gravity anomalies and segmentation of the continental margin offshore west africa the free air edge effect anomaly at passive continental margins provides constraints on the thermal and mechanical properties of the lithosphere in extensional regions like many margins the edge effect high offshore gabon west africa is located at the shelf break in the region of maximum sediment thickness gabon differs however from other margins in that the edge effect low is displaced from over the continental rise to the slope the origin of the low has been investigated by backstripping the sediments at the margin we have used the backstrip to calculate the amount of crustal thinning assuming airy isostasy such a model implies zero strength during rifting alternatively this amount of crustal thinning can be explained by a strong margin and a depth of necking that exactly balances the upward and downward forces that act on the crust during rifting we have developed a method which takes into account variations in the strength during rifting and the depth of necking to compute the thinning directly from the sediment backstrip the backstrip and the crustal thinning have been used to calculate the rifting gravity anomaly differences between the backstrip and the present day water depth give the geometry of the sediment load and hence the contribution to the gaavity anomaly of sedimentation by comparing the combined rifting and sedimentation anomaly to the observed free air gravity anomaly we have been able to constrain the flexural rigidity and equivalent elastic thickness te of the lithosphere during and after rifting the best fitting model is one in which the low is caused by a 125 km wide zone of relatively thin i e 15 km continental crust which was weak i e 0 te 10k during rifting and has remained so since then we speculate that the weak zone extends for 350 400 km along strike of the gabon margin and that passive margins may be highly segmented as regards their long term strength

evolution of gold bearing veins in dykes of the woods point dyke swarm victoria the woods point dyke swarm comprises hundreds of narrow subparallel igneous dykes and dozens of pipe shaped dyke bulges within strongly deformed early palaeozoic turbidites of the melbourne trough porpylitic alteration accompanied dyke emplacement and was followed by microfracturing induced by high fluid pressures involving co2 of magmatic origin as the dykes solidified further stress caused through going faults having ladder and other patterns isotopic studies suggest that metamorphically or geothermally derived solutions filled the faults and other fractures with quartz and carbonate and altered immediately adjacent dyke rock however earlier formed vein and wall rock carbonates retained their magmatic isotopic composition fluid inclusions indicate vein deposition began at approximately 400 c with salinities up to 9 weight percent nacl nine sulfide minerals and gold were deposited in the veins after ankerite sericite and albite while quartz deposition continued through all stages sulfur isotopic determinations indicate the vein sulfur could not have been derived from adjacent sedimentary rocks nor exclusively from the dykes metamorphic waters of marine origin is a viable source for sulfur saline and co2 rich alkaline solutions reacted with the dyke wall rocks and probably evolved chemically prior to deposition of gold vug carbonates deposited by meteoric water that leached vein carbonates mark the end of vein formation

gravity anomalies and segmentation at the east coast usa continental margin summary the free air gravity edge effect anomaly at rifted continental margins has generally been attributed to the transition between thick continental and thin oceanic crust while crustal thinning is a major contributor recent studies suggest that sediment loading and magmatism may significantly modify the edge effect anomaly and cause it at some margins to be highly segmented along their strike in this paper we use a combined 3 d flexural backstripping and gravity anomaly modelling technique to determine the role that sediment loading has played in controlling the segmentation of atlantic type continental margins we focus on the east coast usa since a substantial amount of high quality seismic reflection and refraction gravity and magnetic data already exists for this margin by calculating the gravity anomaly associated with rifting and sediment loading and iteratively comparing it to the observed free air anomaly we have determined the best fit elastic thickness te structure of the margin we show that 0 10 5 a integrated strength of the lithosphere these results imply that weak regions abut strong ones at the east coast usa margin te generally decreases with increase in the amounts of crustal thinning and the flexed basement curvature k suggesting it is controlled at least in part by the mechanical structure of the pre rift lithosphere and yielding due to flexural loading however there is considerable scatter suggesting other factors such as along strike changes in crustal composition irrespective we show that an isostatic anomaly that takes into account the best fit te distribution is significantly reduced in spectral power compared with one which is computed assuming only airy compensation i e te 0 km this is not to imply that rifting and sediment loading completely accounts for the anomalies along strike and across strike the east coast usa margin to the contrary significant isostatic anomaly highs and lows persist especially in inner and middle shelf regions one of the most prominent is an arcuate 670 km long high with flanking lows offshore carolina that we attribute to magmatism during the initial stages of continental break up

origin ore forming fluid evolution and timing of the logrosan sn w ore deposits central iberian zone spain the logrosan sn w ore deposits in the metallogenic sn w province of the european variscan belt consist of endo and exogranitic greisen type and quartz cassiterite veins associated with a s type granite mineral characterization fluid inclusion study isotope geochemistry and ar ar geochronology have been combined in order to reconstruct the conditions for sn w mineralization the endo and exogranitic mineralization must have been developed in a relatively long lived system 308 303 ma during or soon after the emplacement of the logrosan related granite at ca 308 ma the mineralizing fluids are characterized by complex aqueous and volatile h2o n2 co2 ch4 nacl fluid inclusions microthermometry and raman analyses indicate that fluid composition evolved from n2 ch4 to n2 rich followed by co2 rich fluids with varying amounts of h2o the presence of n2 and ch4 suggests the interaction with fluids derived from the nearby metasedimentary host rocks a model of host rock interaction assimilation and mixing of metamorphic and magmatic fluids resulting in change of the redox conditions is proposed for tin deposition later sulfide minerals were precipitated as a result of pressure and temperature release

crustal anatomy of a transform continental margin abstract this paper describes new deep seismic reflection results across the major transform margin southwest of the grand banks off eastern canada an interpretation of these results is presented supplemented with previous seismic refraction gravity and magnetic measurements the results show a fairly sharp transition between oceanic and continental regions at the transform margin a narrow zone about 40 km wide separates the two regions and is interpreted to be a zone in which shearing has destroyed the original fabric of the continental crust faults in this region extend throughout the crust and there is some evidence that a major strike slip fault may extend some tens of kilometres into the upper mantle magmatism may have further modified the crust in this shear zone which is bounded on the oceanic side by a small seamount the continental crust is thinned by a factor of 3 or more in this zone which may be due to erosion or to flow in the lower crust during shearing the oceanic crust does not appear to thin significantly as the transform margin is approached crustal scale dipping reflectors over a confined region below the shelf are interpreted to represent the contact between two appalachian terranes the meguma and avalon terranes this supports earlier interpretations of a large magnetic lineament in the region as marking this boundary these crustal seismic data crossing a large transform margin are the first of their kind and will help us to constrain evolutionary models of transform margins the picture they provide is different in many respects from that across rifted margins and similar in others in some cases it might be difficult to distinguish between these two classes of atlantic type margins on the basis of crustal structure

the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

evidence for transform margin evolution from the ivory coast ghana continental margin the relative motions between lithospheric plates can be reduced to three major types convergence divergence and transform motion convergence leads to an active margin and eventual collision divergence results in the construction of a rifted passive margin transform motion generates a transform or sheared margin although passive margins have been extensively studied1 2 and many models have been proposed for their origin and subsequent evolution3 5 little is known about transform margins with the exception of a few studies of their morphologies6 7 shallow structures or crustal sections8 9 here we present the main conclusions derived from a recent study of the northern gulf of guinea margins particularly off the eastern ivory coast and ghana where the continental margin is one of the best preserved examples of an extinct transform margin10 12 our observations support a four stage model for transform margin evolution technically active transform contacts first between normal continental crusts and then between thinned margins induce characteristic structures such as pull apart grabens and shear folds the next stage in which thermal exchange between oceanic and continental lithos pheres controls a complex subsidence is followed by the transition to a true intra oceanic fracture zone

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

crustal structure at the blake spur fracture zone from expanding spread profiles we present results from wkbj and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles esps along flow lines parallel to the blake spur fracture zone across 140 ma atlantic crust these profiles provide detailed constraints on variations in crustal structure at the fracture zone seven profiles on either side of the fracture zone show a normal structure for old oceanic crust a 2 3 km thick upper layer with a steep velocity gradient is underlain by 4 5 km of crust with velocities of 6 5 7 2 km s and low gradient and a sharp transition to upper mantle velocities of 8 km s several velocity discontinuities were detected within the upper 2 3 km but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles this structure shows little regional variation toward the fracture zone despite the small magnetic anomaly offset 12 km and the indistinct topographic signature of the fracture zone three esps within a 10 20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure relative to normal oceanic crust a 2 4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7 2 7 6 km s and high poisson s ratio probably at least 0 29 which is consistent with 15 30 serpentinization of upper mantle peridotites this interpretation requires the action of off axis hydrothermal circulation penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km the original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent normal crust which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments

from continental extension to seafloor spreading crustal structure of the goban spur rifted margin southwest of the uk we present a 169 km wide angle velocity model across the goban spur rifted margin southwest of the uk a 120 km wide intermediate region is identified between the first clear seafloor spreading magnetic anomaly anomaly 34r and thinned continental crust where velocities increase from 4 5 km s1 to 6 8 km s1 in the top 4 km beneath acoustic basement at depth it can be divided into a region where a 1 5 km thick high velocity layer 7 2 7 6 km s1 exists and a region where this layer is absent but velocities of 7 km s1 are present wide angle pmp arrivals are observed across the whole of the intermediate region but a normal incidence reflection moho is not present based on these velocities and combined with seismic reflection gravity and magnetic modelling along this transect we interpret the intermediate region to consist of a 70 km wide zone of exhumed mantle with high poisson s ratio at top basement 0 34 an extremely low topographic expression and a high velocity deep layer empirical relationships between velocity and degree of serpentinization suggest that serpentinite content decreases with depth from 100 per cent at top basement to 25 per cent from 5 7 km into basement the observed magnetic anomaly is best fit by a thin magnetized layer 1 km of magnetization 2 3 a m1 the intense magnetization may be due to magnetite formation during a prolonged interaction of serpentinite with sea water between this exhumed mantle and anomaly 34r is a 50 km wide region raised 400 m above the adjacent exhumed mantle consisting of a series of basement ridges poisson s ratio is not well defined in this region which may be composed of anomalous oceanic crust or exhumed mantle

the structure of the rockall trough imaged by deep seismic reflection profiling westline is a 450 km long near normal incidence seismic reflection profile crossing the rockall trough from the rockall bank to the irish coast beneath rockall bank and the irish shelf unreflective upper crust overlies reflective lower crust and upper mantle crustal reflectors beneath the irish shelf are interpreted as caledonian the nw slope of the trough is dissected by a series of east dipping faults forming small sediment filled half grabens the steep se slope is formed by the erris ridge the floor of the trough is formed from rotated blocks separated by mainly east dipping faults wedges of sediment fill half grabens formed by this faulting there is no evidence for oceanic crust or a 9reflection moho9 within the trough the floor and slopes of the trough are blanketed by c 4 km of parallel bedded sediments the lower part of the parallel bedded sequence is intruded by sills and dykes that do not extend above a minor unconformity it is suggested that the bulk of the sediment preserved in the rockall trough accumulated during and following a major rifting event in the early or mid cretaceous

the ocean continent boundary off the western continental margin of iberia crustal structure west of galicia bank a seismic refraction transect across the galicia bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin parallel peridotite ridge pr immediately west of the pr oceanic crust is only 2 5 3 5 km thick but farther west oceanward it thickens to 7 km the pr caps a similar to 60 km wide lens shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust when superimposed on a reflection time version of the velocity model the s reflector is clearly intracrustal at its east end westward s cuts down to lower crustal levels eventually coinciding with the top of the serpentinized peridotite lens original crust mantle boundary these observations render almost impossible the seafloor exposure of the pr by s acting as a top to the west detachment fault numerical models of melting and borehole subsidence information constrain our rifting model the easternmost continental crust experienced a total stretching factor of 4 3 most likely in two stages it probably occurred over similar to 25 m y with the highest fate of stretching at the beginning of the main earlier rift phase valanginian 141 135 ma the 3 4 7 km thick continental crust depending on whether serpentinized peridotite is assigned to crust or mantle which may include melt products requires stretching factors 6 more than 11 7 and a rift duration of more than 25 13 m y the thin oceanic crust immediately west of the pr is explained by conductive cooling of the mantle during the long prebreakup stretching phase which temporarily caused reduced melting immediately after breakup

crustal geometry of the northeastern gulf of aden passive margin localization of the deformation inferred from receiver function analysis summary here we use receiver function analysis to retrieve crustal thickness and crustal composition along the 35 my old passive margin of the eastern gulf of aden our aims are to use results from the 3 d seismic array to map crustal stretching across and along the aden margin in southern oman the array recorded local and teleseismic events between 2003 march and 2004 march seventy eight events were used in our joint inversions for vp vs ratio and depth the major results are 1 crustal thickness decreases from the uplifted rift flank of the margin towards the sheba mid ocean ridge we found a crustal thickness of about 35 km beneath the northern rift flank this value decreases sharply to 26 km beneath the post rift subsidence zone on the salalah coastal plain this 10 km of crustal thinning occurs across a horizontal distance of less than 30 km showing a localization of the crustal thinning below the first known rifted block of the margin 2 a second rift margin transect located about 50 km to the east shows no thinning from the coast to 50 km onshore the lack of crustal thickness variation indicates that the maximum crustal stretching could be restricted to offshore regions 3 the along strike variations in crustal structure demonstrate the scale and longevity of the regular along axis rift segmentation 4 extension is still observed north of the rifted domain 70 km onshore from the coast making the width of margin larger than first expected from geology 5 the crust has a felsic to normal composition with a probably strong effect of the sedimentary layer on the vp vs ratio comprised between 1 67 and 1 91

the origin and evolution of mineralizing fluids in a sediment hosted orogenic gold deposit ballarat east southeastern australia the hydrothermal fluids responsible for gold mineralization at the ballarat east gold deposit the second largest orogenic gold deposit in the western lachlan orogen are thought to have links to a variety of processes including metamorphism sedimentation and or magmatism in the current study noble gases and halogens have been used as fluid tracers to reevaluate the origin and evolution of the gold related fluids at the ballarat east deposit gold bearing quartz and carbonate veins from the ballarat east contain low salinity 4 wt nacl equiv aqueous h2o and mixed h2o co2 fluid inclusions these fluid inclusions have variable molar br cl values of between 1 2 10 3 and 2 9 10 3 and i cl values between 150 10 6 and 500 10 6 and br is strongly correlated with i defining a mixing line with a br i ratio of 5 6 the fluid inclusions have 40ar 36ar ratios ranging from 322 close to the atmospheric 40ar 36ar ratio of 296 up to a maximum of 4503 40ar is strongly correlated with cl and defines a mixing line with a 40are cl ratio of 4 6 10 4 40are denotes excess 40ar the fluid inclusions contain 5 1 to 32 ppm 40are by mass and exhibit minimum 36ar concentrations ranging from 3 1 to 11 ppb which exceed air saturated water asw levels by several parts per billion asw 1 3 2 7 ppb fluid inclusion 84kr 36ar and 130xe 36ar values are uniformly enriched in kr and xe relative to air but exhibit limited variation these data provide strong evidence for the involvement of two noble gas and halogen reservoirs this data is compatible with a deeply sourced fluid possibly originating by devolatilization of altered volcanic rocks e g basalts that acquired additional noble gases and organic br plus i by interaction with sedimentary rocks including organic rich shales that are found beneath and surrounding the deposit the data are also consistent with mixing deeply sourced metamorphic fluids with sedimentary formation waters however both interpretations favor the involvement of metamorphic fluids and sedimentary components and highlight the significance of fluid rock interaction as controls on fluid compositions in victorian deposits the data are compatible with genetic models for orogenic gold in which gold mineralization was initiated by metamorphic devolatilization in the lower crust and was linked to lachlan orogenesis at ca 440 ma

crustal structure of the central nova scotia margin off eastern canada summary the central nova scotia margin off eastern canada is located at a transition from a volcanic margin in the south to a non volcanic margin in the north in order to study this transition a wide angle refraction seismic line with dense airgun shots was acquired across the central nova scotia margin the 500 km long transect is coincident with previous deep reflection profiles across the lahave platform and extending into the sohm abyssal plain a p wave velocity model was developed from forward and inverse modelling of the wide angle data from 21 ocean bottom seismometers and coincident normal incidence reflection profiles the velocity model shows that the continental crust is divided into three layers with velocities of 5 5 6 9 km s 1 the maximum thickness is 36 km a minor amount 5 km of thinning occurs beneath the outer shelf while the major thinning to a thickness of 8 km occurs over the slope region the seaward limit of the continental crust consists of 5 km thick highly faulted basement there is no evidence for magmatic underplating beneath the continental crust on the contrary a 4 km thick layer of partially serpentinized mantle 7 6 7 95 km s 1 begins beneath the highly faulted continental crust and extends 200 km seawards forming the lower part of the oceancontinent transition zone the upper part of the transition zone consists of the highly faulted continental crust and 4 to 5 km thick initial oceanic crust the continent ocean boundary is moved 50 km farther seawards compared to an earlier interpretation based only on reflection seismic data the oceanic crust in the transition zone consists of layer 2 and a high velocity lower crustal layer layer 2 is 1 3 km thick with velocities of 5 6 6 0 km s 1 the high velocity lower crustal layer is 1 2 km thick with velocities of 7 25 7 4 km s 1 suggesting a composite layer of serpentinized peridotite and gabbroic layer 3 oceanic crust with normal thickness of 5 7 km and more typical layer 3 with velocities of 6 95 7 3 km s 1 is observed at the seaward end of the profile

life history reconstruction of modern and fossil sockeye salmon oncorhynchus nerka by oxygen isotopic analysis of otoliths vertebrae and teeth implication for paleoenvironmental reconstructions we evaluate the use of oxygen isotope values of biogenic apatite for tracking freshwater to marine migration in modern and fossil pacific sockeye salmon oxygen isotope analyses of otoliths vertebrae and teeth of three anadromous modern sockeye salmon from alaska establish a basis for the interpretation of fossil vertebrae and tooth apatite from pleistocene sockeye salmon of the skokomish river valley washington high resolution 18 o profiles in salmon otoliths provide at a monthly resolution a detailed record of individual history including continental rearing migration to sea seasonal variation in sea surface temperatures during marine life and spawning migration before capture pacific salmon teeth are constantly renewed with the last set of teeth forming under the influence of freshwater therefore they do not allow inference concerning sea run versus landlocked life history in fossil salmon salmon vertebrae are also ambiguous indicators of life history regarding fresh versus marine water because centra are minimally ossified in the freshwater stages of life and the outermost layer of vertebral bone might be resorbed to provide nutrients during the non feeding phase of the spawning migration therefore 18 o values of accretionary growth rings in sea run salmon vertebrae are dominated by the marine signal only if they are not diagenetically altered in freshwater deposits in pleistocene sockeye reported here neither the teeth nor vertebral apatite present clear marine 18 o values due to the combined effects of tooth replacement and diagenetic alteration of bone and dentine 18 o po4 values of fossil vertebrae are intermediate between 18 o po4 values of enamel and basal tooth dentin assuming a similar rate of isotope exchange of vertebrae and dentine with freshwater during diagenesis these results are interpreted to reflect formation of the teeth under the influence of freshwater and formation of the vertebrae under the influence of oceanic water our approach demonstrates that when appropriate knowledge of tissue formation is

characterizing the galicia bank southern iberia abyssal plain rifted margin segment boundary using multichannel seismic and ocean bottom seismometer data 1 we present multichannel seismic reflection and ocean bottom seismometer reflection refraction data from ise 9 a margin parallel north south oriented profile 200 km west of the portuguese coast ise 9 images the boundary between two distinct segments of the iberia nonvolcanic rifted margin galicia bank gb and the southern iberia abyssal plain siap the bathymetric contrast between gb 2 km depth and siap 4 5 km depth spans only 25 km the crustal thickness transition however spans 137 km from 13 18 km thick beneath gb to 2 km thick beneath siap we define this crustal thickness transition as the segment boundary crustal structure along the segment boundary tilted blocks bounded by normal faults is surprisingly similar to crustal structure observed along orthogonal east west profiles of the iberia margin the apparent north south extension is similar in magnitude to previously calculated east west extension implying an overall northeast southwest extension however paleoreconstructions and rift basin orientations constrain lithospheric extension to a nearly east west direction we speculate that north south extension is limited to the crust and is caused by large scale mass wasting sometime between the tithonian and valanginian this rotational slump spans the 137 km wide modern segment boundary emplacing gb continental crust directly onto exhumed serpentinized mantle of siap palinspastic reconstruction restores the southern edge of the blocks to coincide within 10 km of a steep moho transition and a near vertical fault which extends through the slump and offsets moho we interpret this location as the preslump segment boundary that accommodated transform motion between the two rifted margin segments

crustal structure across the southwest newfoundland transform margin a seismic refraction survey providing deep crustal structure information of the continent ocean boundary across the south west newfoundland transform margin was carried out using large air gun sources and ocean bottom seismometer receivers continental crust 30 km thick beneath the southern grand banks p wave velocity 6 2 6 5 km s thins oceanward to a 25 km wide transition zone in the transition zone paleozoic basement of the grand banks 5 5 5 7 km s is replaced by a basement of oceanic volcanics and synrift sediments 4 5 5 5 km s seaward of the transition zone the crust is oceanic in character with a velocity gradient from 4 7 to 6 5 km s and a thickness of 7 8 km oceanic layer 3 is absent no significant thickness of intermediate velocity 7 km s material is present at the continent ocean transition indicating that no under plating of continental crust has taken place the continent ocean transition across the transform margin is much narrower than across rifted margins supporting the

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

recent rapid uplift in the bolivian andes evidence from fission track dating apatite and zircon fission track cooling ages constrain the tertiary cooling and uplift history of the eastern cordillera and altiplano of bolivia fission track data are from two triassic plutons and surrounding paleozoic metasedimentary rocks in the eastern andes north of la paz zircon cooling ages indicate that the roof of the huayna potosi pluton was emplaced in the zircon partial annealing zone and that the zongo pluton was emplaced entirely in the zircon total annealing zone apatite cooling ages for both plutons record uplift in the past 5 15 m y and zircon cooling ages from the zongo pluton reflect uplift in the past 25 45 m y uplift rates calculated by these apatite and zircon cooling ages suggest that uplift rates were 0 1 0 2 mm yr between 20 and 40 ma and increased significantly between 10 and 15 ma by 3 ma uplift rates may have been as high as 0 7 mm yr

rock buffered fluid rock interaction in deformed quartz rich turbidite sequences eastern australia quartz vein 18o 16o ratios across a 500 km transect through the lachlan fold belt of southeastern australia are remarkably uniform 1 1 5 permil at both local centimeter to meter and regional over 104 km2 scales they define isotopic zones that correlate with the tectonic divisions of the cambrian through devonian dominantly ordovician quartz rich turbidites determined by regional mapping from west to east these divisions are 1 stawell 18o 14 7 1 1 47 samples 2 bendigo ballarat 18o 17 5 1 3 204 samples 3 melbourne 18o 19 0 1 6 80 samples 4 tabberabbera 18o 16 3 1 9 12 samples and 5 omeo 18o 14 4 1 0 26 samples isotope profiles across zone boundaries and intrazone fault zones particularly within the bendigo ballarat zone show steps indicating abrupt changes across faults with little or no evidence of fluid mixing within the fault zones 18o values of veins are insensitive to relative age type of vein and immediate host rock lithologies the 18o values of coexisting vein and host rocks show nonequilibrium relationships which can be explained in terms of rock buffering under conditions of low fluid rock ratios water rock 1 limited d h determinations on fluid inclusions fall mainly in the range 70 and 100 with one value as low as 140 these low deuterium values when considered in the context of paleolatitude may have been inherited from deutrium depleted detrital minerals and do not necessarily require the direct penetration of meteoric fluids to midcrustal depths the work suggests that vein formation is possible in regions where the integrated water rock w r ratio is very low w r 1 as long as a pervasive fluid phase is present this fluid appears to achieve a steady state isotopic composition on scales of hundreds of meters and once a quasi steady state has been reached the isotopic compositions of the resultant quartz veins are rather insensitive to the diachronous and nonisothermal conditions under which vein growth occurs advective cycling of the fluid on this scale is by episodic dilatancy pumping in fracture networks associated with localized faulting

formation of gold deposits review and evaluation of the continuum model abstract the continuum model has been one of the more widely advocated genetic models for gold only deposits such as those found in archaean greenstone belts it postulates that hydrothermal gold deposits were formed throughout a 20 25 km vertical crustal profile from temperatures above 700 c to below 180 c and that the deposition occurred synchronous with the peak of metamorphism the continuum model is reviewed at this stage because we believe that it does not successfully account for many aspects of gold deposit formation the most obvious shortcoming is in considering ore deposits found in rocks of uppermost amphibolite and granulite facies domains where temperatures were appropriate for partial melting the main chemical condition that favours melting is access to h2o from either aqueous fluid or the breakdown of hydrous minerals a major gold forming hydrothermal event at the peak of high grade metamorphism as implied by the continuum model is incompatible with partial melting of wallrocks instead of forming a hydrothermal gold deposit an aqueous fluid introduced during partial melting would be consumed to produce further melt five gold deposits are documented from high metamorphic grade domains within four separate archaean cratons one is the type example used in the continuum model the other four have been significant producers partial melting has been recorded in the wallrocks adjacent to gold mineralisation at big bell hemlo challenger and renco gold deposits in the sulphide rich ore itself at challenger and hemlo deposits and as cross cutting dykes and migmatites at griffins find for these five deposits and indeed in general the continuum model does not easily account for aspects of the fluid source fluid composition gold transport and metal deposition the evidence that has been used to support deposit formation at the peak of high grade metamorphism as required by the continuum model is generally compatible with one or more alternative genetic models textural and structural siting of the gold high variance mineral assemblages proportion of various gold host rocks and relative element enrichments at high metamorphic grade can be accounted for by gold formation at greenschist facies conditions followed by progress of the deposit to high metamorphic grade metamorphic model moreover the type examples in the continuum model of deposits in sub greenschist facies domains appear to be incorrectly ascribed and are likely to be in greenschist facies domains the continuum model does not adequately explain archaean gold deposits

crustal structure of the goban spur rifted continental margin ne atlantic summary we present results that constrain the crustal structure across a typical non volcanic rifted continental margin the goban spur in the ne atlantic traveltime and amplitude modelling of wide angle ocean bottom seismograph profiles shows that the continental crust thins in a seaward direction to about 7 km before it breaks to allow a new oceanic spereading centre to develop the crustal velocity structure the stretching factors and the density distribution inferred from gravity anomalies are consistent with bulk pure shear stretching for the development of the rifted continental margin the seismic velocity structure of the thinned continental crust suggests that thinning was accompanied by only limited igneous intrusion and extrusion in the upper crust the adjacent oceanic crust is abnormally thin 5 4km this is explained most readily by accretion at a very slow spreading centre approx 11 mm yr full rate although additional cooling by conductive heat loss from the parent mantle as it rose slowly beneath the stretched and thinned continental lithosphere together with minor igneous intrusion into and surface flows onto the adjacent continental crust may also have reduced the oceanic igneous thickness immediately adjacent to the continent ocean transition

possible intrusion related gold systems in the western lachlan orogen southeast australia several gold deposits occurring in the western lachlan orogen have geological geochemical and geochronological characteristics that distinguish them from typical vein hosted orogenic gold deposits of the central victorian gold province the latter are responsible for more than 90 percent of primary hard rock gold production from this region and are generally considered to represent the only economically significant type of gold deposit in the western lachlan orogen atypical gold occurrences at malmsbury myrtle creek mount piper and the wonga deposit in the stawell goldfield are characterized by a close spatial and temporal association with posttectonic felsic intrusions disseminated to stockwork style mineralization alteration dominated by sericitization sulfidation silicification carbonatization and tourmalinization and associated complex au mo w bi te cu the deposits have a number of features in common with intrusion related gold deposits elsewhere in phanerozoic orogenic belts although production from this type of gold mineralization in the western lachlan orogen has been small compared to orogenic gold deposits the possible existence of intrusion related gold deposits has potentially important implications for exploration in this region and also provides significant clues to the tectonic framework and paleozoic metallogeny of eastern australia

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

petrogenetic significance of au bi te s associations the example of maldon central victorian gold province australia abstract mineralization at maldon victorian goldfields australia is part of the western lachlan foldbelt and is hosted by deformed lower ordovician metasediments intruded by the late devonian harcourt granite gold bi te s associations in the deposit include four gold bearing minerals maldonite native gold jonassonite and aurostibite and a range of sulphotellurides from the tetradymite group all of which are paragenetically tied to native bismuth the simpler phase associations involving bismuth maldonite and one or the other bi sulpho tellurides resemble the equivalents of eutectics in the system au bi te and hint at crystallisation from melts the complex associations include symplectites of gold and bismuth resulting from replacement of maldonite and are dominated by co existing sulphur bearing species joseite a joseite b an unnamed phase with bulk composition approximating to bi 3 te s 2 and jonassonite the complexity of such assemblages is also underpinned by chemical structural inhomogeneity within the unnamed phase i e nanoscale domains of bi 14 te s 9 and bi 5 te s 3 as documented from electron diffractions obtained from a fib tem foil although such lattice scale intergrowths reveal polysomatic disorder that can be associated with replacement of one telluride by another the structural modules in the overall stacking sequence indicate formation under local equilibrium conditions the mineralogical complexity of all au bi te s associations can be modelled in terms of interaction between simpler assemblages and sulphur bearing fluids three stages all involving gold minerals are recognised 1 bismuth maldonite hedleyite 2 bi sulphotellurides jonassonite and decomposition of maldonite gold bismuth and 3 bismuthinite gold from decomposition of maldonite or jonassonite the last stage is coincident with chloritisation and occurs during or following granite emplacement thermodynamic modelling of maldonite replacement by bismuthinite and gold indicates conditions to overlap with the pyrite hematite buffer at 258 c if neutral fluids are involved the general replacement model for the deposit shows that local scale reworking of maldonite and resultant gold remobilisation contributed to the apparent abundance of native gold this interpretation is consistent with the protracted geological history at maldon where granite intrusion post dates an orogenic event initial sulphur poor assemblages crystallised from melts predating granite emplacement the latest sulphidation event is attributed to retrograde fluids in the contact aureole whereas the earlier one could also have been produced during the multi stage deformation although observed assemblages and textures are the product of a complex sequence of overprinting there is no need to invoke input of gold from more than a single fluid generation

fluid inclusion and stable isotope constraints c o h on the dagbasi fe cu zn skarn mineralization trabzon ne turkey abstract the dagbasi fe cu zn skarn mineralization developed along the contact between the block and lens shaped limestones of the lower cretaceous berdiga formation and the upper cretaceous dagbasi granitoid the exoskarn type mineralization is characterized by prograde stage garnet and pyroxene while the retrograde stage is characterized by epidote tremolite actinolite and chlorite quartz and calcites were observed in both stages of the skarn development the ore minerals mainly consist of magnetite and hematite with a lesser amount of pyrrhotite pyrite chalcopyrite sphalerite and minor galena the homogenization temperatures th and salinity values of the prograde stage halite bearing fluid inclusions are in the range of 412 514 c and 48 8 61 8 wt nacl equ respectively the second stage liquid and vapor rich fluid inclusion assemblage reveals that boiling at temperatures of 353 458 c took place after the formation of halite bearing fluid inclusions final stage liquid rich fluid inclusions were characterized by low th 160 and 327 c and salinity values 0 5 and 6 2 wt nacl equ the decreasing salinity trend of the fluid inclusions versus th indicated that meteoric water was involved in the hydrothermal solutions eutectic temperatures te of the prograde stage fluid inclusions were found to be cacl2 dominated while retrograde stage inclusions contained different salt combinations rather than a specific salt type the minimum trapping pressures of the early stage brine fluid inclusions were calculated to be between 710 and 884 bar while later stage inclusions had much lower trapping pressures between 195 and 445 bar the 18o isotopes of prograde stage quartz garnet and pyroxenes are close to the composition of the hydrothermal solutions of magmatic sources moreover retrograde stage quartz epidote tremolite actinolite and calcite minerals and their equilibrated solutions were found to be highly depleted by 18o isotopes therefore the fluid inclusion and stable isotope constraints suggest that the hydrothermal solutions of magmatic origin were responsible for the prograde skarn stage while a mixture of magmatic and meteoric solutions were responsible for the ore formation in a shallow skarn environment

the crustal structure across the transform continental margin off ghana eastern equatorial atlantic forward modeling of a suite of wide angle seismic lines across the transform continental margin at the eastern end of the romanche fracture zone off ghana has shown the transition from continental to oceanic crust to be confined to a narrow 6 to 11 km wide zone located at the foot of the steep continental slope the structure of the adjacent oceanic and continental crusts has also been resolved these results are confirmed and enhanced by gravity and magnetic models the crust of the ocean continent transition zone is characterized by high velocities 5 8 7 3 km s 1 a high density 3 10 mg m 3 and high magnetizations 1 1 1 25 a m 1 this rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin the oceanic crust within 70 km of the margin is abnormally thin compared to normal atlantic oceanic crust and shows an average thickness of just 4 4 km we suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded on at least three sides by cold continental lithosphere the preferred seismic model also shows a new fracture zone 30 km southeast of the ocean continent transition which is characterized by low velocities in the upper crust a lens shaped layer with velocities of 7 2 7 4 km s 1 at the base of the crust and a crustal thickness of just 3 4 km the continental crust appears largely unaffected by the proximity of the adjacent oceanic crust there is no evidence for underplating of the continental crust adjacent to the ocean continent transition zone

evidence for unusually thin oceanic crust and strong mantle beneath the amazon fan we used seismic and gravity data to determine the structure of the crust and mantle beneath the amazon fan seismic data suggest that the crust is of oceanic type and is unusually thin 4 km compared to elsewhere in the atlantic we attribute the thin crust to ultraslow seafloor spreading following the breakup of south america and africa during the early cretaceous gravity data suggest that the fan was emplaced on lithosphere that in creased its elastic thickness te and hence strength following rift ing the increase from 10 km to 40 km is greater however than would be expected if te were determined by a single controlling isotherm based on a cooling plate model hence we conclude that the amazon fan has been emplaced on and is supported by un usually thin oceanic crust and strong mantle

ch4 n2 in the maldon gold deposit central victoria australia abstract the maldon gold deposit in central victoria has geological geochronological and fluid chemistry characteristics that distinguish it from typical vein hosted orogenic gold deposits in this region the deposit lies within the thermal aureole of the late devonian harcourt granite and associated granitic dykes that postdate regional metamorphism 445 ma and large gold deposits such as bendigo the fluid inclusions are characterised by the presence of non aqueous i e carbonic fluids which exhibit complex freezing and heating behaviour as well as mixed co 2 low salinity aqueous fluids mostly 10 wt nacl eq raman analysis indicates that carbonic inclusions can vary from co 2 rich to ch 4 n 2 rich furthermore higher salinity fluid inclusions containing 20 22 wt nacl eq occur locally overall fluid inclusions in the k feldspar zone are much less abundant by volume than those in the cordierite zone probably due to recrystallisation suggesting limited magmatic fluid input the harcourt granite is a moderately reduced i type granite and it is suggested that the retrograde reduced fluids e g ch 4 n 2 rich formed within the thermal aureole of the granite and associated dykes during contact metamorphism are not part of the regional mineralising fluid system which was dominated by deeply derived co 2 low salinity aqueous fluids of metamorphic origin thus the maldon deposit is an orogenic gold deposit that was metamorphosed and or remobilised during the emplacement of post orogenic intrusions

from rifting to spreading in the eastern gulf of aden a geophysical survey of a young oceanic basin from margin to margin a geophysical survey in the eastern gulf of aden between the alula fartak 52 e and the socotra 55 e transform faults was carried out during the encens sheba cruise the conjugate margins of the gulf are steep narrow and asymmetric asymmetry of the rifting process is highlighted by the conjugate margins horst and graben in the north and deep basin in the south two transfer fault zones separate the margins into three segments whereas the present day sheba ridge is divided into two segments by a transform discontinuity therefore segmentation of the sheba ridge and that of the conjugate margins did coincide during the early stages of oceanic spreading extensive magma production is evidenced in the central part of the western segment anomaly 5d was identified in the northern and southern parts of the oceanic basin thus confirming that seafloor spreading in this part of gulf of aden started at least 17 6 ma ago

distribution timing and causes of andean deformation across south america the andean orogeny in south america has lasted over 100 ma it comprises the peruvian incaic and quechuan phases the nazca and south american plates have been converging at varying rates since the palaeocene the active tectonics of south america are relatively clear from seismological and global positioning system gps data horizontal shortening is responsible for a thick crust and high topography in the andes as well as in se brazil and patagonia we have integrated available data and have compiled four fault maps at the scale of south america for the mid cretaceous late cretaceous palaeogene and neogene periods andean compression has been widespread since the aptian the continental margins have registered more deformation than the interior for the peruvian phase not enough information is available to establish a tectonic context during the incaic phase strike slip faulting was common during the quechuan phase crustal thickening has been the dominant mode of deformation to investigate the mechanics of deformation we have carried out 10 properly scaled experiments on physical models of the lithosphere containing various plates the dominant response to plate motion was subduction of oceanic lithosphere beneath continental south america however the model continent also deformed internally especially at the margins and initial weaknesses

crustal structure beneath orphan basin and implications for nonvolcanic continental rifting a wide angle seismic profile was acquired across the northeast newfoundland margin seaward to orphan knoll the profile is complemented by existing multichannel reflection data two online deep wells and other deep seismic reflection and refraction lines modeling of data from 15 ocean bottom seismometers provides the seismic velocity structure for a significant portion of the 400 km wide stretched continental crust on this margin prerift metasedimentary rocks have a topographic relief of up to 5 km with a velocity of 5 2 km s increasing to 5 9 km s unaltered crystalline crust below the metasedimentary section has a velocity range of 6 1 6 5 km s most of the lower crust landward of orphan knoll has a typical continental lower crustal velocity 6 8 7 0 km s and the lower crust shallows significantly beneath orphan knoll our model shows no evidence for the 5 km thick 7 35 km s layer modeled from data of an earlier experiment and interpreted as magmatic underplating but instead indicates that thinned continental crust extends seaward for 360 km without underplating implying nonvolcanic rifting for the ne newfoundland margin we suggest that continental stretching persisted from 180 ma to chron 34n time 101 ma when the final breakup between canada and western europe took place leaving a 400 km wide zone of thinned continental crust underneath the shelf and deep water area of the orphan basin on the landward side of the basin gravity modeling indicates a zone of very thin 6 8 km crust possibly a failed rift center formed as a result of the northward progression of nonvolcanic rifting between canada and europe a possible sub moho reflector suggests generally uniform stretching of the crust and upper mantle and the absence of volcanics and underplating is in contrast to the observations on related rift basins of the grand banks

the geology and gold deposits of the victorian gold province abstract the palaeozoic succession of victoria represents a major world gold province with a total production of 2500 t of gold i e 78 million oz on a global scale central victoria represents one of the most gold mineralized areas outside the witwatersrand of south africa and remains the prime example of a slate belt gold province also known as turbidite hosted or shale greywacke gold province production came from quartz veins 1000 t modern placers 1200 t and palaeoplacers 300 t up to 7000 mines produced gold from quartz veins however the 168 mines of this type which produced over 1 t au contributed 68 of the primary gold production twelve goldfields have each produced 30 t au approximately one million ounces from all sources with bendigo 697 t ballarat 408 t and castlemaine 173 t being the largest these twelve goldfields have contributed 71 of the combined primary and secondary production gold mining commenced in 1851 declined dramatically around 1914 and has not increased significantly since the palaeozoic succession of victoria is part of an extensive fold belt along much of eastern australia i e the tasman orogenic belt in victoria the succession is dominated by cambrian to early devonian clastic metasedimentary rocks that have undergone major upright deformation and low grade metamorphism culminating in the middle devonian tabberabberan orogeny north south inliers of cambrian metavolcanic and metasedimentary rocks associated with regional fault zones divide the province into several geologically distinct zones in central victoria there are particularly well developed acid volcanic complexes of late devonian age and synchronous peraluminous granites which represent high temperature crustal melting tertiary to quaternary basalts have covered some of the palaeozoic gold deposits and have also covered several rich gold placer deposits of cainozoic age larger goldfields including bendigo and ballarat occur in ordovician slate and greywacke sequences but a wide range of metamorphosed sedimentary and igneous rock types are host to goldfields most if not all primary deposits demonstrate structural control although the controlling structural features differ between deposits wallrock alteration is strongly influenced by host rock in metasedimentary rocks it is limited and subtle but it is more pervasive in mafic and felsic igneous rocks carbonates muscovite and pyrite are the most widespread alteration minerals in these rocks representing co 2 k and s addition enrichment of as within gold deposits is common with sb important in some small deposits whereas base metals are only abundant locally and bi w mo and te show strong spatial association with granites and are rarely associated with economic gold deposits granites are an important component of the palaeozoic succession in victoria no large deposits are within granites but a few goldfields including maldon 65 t au are within contact aureoles of either s or i type granites of differing degrees of fractionation and silica content a single stage but protracted and possibly diachronous period of gold introduction near the time of the middle devonian tabberabberan deformation can explain geological relationships at many gold deposits the possibility that this event was slightly older i e silurian or early devonian in the west of the state cannot be precluded from the available evidence the mineralizing event appears linked to the same thermal event as diorite lamprophyre dykes and graite intrusions acidic and basaltic volcanism deformation and regional metamorphism the limited amount of fluid chemistry data from victorian gold deposits suggests low salinity fluids which contained co 2 and variable methane with homogenization temperatures from 150 to 300 c the sulphur isotopic compositions of pyrite are mostly 1 to 5 with some values slightly above 10 mineralizing temperatures are inferred to be near 300 c and pressure around 1 2 kbar sulphide rich gold deposits in the east of victoria many of which contain significant base metals contrast with the rest of the province which contains mostly gold deposits with lower sulphide content and this eastern area should possibly be viewed as separate from the victorian gold province per se the benefits to victorian gold mining coming from the 1980 s global gold boom have been insignificant compared to the historical importance of the victorian gold province there seems little justification for assuming that all the large deposits have been discovered and there is reason to believe new deposit styles may emerge to complement the limited current exploration activity which is focussed on the search for extensions and repeats of known deposit types a major advance in scientific understanding of these oft neglected deposits could kindle a major exploration and production revival

p t x conditions of hydrothermal fluids and precipitation mechanism of stibnite gold mineralization at the wiluna lode gold deposits western australia conventional and infrared microthermometric constraints the wiluna lode gold deposits are located in the archean wiluna greenstone belt in the northern sector of the norseman wiluna belt in the yilgarn craton of western australia they are hosted in subgreenschist facies meta basalts and controlled by the wiluna strike slip fault system and associated shear veins and breccias the 13 individual lode gold deposits have produced around 115 t au from 1901 to 1946 and 1986 to today historically they also produced 38 3 t as and 3 5 t sb gold formed in two stages stage 1 gold pyrite arsenopyrite is finely disseminated in the wallrock and breccia fragments whereas stage 2 gold stibnite is located in massive shear veins and breccia matrix as fracture fill and in banded colloform textured veins stibnite gold orebodies only occur in some of the deposits e g moonlight and northern part of the west lode and also display a restricted vertical extent being preserved only in the uppermost 200 m of stibnite bearing lodes petrographic conventional and infrared microthermometric and laser raman analysis on stibnite bearing quartz veins and breccias reveal that the antimony and gold rich hydrothermal fluid was of mixed h2o nacl co2 ch4 type microthermometric measurements reveal maximum homogenization temperatures of 340 c average 290 25 c and a wide range of salinities between 0 2 and 23 eq wt nacl aqueous carbonic fluid inclusions contain variable xco2 ch4 0 03 to 0 82 with the carbonic phase containing a maximum xch4 of 0 21 combined petrographic and microthermometric evidence suggests that the fluid inclusion properties reflect fluid immiscibility of a low salinity medium xco2 ch4 homogeneous parent fluid at about 290 c and pressures between 700 and 1 700 bar fluid immiscibility was triggered by cyclic pressure release during fault zone movement the decompression adiabatic cooling of the hydrothermal fluids shifted the ore fluid to lower temperatures significantly reduced the degree of stibnite undersaturation and caused stibnite to precipitate the deposition of stibnite reduced the ore fluid h2s concentration thereby destabilized gold bisulfide complexes in solution and caused gold precipitation locally this mechanism explains the intimate spatial association of stibnite and gold in quartz veins and breccias in the stibnite gold orebodies at wiluna

stable isotope o h s c and n systematics of quartz vein systems in the turbidite hosted central and north deborah gold deposits of the bendigo gold field central victoria australia constraints on the origin of ore forming fluids the central and north deborah gold deposits are located in one of 15 mining centers of the bendigo gold field central victoria australia a major world class gold province developed in a turbidite terrane gold mineralization occurs primarily within domes and reverse fault systems in multiply deformed lower ordovician metaturbidites deformation and thrusting occurred between 450 and 420 ma coevally with regional metamorphism at zeolite to greenschist facies in the central and north deborah deposits six stages of quartz veining each with characteristic geometry and mineral paragenesis developed sequentially stages 1 2 and 4 are gold bearing whereas stages 3 5 and 6 are barren gold related quartz veins were deposited at temperatures of 350 25 c based on oxygen isotope geothermometry of silicate phases quartz muscovite and sulfur isotope compositions of coexisting sulfide mineral pairs sphalerite galena this result is consistent with temperatures inferred from fluid inclusions the 18o values of gold bearing vein quartz and coexisting hydrothermal muscovite are uniform within and between the six vein stages ranging from 15 9 to 17 2 per mil and 11 6 to 12 8 per mil respectively the d values of hydrothermal muscovites are also uniform at 68 to 55 per mil ore fluids have calculated 18o values of 8 to 11 per mil and d values of 37 to 17 per mil pyrite of synsedimentary diagenetic origin in slate and sandstone displays sulfur isotope compositions of 23 5 to 8 0 per mil and 0 1 to 11 9 per mil respectively sulfides from gold bearing quartz veins have a range of 7 4 to 8 1 per mil median ca 2 5 the relatively uniform vein pyrites indicate a large uniform sulfur source most likely from desulfidation reactions of sulfide minerals interaction of the ore forming fluid with depleted sulfides at the site of deposition may have shifted the sulfur isotope signatures of source sulfides to more negative values carbon isotope compositions of carbonates range from 5 1 to 9 0 per mil median ca 6 5 nitrogen concentrations and nitrogen isotope compositions of hydrothermal muscovite from quartz veins are 652 to 895 ppm and 2 84 to 4 49 per mil respectively collectively the data define a band in n content vegh rsus 15n coordinate space from higher n and lower 15n in the phanerozoic metasedimenary hosted central and north deborah gold deposits to lower n and higher 15n in predominantly meta igneous hosted neoarchean gold provinces collectively the h o s c and n isotope compositions of ore related hydrothermal minerals indicate that the dilute aqueous carbonic and n bearing ore forming fluids were generated from metamorphic dehydration reactions at deeper crustal levels and were variably influenced by exchange with wall rocks during transport to the site of deposition this genetic model for the central and north deborah gold deposits is consistent with other turbidite hosted lode gold deposits and counterparts in volcanic plutonic terranes in contrast these data are not consistent with either mantle derived fluids or magmatic fluids nor do they support a deeply circulated meteoric surface water model for vein formation

mode of extension and rifting history of upper tiburon and upper delfin basins northern gulf of california 1 the crustal structure of the northern gulf of california transtensional margin has been investigated by a 280 km long nw se profile including deep multichannel seismic reflection and densely sampled refraction wide angle reflection seismic information combined with gravity modeling the seismic and gravity modeling constrains two thinned crustal areas corresponding to the upper delfin and the upper tiburon basins on both sides of the profile toward the baja california peninsula and the mexico mainland a progressive thickening of the continental crust is observed our results indicate that the crustal thickness is 19 km below the coastline and it decreases to 14 and 17 km below the upper delfin and upper tiburon basins respectively in the area between both basins the crust thickens to 19 5 km there are significant lateral thickness variations for the different levels of the crust the interpreted structure is consistent with the existence of an aborted rift below the upper tiburon basin prominent dipping reflections in the multichannel data under upper tiburon basin and the ridge between upper tiburon and upper delfin basins can be explained as a mylonite like zone related to a detachment fault this interpretation suggests that the structural evolution of upper tiburon basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust the mode and locus of extension have evolved from a core complex in upper tiburon to a narrow rift mode in upper delfin basin

structure and variability of oceanic crust on the flanks of the east pacific rise between 11 and 13 n we use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0 5 m y old crust in the rivera ocean seismic experiment rose area of the east pacific rise between 11 and 13 n there are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron amplitude patterns recorded on the individual lines however suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200 km segment of lithosphere on the basis of the modeling of amplitude patterns at ranges of 5 10 km and of comparisons with the upper crust sampled at deep sea drilling project hole 504b and in ophiolite complexes layer 2 can be divided into three units 2a corresponds to extrusive volcanics with a steep velocity gradient 2b to extrusives with a near zero velocity gradient and 2c to a complex region of transition from extrusives to sheeted dikes the total thickness of layer 2 varies by about 0 7 km over the length of the line another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3 the variability of these two amplitude patterns provides direct evidence that the along strike structure of oceanic crust in the rose area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information

character and kinematics of faults within the turbidite dominated lachlan orogen implications for tectonic evolution of eastern australia reply fault zones within turbidite dominated orogenic systems typified by the lachlan orogen of eastern australia are characterised by higher than average strain and intense mica fabrics transposition foliation and isoclinal folds poly deformation with overprinting crenulation cleavages and steeply to moderately plunging meso and micro folds they have a different character compared to the brittle ductile fault zones of classic foreland fold and thrust belts such as the appalachians and the canadian rocky mountains multiple cleavages and transposition layering record a progressive shear related deformation history an intense mica fabric evolves initially during shortening of the overlying sedimentary wedge but is progressively modified during rotation and emplacement to higher structural levels along the steep parts of inferred listric faults the deformed wedge outside the fault zones generally undergoes one phase of deformation shown by a weak to moderately developed slaty cleavage which is parallel to the axial surface of upright subhorizontally plunging chevron folds other faults within the turbidites of the lachlan orogen include the steep zones of ductile strike slip deformation that bound a centrally located high t low p metamorphic complex characterised by s c mylonites these ductile shear zones indicate a southward passage of the metamorphic complex as a crustal wedge with emplacement to higher structural levels along a leading edge ductile thrust fault ar ar dating constrains the timing of regional deformation to be mostly late ordovician through silurian across the lachlan orogen faults in the low grade turbidite sequences record the kinematic evolution of the developing lachlan orogen and indicate progressive deformation associated with simultaneous eastward propagating and migrating deformation fronts in both the western and eastern parts of the fold belt these deformation fronts are related to accretionary style deformation at the leading edges of overriding plates in an inferred southwest pacific type subduction setting from the late ordovician to the mid devonian along the former gondwana margin the fault zones effectively accommodate and preserve movements within the structurally thickening migrating and prograding accretionary wedge

new constraints on fluid sources in orogenic gold deposits victoria australia fluid inclusion microthermometry raman spectroscopy and noble gas plus halogen geochemistry complemented by published stable isotope data have been used to assess the origin of gold rich fluids in the lachlan fold belt of central victoria south eastern australia victorian gold deposits vary from large turbidite hosted orogenic lode and disseminated stockwork gold only deposits formed close to the metamorphic peak to smaller polymetallic gold deposits temporally associated with later post orogenic granite intrusions despite the differences in relative timing metal association and the size of these deposits fluid inclusion microthermometry indicates that all deposits are genetically associated with similar low salinity aqueous co2 bearing fluids the majority of these fluid inclusions also have similar 40ar 36ar values of less than 1500 and 36ar concentrations of 2 6 58 ppb by mass that are equal to or much greater than air saturation levels 1 3 2 7 ppb limited amounts of nitrogen rich fluids are present at a local scale and have the highest measured 40ar 36ar values of up to 5 700 suggesting an external or distinct source compared to the aqueous fluids the predominance of low salinity aqueous carbonic fluids with low 40ar 36ar values in both orogenic and intrusion related gold deposits is attributed to fluid production from common basement volcano sedimentary sequences and fluid interaction with sedimentary cover rocks turbidites aqueous fluid inclusions in the stawell magdala deposit of western victoria including those associated with n2 preserve mantle like br cl and i cl values in contrast fluid inclusions in deposits in the eastern structural zones which contain more abundant shales have elevated molar i cl ratios with maximum values of 5 170 10 6 in the melbourne zone br i ratios in this zone range from 0 5 to 3 0 that are characteristic of fluid interaction with organic rich sediments the maximum i cl and characteristic br i ratios provide evidence for organic br and i released during metamorphism of the shales therefore the regional data provide strong evidence for the involvement of sedimentary components in gold mineralisation but are consistent with deeper metamorphic fluid sources from basement volcano sedimentary rocks the overlying sediments are probably involved in gold mineralisation via fluid rock interaction

south atlantic fits and intraplate boundaries in africa and south america abstract the misfit problems raised by the pre drift reconstructions of the south and equatorial atlantic compel us to resort to intraplate deformations it is shown that acceptable deformations of the african plate in line with the benue trough and the transafrican rifts and shear zones provide a far from satisfactory solution to the geometrical misfit problems in the south atlantic these problems can instead be alleviated by considering a degree of deformation of the south american plate along an intraplate boundary in line with the parana province the rio grande rise and the walvis ridge in spite of the scarcity of field observations in this area of south america we show that such a deformation cannot be ruled out but it will have to be associated with deformations in the equatorial atlantic and in other areas of the african and south american plates

segmentation in gravity and magnetic anomalies along the u s east coast passive margin implications for incipient structure of the oceanic lithosphere segmentation is a characteristic feature of seafloor spreading along the global mid ocean ridge system while segmentation of active spreading centers has been the focus of much recent research the process by which a rifted continental margin develops into a segmented mid ocean ridge is still poorly understood in this study we investigate the segmentation character of the u s east coast margin through a modeling study of the margin basement structure magnetics and gravity anomalies the east coast margin is of particular interest because it is one of several rifted continental margins that display thick sequences of high seismic velocity igneous crust presumably formed during rifting the east coast magnetic anomaly ecma a distinct total field magnetic high running offshore along the margin is commonly located seaward of the thickest sections of the high velocity crust and displays segmentation on length scales 100 120 km similar to the segmentation observed at the mid atlantic ridge mar isostatic gravity anomalies were calculated by removing from free air gravity the predicted effects of seafloor sediments and a crust mantle model assuming local isostatic compensation the resultant residuals show a corridor of high anomaly running along the margin situated close to the maximum thickness of the high seismic velocity crust as determined from the two available seismic refraction lines reduction to the pole r t p of the total field magnetic anomaly shows that after the removal of skewness from the ecma the location of the isostatic gravity high is closely correlated to the ecma the isostatic gravity high is also segmented but in two distinct wave bands 100 150 km and 300 500 km the short wavelength 100 150 km segmentation in the r t p magnetic and isostatic gravity anomalies is similar in wavelength to segmentation in magnetization and mantle bouguer anomaly observed along the present day mar the 300 500 km segmentation in the along margin isostatic gravity anomaly is similar in length scale to both intermediate wavelength tectonic segmentation observed in the south atlantic and variations in lithospheric strength observed along the african margin furthermore two of the intermediate wavelength 300 500 km isostatic gravity lows correspond to the early traces of the kane and atlantis fracture zones suggesting that these two fracture zones may define boundaries of a single tectonic corridor in the north atlantic we hypothesize that the direct cause of the intermediate wavelength segmentation may be along margin variations in both the amount of underplated igneous crust and the strength of the lithosphere although the relative importance of these two effects remains unresolved our results imply that segmentation is an important feature of margin development and that segmentation at mature oceanic spreading centers may be directly linked to segmentation during continental rifting

mineral deposits of canada a synthesis of major deposit types district metallogeny the evolution of geological provinces and exploration methods wayne d goodfellow editor 1 068 pp geological association of canada mineral deposits division special publication no 5 2007 hardcover isbn 13 978 1 897095 24 9 price members cdn 60 00 non members cdn 80 00 following in the footsteps of other landmark reviews of major ore producing countries such as australia hughes 1990 solomon and groves 1994 and south africa anhaeusser and maske 1986 this substantial 1 000 pages monograph is a great achievement that should be in every geologic research library in the world in many ways it sets a new standard most notably in that all figures and photographs are in color and that the entire volume is available in electronic form both as an included dvd and mostly on the gac website it is common in such compendia to have a spectacular cover photo or frontispiece to catch the reader s attention and this book does not disappoint the frontispiece photo is of a 61 8 kg sample from the dome mine timmins ontario and is stated to contain 20 wt percent gold at today s prices the gold alone would be worth 1 5 million dollars

no mass independent sulfur isotope fractionation in auriferous fluids supports a magmatic origin for archean gold deposits the source of the auriferous fluids that formed archean greenstone gold deposits remains controversial metamorphic fluids fluids derived from felsic magmas mantle fluids and meteoric water have all been suggested we report the results of ion microprobe sensitive high resolution ion microprobe stable isotopes multiple sulfur isotope analyses 32 s 33 s 34 s conducted to constrain the source of sulfur in four archean gold deposits the analyses are of pyrite samples that are genetically related to the gold mineralization from the bellerophon victory and wallaby deposits in the eastern goldfield superterrane of the yilgarn craton western australia together with sulfide from black shale from the golden mile deposit and st ives gold camp ore associated pyrite from the beattie gold deposit of the abitibi greenstone belt of the superior craton in canada was measured to test whether the results obtained from the yilgarn craton apply to other archean gold deposits sulfur analyses from sulfide from shale show mass independent fractionation mif with 33 s as high as 1 8 whereas those from the 4 archean gold deposits are dominated by mass dependent fractionation with only a minor contribution of mif sulfur in this respect archean gold deposits contrast with archean nis deposits which are dominated by mif sulfur the absence of mif in the pyrite from the four archean gold deposits analyzed in this study is consistent with the auriferous fluid originating from a felsic magma or mantle source but inconsistent with the metamorphic and meteoritic fluid hypotheses

a geochronological framework for orogenic gold mineralisation in central victoria australia new 40ar 39ar geochronological data support and significantly expand upon preliminary age data that were interpreted to suggest an episodic and diachronous emplacement of gold across the western lachlan fold belt australia these geochronological data indicate that mineralisation in the central victorian gold province occurred in response to episodic eastward progressing deformation metamorphism and exhumation associated with the formation of the western lachlan fold belt initial gold formation throughout the stawell and the bendigo structural zones can be constrained to a broad interval of time between 455 and 435 ma with remobilisation of metals into new structures and or new pulses of mineralisation occurring between 420 and 400 ma and again between 380 and 370 ma linked to episodic variations in the regional stress field and during intrusion of felsic dykes and plutons this separation of ages is incompatible with the view that gold emplacement in the western lachlan fold belt was the result of a single orogen wide event during the devonian a distinct phase of gold mineralisation characterised by elevated cu mo sb or w is associated with both late silurian to early devonian 420 to 400 ma and middle to late devonian 380 to 370 ma magmatism when crustal thickening and shortening during the ongoing consolidation of the western lachlan fold belt led to extensive melt development in the lower crust and resulted in widespread magmatism throughout central victoria these 420 to 400 ma and 380 to 370 ma occurrences best exemplified by the wonga deposit in the stawell structural zone and many of the woods point deposits in the melbourne structural zone but also evidenced by occurrences at fosterville and maldon in the bendigo structural zone clearly formed synchronous with or post date the emplacement of plutons and dykes and thus are spatially if not genetically related to melt generation at depth this later magmatic associated and polymetallic type of gold mineralisation is economically subordinate to the earlier metamorphic associated type of gold deposition in the stawell and bendigo structural zones but tends to be the dominant style in the melbourne zone these new geochronological constraints together with zircon u pb data from felsic intrusive rocks of known relationship to gold mineralisation demonstrate that initial hydrothermal alteration associated with gold emplacement in the western lachlan fold belt was metamorphic related predating the emplacement of granite plutons by as much as 80 million years this timing differs from other important orogenic gold districts where gold deposition is closely associated spatially with felsic magmatism the early introduction of metamorphically derived fluids well before magmatism may reflect variations in the timing of peak metamorphic conditions at different crustal levels in an accretionary prism undergoing simultaneous deformation and erosion consequently no genetic link exists between the main phase s of gold mineralisation and magmatism in the central victorian gold province with the exception of formation of a minor magmatism related and geochemically distinct mineralisation style at about 420 to 400 ma and again at about 380 to 370 ma the apparent spatial relationship between gold mineralisation and felsic intrusions is merely the result of melts and fluids being channelised along the same structures

oblique rifting and segmentation of the ne gulf of aden passive margin 1 the gulf of aden is a young obliquely opening oceanic basin where tectonic structures can easily be followed and correlated from the passive margins to the active mid oceanic ridge it is an ideal laboratory for studies of continental lithosphere breakup from rifting to spreading the northeastern margin of the gulf of aden offers the opportunity to study on land the deformation associated with oblique rifting over a wide area encompassing two segments of the passive margin on either side of the socotra fracture zone exhibiting distinct morphologic stratigraphic and structural features the western segment is characterized by an elevated rift shoulder and large grabens filled with thick synrift series whereas the eastern segment exhibits low elevation and is devoid of major extensional structures and typical synrift deposits though the morphostructural features of the margin segments are different the stress field analysis provides coherent results all along the margin four directions of extension have been recognized and are considered to be representative of two tensional stress fields with permutations of the horizontal principal stresses 2 and 3 the two dominant directions of extension n150 e and n20 e are perpendicular to the mean trend of the gulf of aden n75 e and parallel to its opening direction n20 e n30 e respectively unlike another study in the western part of the gulf our data suggest that the n150 e extension stage is older than the n20 e extension stage these conflicting chronologies which are nowhere unambiguously established suggest that the two extensions coexisted during the rifting on land data are compared with offshore data and are interpreted with reference to oblique rifting the passive margin segmentation represents a local accommodation of the extensional deformation in a homogeneous regional stress field which reveals the asymmetry of the rifting process the first order segmentation of the sheba ridge is inherited from the prior segmentation of the passive margin

n2 and co2 in deep crustal fluids evidence from the caledonides of norway abstract high pressure metamorphic rocks eclogites and high pressure granulites occur along the entire length of the norwegian caledonides and have formed from a variety of protoliths in some cases the relationship between protoliths high pressure rocks and their later retrogression products have been preserved in situ fluid inclusion data suggest a simple correlation between metamorphic grade and metamorphic fluid composition 1 eclogites and high pressure granulites contain n 2 bearing fluids pure n 2 or mixtures with co 2 or h 2 o with x n 2 5 in some eclogite facies rocks co 2 1bn 2 inclusions are associated with aqueous brine inclusions ca 30 wt nacl the two compositions representing immiscible fluids at peak metamorphic conditions 2 granulite facies protoliths and eclogites reworked in the granulite facies contain pure co 2 or co 2 dominated fluids with less than 2 5 n 2 3 rocks retrograded in the amphibolite facies contain h 2 o 1bnacl fluids immiscibility between brine and anhydrous n 2 1bco 2 fluid and between anhydrous fluid and waterbearing aluminosilicate melt have taken place in some eclogites during high grade metamorphism nitrogen may be incorporated in minerals as nh 4 substituting for k or it may occur as n 2 in the free fluid phase the partitioning of nitrogen between minerals and fluids depends upon the water activity and oxygen fugacity during metamorphism low a h 2 o and or high f o 2 partitioning nitrogen to the fluid phase a rock interacting with a carbonic fluid at granulite facies pt conditions will be depleted in mineralogically bound nitrogen in cases where the protoliths of high pressure rocks have been through a previous granulite facies event a local source for the nitrogen contained in high pressure fluid is therefore unlikely

the genesis and tectonic control on archaean gold deposits of the western australian shield a metamorphic replacement model abstract gold deposits occur in greenstone belts world wide and contribute to anomalously high gold production from archaean terranes as in other cratons archaean gold mineralization of western australia represents a complex array of deposit styles despite this most deposits are clearly epigenetic and large deposits have a number of features in common including their strong structural controls distinctive wallrock alteration fe sulphide k mica albite ca mg fe carbonates consistent metal associations au ag as sb w b low base metals commonly fe rich host rocks great depth extension and lack of appreciable vertical zonation these shared characteristics combined with their ubiquitous occurrence indicate that archaean gold deposits had a common origin related to the tectonic evolution of greenstone belts auriferous hydrothermal systems were broadly synchronous with regional metamorphism and emplacement of synkinematic granitoids and felsic porphyry intrusions although these gold systems involved low salinity lowdensity reduced near neutral h 2 o co 2 fluids carrying gold as reduced sulphur complexes the origin of the fluids is equivocal most timing evidence and stable isotope data cannot distinguish metamorphic from magmatic granitoid or felsic porphyry orggins but the lack of consistent spatial relationships between specific volumetrically significant intrusive phases and large gold deposits in a number of cratons strongly favours metamorphic derivation of fluids the metamorphic replacement model for gold mineralization involves devolatilization of the lower portions of the greenstone pile with high geothermal gradients inhibiting significant melting co 2 possibly formed by the decarbonation of early alteration related to mantle degassing along crustal scale synbasinal fault zones auriferous fluids were channelled along greenstone scale faults in part developed during reactivation of crustal scale faults in a strike slip regime gold deposition occurred largely under greenschist facies conditions about 300 400 c 1 2 kb in response to decreasing gold solubility with declining temperature however a major control on gold deposition was fluid wallrock interaction many large deposits formed by sulphidation of fe rich host rocks with synchronous deposition of fe sulphides and gold however the variable nature of gold depositing reactions including lowering of fo 2 and ph allowed a multitude of small and some large deposits to form wherever that fluid circulation occurred in consequence several of the relatively small deposits currently worked from open pit are hosted by ultramafic and felsic rocks there are few constraints on the source of components au s k co 2 added to gold deposits but even giant deposits such as the golden mile kalgoorlie could have formed from a realistic greenstone source volume ca 8 8 5 km convective circulation of fluids could have contributed to the generation of high fluid rock ratios on the regional scale the markedly heterogeneous distribution of large gold deposits gold productivity and host rocks to deposits can be accommodated by the metamorphic replacement model the most favourable conditions for development of auriferous hydrothermal systems operated in younger ca 2 7 0 1 ga rift phase greenstones where greatest extension and crustal thinning produced high geothermal gradients crustal scale synbasinal faults and rapid extrusion and burial of volcanics including abundant komatiites iron rich tholeiitic basalts and dolerites were preferred host rocks for large gold deposits the least favourable conditions existed in older ca 3 5 3 4 ga platformphase greenstones where gentle sagging on submerged continental crust produced eruption of mainly mafic volcanics with few komatiites commonly in very shallow water environments this allowed intense synvolcanic alteration of both gold source rocks and potential host rocks the generally smaller gold deposits formed mainly in ultramafic or greywacke hosts younger ca 3 0 ga platform phase greenstones appear intermediate in nature but unlike other greenstones have significant epigenetic gold deposits in originally oxide facies bif which were deposited on relatively deep water platforms similar controls appear to exist on a world scale with gold mineralization peaking at ca 2 7 0 1 ga in response to development of major rift zones in thickened relatively mature continental crust interestingly the giant witwatersrand goldfield formed at about the same time

the s reflector west of galicia the seismic signature of a detachment fault the so called s reflector is a group of bright continuous reflections underlying the landward tilted fault blocks of the west galicia rifted margin s has been interpreted as the brittle ductile transition the lop of an intrusion a detachment fault and the crust mantle boundary to constrain the internal structure of the reflector we have carried out detailed analyses of these reflections we compare the waveforms of the seafloor reflection and its first multiple both to determine the amplitude of the seafloor reflection and to show hat the seafloor is in effect a spike in the reflectivity series so that the seafloor reflection can be used as the far field wavelet including both source and receiver ghosts we compare he waveform of the seafloor and 5 and show that within the resolution of our data s is a reflection from a step increase in acoustic impedance this result is confirmed through complex trace analysis and in particular the determination of the apparent polarity of s and the examination of the instantaneous frequency function s is consistently positive polarity and shows no significant frequency anomaly simple modelling shows that s is very unlikely to be a reflection from a thin layer we thus conclude that s is probably a single steplike interface from the varying frequency content of the data we determine a value for the effective q between s and the seafloor and use this to assess the amplitude loss due to attenuation and scattering we use a comparison between the seafloor and the s reflection to constrain the amplitude of s and estimate a reflection coefficient for s of at least 0 2 in places decreasing landwards by analogy with structures developed in the highly extended regions of the western united states we consider that the most likely interpretation of s is as a sharp west dipping detachment fault separating a granitic upper plate from a higher velocity lower plate locally probably serpentinized mantle

a fluid inclusion study of au bearing quartz vein systems in the central and north deborah deposits of the bendigo gold field central victoria australia the central and north deborah gold deposits with a production of about 7 tons au are located in one of 15 domes in the bendigo gold field central victoria australia an important mesothermal lode gold province the gold bearing quartz vein systems are hosted predominantly by reverse faults in lower ordovician turbidites which underwent lower greenschist facies metamorphism in the central and north deborah mines there are six stages of quartz veins that developed in the following sequence 1 laminated veins with a metal association of au as sb pb zn ni 2 spurs au 3 massive barren veins main dilation stage 4 brecciated veins with au as 5 quartz ankerite veinlets and 6 late pure quartz or calcite veinlets of the three mineralized vein stages 1 2 and 4 only the first one predated or was coeval with the main deformation event whereas stages 2 and 4 postdated the main deformation the vein structures indicate that vein development was controlled by repeated fluctuations of fluid pressure inducing hydraulic fracturing fluid inclusions in vein quartz contain c o h fluids of variable compositions three main types of fluid inclusions are recognized at room temperature type i two phase primary ch4 v h2o l fluid inclusions type ii two or three phase primary pseudosecondary co2 v or l ch4 v or l h2o l fluid inclusions including iia ch4 co2 h2o and iib co2 h2o fluid inclusions and type iii h2o v h2o l fluid inclusions type iii fluid inclusions are further divided into three groups iiia primary pseudosecondary vapor rich iiib primary pseudosecondary liquid rich and iiic secondary liquid rich data from fluid inclusion distribution microthermometry and raman spectroscopy indicate that fluids associated with au mineralized quartz veins stages 1 2 and 4 have moderate salinity ranging from 0 to 12 wt percent nacl equiv modeled salinities around 7 8 wt nacl equiv these veins formatted at temperatures from 325 to 375 c and pressures of 200 to 300 mpa with emplacement depths of about 8 to 12 km fluids associated with barren quartz veins stages 3 5 and 6 have a low salinity of about 0 to 3 wt percent nacl equiv modeled salinities about 2 3 wt nacl equiv and lower temperatures there is evidence of fluid immiscibility in all vein stages and mixing between fluids of the same souce but differing fo2 may also have influenced gold deposition cox et al 1995 hydrothermal ore forming fluids responsible for gold mineralization are in the ch2 co2 h2o nacl system whereas those of massive barren veins stage 3 and veinlets stages 5 and 6 are in the co2 h2o nacl and h2o nacl systems respectively the carbonic phases of type i and ii fluid inclusions range from pure ch4 through mixed ch4 and co2 to pure co2 gold ore forming fluids were weakly acidic with f o2 estimated to have been 10 25 to 10 37 bars and generally above the co2 ch4 and below the so2 h2s buffer boundaries throughout gold mineralization the relatively reduced mineralizing fluids are similar to those of other turbidite hosted lode gold deposits and counterparts in volcanic plutonic terranes

continent ocean transition on the voring plateau ne atlantic derived from densely sampled ocean bottom seismometer data 1 a 180 km long seismic wide angle profile was acquired across the voring plateau ne atlantic using a tuned air gun array and three component ocean bottom seismometers deployed with 5 km spacing the seismic p wave data have been modeled by ray tracing inversion and the model has been constrained by s wave and gravity modeling the data and modeling have allowed us to depict the crustal structure and nature of the continent ocean transition cot in a classical volcanic margin case the p wave velocity near the top of the main crustal layer is estimated to 6 0 km s landward of the 25 km wide cot the seaward increase to 6 5 km s in the cot is conformable with heavily intruded continental crust within this zone farther seaward the velocity increase to 6 9 km s in the same layer suggests the presence of oceanic crust the abundant magmatism landward of and within the cot is primarily observed as extrusives forming wedges of seaward dipping reflectors and mafic lower crustal intrusions underplating the maximum thickness of the oceanic crust is measured to 23 5 km at the seaward termination of the cot

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

demerara plateau the structure and evolution of a transform passive margin summary the amazon cone experiment acquired two transects profile sd a across the demerara plateau part of the french guiana northeast brazil continental margin to better understand rift and transform style margin evolution profile a images an intermediate type margin formed as a result of trans tensional extension in this paper we describe the modelling of wide angle and multichannel seismic and gravity data from profile d to reveal whole crustal structure and features exhibiting transform characteristics combining these results with other studies in the region and comparing our results with young rift analogues we develop a model of along margin segmentation that explains the evolution of the west equatorial atlantic interpretation of the velocity depth model for profile d shows a 35 37 km thick continental crust which thins to 10 11 km over a distance of 320 km this thinning is accommodated in two regions the narrowest region associated with the ocean continent transition is interpreted to have formed by dextral shearing of the margin along major transform zones that accommodated the initial break up geometry of the central atlantic given the orientation of the margin relative to local fracture zone traces it is likely that the second region of thinning located 162 km landward of the ocean continent transition is a result of rifting suborthogonal to the profile orientation there is no evidence of rotated faulted blocks half graben structures or riftrelated magmatism manifest as either seaward dipping reflectors or as a high velocity region within the lower crust the demerara plateau is therefore interpreted as a margin segment comprising thinned continental crust bound to the north and south by transform type zones in which trans tensional extension is accommodated in contrast to profile a modelling suggests that the eastern margin exhibits a relatively broad region of crustal thinning associated with extension consistent with a rift type setting offshore unusually thin oceanic crust of 3 3 5 7 km thickness is identified which is consistent with similar observations further south in the absence of identifiable magnetic anomalies best estimates of the initial half spreading rate of 20 mm yr 1 suggest that the thin crust throughout the region is unlikely to be a result of ultra slow spreading but instead it is most likely due to a reduced magma supply at numerous long lived transform faults reflected by those presently offsetting the mid atlantic ridge in this equatorial setting

emplacement depth and carbon dioxide rich fluid inclusions in intrusion related gold deposits phanerozoic intrusion related gold deposits have many consistent characteristics such as a spatial temporal and geochemical association with moderately reduced predominantly ilmenite series i type intrusions the deposits exhibit a range of characteristics that vary over a wide range of emplacement depths 7 km deposits in shallow crustal settings 5 km have characteristics of mesothermal environments and are hosted by plutons containing sheeted veins greissen and disseminated gold fluid characteristics also vary with depth deposits in shallow environments contain high temperature 350 c immiscible brine 30 wt nacl equiv and low salinity 5 wt nacl equiv vapor that commonly contains carbon dioxide deposits in deeper environments contain abundant low salinity carbon dioxide rich aqueous fluids 10 wt nacl equiv that in some deposits are postdated by moderate to high salinity brines 10 40 wt nacl equiv these contrasting fluid types are interpreted to be magmatic in origin and are the result of the complex interplay between exsolution of different volatiles carbon dioxide water and chlorine from felsic magmas emplaced at different crustal levels experimental studies have shown that carbon dioxide will exsolve from felsic magmas at much higher pressures than water and chlorine due to its lower solubility in the melt thus deposits formed from felsic magmas in deeper environments will contain abundant carbon dioxide rich fluids subsequent fluids released from the magma are likely to be water rich and more saline felsic magmas in shallow environments will contain lower concentrations of carbon dioxide that will partition into the vapor phase during phase separation between brine and vapor

mechanism of thin skinned detachment in the amazon fan assessing the importance of fluid overpressure and hydrocarbon generation abstract since about 10 ma a wedge of clastic sediment up to 10 km in thickness has accumulated near the mouth of the amazon river forming the amazon fan thin skinned extensional and contractional structures root into a common basal detachment hundreds of kilometers long extensional faults have formed landward of the prograding delta slope and contractional folds and faults have formed seaward of it by plan view restoration we show that extension and contraction balance we therefore infer that slope instability has provided the driving force for thin skinned deformation on seismic data the detachment appears as a sharp surface within late cretaceous strata however there is no evidence at that level for soft layers such as evaporites or muds abnormal fluid pressure provides a more reasonable mechanism for the detachment at depths of several thousand meters deep wells have encountered fluid overpressures amongst the possible causes of abnormal fluid pressures are compaction and hydrocarbon generation although compaction may have contributed to fluid overpressures encountered in wells we argue that hydrocarbon generation provides the best explanation for detachment in cretaceous rocks which were old and lithified when it occurred in particular the broadly triangular area of detachment coincides with an inferred pod of cenomanian turonian source rock which is interpreted to have been in the gas window during detached deformation we have developed new techniques of sandbox modelling to investigate the geometries of fault patterns produced by fluid flow through porous media for the amazon fan we have been able to reproduce the structural pattern on the assumption that fluid overpressures existed over a broadly triangular area beneath the detachment

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

structural and geochemical controls on the development of turbidite hosted gold quartz vein deposits wattle gully mine central victoria australia gold quartz vein deposits in the wattle gully mine arefault related structures developed in a low grade metamorphosed ordovician quartz rich flysch sequence the major mineralization is associated with a high angle reverse fault wattle gully fault that has a maximum displacement of 40 m over 10 6 metric tons t of vein quartz averaging 10 5 g t au occur within a dilatant jog in this structure fold geometry has controlled fault geometry and resulted in the localization of the jog segment where the wattle gully fault traverses an east dipping fold limb to link two more steeply dipping bedding parallel fault sediments on adjacent west dipping fold limbs gold mineralization is also associated with several minor east clipping reverse faults and a bedding parallel fault whose development is related to flexural slip during fold tightening mineralization occurs within fault fill veins and extension veins adjacent to faults the geometry and internal structures of veins indicate that vein growth has been controlled by repeated fluctuations in fluid pressure and shear stress associated with fault valve behavior vein mineralogy is dominated by quartz with minor carbonates chlorite white mica albite and apatite arsenopyrite and pyrite are the major sulfide phases but sphalerite galena chalcopyrite and pyrrhotite are also present the highest au grades tend to be localised in vein systems that are in proximity to carbonaceous slates wall rock alteration is minor and restricted mainly to narrow zones adjacent to major vein systems sulfur isotope fractionation between galena and sphalerite indicates mineralization occurred at temperatures around 300 degrees c the delta 34 s compositions of pyrite and arsenopyrite are nearly uniform throughout the mine 1 5 2 8 but are generally not in equilibrium with the delta 34 s values of associated base metal sulfides 0 7 5 3 the delta 18 o values for vein quartz vary between 15 6 and 18 0 per mil and correspond to delta 18 o values