strata yet algal mats are common in certain modern environments originating primarily in moist settings and in relatively quiescent shallow water settings such structures may be of some value in interpreting ancient depositional environments

mechanisms of olivine dissolution by rock inhabiting fungi explored using magnesium stable isotopes abstract to unravel the dissolution mechanisms of olivine by a rock inhabiting fungus we determined the stable isotope ratios of mg on solutions released in a laboratory experiment we found that in the presence of the fungus knufia petricola the olivine dissolution rates were about seven fold higher 1 04 10 15 mol cm 2 s 1 than those in the abiotic experiments 1 43 10 16 mol cm 2 s 1 conducted under the same experimental condition ph 6 25 c 94 days measured element concentrations and mg isotope ratios in the supernatant solutions in both the biotic and the abiotic experiment followed a dissolution trend in the initial phase of the experiment characterized by non stoichiometric release of mg and si and preferential release of 24mg over 26mg in a later phase the data indicates stoichiometric release of mg and si as well as isotopically congruent mg release we attribute the initial non stoichiometric phase to the rapid replacement of mg2 in the olivine with h along with simultaneous polymerization of si tetrahedra resulting in high dissolution rates and the stoichiometric phase to be influenced by the accumulation of a si rich amorphous layer that slowed olivine dissolution we attribute the accelerated dissolution of olivine during the biotic experiment to physical attachment of k petricola to the si rich amorphous layer of olivine which potentially results in its direct exposure to protons released by the fungal cells these additional protons can diffuse through the si rich amorphous layer into the crystalline olivine our results also indicate the ability of k petricola to dissolve fe precipitates in the si rich amorphous layer either by protonation or by fe iii chelation with siderophores such dissolution of fe precipitates increases the porosity of the si rich amorphous layer and hence enhances olivine dissolution the acceleration of mineral dissolution in the presence of a rock dissolving fungus further suggests that its presence in surficial co2 sequestration plants may aid to accelerate co2 binding

late eburnean granitization and tectonics along the western and northwestern margin of the archean kenema man domain guinea west african craton abstract recent mapping of the eastern part of guinea has revealed a vast plutonic belt that rims the margin of the archean kenema man craton and that is made up of a variety of granitic rocks granodiorite biotite granite monzogranite two mica granite granodiorite with common clinopyroxene being the most abundant single zircon dating by stepwise pb evaporation carried out on characteristic rock types granodiorite monzogranite two mica granite reveals their age to be late eburnean c 2090 2070 ma and indicates that their emplacement occurred within a relatively short time interval 20 ma the granodiorite biotite granite and monzogranite of the plutonic belt all present a calc alkaline and highly potassic nature and show numerous analogies with the neogene calc alkaline rocks of the central andes there is evidence to suggest strong contamination from the archean crust these analogies make it possible to envisage the emplacement of the plutonic belt directly above a subduction zone the occurrence of peraluminous two mica granite at the northern extension of the plutonic belt in the siguiri basin may suggest that the convergence here was associated with the local melting of metasedimentary rocks in the deep part of the basin contemporaneously with its emplacement the late eburnean plutonic belt accommodated regional convergence by major wnw ese sinistral movements along the northwestern margin of the archean block horizontal shortening towards the wsw is envisaged at the scale of eastern guinea in relation with these kinematics the late eburnean plutonic belt of eastern guinea is a key element of the archean proterozoic transition zone in the southern part of west africa evidence along the western and northwestern border of the archean kenema man domain leads us to interpret this transition zone as an active margin along which strike slip tectonism accommodated regional convergence towards the end of the eburnean cycle

stromatolites the challenge of a term in space and time abstract several definitions of stromatolites are discussed and kalkowsky s most important statements about stromatolites are translated from german to english a new definition for stromatolites is proposed namely stromatolites are laminated rocks the origin of which can clearly be related to the activity of microbial communities which by their morphology physiology and arrangement in space and time interact with the physical and chemical environment to produce a laminated pattern which is retained in the final rock structure unconsolidated laminated systems clearly related to the activity of microbial communities and often called recent stromatolites or living stromatolites are defined as potential stromatolites the main microbial activities which are important in the formation of potential stromatolites and stromatolites are described and examples of stromatolites of various types including iron stromatolites are also described

structural and gold mineralizing evolution of the world class orogenic mana district burkina faso multiple mineralizing events over 150 million years abstract the mana district located in the northern part of the birimian hounde greenstone belt in western burkina faso is a world class paleoproterozoic orogenic gold district 8 moz including five gold deposits fofina nyafe siou wona kona and yaho these deposits are located in specific lithostratigraphic domains and gold is controlled by various structural features deposit and regional scale mapping intrusion age and geochemistry as well as airborne aeromagnetic and electrical resistivity geophysical data were used to decipher the tectonic evolution of each gold deposit and the district five deformational and four gold mineralizing events were recognized the first deformation event d1md e w oriented shortening affected the metamorphosed volcanic and sedimentary rocks of the lower birimian group this early deformation episode was correlated with the formation of gently n plunging folds f1md and n s striking thrusts faults coeval with emplacement of the pre to synkinematic wona kona and siou plutons dated at 2172 ma under greenschist facies metamorphism the quartz carbonate veins v1md at fofina and siou formed during d1md at eoeburnean time manifesting the first gold event at approximately 2172 ma the following deformation event d2md e w oriented extension is associated with the deposition of the upper birimian group mana basin overlying the lower birimian group the geometry of the mana basin is controlled by the mana and maoula shear zones the tarkwaian type rock formation overlying the upper birimian group controlled by the wona kona and siou shear zones is constrained at the end of d2md or at the beginning of the d3md event with a maximum deposition age at 2113 ma the third deformation event d3md e w to wnw ese transpression affected the entire supracrustal rock such event is correlated with the formation of map scale f3md folds and dextral shear zones during the eburnean orogeny 2113 2090 ma a second gold mineralizing event occurred during d3md and is manifested by quartz carbonate veins v3md and disseminated sulfides at the yaho fofina and nyafe and possibly wona kona deposits the fourth deformation event d4md nnw sse transpression is correlated with sinistral shearing along the major transcurrent faults and the development of asymmetric nne striking folds f4md associated with vertical fold axes syn d4md mineralization is characterized mainly by a strong silicification si4md with disseminated pyrite and arsenopyrite along the wona kona shear zone and by tiny quartz carbonate veinlets v4md this event is considered the main gold bearing event in the western margin of the mana district the fifth and last deformation event d5md is brittle in character and was responsible for the formation of e w subvertical crenulation cleavages and reverse faults under overall n s shortening this late deformation event is tentatively associated with a last gold event recorded as free gold associated with muscovite in brittle fractures developed in competent orebodies at the wona kona and siou deposits this event could be as young as 2022 ma the age obtained from ar ar datation of muscovite schists at the wona kona deposit our main contribution is that we decipher multiple gold mineralizing events at the district scale based on deposit and regional scale mapping it is interpreted that gold was introduced as early as 2172 ma and possibly as late as 2022 ma during at least 3 or even 4 shortening tectonic events in a timeframe not yet recognized at the district scale for all the birimian belts

microbial processes in the negev phosphorites southern israel microscopic and sem observations indicate that cyanobacterial activity was important during the formation of the campanian negev phosphorites one aspect of such activity is the development of phosphorites through the binding of phosphate particles these sediments often demonstrate a conspicuous smooth cryptalgal lamination the algal binder was fossilized as intertwined phosphatic sheaths that coat and seal off the phosphate grains different types of sheaths representing the remnants of an algal community were found thick apatite overgrowth commonly deforms the microbial fossils converting them into barely recognizable forms fabrication of phosphate particles is another aspect of the microbial activity phosphate algal coated grains are a common constituent of the negev phosphorites the algal coating consists of stacked phosphatic tubes and colonies of coccoid unicells with an apatite infilling fragmented phosphatic sheath bundles form another type of phosphate particle produced by algae obliteration of algal structures is common resulting in undifferentiated groundmasses of cryptocrystalline apatite the extremely close connection between microbial structures and phosphatization processes suggests cyanobacterial participation in phosphorus enrichment

fault valve behaviour in optimally oriented shear zones an example at the revenge gold mine kambalda western australia abstract quartz vein systems developed in and adjacent to shear zones host major gold deposits in the kambalda region of the norseman wiluna greenstone belt at the revenge mine two groups of mineralised reverse shear zones formed as conjugate near optimally oriented sets during ese subhorizontal shortening adjacent to a major transpressional shear system the shear zones developed at temperatures of about 400 c in a transitional brittle ductile regime deformation was associated with high fluid fluxes and involved fault valve behaviour at transiently near lithostatic fluid pressures during progressive evolution of the shear system early brittle and ductile deformation was overprinted by predominantly brittle deformation brittle shear failure was associated with fault dilation and the formation of fault fill veins particularly at fault bends and jogs a transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses the formation of subhorizontal stylolites locally subvertical extension veins and minor normal faults in association with thrust faulting indicates episodic or transient reorientation of the near field maximum principal stress from a subhorizontal to a near vertical attitude during some fault valve cycles local stress re orientation is interpreted as resulting from near total shear stress release and overshoot during some rupture events previously described fault valve systems have formed predominantly in severely misoriented faults the shear systems at revenge mine indicate that fault valve action and associated fluctuations in shear stress and fluid pressure can influence the mechanical behaviour of optimally oriented faults

mechanisms controlling the silicon isotopic compositions of river waters it has been proposed that silicon si isotopes are fractionated during weathering and biological activity leading to heavy dissolved riverine compositions in this study the first seasonal variations of stable isotope compositions of dissolved riverine si are reported and compared with concomitant changes in water chemistry four different rivers in switzerland were sampled between march 2004 and july 2005 the unique high resolution multi collector icp ms nu1700 has been used to provide simultaneous interference free measurements of 28si 29si and 30si abundances with an average limiting precision of 0 04 on 30si this precision facilitates the clarification of small temporal variations in isotope composition the average of all the data for the 40 samples is 30si 0 84 0 19 1 sd despite significant differences in catchment lithologies biomass climate total dissolved solids and weathering fluxes the averaged isotopic composition of dissolved si in each river is strikingly similar with means of 0 70 0 12 for the birs 0 95 0 22 for the saane 0 93 0 12 for the ticino and 0 79 0 19 for the verzasca however the 30si undergoes seasonal variations of up to 0 6 comparisons between 30si and physico chemical parameters such as the concentration of dissolved si and other cations the discharge of the rivers and the resulting weathering fluxes permits an understanding of the processes that control the si budget and the fate of dissolved si within these rivers the main mechanism controlling the si isotope composition of the mountainous verzasca river appears to be a two component mixing between the seepage of soil ground waters with heavier si produced by clay formation and superficial runoff associated with lighter si during high discharge events a biologically mediated fractionation can be excluded in this particular river system the other rivers display increasing complexity with increases in the proportion of forested and cultivated landscapes as well as carbonate rocks in the catchment in these instances it is impossible to resolve the extent of the isotopic fractionation and contributed flux of si contributed by biological processes as opposed to abiotic weathering the presence of seasonal variations in si isotope composition in mountainous rivers provides evidence that extreme changes in climate affect the overall composition of dissolved si delivered to the oceans the oceanic si isotope composition is very sensitive to even small changes in the riverine si isotope composition and this parameter appears to be more critical than plausible changes in the si flux therefore concurrent changes in weathering style may need to be considered when using the si isotopic compositions of diatoms sponges and radiolaria as paleoproductivity proxies

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

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

biogenic structures and micrite in stalactites from grand cayman island british west indies stalactites from modern and old caves developed in the bluff formation of grand cayman island contain laminae and bulbous masses of micrite intercalated with sparry calcite the micrite as well as some of the sparry calcite around it contains small up to 25 m long ovate to spherical bodies that have a high concentration of either manganese or iron such bodies may be of bacterial origin the micrite contains numerous calcified filaments that are probably of algal origin calcification of the filaments occurred either during life or shortly after death of the algae the algae played an important role in trapping and binding the micrite furthermore the algae may be directly or indirectly responsible for the formation of much of the micrite

west africa gold preface west africa with an approximate 10 000 metric ton t gold endowment is one of the world s great gold provinces and the largest paleoproterozoic gold producing region the gold resources are concentrated within the 2250 to 2000 ma greenstone belts of the man leo shield forming the southern part of the west african craton the gold endowment of the eburnean gold deposits mainly orogenic type equals that of the world s other great precambrian lode gold provinces however the exploration and mining is much less mature in west africa than it is in other great precambrian gold provinces with much of the development throughout the man leo shield taking place during just the past 25 years the contributions included in this west africa gold special issue of economic geology collectively document the paleoproterozoic gold endowment observed in the southern part of the west african craton from west mali guinea to east burkina faso ghana from large to deposit scales and throughout the eburnean orogeny goldfarb et al 2017 present a broad scale overview of the paleoproterozoic gold endowment of man leo shield southern part of the west african craton the eburnean orogeny generated the world s most significant paleoproterozoic gold accumulation in the greenstone belts of ghana mali burkina faso and adjacent countries this paper documents successively the geology of the region gold deposit distribution in the man leo shield the

three dimensional modelling of alteration zones based on geochemical exploration data an interpretable machine learning approach via generalized additive models abstract the inference of deep alterations plays an important role in the analysis of deep geological processes and the targeting of buried mineral resources however the construction of three dimensional 3d models of alteration zones based on geochemical data is challenging due to the diverse geological processes and scarcity of information related to deep alterations in this paper we propose an interpretable machine learning method to infer 3d geological models of alteration zones based on geochemical data from the dayingezhuang area china to expose the information hidden in geochemical data that is indirectly and weakly related to alteration zones a generalized additive model gam is used it learns a nonlinear intelligible model associating geochemical anomalies extracted by pca with alteration features the model learned via gam and is then combined with a bayesian framework to infer the thickness of alterations in a no drilling area thus enabling 3d modelling of deep alteration zones with such an intelligible model geologists are capable of inspecting the reliability of the model and better understanding the association between the distribution of geochemical elements and deep alterations the inference results show that the relationship between geochemical anomalies and alteration depth is significant while the relationship with alteration thickness is complex the results of this study provide the underlying insights needed to guide the analysis of deep geological mineralization processes

freshwater oncolites created by industrial pollution onondaga lake new york abstract onondaga lake is a moderately saline eutrophic lake characterized by waters rich in calcium sodium chloride and bicarbonate large quantities of caco3 that are precipitated in the lake result from excess calcium supplied as calcium chloride wastes produced by soda ash manufacturing to lake waters that are at or near saturation with respect to caco3 from solution of carbonate rocks in the drainage basin beaches along the leeward northeastern shore of the lake are composed almost entirely of oncolites ranging from a few millimeters to several centimeters in maximum dimension offshore in 1 2 m of water the oncolites are biscuit shaped concretions as much as 15 cm in diameter the oncolites consist mainly of low magnesium calcite but dissolution of the carbonate with dilute acid results in a mass of blue green algal filaments of the same approximate size and shape as the original oncolite most oncolites have an obvious nucleus the most common nucleus is the hollow stem and cortication tubules of charophytes charophytes do not occur in onondaga lake today although they are common in other limestone bedrock lakes in central new york state charophytes probably were eliminated by the marked increases in salinity of the lake that resulted from the introduction of soda ash manufacturing on the lake shores around 1880 which means that growth of the oncolites began at least 100 years ago

a first look at the distribution of the stable isotopes of silicon in natural waters the first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported both marine and riverine 30si are more positive than 30si of igneous rocks suggesting isotopic fractionation during weathering and clay formation and or biomineralization the 30si value for dissolved silicic acid from several ocean basins is 1 1 0 3 n 69 more positive values are found in surface waters due to discrimination against 30si during biogenic opal formation by diatoms the deep pacific ocean is depleted in 30si relative to the deep atlantic by roughly 0 4 30si of riverine dissolved silicon averages 0 8 0 3 n 8 the biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea the average 30si value of inputs of silicon to the ocean most likely falls between 0 3 and 0 9 and that of the outputs falls between 0 9 to 1 9 probably in the neighborhood of 1 0

short lived eburnian orogeny in southern mali geology tectonics u pb and rb sr geochronology abstract this work is based on a comprehensive field survey including mapping of the massigui degree sheet in southern mali the investigated area belongs to the northern part of the man shield which is mainly composed of lower proterozoic volcano sedimentary sequences birimian and of large areas of granitoids the whole being affected by the eburnian orogeny structural and geochronological u pb and rb sr results from this study point to a short period of time 30 ma for the eburnian orogenesis between 2100 and 2070 ma the events recorded later 1984 30 ma being due to tectonic reactivations on the other hand no trace of events older than 2100 ma have been found coupled with geochemical and rb sr isotopic systematics this leads to invoke a fundamental link between birimian volcanics and eburnian granitoids both being high k calc alkaline in composition and coming from a single evolving mantle source in a palaeo environment which has some of the characteristics of modern subduction including back arc or docking zones the main regional structure of the area strikes roughly nne ssw with an important strike slip component escorted by a greenschist metamorphism the deformation is the most intense in the n40 to n10 5 km large banifin shear zone where metamorphism reached the lower amphibolite facies here one observes subvertical strong foliation non cylindrical folding locally sheath folds and subhorizontal pencil lineations the deformation being regionally recorded by light subisoclinal folds with subvertical axes compatible with a sinistral movement this regional shear tectonics d2 2074 9 8 ma is preceded by an isoclinal deformation d1 bracketed between 2098 5 ma and d2 only known as relics from which it likely derived a later phase d3 occurred along the banifin shear belt at a more superficial level even if still ductile generated by a dextral movement and having induced retrograde metamorphism this last tectonic phase which is not accompanied by plutonism nor volcanism has been dated at 1982 30 ma being responsible for the complete resetting of the rb sr isotopic system in the plutons present in the banifin corridor

self sedimented diatom mats as agents of exceptional fossil preservation in the oligocene florissant lake beds colorado united states microbial mats play a major role in the formation of exceptionally preserved fossil deposits by overgrowing and binding organic remains and sedimentary particles this minimizes hydrodynamic and biological disruption of dead organisms and sedimentary laminae but published works all implicate prokaryotic cyanobacteria as the microbial agent however exceptionally well preserved macrofossils of the oligocene florissant lake beds colorado united states are enveloped in matted aggregations of mucous secreting pennate diatom frustules it is suggested that the macrobiota became entrapped in mucous secreting mats of surface water blooms of planktonic diatoms in lake florissant as the mats and the incorporated macrobiota were sedimented out of the water column the mucosic mats and their associated bacterial communities arrested decay and promoted preservation of refractory tissues thus by a completely different mechanism the diatom mats fulfilled the same preservational role previously suggested for cyanobacterial mats this hitherto unrecognized mode of preservation may be an important causative factor in the formation of exceptionally preserved lacustrine fossil biotas

natural variations of δ30si ratios during progressive basalt weathering hawaiian islands abstract silicon stable isotopes can be used to trace the biogeochemical pathways of si as it moves from its continental sources to its sink in ocean sediments along the way si is incorporated into clay minerals taken up by plants where it forms plant opal and leached into rivers the major land to ocean conduit compared to igneous rocks the waters that drain continents are enriched in heavy si isotopes but the mechanisms that control fractionation have not been elucidated we studied si isotope fractionation along a 4 million yr basaltic soil chronosequence on the hawaiian islands using the natural context of these samples in combination with laboratory experiments we demonstrate that the isotopic composition of dissolved si in weathering systems is determined by the combined effects of rock disintegration clay mineral neosynthesis and si biocycling weathering preferentially releases 28si into solution whereas secondary mineral formation preferentially removes 28si from solution in humid environments leached soils have lost large amounts of this soluble si thus creating a net loss of 30si from the entire soil system as soils develop and greater fractions of si reside in neoformed clay minerals 30sibulk soil values change progressively toward more negative values basalt 30si values are about 0 5 but older soils have 30si values up to 2 5 the difference between the solid and solution 30si values remains more or less constant with progressive weathering and therefore soil water from older soils has a more negative 30si composition in the upper horizons of the hawaiian soils this weathering driven 30si shift is modified by the addition of unweathered primary minerals via dust carrying 30si values of about 0 5 and by biocycling of si via plants producing negative 30si values in phytoliths and positive 30si values in soil solutions derived from upper horizons due to the high concentrations of dissolved si in these near surface layers rivers have more positive 30si values than predicted based on the weathering status of the lower horizons when combined with published 30si values from large rivers worldwide we find that the results from hawaii point to weathering control of si isotopes delivered to the oceans and thus to an important continent ocean linkage that warrants further investigation

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

giant subtidal stromatolites forming in normal salinity waters we report here the discovery of giant lithified subtidal columnar stromatolites 2 m high growing in 7 8 m of clear oceanic water in current swept channels between the exuma islands on the eastern bahama bank they grow by trapping ooid and pelletal carbonate sand and synsedimentary precipitation of carbonate cement within a field of giant megaripples the discovery is important to geologists and biologists because similar organo sedimentary structures built by a combination of cementation and the trapping of sediment by microbes were the dominant fossil types during the precambrian stromatolites are thought to have been responsible for the production of free oxygen and thus the evolution of animal life1 2 until the discovery of small lithified subtidal columnar stromatolites in the bahamas3 the only subtidal marine examples known to be living while undergoing lithification were in the hypersaline waters of hamelin pool at shark bay western australia4 7 shark bay stromatolites range from intertidal to the shallow subtidal with the larger columns reaching 1 m in height the shark bay stromatolites have strongly influenced geological interpretation by analogy many ancient stromatolites have been considered to have grown in intertidal and or hypersaline conditions8 although hypersalinity was not a necessity for growth during the precambrian because grazing metazoan life had not then evolved

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

alteration of eocene lamprophyres in the zhenyuan orogenic gold deposit yunnan province china composition and evolution of ore fluids abstract the zhenyuan orogenic gold deposit 100 t au located in the northern part of the ailaoshan shear zone assz in south china is characterized by extensive alteration and mineralization in a variety of host rocks in the deposit abundant eocene lamprophyres have been infiltrated by ore fluids to produce carbonate veins with zones of disseminated fine grained gold bearing pyrite and arsenopyrite on both margins of the veins primary phlogopite k feldspar and magnetite have been altered to carbonate sericite and fuchsite carbonate minerals and sericite in altered lamprophyre have higher mg contents than in other host rocks reflecting the primitive geochemistry of the lamprophyres since carbonatization is the primary alteration type in the lamprophyres whose main primary components are low ca phlogopite and k feldspar the cao content of the lamprophyres reflects their degree of alteration as alteration increases incompatible element ratios remain essentially constant whereas sr is enriched with ca in addition au with co ni as ag sb and bi are enriched reflecting the addition of these metal elements during alteration of the lamprophyres altered lamprophyres have slightly lower 143nd 144nd t 0 51240 0 51253 and substantially higher 87sr 86sr t 0 70716 0 71234 than unaltered or weakly altered lamprophyres indicative of crustal contamination the auriferous ore forming fluids are thus interpreted to have migrated upwards and interacted with the sedimentary host rocks before altering the lamprophyres lamprophyres commonly show a close spatial and commonly temporal association with orogenic gold deposits worldwide at zhenyuan the lamprophyres shared a common deep source to the auriferous ore fluids but did not themselves contribute to this fluid or its gold content

taphonomy of insects in carbonates and amber abstract the major taphonomic processes that control insect preservation in carbonate rocks limestones travertines and nodules are biological insect size and wingspan degree of decomposition presence of microbial mats predation and scavenging environmental water surface tension water temperature density and salinity current activity and diagenetic authigenic mineralisation flattening deformation carbonisation the major taphonomic processes that control the preservation of insects in fossil resins amber and copal are different but can be considered under the same headings biological presence of resin producers size and behaviour of insects environmental latitude climate seasonality resin viscosity effects of storms and fires soil composition and diagenetic resin composition insect dehydration pressure carbonisation thermal maturation reworking oxidation these taphonomic processes are geographically and temporally restricted and generate biases in the fossil record nevertheless where insects occur they may be abundant and very diverse taphonomic processes may impact on phylogenetic and palaeobiogeographic studies in determining the timing of the origin and extinction of insect groups and in identifying radiations and major extinctions taphonomic studies are an essential prerequisite to the reconstruction of fossil insect assemblages to interpreting the sedimentary and environmental conditions where insects lived and died and to the investigation of interactions between insects and other organisms

aragonite laminae in hot water travertine crusts rapolano terme italy small 5 30 m aggregates of aragonite needles occur in calcite crystal crusts of present day hot water slope travertines at rapolano terme in tuscany italy the aggregates are mainly concentrated in irregular wispy and dark laminae which cross cut calcite crystal feathers to create a pervasive millimetre scale banded appearance in the deposit they also occur less commonly scattered irregularly through the calcite layers the aragonite needle aggregates are in the form of crosses fascicles sheaf shaped bundles or dumbbell shaped rosettes and spherulites locally irregular masses of needles also occur the fascicles rosettes and spherulites have hollow centres which resemble microbial components fungal spores bacterial colonies and pollen suggesting that the aragonite crystals are biotically nucleated the lamination is interpreted to reflect diurnal control stimulation of microbial activity during daylight concentrates cells in laminae and promotes aragonite calcification calcite feather crystals although traversed by the aragonite aggregate laminae have a clear appearance under the light microscope they form more or less continuously through the diurnal cycle by abiotic precipitation the constant association of aragonite with organic nuclei irrespective of whether the latter are in laminae or scattered through the calcite layers supports a biotic control on aragonite formation lamination in pleistocene travertines is superficially similar to that in the present day deposits but is diagenetically altered in the pleistocene deposits the calcite feathers appear dark under the light microscope and the aragonite aggregates where they are not altered to dark calcite are dissolved together with parts of the adjacent spar calcite and therefore appear light coloured

combined oxygen and silicon isotope analysis of diatom silica from a deglacial subarctic pacific record we present an sif4 separation line coupled to a laser fluorination system which allows for an efficient combined silica d18o and d30si analysis 50 min per sample the required sample weight of 1 5 2 0mg allows for high resolution isotope studies on biogenic opal besides analytical tests the new instrumentation set up was used to analyse two marine diatom fractions 63 mm 10 20mm with different diatom species compositions extracted from a bolling allerod holocene core section md01 2416 north west nw pacific to evaluate the palaeoceanographic significance of the diatom isotopic signals and to address isotopic effects related to contamination and species related isotope effects vital and environmental effects while d30si offsets between the two fractions were not discernible supporting the absence of species related silicon isotope effects systematic offsets occur between the d18o records although small these offsets point to species related isotope effects as bias by contamination can be discarded the new records strengthen the palaeoceanographic history during the last deglaciation in the nw pacific characterized by a sequence of events with varying surface water structure and biological productivity with such palaeoceanographic evolution it becomes unlikely that the observed systematic d18o offsets signal seasonal temperature variability this calls for reconsideration of vital effects generally excluded to affect d18o measurements

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

halloysite as a kinetically controlled end product of arid zone basalt weathering abstract mineralogical and isotope results paired with field observations suggest that halloysite is the favored albeit metastable aluminosilicate end product of arid zone basalt weathering on kohala hawai i and that the formation of smectite has been inhibited by kinetic factors soils sampled on constructional lava flows having ages ranging from 4 to 350 ka provide a chronosequence that has had minimal physical disturbance thus allowing us to interpret chemical and mineralogical changes as the result of a time dependent process the halloysite content of the soil increases with increasing soil age its growth is at the expense of allophane which in turn forms at the expense of primary minerals these mineral relationships suggest that halloysite has been forming continuously throughout the lifetime of the soil smectite the thermodynamically stable phyllosilicate phase predicted by soil solution composition is not found in these arid soils we determined that our soil system is controlled by kinetic factors and that therefore thermodynamic predictions do not reflect reality the main factors favoring kinetic control of halloysite formation are intense but short wet periods followed by prolonged extremely dry seasons and microenvironmental conditions leading to immediate uptake of released al by the halloysite precursor mineral allophane the 18o relationship between present day soil water and halloysite formed over the last 170 ka was used as a tracer of past soil conditions results from a reconstruction of paleo climate and soil conditions combined with 18o data observed mineral relationships along the 350 ka chronosequence and field evidence for long term aridity rule out the possibility that the arid side of kohala was affected by prolonged periods of higher rainfall that could have produced more dilute soil waters and therefore altered mineralogical stabilities we conclude that pedogenic halloysite has been continually forming from the early stage of soil formation and that it has consistently formed under isotopic equilibrium conditions indicating that halloysite 18o compositions imply paleoclimatic conditions over the time of its formation that are similar to today s

micro scale tracing of fe and si isotope signatures in banded iron formation using femtosecond laser ablation we have detected micrometre scale differences in fe and si stable isotope ratios between coexisting minerals and between layers of banded iron formation bif using an uv femtosecond laser ablation system connected to a mc icp ms in the magnetite carbonate chert bif from the archean old wanderer formation in the shurugwi greenstone belt zimbabwe magnetite shows neither intra nor inter layer trends giving overall uniform d 56 fe values of 0 9 but exhibits intra crystal zonation bulk iron carbonates are also relatively uniform at near zero values however their individual d 56 fe value is highly composition dependent both siderite and ankerite and mixtures between both are present and d 56 fe end member values are 0 4 for siderite and 0 7 for ankerite the data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by fe iii reducing bacteria or more likely an abiotic reaction of organic carbon and fe iii during low grade metamorphism si isotope composition of the old wanderer bif also shows significant variations with d 30 si values that range between 1 0 and 2 6 for bulk layers these isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal rich waters interestingly fe and si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures moreover the changes of fe and si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal rich water which govern the rates of fe and si precipitation and therefore also the development of layering during periods of low hydrothermal activity precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate chert layers during periods of intensive hydrothermal activity precipitation rates of ferric oxyhydroxide were high and subsequent diagenesis triggered only partial reduction forming magnetite carbonate chert layers we are confident that our micro analytical technique is able to detect both the solute flux history into the sedimentary bif precursor and the bif s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors

classification and environmental models of cool freshwater tufas abstract tufa deposits are common in the quaternary record facies developments are dependent upon localised carbonate precipitation generally these take the form of phytoherm build ups together with associated detrital deposits derived from disagragation of the former phytoherm cements differ from speleothems and floe calcite accumulations in being biologically mediated or modified this paper recognises the following five types perched spring line cascade fluviatile paludal and lacustrine facies each type is composed of distinctive constructional and detrital sediment associations these unique sediment combinations are the product of an interplay between angle of slope water discharge rate standing water bodies phytal and microbial colonization and ambient temperature the proposed models will be of value in the interpretation of ancient and partially exposed tufas

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

lower permian oncolites from south china implications for equatorial sea level responses to late palaeozoic gondwanan glaciation the oncoid bearing chuanshan formation is a regionally extensive carbonate deposit of predominantly asselian to early sakmarian early permian age in south china occupying an area of some 500 000 km2 throughout south china the oncoid bearing horizons are generally stable and broadly comparable in lithology fossil content and the morphology of the oncoid grains four types of microfacies are recognized from the oncolite succession and overall they suggest a moderate to high energy wave agitated shallow marine carbonate platform environment an analysis of the stratigraphic distribution of oncoid grain size density thickness and the bedding structures of the oncolite beds and the number of coating laminae indicate the presence of metre scale cyclothems suggestive of possible high frequency cycles of sea level fluctuation compared to carbonate successions above and below that lack oncolites and in conjunction with evidence from sequence stratigraphic and isotopic geochemical analyses of coeval carbonate deposits in south china and elsewhere the origin of the chuanshan oncolites is linked to a drastic drop in global sea level at the pennsylvanian permian boundary that can be correlated closely in timing with the zenith of the late palaeozoic gondwanan glaciation it is further suggested that the eustatic changes apparent from the deposition of the chuanshan oncolites and similar coeval deposits in lower palaeolatitudes were coupled with and influenced by the contemporaneous high latitude gondwanan glaciation the largest and longest known such event in phanerozoic earth history

silicon isotope compositions of dissolved silicon and suspended matter in the yangtze river china abstract silicon isotope compositions of main channel samples of the yangtze river were systematically investigated along with their chemical compositions the concentration of suspended matter in the yangtze river tends to decrease from the upper reaches to the lower reaches corresponding to settling of the sediments in the lakes and reservoirs due to reduction of the velocity of water flow the silica contents of suspended matter vary from 52 1 to 56 9 and their 30si values vary from 0 to 0 7 both similar to those of shales from the upper to lower reaches the silica contents of suspended matter tend to increase whilst their 30si values tend to decrease both trends reflect the increase of clay minerals and decrease of carbonates in suspended matter the concentrations of dissolved silicon vary from 97 to 121 mol l and their 30si values vary over a wide range from 0 7 to 3 4 from the upper to lower reaches dissolved silica concentrations tend to decrease and their 30si values tend to increase these trends mainly reflect the change of chemical and isotopic characteristics of the tributaries from the upper to lower reaches the major factors responsible for these changes may be the high meteoric precipitation and significant silicon absorption by grass in wetlands and rice in paddy fields in drainage areas of the middle and lower reaches there is no correlation between 30si of dissolved silicon and that of suspended matter the 30sidiss spm values vary over a wide range of 1 0 3 7 indicating that 1 they are out of isotopic equilibrium 2 dissolved silicon and the associated suspended matter do not belong to one physico chemical system and 3 isotopic exchange rate between them is very slow the 30si value of dissolved silicon output from the yangtze river to the east sea is estimated to be 3 0 much higher than the values reported for the amazon and congo rivers this increases the 30si range of dissolved silicon in the world s rivers from 0 4 1 2 to 0 4 3 4

morphogenetic impact of microbial mats on surface structures of kimmeridgian micritic limestones cerin france for several years palaeoecological research has been conducted on micritic limestones of late kimmeridgian age in the southern jura mountains the sedimentary environment is that of a lagoon with an irregular bottom which was repeatedly exposed between two stages of lime deposition a microbial mat grew over the muddy surface giving cohesion to the sediment restraining erosion and preserving fossil remains and reptile tracks various structures at the microbial mat surface can be observed crescentic wavy radial wavy torn petee and mixed structures they imply the presence of desiccation periods and a slight bottom slope leading to a downward sliding of the microbial mat such features may also be generalised to explain superficial microbial structures of other shallow carbonate mud environnements

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

ultrastructure of a microbial mat generated phosphorite optical and sem observations of phosphorites reveal that mineralization is concentrated on ooids and micro oncoids the coated grains occur within microbial mats microbial mats represent the formational environment of the ooids and oncoids both coated grains and mats exhibit similar filamentous micro organisms the mat filaments show no fixed orientation and they merge with the concentrically oriented filaments of the coated grains the branching nature and chlamydospore like structures of filaments suggest that both mat and coated grains have been formed by fungi some coated grains appear to have been slightly disturbed and sometimes mobilized from their sites of formation due to separation from the parent mat resulting perhaps from contraction fragmentation the voids so created within the mat had been later filled with either micrite or sparite

recent freshwater ooids and oncoids from western lake geneva switzerland indications of a common organically mediated origin abstract the shallow water sediments of western lake geneva are composed mainly of sand size coated grains ranging from oncoids to ooids in surficial sediments the coatings consist of anhedral microcrystals of low mg calcite and often show a well developed intercrystalline porous network containing bacteria and residues of degraded mucus nucleation occurs in close association with organic films formed by mucus produced by filamentous cyanobacteria and diatoms similarities between oncoid and ooid cortex crystal size shape and arrangement indicate that the processes controlling the building of these types of particle are identical in subsurface sediments the microstructure of the coatings changes considerably the intercrystalline porosity is reduced by aggrading neomorphism and the micritic porous and organic rich coatings are transformed into a dense xenotopic mosaic with scattered molds of filamentous cyanobacteria this occurrence of ooidal oncoidal sands in lake geneva may help in interpreting ancient lacustrine formations and brings additional information on the processes of genesis and diagenesis of coated grains

decoupled sources of the 2 3 2 2ga yuanjiacun banded iron formation implications for the nd cycle in earth s early oceans abstract the recognized worldwide gap in bif deposition between 2 4 and 2 0 billion years ago has long been considered as an obstacle to fully determining the geochemical composition of seawater at that time however the recently dated 2 3 2 2 ga yuanjiacun banded iron formation bif in the north china craton ncc offers a possibility to redress these uncertainties shale normalized rare earth element yttrium ree y patterns of the bif and interlayered meta chert samples show features characteristic of other archean and paleoproterozoic bifs with hree enrichment relative to lree positive la and eu anomalies and superchondritic y ho ratios comparable to modern seawater very low al 2 o 3 e nd t 3 5 derived from interaction with a depleted mantle source and associated with high fe fluxes the second is ambient surface seawater e nd t 2 4 which obtained its signature through weathering of the nearby landmasses and associated with high si fluxes our findings suggest that the ree budget of the oceans prior to 2 3 ga was generally dominated by hydrothermal circulation of seawater through depleted mantle derived source rocks however where evolved local continental crust and or an enriched mantle source were present this positive mantle nd signal became discernible in the bifs by comparing the nd isotopic features of similar aged bifs and marine carbonates it is concluded that similar to modern oceans the early precambrian ocean was not well mixed with respect to its nd isotopic composition

refractory element mobility in volcanic soils refractory trace element concentrations in strongly weathered hawaiian soils ranging in age from 20 to 4100 ka are highly elevated over parent rock values due to extensive mass loss of more soluble major elements during pedogenesis nb and ta exhibit virtually no mobility soil nb ta ratios are within the range of fresh bedrock even when soil nb concentrations are residually enriched by a factor of 10 in contrast al zr and hf are depleted relative to nb in surface soil horizons but are enriched at depth clearly indicating mobility of these elements variations in th nb ratios in soil profiles indicate significant th mobility within the soil column however mass balance calculations require that accretion of th enriched asian dust has resulted in a net increase in th in some soils soils developed on a 150 ka rainfall gradient show that the mobility and loss of zr increase with mean annual precipitation

the effects of weathering variability and anthropogenic pressures upon silicon cycling in an intertropical watershed tana river kenya abstract we present the first study of the si isotopic composition of dry season river waters from the tana river kenya data encompasses the entire river basin with samples collected from headwaters to the estuary thereby capturing a salinity gradient in the headwaters the isotopic signature is affected by climate as a result of its control on soil drainage and weathering the 30 si signatures in the basin range from 0 69 up to 2 23 signatures are clearly affected by dams an increase in 30 si ratios of 0 54 and a decrease in the dissolved si dsi concentration by 41 were observed downstream of the masinga dam the largest of a succession of 5 hydroelectric dams this reduction in si load is most likely due to increased diatom productivity as the corresponding change in 30 si signature is consistent with the known fractionation by these organisms the 30 si composition of waters entering the estuary is ca 2 and dsi concentrations are 349 m in the estuary the dsi concentrations decrease linearly following the salinity while the 30 si ratio remains stable indicating the absence of processes affecting the dsi pool for the studied range of salinity

climatic and tectonic controls on chemical weathering in the new zealand southern alps abstract climatic and tectonic controls on the relative abundance of solutes in streams draining the new zealand southern alps were investigated by analyzing the elemental and sr isotope geochemistry of stream waters bedload sediment and hydrothermal calcite veins the average relative molar abundance of major cations and si in all stream waters follows the order ca2 50 si 22 na 17 mg2 6 k 5 for major anions the relative molar abundance is hco3 89 so42 7 cl 4 weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters but nearly all streams have a carbonate dominated ca2 and hco3 mass balance stream water ca sr and 87sr 86sr ratios vary from 0 173 to 0 439 mol nmol and from 0 7078 to 0 7114 respectively consistent with the ionic budget these ratios lie solely within the range of values measured for bedload carbonate ca sr 0 178 to 0 886 mol nmol 87sr 86sr 0 7081 to 0 7118 and hydrothermal calcite veins ca sr 0 491 to 3 33 mol nmol 87sr 86sr 0 7076 to 0 7097 streams draining regions in the southern alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain 10 more ca2 and 30 more sr2 from carbonate weathering compared to streams draining regions in drier more stable landscapes similarly streams draining glaciated watersheds contain 25 more sr2 from carbonate weathering compared to streams draining non glaciated watersheds the highest abundance of carbonate derived solutes in the most physically active regions of the southern alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock which contains trace amounts of calcite estimated to weather 350 times faster than plagioclase in this environment in contrast regions in the southern alps experiencing lower rates of uplift and erosion have a greater abundance of silicate versus carbonate derived cations these findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters using the southern alps as a model for assessing the role of active tectonics in geochemical cycles this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater

lithospheric scale gravitational flow the impact of body forces on orogenic processes from archaean to phanerozoic in the archaean the combination of warmer continental geotherm with a lighter sub continental lithospheric mantle suggests that gravitational forces played a more signifi cant role in continental lithospheric deformation to test this hypothesis we compare the evolution of the deformation and the regional state of stress in archaean like and phanerozoic like lithospheres submitted to the same boundary conditions in a triaxial stress field with imposed convergence in one direction for plausible physical parameters thickening of normal to cold phanerozoic lithospheres produces relatively weak buoyancy forces either extensional or compressional in contrast for archaean continental litho spheres or for anomalously warm phanerozoic lithospheres lateral gravitationally driven flow prevents significant thickening this conclusion is broadly consistent with 1 the rela tive homogeneity of the erosional level now exposed at the surface of archaean cratons 2 the sub aerial conditions that prevailed during the emplacement of up to 20 km of green stone cover 3 the relatively rare occurrence in the archaean record of voluminous detrital sediments 4 the near absence of significant tectonic metamorphic and magmatic age gra dients across archaean cratons 5 the relative homogeneity of strain across large areas and 6 the ubiquitous presence of crustal scale strike slip faults in many late archaean cratons

accretion of asian dust to hawaiian soils isotopic elemental and mineral mass balances hawaiian soils contain a mixture of material derived from in situ weathering of parent material plus atmospheric inputs including sea salt aerosols and asian dust we use soil mineralogy and radiogenic isotope geochemistry sr and nd to evaluate the impact of asian dust on a chronosequence of hawaiian soils dust becomes an important constituent of soils 20 ky and older near surface 50 cm depth horizons contain as much as 30 quartz a mineral absent from local parent material basaltic sr and nd isotope signatures in these horizons have been completely overprinted by asian dust signatures with 87sr 86sr ratios as high as 0 723 and end values as low as 7 ree patterns in these soils are indistinguishable from that of average upper continental crust quartz abundance and nd isotopes provide two independent tracers of long term dust additions to the chronosequence soils the two tracers indicate a minimum long term average dust accretion rate of 125 mg cm 2 ky 1 at the 150 ka chronosequence site roughly a factor of three higher than holocene dust accumulation rates estimated from north pacific sediment cores we find that the mass of dust preserved in these soil profiles does not increase with age in soils 150 ka and older requiring a loss mechanism for accreted dust on the basis of the geomorphic stability of these sites observed preferential loss of dust derived mica relative to quartz and estimates of soil si leaching rates we argue that chemical weathering is the dominant loss mechanism for dust from these soils dust has a profound effect on the budgets of elements that are susceptible to leaching losses dust becomes the dominant source of soil nutrients si and p in the oldest most intensely weathered soils we calculate a dust derived p input flux of 0 8 mg cm 2 ky 1 and a dust derived si input flux of 35 mg cm 2 ky 1 si leaching fluxes are high 1400 mg cm 2 ky 1 in the youngest 2 ka soils and drop systematically with soil age to a value that closely balances the dust derived si input flux by 4100 ka extremely refractory elements such as nb which are concentrated in soils by residual enrichment processes are much less readily impacted by dust addition although dust has had a pronounced impact on sr and nd isotopic budgets and on soil mineral composition at the 150 ka site dust cannot have contributed 4 of the total nb contained in this soil profile

structural event framework for the eastern yilgarn craton western australia and its implications for orogenic gold abstract the nnw trending tectonic grain of the eastern yilgarn craton eyc was established as a result of predominantly ene wsw directed extension d1 and d3 and e ene w wsw d2 d4 to ne sw directed d5 contraction the result has been a succession of nnw striking temporally discrete fabric elements which can be difficult to interpret reliably at any single location despite this many past workers interpreted the nnw striking fabric as the result of only one regional contractional event and used it as a marker for correlating structural events across the region in order to unravel the complexity this paper presents a new sixfold d1 d6 deformation nomenclature based on 10 000 new mesoscale structural observations including their kinematic analysis and cross cutting relationships these mesoscale data were referenced with regional 3d map patterns stratigraphic magmatic metallogenic considerations and deep seismic reflection images this integrated geodynamic architectural approach is applicable to solving structural event histories in other polydeformed terrains gold mineralisation occurred during the first five events but was particularly concentrated from d3 onwards the d3 event marked the most profound change in the tectonic evolution of the eyc with changes in greenstones granites and tectonic mode lithospheric extension and core complexes with the first significant gold deposited within extensional shear zones that dissect the crust later contraction d4 was imposed at a high angle to the previously established anisotropic architecture the outcome was the creation of a new dynamic permeability framework which resulted in gold mineralisation during nnw striking sinistral strike slip faulting and associated thrusting a further stress switch d5 further modified the architecture resulting in n to nne striking dextral strike slip faulting and the final period of gold mineralisation before late stage extension d6

magnesium isotope systematics of the lithologically varied moselle river basin france abstract magnesium and strontium isotope signatures were determined during different seasons for the main rivers of the moselle basin northeastern france this small basin is remarkable for its well constrained and varied lithology on a small distance scale and this is reflected in river water sr isotope compositions upstream where the moselle river drains silicate rocks of the vosges mountains waters are characterized by relatively high 87sr 86sr ratios 0 7128 0 7174 in contrast downstream of the city of epinal where the moselle river flows through carbonates and evaporites of the lorraine plateau 87sr 86sr ratios are lower down to 0 70824 magnesium in river waters draining silicates is systematically depleted in heavy isotopes 26mg values range from 1 2 to 0 7 relative to the value presently estimated for the continental crust and a local diorite 0 5 in comparison 26mg values measured in soil samples are higher 0 0 this suggests that mg isotope fractionation occurs during mineral leaching and or formation of secondary clay minerals on the lorraine plateau tributaries draining marls carbonates and evaporites are characterized by low ca mg 1 5 3 2 and low ca sr 80 400 when compared to local carbonate rocks ca mg 29 59 ca sr 370 2200 similar to other rivers draining carbonates the most likely cause of the mg and sr excesses in these rivers is early thermodynamic saturation of groundwater with calcite relative to magnesite and strontianite as groundwater chemistry progressively evolves in the aquifer 26mg of the dissolved phases of tributaries draining mainly carbonates and evaporites are relatively low and constant throughout the year from 1 4 to 1 6 and from 1 2 to 1 4 respectively within the range defined for the underlying rocks downstream of epinal the compositions of the moselle river samples in a 26mg vs 87sr 86sr diagram can be explained by mixing curves between silicate carbonate and evaporite waters with a significant contribution from the vosgian silicate lithologies 70 temporal co variation between 26mg and 87sr 86sr for the moselle river throughout year is also observed and is consistent with a higher contribution from the vosges mountains in winter in terms of runoff and dissolved element flux overall this study shows that mg isotopes measured in waters rocks and soils coupled with other tracers such as sr isotopes could be used to better constrain riverine mg sources particularly if analytical uncertainties in mg isotope measurements can be improved in order to perform more precise quantifications

mechanism diversity of the loma prieta aftershocks and the mechanics of mainshock aftershock interaction the diverse aftershock sequence of the 1989 loma prieta earthquake is inconsistent with conventional models of mainshock aftershock interaction because the aftershocks do not accommodate mainshock induced stress changes instead the sense of slip of the aftershocks is consistent with failure in response to a nearly uniaxial stress field in which the maximum principal stress acts almost normal to the mainshock fault plane this orientation implies that i stress drop in the mainshock was nearly complete ii mainshock induced decreases of fault strength helped were important in controlling the occurrence of after shocks and iii mainshock rupture was limited to those sections of the fault with preexisting shear stress available to drive fault slip

genetic links among fluid cycling vein formation regional deformation and plutonism in the juneau gold belt southeastern alaska gold bearing quartz vein systems in the juneau gold belt formed within a 160 km long by 5 to 8 km wide zone along the western margin of the coast mountains alaska vein systems are spatially associated with shear zones adjacent to terrane bounding mid cretaceous thrust faults analysis of vein orientations and sense of shear data define a stress configuration with greatest and least principal axes oriented subhorizontally with northeast southwest trends and subverticaly respectively this local stress configuration is compatible with the far field plate configuration during eocene time isotopic ages of vein formation indicate that fluid cycling occurred between 56 5 and 52 8 ma and are consistent with a genetic link between veining and a change in plate motion in early eocene time veining was also synchronous with the latter stages of rapid exhumation and voluminous plutonism immediately inboard of the gold belt we propose a model in which interacting tectonic events facilitated fault valve action and vein development along now exhumed shear zones

impact des ecosystemes microbiens sur la sedimentation impact of microbial ecosystems on sedimentation abstract the microbial communities which often give rise to the fine lamination in sediments are very diverse both in species composition e g phototrophic and chemotrophic bacteria fungi algae and metabolism e g oxygenic and anoxygenic photosynthesis organic and inorganic chemosynthesis anaerobic and aerobic respiration organic and inorganic fermentation these microbial ecosystems have separated for more than 3 5 ga 3500 m y they intervene in the biostructuration of sediments by active mineral deposition particle trapping and sediment stabilisation they build bioconstructions stromatolites oncolites oolites and microbial bioherms further they are active in a number of biogeochemical biomineralisation processes including phosphatogenesis carbonate genesis and the formation of iron ore and sulfur deposits and petroleum and kerogen they are also active agents in the biocorrosion and bioerosion of sedimentary rocks under marine and terrestrial conditions since they have an enormous capacity for proliferation microbial laminated communities invade new habitats practically instantaneously where suitable energy sources and gradients for electron flow are available in addition such microbial systems are excellent and sensitive paleoecological indicators in order to study their impact on sedimentation and fossilisation a close collaboration between geologists and microbiologists is needed only in this way can the nature of fossil microbial ecosystems and the metabolisms that were at work be recognized and reconstructed

extension and gold mineralisation in the hanging walls of active convergent continental shear zones abstract orogenic gold bearing quartz veins form in mountain belts adjacent to convergent tectonic boundaries the vein systems hosted in extensional structures within compressively deformed rocks are a widespread feature of these orogens in many cases the extensional structures that host gold bearing veins have been superimposed on and locally controlled by compressional structures formed within the convergent orogen exploring these observations within the context of a three dimensional mechanical model allows prediction of mechanisms and locations of extensional zones within convergent orogens our models explore the effect of convergence angle and mid crustal strength on stress states and compare them to the southern alps and taiwan the dilatation zones coincide with the highest mountains in the hanging walls of major plate boundary faults and can extend as deep as the brittle ductile transition extensional deformation is favoured in the topographic divide region of oblique orogens with mid lower crustal rheology that promotes localisation rather than diffuse deformation in the near surface topography influences the stress state to a depth approximately equal to the topographic relief bringing the rock closer to failure and rotating 1 to near vertical the distribution of gold bearing extensional veins may indicate the general position of the topographic divide within exhumed ancient orogens

from a black to a gray box a mass balance interpretation of pedogenesis we utilize chemical elements as tracers in a mass balance analysis that provides functional relationships among soil chemical composition bulk density and volume change in relation to parent material these analytical functions are based on the principle of conservation of mass and include a term quantifying mass flux into out of the soil and between horizons we apply the technique to the oldest member of a chronosequence developed on marine terraces in northern california it is an alfisol that evolved from beach sand to its present state nearly one third of its weight is composed of secondary clay minerals in about 240 ky aside from large increases in organic carbon desilication is the dominant factor in soil evolution 29 50 83 g cm 2 si was leached from the beach sand during pedogenesis the rate of desilication is roughly 2 1 t km 2 yr 1 0 21 g cm 2 ky 1 an order of magnitude slower than that implied by the si denudation rate calculated for the mattole river watershed weathering of primary minerals and synthesis of secondary minerals is relatively well advanced suggesting that the rate of desilication may be declining the local beach is composed of quartz and sodic plagioclase with smaller amounts of chlorite mica and kaolinite the soil has substantially different mineralogy sand is dominated by quartz and clay is dominated by kaolinite halloysite chloritic intergrades and gibbsite bases are also leached though the total mass was much less 57 2 55 g cm 2 of na in the beach sand was lost as plagioclase weathered by focusing on elemental and mineralogical gains and losses we emphasize the essential connection between the pedologic environment and the external hydrochemical environment

palaeoseismic events recorded in archaean gold quartz vein networks val d or abitibi quebec canada archaean gold quartz vein deposits are commonly hosted in high angle reverse shear zones and are interpreted to have formed in a regime of horizontal compression and high fluid pressure environment this paper presents the results of a combined structural and fluid inclusion study on three gold quartz vein deposits of the val d or area abitibi quebec consisting of subhorizontal extensional veins and e w steeply dipping shear veins crack seal structures tourmaline fibres stretched quartz crystals and open space filling textures indicate that the subhorizontal veins formed by hydraulic fracturing under supralithostatic fluid pressure co2 rich and h2o nacl fluid inclusions interpreted as two coexisting immiscible fluids occur typically in microcracks of different orientations interpreted to have formed in the 1 2 plane horizontal co2 rich fluid inclusion planes are contemporaneous with the opening of these veins 3 vertical vertical h2o nacl fluid inclusion planes as well as some microstructures such as deformed minerals indicate that the same extensional veins have experienced episodic vertical shortening 3 horizontal alternating with the opening events deformation and slip opening also occurred in shear veins in which preferred orientation of fluid inclusion planes is not clear except that the h2o nacl fluid inclusion planes tend to be oriented at high angles to the slip direction the successive opening and collapse events in subhorizontal extensional veins are correlated with deformation and slip opening events in shear veins respectively and are attributed to cyclic fluid pressure fluctuations in the system they are thus consistent with the fault valve model sudden drop in fluid pressure from supralithostatic to lower values induces fluid unmixing and occurs immediately post failure following seismic rupturing along the shear zone sealing of the shear veins allows the fluid pressure to build up again and the subhorizontal extensional veins to form the intermittent change in orientation of 3 in horizontal extensional veins is ascribed to a total shear stress release along the neighbouring shear veins after rupture the complex internal structures of the gold quartz veins provide an excellent record of archaean seismic activity along high angle reverse faults

travertines depositional morphology and the bacterially constructed constituents abstract investigation of travertine accumulations throughout central italy and the west central u s has shown that the carbonate is precipitated in response to both inorganic and organic processes individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent all of the carbonate is low magnesian calcite water chemistry temperature and morphology of the accumulation greatly influence the constituents comprising these deposits harsh environmental situations favor inorganic deposits while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material morphological variations of travertine deposits recognized include 1 waterfall or cascade 2 lake fill 3 sloping mound fan or cone 4 terraced ound and 5 fissure ridge gross morphology internal stratification and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations exceeding 90 of the framework grains comprising some of the lake fill deposits bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents the bacteria are primarily rods generally 0 2 m in diameter and less than 1 0 m in length the rods readily decay resulting in calcite crystals loaded with micropores the basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 m in diameter enclosed in a single crystal of calcite aggregates of these crystals produce a variety of deposits including 1 crudely laminated carbonate mud 2 finely laminat d layers of mud 3 intraclasts 4 foam rock and 5 shrubs the shrubs are most striking commonly forming layers 1 3 cm thick but also producing bacterial pisoids at some locales bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit the shrub layers are the result of flourishing summer growth of bacteria and furthermore show remarkable daily laminae 0 1 0 5 mm thick the importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits

the impact of climate on the biogeochemical functioning of volcanic soils abstract rainfall and the amount of water available to leach ions from soil are among the most important features determining mineral weathering secondary mineral synthesis and soil chemical properties along an arid to humid climosequence on kohala mountain hawaii we sampled 16 soil profiles and found that weathering and soil properties change in a nonlinear fashion with increased rainfall the lavas are influenced by a strong rain shadow with mean annual precipitation map averaging 160 mm near the coast and rising to 3000 mm near the summit a temperature decline from 24 to 15 c with increasing elevation is matched by lower potential evapotranspiration et a water balance model monthly precipitation minus monthly et defines three broad climate zones along the sampling transect an arid zone with moisture deficit in every month an intermediate zone with moisture deficit during low rainfall summer months and moisture surplus during high rainfall winter months and a humid zone with moisture surplus during every month the annualized water balance can be ratioed with the integrated porosity of the top meter of soil to provide a leaching index the index reaches 1 total filling of the pore space on an annual basis at about 1400 mm map index values 1 imply intense leaching conditions because of pore water replacement in these 170 ka soils leaching losses of soluble base cations and si are nearly complete at index values 1 whereas only 60 of al has been lost at index values 1400 mm soil buffering capacity has been totally exhausted leading to low ph and low ecec the nonlinear decline in ecec is irreversible under natural conditions base cation depleted soils will remain so even if the climate shifts to drier conditions in contrast a climate shift to wetter conditions can drastically modify surface chemical properties existing in the drier soils as weathering depletes primary minerals elements are lost to leaching and surface chemistry is modified there is a time dimension implied in climate gradient studies soils forming in recently rejuvenated landscapes contain more primary minerals and should experience loss of buffering capacity at higher rainfall loss of buffering capacity means that biological acidity will move more deeply into the vadose zone or into the aquatic system the details of this transfer depend on present and past climate and the age and erosional stability of landscapes

geochemistry of the hawi lavas kohala volcano hawaii hawi lavas form the late stage alkalic cap on kohala volcano and range in composition from hawaiite to trachyte new detailed field mapping of kohala and reinterpretation of previously published age data suggest that there was no significant eruption hiatus between the hawi and underlying pololu shield lavas as was previously suggested mineral and whole rock chemical data are consistent with a crystal fractionation origin for the hawaiite to trachyte compositional variation observed within the hawi lavas plagioclase clinopyroxene ti magnetite olivine and apatite fractionation are needed to explain this variation the clinopyroxene fractionation may have occurred at moderate pressure because it is virtually absent in these lavas and is not a near liquidus phase at pressures of less than 8 kb plagioclase would be buoyant in the hawi hawaiite magmas so a mechanism like dynamic flow crystallization is needed for its fractionation and to account for the virtual absence of phenocrysts in the lavas hawi lavas are distinct in sr and nd isotopic ratios and or incompatible element ratios from the pololu lavas thus they were derived from compositionally distinct sources compared to other suites of hawaiian alkalic cap lavas hawi lavas have anomalously high concentrations of phosphorus and rare earth elements these differences could be due to greater apatite content in the source for the hawi lavas

microbial structures in oolitic iron formations ferriferous ooids and microoncoids occur in many sedimentary iron formations these ferriferous coated grains are found in fine grained carbonate rich groundmasses iron mineral encrusted microbiota are observed in both the coated grains and the groundmass the branching nature and other morphological features of the microorganisms suggest fungal origins for the oolitic iron ores particularly of the lower jurassic lorraine minette the similarity of the microbial structures in coated grains and their groundmasses suggests that both had developed within microbial mats growing under calm environmental conditions the contribution of stromatolitic marine fungal mats to the fast extraction and immobilization of iron and thus to the genesis of iron ores is demonstrated observations on well preserved freshwater derived fungal stromatolites of the tertiary and laboratory experiments with organotrophic fungal mats confirm the findings on the jurassic and imply a general role of microorganisms in the formation of such deposits

structural controls on an orogenic gold system the world class siguiri gold district siguiri basin guinea west africa orebodies in the siguiri district a world class paleoproterozoic orogenic gold camp located in the birimian of northeastern guinea are typically represented by cryptic subvertical damage zones that host a high density of mineralized veins although no large regional fault system was recognized observations from five representative deposits of the siguiri district sanu tinti bidini kami kosise and sintroko pb1 show that these orebodies are locally controlled by incipient structures and spread across three distinct structural and lithostratigraphic domains two shale dominated peripheral domains adjoin a central domain whose lithostratigraphy is dominated by medium to coarse grained graywacke and this domain hosts the bulk of the gold endowment of the district the three domains exhibit similar structural elements that can be described within a four stage deformation scheme the first deformation event d 1s is poorly constrained and interpreted to have been an n s compressional event it included development of minor folds with w to wnw gently plunging fold axes without a clear axial planar cleavage the main and second deformation event d 2s is interpreted to have been associated with e w to ene wsw directed compression the d 2s event was responsible for forming the dominant n trending structural grain of the district and creating interference patterns between f 1s and f 2s folds the third event d 3s developed progressively from d 2s compression into an early d 3s e w to enewsw directed transpression and a late d 3s nnw sse directed transtension responsible for most of the gold mineralization in the siguiri district the fourth and last event d 4s was an nw se oriented compressional event responsible for the localized overprinting of veining by a steep to shallowly dipping nne ssw ductile cleavage late d 3s gold bearing mineral occurrences formed along subvertical n s reverse faults ne trending dextral shear zones wnw trending sinistral faults and e trending normal relay faults developed or reactivated early d 3s mineralization is expressed as mineralized shear zones or subvertical damage zones characterized by a 10 to 15 m wide zone of dense quartz carbonate sulfide veining or disseminated gold bearing sulfides the mineralized veins consistently strike ene wsw are steeply dipping and commonly have a conjugate geometry at the mesoscale finite strain analysis of deformation including analysis of folds faults and conjugate mineralized vein sets is consistent with a stress switch from a compressional d 2s to transpressional deformation termed early d 3s results of paleostress analysis on conjugate mineralized vein sets that formed late during d 3s indicate that the stress field ranged from extensional to strike slip sometimes within the same vein locality the late d 3s deformation is interpreted to have been a transtensional event the first change in the orientation of the principal stress axes is related to a switch from a far field dominated to a body force dominated stress field reducing the deviatoric component on the stress tensor the second change in the orientation of the principal stress axes from early d 3s transpression to late d 3s transtension suggests that 1 and 2 were similar in magnitude which facilitated localized stress switches in the siguiri district the early d 3s and late d 3s stress switches which occurred at both a local and regional scale enhanced the fracture permeability and were critical for the establishment of active fluid pathways leading to the formation of a world class gold system

deep mid water bacterial mats from anoxic basins of the eastern mediterranean peculiar pellicles have been recorded in the extant anoxic hypersaline basins of the bannock area and the western strabo trench eastern mediterranean discrete layers of pellicles were found in 29 cores they are oriented parallel to bedding planes or are folded within the sandy base of turbidites and within slumped sediments in addition similar pellicles occur at the surface of or interlocked within gypsum crystals recovered from the bannock area the pellicles are 0 5 3 mm thick and dark greenish grey in colour they foliate into very small 1 mm undulating and anastomosing microlaminae entrapping abundant biogenic and inorganic particles the amorphous organic matter of the pellicles is most probably produced by bacterial masses linked to mineralization processes of the organic debris anaerobic sulphate reducing bacteria desulphovibrio are abundant and the presence of methane bacteria is likely detailed analyses with the light microscope and scanning electron microscope yielded differences and similarities concerning ultrastructure content abundance and morphotypes of the bacterial pellicles from the bannock area and the western strabo trench a consistently well developed ultrastructure comprising thin microlaminae characterizes the bannock pellicles whereas the ultrastructure of the tyro and poseidon pellicles is usually not organized siliceous microfossils are abundant in samples from both sites but centrate diatoms prevail in the bannock pellicles whereas pennate diatoms are overwhelmingly dominant in the tyro and poseidon pellicles calcareous micro and nannofossils together with pyrite framboids are more abundant in samples from the bannock area bacteria are extremely abundant in samples from the bannock area and three different morphological types have been distinguished only one type of bacteria was commonly observed in pellicles from the western strabo trench basins pellicles form at the normal seawater brine interface as substantiated by transmittance profiles and bacteriological investigations therefore the term deep mid water bacterial mats is proposed herein for these peculiar layers after bacterial mats grow at the interface they are included within the sediments by following one of two different paths a when pelagic sedimentation prevails pellicles sink to the basin floor due to the increasing load of biogenic and inorganic detritus and b under a tectonically active regime turbidity currents and slumps can disrupt the interface and destroy the floating mats in the latter case fragments of pellicles are transported to the bottom within the gravity flows

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

the relationship between the structural orientation and the gold mineralisation of quartz sulphide veins in the navachab gold deposit namibia abstract this study uses a representative dataset of more than 500 vein samples from the navachab gold deposit containing information on the gold concentration structural orientation vein thickness alteration and host rock type to investigate small scale controls on the grade distribution in the veins in order to obtain a better understanding of the architecture of the navachab ore bodies we calculated mean gold concentrations for each vein set in different parts of the mine we then examine the preferential mineralisation of certain vein sets by setting this grade information into context to other geological parameters such as the structural geology of that particular part of the mine as well as the above mentioned factors structural orientation of the vein vein thickness alteration and host rock type our main findings are that vein sets oriented favourably to the stress field and at high angles to the bedding are better mineralised than veins sets oriented at low angles to the bedding other factors do not exert a great influence on the gold concentration in the veins we propose that the gold grade in the veins is influenced by the effectiveness of fluid flow into the veins and by time rates during vein propagation and fluid flow

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

stress controls on intrusion related gold lodes wonga gold mine stawell western lachlan fold belt southeastern australia the wonga gold lodes 300 000 oz au in the victorian gold fields of eastern australia formed at a depth of less than 7 km within the seismogenic zone the wonga gold deposit is hosted by mudstone dominant lithofacies that define the core of the regional synform these are underlain by sandstone dominant lithofacies that are interlayered with mafic schist the lodes occur within the contact aureole of a late stage granitic body that intrudes an earlier phase of the fractionated pluton in contrast to the nearby 440 ma magdala orogenic gold lodes 5 moz au this smaller deposit formed at ca 400 ma and has been classified as intrusion related the orogenic and intrusion related gold deposits formed at different stages in the structural and magmatic evolution of the fold belt and at different crustal levels compared to the magdala lodes the wonga lodes have a close temporal and spatial relationship with the intrusive rocks and a distinct structural style and mineralogy including acicular arsenopyrite stibnite gold telluride alloys bismuth silver and late stage molybdenite the wonga lodes formed between two distinct phases of regional faulting and the kinematics of the mineralized structures cannot be correlated with any preserved far field regional paleostresses instead they are inferred to reflect the local state of stress at the point that magmatic related fluid overpressure occurred pervasive fluid overpressure drove deformation across a broad zone producing massive mineralized quartz vein structures that included angular clasts of wall rock a discrete overpressure event is suggested rather than repeated crack seal behavior the lodes represent an early stage in the development of a fault zone characterized by a series of discontinuous discrete shear failures the geometry of the lodes is strongly dependent on structural position and rheology northeast dipping preexisting weaknesses such as dike margins and variably developed crenulation cleavages produced localized shear failure defined by dextral reverse slip structures the hanging wall structures 30 to 60 se dipping extension veins and extensional shear veins the link structures diverge from these creating en echelon lode geometries there is a sudden change over the top of a regional plunge reversal defined by the dominant fold axes from shear failure the hanging wall structures to extensional shear and then extensional failure the links this occurs along strike in the same structural corridor and within the same stratigraphic unit the explicit finite difference code flac was used to model the local state of stress at the point where an influx of magmatic fluids raised the pore fluid pressure the model predicts that a zone of slightly lower differential stress was associated with the plunge reversal and this variation in differential stress is interpreted to have caused the drastic change in lode style the modeling highlights that only subtle differences in the elastic moduli of the rocks are required to produce the observed stress geometry

recent stromatolites in landlocked pools on aldabra western indian ocean abstract stromatolites occurring in three areas of landlocked pools on aldabra show distinctive morphologies petrography and biological characteristics related both to water chemistry and the physical characteristics of sites they are formed apparently by indirect precipitation of high magnesium calcite in association with growths of cyanobacteria and cladophora the three contrasted areas are dominated respectively by phormidium lyngbya and pleurocapsa by entophysalis lyngbya and pleurocapsa and by pleurocapsa dichothrix gypsophila and schizothrix reflecting an increasing concentration and increasing dissolved carbonate activity of pool waters centimeters to decimeters of carbonate encrustations are made up of irregular laminate columns porous layered coatings of planar or cerebroid form including oncolites and finely laminate branching columns laminae are typically alternations of dense and porous micrite with in one locality radiating fibrous sparry crystal growths although exclusively associated with cyanobacteria calcification typically begins several millimeters beneath living cells with few examples of carbonate coated unicells or filaments micrite consists of randomly arranged particles of varying size products of sparse homogeneous nucleation followed by both overgrowth and dissolution coarse cements of contrasted morphologies develop within internal cavities resembling the endostromatolites described by others chemical data indicate that waters in all three pools are supersaturated with respect to caco 3 calcite this is adequate but is not in itself sufficient to bring about precipitation of carbonate which is site specific cyanobacteria are not directly calcified but metabolic activities may promote precipitation through mechanisms such as extraction of co 2 or by the generation of specific organic molecules which promote nucleation or colonies may provide a closed passive environment in which other bacteria or inorganic processes bring about crystallization the precise path is not known but the reduction of possibilities carries implications both for the accretion of other stromatolites and the crystallization of carbonate in deposits such as tufas

sadiola hill a world class carbonate hosted gold deposit in mali west africa the 8 moz sadiola hill gold deposit is located in the kedougou kenieba inlier a window of deformed ca 2200 to 2050 ma rocks that crop out in eastern senegal and western mali the geology of the inlier differs from other paleoproterozoic granite greenstone belts and sedimentary basins by the abundance of carbonate rocks the sadiola hill gold deposit occurs within 3 km of the senegal mali shear zone and country rocks in the region have undergone polycyclic deformation the lithostratigraphy of the open pit consists of impure limestones overlain by younger sequences of detrital sedimentary rocks which include arenite wacke and siltstone the sedimentary rocks have been subjected to regional greenschist facies metamorphism and have been intruded by multiple generations of synkinematic calc alkaline stocks and dikes the bulk of the ore is hosted within a n s to nnw trending 10 to 50 m wide brittle ductile dilational shear zone defined as the sadiola shear zone gold lodes also occur along an array of steep nne trending shear zones geometric and kinematic analyses indicate that the ore hosting structures were undergoing sinistral displacement at the time of mineralization locally defined as the d3s nnw sse shortening event hydrothermal alteration is polyphase and includes an early high temperature calc silicate phase i e porphyroblastic growth of actinolite tremolite followed by a potassic phase i e biotite calcite quartz k feldspar tourmaline actinolite that was synchronous with ore mineral deposition paragenetic studies reveal a multistage ore development that includes an as rich sulfide stage followed by an au sb stage the ore is associated with a metal enrichment suite of au as sb cu fe w mo ag bi zn pb te the sadiola hill deposit shares a similar relative timing and structural setting to that of other world class orogenic gold systems in the west african craton e g ashanti loulo with gold deposition occurring during a period of transcurrent tectonics after the cessation of region wide compressional deformation the high temperature paragenesis at sadiola hill is atypical of gold mineralization in the craton and indicates that the late eburnean tectono magmatic activity between ca 2080 and 2060 ma played an important role in the dynamics of hydrothermal fluid circulation along the senegal mali shear zone