regional structural controls of gold mineralisation bendigo and castlemaine goldfields central victoria australia the bendigo and castlemaine goldfields are classic examples of structurally controlled orogenic gold deposits in the bendigo zone of central victoria se australia detailed mapping and biostratigraphic interpretation has led to a better understanding of the regional structural controls of this type of gold quartz mineralisation mineralised quartz veins are hosted by the castlemaine group an early to middle ordovician turbidite succession at least 3 000 m thick gold deposits are controlled by low displacement faults that are clustered into several belts the goldfields indicating a regional structural control the timing of mineralisation overlapped with that of the major period of deformation including folding cleavage development and regional faulting the bendigo and castlemaine goldfields are located in an area termed the whitelaw thrust sheet bounded by two unmineralised high displacement regional scale faults mapping has revealed an interrelationship between the regional scale faults regional structural style and goldfield location the goldfields lie immediately west of the boundary between the upper and lower portions of the thrust sheet and are characterised by symmetric folds with sub horizontal to synclinal enveloping surfaces relatively low co axial strains and moderate cleavage development the non gold bearing areas immediately east of each goldfield correspond with the lower part of the whitelaw thrust sheet and are characterised by higher non coaxial strains stronger cleavage and folds with wide west dipping limbs giving rise to easterly vergent sections and steeply west dipping enveloping surfaces that mineralisation was an integral part of the thin skinned style of deformation in the central bendigo zone is indicated by timing relationships and the interrelationship between local scale mineralised structures and regional scale features such as large displacement unmineralised faults regional variations in fold style and overall thrust sheet geometry the work supports previous models that suggest mineralised fluids were focussed along a linked system of deep seated faults the primary conduits may have been major regional scale intrazone faults which are inferred to sole into detachments near the base of the castlemaine group it is proposed that these structures linked with minor intrazone faults and then with networks of low displacement mineralised faults that were strongly controlled by folds the location of minor intrazone faults was probably controlled by internal thrust sheet geometry the distribution of gold deposits and of gold production suggests that maximum fluid flow was concentrated along the eastern margins of networks of low displacement faults

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

a brittle failure mode plot defining conditions for high flux flow stress and fluid pressure conditions for the initial formation within intact rock of faults shear fractures extension fractures and extensional shears and for the reshear of existing faults may all be represented on a generic failure plot of differential stress 1 3 versus least effective compressive stress 3 3 pf scaled to nominal tensile strength t half the cohesive strength plots of this kind may be used to define maximum sustainable overpressure in different tectonic environments and the structural conditions under which the flow of large fluid volumes may occur through fault fracture meshes containing gaping extension and extensional shear fractures they may also be adapted to different tectonic regimes and correlated to depth for particular fluid pressure conditions high flux flow through distributed fault fracture meshes requires the tensile overpressure condition 3 3 which can only be achieved in the absence of throughgoing cohesionless faults that are well oriented for frictional reactivation in the prevailing stress field high flux flow through distributed fault fracture meshes intrinsically a transient pulsing phenomenon may therefore occur as follows 1 in effectively intact low pemeability crust devoid of throughgoing favorably oriented faults 2 where existing faults have become severely misoriented in the prevailing stress field and 3 where existing faults have regained cohesive strength through hydrothermal cementation

near equatorial eddies in the north atlantic surveys of the large scale near equatorial eddies 400km across off the northeast coast of south america with expendable bathythermographs from ships of opportunity and aircraft indicate that a large eddy 6n 11n to the north and a southern eddy like feature 2n 6n probably are a permanent part of the circulation both features were observed during the periods of seasonal extremes march and september although during the latter period the structure of the eddies had enlarged and deepened indicating a pronounced seasonal variability

dynamics of continental accretion three dimensional dynamic computer models show how accretionary tectonic margins evolve from the initial plate collision state through a period of plate margin instability and then re establish a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones where one plate dives under another become congested when they try to accommodate buoyant exotic crust louis moresi et al describe new numerical models of continental accretion that follow the entire process from the initial collision state through a period of plate margin instability to the re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones become congested when they try to consume buoyant exotic crust the accretionary mountain belts orogens that form at these convergent plate margins have been the principal sites of lateral continental growth through earth s history modern examples of accretionary margins are the north american cordilleras and southwest pacific subduction zones the geologic record contains abundant accretionary orogens such as the tasmanides1 along the eastern margin of the supercontinent gondwana and the altaides which formed on the southern margin of laurasia2 in modern and ancient examples of long lived accretionary orogens the overriding plate is subjected to episodes of crustal extension and back arc basin development often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4 5 6 7 coincident with accretion of exotic crust here we present three dimensional dynamic models that show how accretionary margins evolve from the initial collision through a period of plate margin instability to re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region the complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction we find geological and geophysical evidence for this process in the tasmanides of eastern australia and infer that this is a recurrent and global phenomenon

teleconnections of the southern oscillation in the tropical atlantic sector in the osu coupled upper ocean atmosphere gcm abstract the oregon state university coupled upper ocean atmosphere gcm has been shown to qualitatively simulate the southern oscillation a composite analysis of the warm and cold events simulated in this 23 year integration has been performed during the low phase of the southern oscillation when warm anomalies occur in the eastern pacific the model simulates for the atlantic region during march may 1 a deficit of precipitation over the tropical south american continent 2 caribbean and gulf of mexico sea level pressure and sea surface temperature are in phase with the eastern pacific anomalies while those east of the nordeste region are out of phase and 3 northeast trade winds are anomalously weak and southwest trade winds are anomalously strong as inferred from surface current anomalies the anomalies in the oceanic processes are induced by perturbations in the atmospheric circulation over the atlantic and are coupled to changes in the walker circulation during the high phase of the simulate

oroclines in the tasmanides abstract oroclines are features of orogenic belts that exhibit curvature in plan on a map scale with the additional requirement that this curvature resulted from relative rotation of the limbs around a vertical or near vertical axis much debate has centred on contrasts between thin skinned and thick skinned systems and on the question of whether the structural fabric was imposed prior to or synchronous with the development of curvature leading to a variety of inconsistent terminology a simple hierarchy in which all map scale curved orogenic features which pass a test for rotation are termed oroclines as distinct from primary arcs and in which these are then subdivided by timing into progressive synchronous fabric and curvature and secondary pre existing fabric is then rotated and by mechanism into thin or thick skinned is applied to a suite of curved orogenic features in the tasmanides of eastern australia this analysis includes the first formal application of the orocline test palaeomagnetic or palaeocurrent directions plotted against structural strike to the tasmanides the neoproterozoic to early mesozoic tasmanides comprise four orogens each of which contains curved orogenic features which are highlighted by extensive aeromagnetic datasets the delamerian orogen developed on the gondwana margin exhibits two curved structures in the neoproterozoic to cambrian adelaide fold belt termed the fleurieu and nackara arcs previous structural interpretations have indicated that the nackara arc is non rotational and hence not oroclinal re examination of existing palaeomagnetic data including a positive palaeomagnetic orocline test shows the nackara arc to in fact be a hinge of the adelaide orocline curvature in the adelaide orocline was dominantly progressive and thin skinned but may have been influenced by secondary rotation of the pre existing partly underlying curnamona craton mega kinks in a buried delamerian arc volcanic chain revealed in aeromagnetic images follow the curvature of the nackara arc suggesting an accompanying thick skinned secondary oroclinal deformation further from the continental margin orogenic curvature in the palaeozoic lachlan orogen has only been widely recognised since the advent of tilt filtered aeromagnetic images and a model of oroclinal folding with two hinges riverina and tambo and strike slip offset during the silurian bindian orogeny has recently been proposed to explain perplexing repetitions of sequence and inversions of structural vergence and directions of sediment provenance palaeomagnetic data indicates rotation of elements within the proposed orocline and a preliminary study indicates a positive palaeomagnetic orocline test for the riverina hinge palaeocurrent data around the tambo hinge provide a positive if scattered orocline test an independently conceived numeric model of the roll back response to congestion of a subduction zone has generated an oroclinal structure which shares many features with the proposed lachlan orocline and deep crustal ambient noise seismic data confirms that the major curved and strike slip features persist throughout the crust the lachlan orocline appears to be secondary and thick skinned orogenic curvature in the largely covered thomson orogen occurs along its southern border with the lachlan orogen the putative olepoloko orocline and around its geometrically complex boundary with the mossman orogen in northern queensland the charters towers orocline rotation of the olepoloko structure cannot be confirmed and competing models for its formation leave open the question of whether this is a primary arc a thin skinned progressive orocline or a thick skinned secondary orocline palaeomagnetic data from the charters towers orocline are limited to two poles but a change in declination between the silurian pole and the devonian pole matches the apparent rotation of the orocline which appears to have been thick skinned and secondary in contrast to the other orogens consensus that at least part of the curvature in the palaeozoic to triassic new england orogen is oroclinal has been broad although not universal there remains substantial debate over the number of hinges from two to four and mechanism palaeomagnetic poles have previously been cited as evidence of rotation of blocks within the orocline but this paper presents the first formal palaeomagnetic orocline test which is positive for the manning and texas hinges a palaeocurrent orocline test of the manning hinge in younger rocks than the palaeomagnetic sample is negative constraining rotation in the southern manning hinge to the carboniferous before 322 ma while rotation in the northern texas hinge appears to be latest carboniferous or permian the existence of a hastings hinge is questionable but if real its rotation also appears to be younger than that of the manning hinge thick skinned secondary rotation of the hinges of the new england orocline appears to have been diachronous

a biogeochemical influence on the secular distribution of orogenic gold orogenic gold deposits are unevenly distributed in geologic time the richest period of orogenic gold deposit formation was the phanerozoic which followed a 1 b y period of relatively meager deposit generation during the proterozoic this proterozoic low gold period developed despite continent building episodes which should have been appropriate for orogenic gold genesis here a brief review of the changes in global geochemistry brought on by evolving interactions between plate tectonics and the biosphere is conducted before the consequences for gold uptake into sedimentary pyrite are discussed it is suggested that the extensive oxygenation of the deep oceans during the second great oxidation event over the period 635 to 510 ma produced conditions where gold was soluble in the deep oceans from that point on regions where bacterial sulfate reduction drove formation of sedimentary pyrite also destabilized this soluble gold promoting its uptake into pyrite and allowing formation of metallogenically enhanced sedimentary sequences these sediments would have become ideal source rocks for gold deposits during subsequent collisional tectonics and metamorphism this biogenic influence on source region enhancement may explain the association during the phanerozoic eon between some of the world s most gold productive regions and sedimentary sequences that contain pyritic carbonaceous rocks

gold quartz veins in metamorphic terranes and their bearing on the role of fluids in faulting gold quartz vein fields in metamorphic terranes such as greenstone belts provide evidence for the involvement of large volumes of fluids during faulting and may be products of seismic processes near the base of the seismogenic regime in the val d or district of the abitibi greenstone belt canada quartz tourmaline carbonate veins form a vein field 30 15 km in the hanging wall of a crustal scale fault zone which was the main channelway for upward migration of the deeply generated fluids the veins occur in small high angle reverse faults and in adjacent horizontal extensional fractures extending up to 75 m in intact rocks they have formed incrementally during active reverse faulting in response to crustal shortening at depths corresponding to those at the base of the seismogenic zone in actively deforming crust detailed structural and fluid inclusion studies provide evidence for generally lithostatic but fluctuating fluid pressures p of the order of 200 mpa and for cyclic stress reversals during vein formation and provide good support for the fault valve model a comparison of vein characteristics with standard earthquake rupture parameters suggests that each slip increment along veins in reverse faults was accompanied by a small earthquake 4 m 3 or less the large vein field thus represents both the extent of fluid dispersion in the hanging wall of a crustal scale channelway and the distribution of small earthquakes integrated over the lifetime of the hydrothermal system it is proposed that such small earthquakes along veins in reverse faults are related to large earthquakes m 6 nucleating near the base of the seismogenic regime along the nearby crustal scale fault either as aftershocks or as a precursory smarm

geologic framework and tectonic evolution in western victoria australia abstract the stawell zone is interpreted to be part of the eastern extension of the adelaide fold belt into victoria the cambro ordovician turbidite sequence of the stawell and glenelg zones rests on a pile of late proterozoic metavolcanic rocks rather than cambrian metavolcanic units that are characteristic of the lachlan fold belt early deformation events d 1 d 3 in the stawell zone pre date granite emplacement and were synchronous with regional fold forming events that accompany thrust movements along discrete detachment surfaces the thrust system follows a ne se trending strike with an e directed translation of the tectonic units the steep 60 predominantly w dipping thrusts represent high strain zones localised in relatively weak cambro ordovician quartz rich turbidites that are sandwiched along the boundaries of the late proterozoic metavolcanics overprinting the early thrust system are d 4 d 6 deformation events that include reverse strike slip and normal faults the grampians extensional basin overlying the late proterozoic to ordovician sequence records a significant change in the tectonic regime operating during the late silurian early devonian it has been subjected to a middle devonian compressional deformation event d 4 with the development of thrust and fold structures this deformation is also superimposed on the thrusted and folded d 1 to d 3 sequence to the east of the grampians half graben producing further thrusts a gradual change from a ne sw to a n s stress field produces oblique strike slip faults normal faults of probably early cretaceous age transect the entire sequence the proterozoic cambrian sequence has been affected by a low p high t mid greenschist facies regional metamorphic event peak metamorphic conditions have been inferred from metavolcanic rocks and mafic units derived from a strongly differentiated tholeiitic suite at stawell these have been calculated to be 1 7 0 7 2 kbar and 450 20 2 c the advective heat input necessary to create this low p high t metamorphic event is attributed to penetrative deformation and strain partitioning between the crust and the mantle lithosphere which in turn causes crustal thickening associated compression and the observed d 1 to d 3 structures on the surface

late quaternary productivity changes in the western equatorial atlantic evidence from 230th normalized carbonate and organic carbon accumulation rates paleoproductivity changes in the western equatorial atlantic have been estimated from carbonate and marine organic carbon accumulation for the last 180 000 yr from a ceara rise sediment core geob 1523 1 accumulation rates were calculated by normalizing to excess 230th activity paleoproductivity from carbonate accumulation was estimated by a correlation between carbonate and organic carbon fluxes derived from sediment traps deployed in oligotrophic waters results indicate minor productivity changes varying around 30 gc m 2 yr 1 for organic carbon derived estimations and around 45 gc m 2 yr 1 for carbonate derived estimations suggesting that the study area remained a low productivity region throughout the time period examined maxima in western atlantic paleoproductivity occurred during the warm substages of glacials and interglacials in contrast to the eastern tropical atlantic where maxima are recorded in the cold substages this contrast might be caused by a deepening of the thermocline and nutricline in the west connected with a synchronous shallowing in the east the carbonate preservation record was examined for the last 380 000 yr based on variations in percent planktonic foraminiferal fragments and percent coarse fraction only minor carbonate dissolution above 3300 m water depth is observed except for three main dissolution events at isotope stages 4 8 4 and 10 considerable carbonate loss during these intervals is attributed to a decreased production of north atlantic deep water which is associated with vertical expansion of southern source deep water

structural evolution of the orogenic gold deposits in central victoria australia the role of regional stress change and the tectonic regime abstract this review synthesizes the main characteristics of the orogenic gold deposits in western and central victoria australia and presents new insights into the relationships between orogen scale deformation and the 12 hard rock gold deposits that have each produced 30 t au we examine five of these large deposits stawell ballarat bendigo fosterville and costerfield from the start of the delamerian orogeny at ca 515 ma to the end of the tabberabberan orogeny at ca 370 ma the eastern continental margin of gondwanan australia experienced both accretionary growth and the development of the lachlan orocline central western victoria lies on the western edge of this oroclinal structure and is partly underlain by a proterozoic continental fragment the selwyn block all of the 30 t orogenic gold deposits in the region lie either on or to the west of the selwyn block from the delamerian orogeny onwards the deformation front progressively moved eastwards and the principal stress direction rotated clockwise from ne sw through e w and ultimately to n s so that the initial faulting and folding created pathways and traps for the subsequent mineralizing fluids through the brittle to brittle ductile crust thus the 455 440 ma benambran orogeny was the major mineralizing event at stawell and resulted in some early mineralization in the bendigo zone the 420 410 ma bindian orogeny gave rise to significant additional mineralization at ballarat bendigo and fosterville while the 385 370 ma tabberabberan orogeny drove mineralization at costerfield and a second bonanza event at fosterville the largest gold deposits formed where the fluid pathways either remained open or reopened in this rotating stress field allowing multiple pulses of gold rich fluids

seasonal cycle of velocity in the atlantic north equatorial countercurrent as measured by surface drifters current meters and ship drifts this report describes the general circulation and seasonal variation of currents in the equatorial atlantic concentrating on the north equatorial countercurrent necc using data collected as part of the seasonal response of the equatorial atlantic and programme francais ocean et climat dans l atlantique equatorial experiments plus historical ship drifts during 1983 1985 the lagrangian circulation was studied by launching and tracking 30 freely drifting drogued buoys in the north equatorial countercurrent and 23 in the south equatorial current sec in addition continuously recording current meters were moored for 20 months at depths from 20 to 300 m near the center of the necc at 6 n 28 w these measurements are the longest series ever obtained in this current and provide information about its seasonal and interannual variations and zonal meridional and vertical structure the seasonal cycle of the necc is very regular from year to year each year the necc starts up in may june it flows eastward across the atlantic with surface speeds of up to 143 cm s in the west extending down to 350 m at 28 w and flowing into both the guinea current and north equatorial current it disappears or reverses from about january june west of 18 w some near surface water of the necc is inferred to downwell and flow equatorward toward the equatorial undercurrent the sec flows into the north brazil current which during spring continues up the coast into the caribbean during fall however the whole north brazil current retroflects or turns back on itself between 45 and 50 w forming the western necc the retroflection establishes a meander pattern in the necc that slowly propagates westward during fall at a speed of 4 cm s the meanders have a displacement of about 300 km in latitude a wavelength of 900 km and meridional velocity fluctuations of up to 100 cm s the swift currents and time dependent meanders in the western necc cause a high eddy kinetic energy 2400 cm2 s2 equivalent to that of the energetic part of the gulf stream

aspects of the reconstruction of the stress history of a fold and thrust belt rhenish massif federal republic of germany abstract by use of faults and striations parts of the deviatoric stress tensors and thereby essential aspects of the paleostress history of the rhenish massif can be reconstructed the stress history is unexpectedly complex even on the local scale the region under investigation is subdivided into various areas each of which exhibits somewhat different sequences of stress states several mutual aspects are evident in lower devonian rocks in the north which show only very slight deformation a pre folding stress state is preserved with a preferentially n s trending extension attributed to the stage of basin development the stage of folding itself during the carboniferous shows strong regional and temporal heterogeneity under frequently altering stress states after an initial stage of a semi homogeneous stress field this development occurs simultaneously with partial stress uncoupling at larger thrust zones with diverging deformation histories of hangingwalls and footwalls movement patterns in the north evidence subsequent clockwise rotation of the stress field which is only weakly represented in the south on the other hand there is clear evidence of a still younger counterclockwise rotation of the stress field which only affects the southern massif this is probably due to the superpositioning of orthogonal relative movements on the southern rim of the fold belt by a dextral strike slip component during final deformation thus indicating late dextral transpression succeeded by dextral transtension the final extensional stage is related to late paleozoic uplift and crustal stretching perpendicular to the strike of the fold belt followed by a change of direction of extension this state of stress obviously extends into the present although complex the various stress conditions are clearly correlated with other fabric elements in particular with cleavage fold geometry and mineralization

structural analysis of faults related to a heterogeneous stress history reconstruction of a dismembered gold deposit stawell western lachlan fold belt australia in the internationally significant victorian goldfields a complex system of faults dismembers the 5 million ounce magdala gold deposit these faults represent a combination of neoformed faults and inherited faults that reflect deformation associated with stress tensors of variable orientation and stress shape ratio the fault geometry is strongly controlled by the pre existing rheology faults have propagated around the flanks of an antiformal basalt dome along earlier ductile cleavages and the margins of porphyry dykes many of the faults do not have andersonian geometries and there is no correlation between the orientation of the faults and the palaeostress directions much of the faulting is associated with the emplacement of porphyry dykes additional gold mineralisation related to plutonism and late stage deformation post dating the intrusion of the stawell pluton systematic mapping of extension veins associated with faults striations and conjugate joint sets allowed the construction of a revised and more robust history of brittle deformation this successfully predicted the offset direction of the currently mined magdala ore body beneath the studied system of faults the use of extension veins was a critical aspect of the analysis if striations on the fault surfaces had solely been used the offset direction of the new golden gift orebody would not have been correctly ascertained the palaeostress history was delineated via use of compression and tension dihedra stress inversion of slip data and calculation of theoretical resolved shear stress for faults with orientations similar to those mapped the calculation of theoretical resolved shear stress directions highlights the importance that the intermediate stress has on the slip direction for faults whose pole does not lie in the plane containing 1 and 3

comparison of eddies off the north brazilian and somali coasts abstract repeated temperature sections using xbts from ships of opportunity along the sea lane off the brazilian coast indicate that a region of mesoscale occurs in which the eddies are particularly well developed in northern summer and fall these are located between approximately 3 10 n are on the order of 200 to 400 km across and may extend to 300 to 400 m in depth relatively fresh surface water apparently from the region of the amazon outflow is often observed in the eddy region similar eddies both in size and latitude are found off the somali coast these are associated with the turning offshore of all or part of the strong northward flowing somali current it is speculated that a somewhat similar circulation may also occur in northern summer in the pacific ocean

on the seasonal variability and eddies in the north brazil current insights from model intercomparison experiments the time dependent circulation of the north brazil current is studied with three numerical ocean circulation models which differ by the vertical coordinate used to formulate the primitive equations the models are driven with the same surface boundary conditions and their horizontal grid resolution isotropic 1 3 at the equator is in principle fine enough to permit the generation of mesoscale eddies our analysis of the mean seasonal currents concludes that the volume transport of the north brazil current nbc at the equator is principally determined by the strength of the meridional overturning and suggests that the return path of the global thermohaline circulation is concentrated in the nbc models which simulate a realistic overturning at 24 n of the order of 16 18 sv also simulate a realistic nbc transport of nearly 35 sv comparable to estimates deduced from the most recent observations in all models the major part of this inflow of warm waters from the south atlantic recirculates in the zonal equatorial current system but the models also agree on the existence of a permanent coastal mean flow to the north west from the equator into the carribean sea in the form of a continuous current or a succession of eddies important differences are found between models in their representation of the eddy field the reasons invoked are the use of different subgrid scale parameterisations and differences in stability of the nbc retroflection loop because of differences in the representation of the effect of bottom friction according to the vertical coordinate that is used finally even if differences noticed between models in the details of the seasonal mean circulation and water mass properties could be explained by differences in the eddy field nonetheless the major characteristics mean seasonal currents volume and heat transports appears to be at first order driven by the strength of the thermohaline circulation

metatholeiites and interflow sediments from the cambrian heathcote greenstone belt australia sources for gold mineralization in victoria metatholeiites and interflow sediments of cambrian age are exposed in the lake cooper quarry located in the northern segment of the heathcote greenstone belt central victoria australia the exposed sequence is 281 m thick and comprises nine massive to pillowed lava flows and seven interflow sequences the metabasalts from the lake cooper quarry have strong petrological and geochemical similarities to normal midocean ridge basalts n morb and to metabasalts from greenstone suites elsewhere in victoria suggesting that they have not been exposed to extensive hydrothermal alteration the interflow sediments in the quarry are subdivided into units comprising finely bedded sulfides layers of sulfidic carbonaceous argillites chert and sedimentary carbonate rocks unusual mineral assemblages in these sediments as well as their trace element concentrations and rare earth element signatures suggest an exhalative volcanogenic origin whereas gold values in the tholeiites from the lake cooper quarry generally remain below 2 to 5 ppb gold contents in the interflow sediments which are also characterized by elevated as cu and zn values range from 5 to 243 ppb and average 67 ppb on the basis of sedimentological features and sulfide petrology accumulation of the ore metals in these sediments is inferred to have occurred during deposition of the exhalative precipitates possibly as a result of extraction and transfer of ore metals from the thick boninite sequence known to occur below the tholeiites in victoria mass balance calculations show that remobilization of gold from the interflow sediments by metamorphic fluids and subsequent redeposition within dilatation zones and structural traps at higher crustal levels during the early to middle paleozoic could account for a significant component of the central victorian gold province thus these exhalative metasediments might have played a vital role in accumulating preconcentrating and providing the gold required to form the world class turbidite hosted lode deposits in the lachlan fold belt of victoria

the relative roles of folding and faulting in controlling gold mineralization along the deborah anticline bendigo victoria australia gold mineralization along the deborah anticline bendigo australia occurs in quartz veins that are categorized into two main groups those associated with folds and those associated with faults the relative chronology of the structural development and quartz vein emplacement is characterized by the initial development of the fold and the subsequent formation of bedding and cleavage parallel veins saddle reefs may have developed at this stage reverse faults and their associated veins formed after folds had locked up major gold hosting structures include saddle reefs reverse faults and composite fold fault structures where reverse faults have disrupted saddle reefs some reverse faults are interpreted to have propagated along preexisting structures such as bedding parallel veins although gold is hosted in fold related structures the majority is found in veins associated with later faults folds and fold related veins strongly controlled the location of these later formed reverse faults and their associated veins a fault based fracture system was superimposed on preexisting dilational sites such as bedding parallel veins and saddle reefs and it is possible that this allowed for the introduction of a larger volume of fluids during faulting than was possible during folding

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

mineral precipitation in the quartz reefs of the bendigo gold deposit victoria australia gold precipitation at bendigo postdates cleavage development and was initiated during folding increased during a reverse faulting stage in bedding parallel quartz veins and culminated in quartz reefs associated with a strike slip faulting event some gold was remobilized along carbonate rich cataclasites associated with later brittle faults understanding the paragenesis of the sulfide and gangue minerals associated with the auriferous quartz reefs is critical to unraveling the cycle of ore genesis from source to sink in this study a petrographic approach closely tied to macroscopic structural observations along with stable isotope and fluid inclusion analyses places constraints on the relative timing of gold mineralization with respect to deformation early sulfide assemblages in bedding and cleavage parallel veins are dominated by pyrite pyrrhotite siderite and are free of visible gold later assemblages in reactivated bedding parallel veins and other fold related veins are characterized by the presence of arsenopyrite ankerite gold fault related veins and the massive quartz reefs are rich in sphalerite and galena with associated gold but lack pyrrhotite small amounts of late stage antimony bearing minerals occur in many vein types and postdate the precipitation of gold decreasing temperature and increasing sulfur activity controlled successively younger sulfide assemblages in the quartz veins stable oxygen isotope data show that quartz from all vein types 18 o 15 9 19 0 homogenized with the host rocks whereas carbon 13 c 14 0 to 4 1 and oxygen 18 o 4 5 24 0 in carbonate have a wider range in values that is interpreted to be a function of decreasing temperature this corresponds with early siderite pyrrhotite and anhedral pyrite and with later ankerite and ferroan dolomite associated with arsenopyrite galena and euhedral pyrite that are crosscut by calcite veins fluid inclusions in quartz veins are predominantly composed of water with carbon dioxide with smaller proportions of nitrogen and methane particularly in the later strike slip faults the sporadic occurrence of methane suggests that there was an open fluid system with incoming fluids from an external source mixing with those in the host rocks although several recent studies have argued that synsedimentary preenrichment may be a significant factor determining the size and distribution of gold in the bendigo orogenic gold deposits we believe there is little direct or deposit scale evidence for a relationship between original metal content in the host rock metasedimentary rocks and the distribution of the gold mineralization in specific structural sites it is suggested that deep seated faults act as conduits for fluid flow and the source for the gold at bendigo the gold is externally derived from deeply sourced auriferous metamorphic fluids that have been focused into discrete structural sites

synchronous compression and extension in east gondwana tectonic controls on world class gold deposits at 440 ma the collision of a seamount or microcontinental block with an arc may produce complex strain distributions and changes in the tectono magmatic setting slab rollback and associated upwelling of asthenosphere occurred ca 440 ma near the margin of east gondwana in response to subduction zone lock up following arc collision ca 455 ma world class porphyry copper gold deposits e g cadia formed in the backarc during extension and magmatism associated with slab rollback upwelling of the asthenosphere also provided the heat engine for generation of hydrothermal fluids the result was elevated fluid pressures and compressional brittle deformation associated with orogenic gold mineralization e g bendigo inboard of the east gondwana margin

vertical structure of the seasonal cycle in the central equatorial atlantic ocean xbt sections from 1980 to 1988 abstract a set of temperature profiles from expendable bathythermographs collected from 1980 to april 1988 along two ship routes transecting the equatorial atlantic from 11 n to 11 s is analyzed to inter the vertical structure of the annual variability of the temperature and the currents in the upper ocean during the average seasonal cycle the vertical isotherm displacements occur earlier below 300 meters than near the surface at most locations within 4 degrees of the equator at the equator the amplitude of the displacements does not decrease with depth in the upper 500 meters this still holds down to 700 meters but there are less data at these depths the lead of the deeper isotherm displacements with respect to those in the upper thermocline implies that there is a contribution to the pressure forces from these layers that is not in phase with the contribution of the upper thermocline this also suggests that the energy source of the seasonal variability is close to the surface dynamic height and

the structural setting and contact metamorphism of the wonga gold deposit victoria australia gold quartz vein deposits in the wonga mine area are fault related structures occur in a low grade regionally metamorphosed cambrian quartz rich turbidite sequence wonga schist interlayered with mafic volcanics and postdate a suite of quartz feldspar porphyry dikes the nonplanar geometry of these vein deposits can be explained by their location within a shear system as en echelon dilations in combination with refraction across rheological interfaces within the sediment package the gold mineralization and related deformation occurred under low to midgreenschist facies conditions the intrusion of the early devonian stawell granite at approximately 400 ma has contact metamorphosed the wonga schist a preexisting suite of porphyry dikes and the first stage of mineralization typically mineralization and alteration occur in discrete dilational openings as a package of mineralized structures related to a d 4 deformation event and fluid injection the first stage of gold mineralization and accompanying alteration was deposited in 350 degrees trending 25 degrees to 50 degrees east dipping fracture sets within a 320 degrees trending shear system eastern and western shears and a 240 degrees trending 40 degrees to 70 degrees southeast dipping extensional fracture set link structures both the eastern and western shears are anastomosed high strain zones up to 50 m in width and are connected at a high angle by linking mineralized structures that begin to dominate as the main mineralized lodes persist with depth where the mineralized link has an east west trend the lodes located within the eastern and western shear zones are oblique shear veins whereas the link structures are large scale tensile fractures in the block between the zones of shear zone hosted mineralization these d 4 features initiated in a regional northeast southwest crustal shortening episode subsequent deformation was then accommodated by sinistral oblique slip during a north south compressional event the mineralized lodes are locally influenced by a set of northwest trending shallow northeast dipping faults and refracted across the preexisting porphyry dikes postdating the stawell granite is a second stage of hydrothermal fluid introduction along some northwest trending northeast dipping faults d 5 event this fluid flow is focused along the reverse faults and gave rise to local retrogression of peak contact metamorphic assemblages producing local carbonate sericite alteration and further gold mineralization subsequent fluid activity has produced carbonate alteration and or minor sulfide mineralization that can be related to either the stawell granite late fractures or a suite of lamprophyre dikes

regional dome evolution and its control on ore grade distribution insights from 3d implicit modelling of the navachab gold deposit namibia abstract we introduce a novel approach to analyse and assess the structural framework of ore deposits that fully integrates 3d implicit modelling in data rich environments with field observations we apply this approach to the early palaeozoic navachab gold deposit which is located in the damara orogenic belt namibia compared to traditional modelling methods 3d implicit modelling reduces user based modelling bias by generating open or closed surfaces from geochemical lithological or structural data without manual digitisation and linkage of sections or level plans instead a mathematically defined spatial interpolation is used to generate 3d models that show trends and patterns that are embedded in large drillhole datasets in our 3d implicit model of the navachab gold deposit distinctive high grade mineralisation trends were identified and directly related to structures observed in the field the 3d implicit model and field data suggest that auriferous semi massive sulphide ore shoots formed near the inflection line of the steep limb of a regional scale dome where shear strain reached peak values during fold amplification this setting generated efficient conduits and traps for hydrothermal fluids and associated mineralisation that led to the formation of the main ore shoots in the deposit both bedding parallel and highly discordant sets of auriferous quartz sulphide veins are interpreted to have formed during the later lock up stage of the regional scale dome additionally pegmatite dykes crosscut and remobilise gold mineralisation at the deposit scale and appear to be related to a younger joint set we propose that kilometre scale active folding is an important deformation mechanism that influences the spatial distribution and orientation of mineralisation in ore deposits by forming structures traps and pathways for fluids at different preferred sites and orientations we also propose that areas that experience high shear strain located along the inflection lines of folds can act as preferred sites for syn deformational hydrothermal mineralisation and should be targeted for regional scale exploration in fold and thrust belts our research also suggests that examination of existing drillhole datasets using 3d implicit modelling is a powerful tool for spatial analysis of mineralisation patterns when combined with fieldwork this approach has the potential to improve structural understanding of a variety of ore deposits

thrust related strain gradients and thrusting mechanisms in a chevron folded sequence southeastern australia an e directed imbricate fan thrust system within a chevron folded monotonous quartz rich turbidite succession shows marked gradients in strain from the upper to lower parts of individual thrust sheets upper parts of thrust sheets show low to moderate strains xz strains 30 1 tight inclined chevron folds ila 30 with predominantly w dipping axial surfaces intense phyllitic cleavage with a strong down dip mineral stretching lineation and long curved tapering quartz pressure shadows on pyrite such differences in fold geometry the intensity of the cleavage and the geometry of quartz fibres in pressure shadows require thrust sheet emplacement to be synchronous with the internal deformation of sheets thrust sheets show a change from an approximately constant volume plane strain coaxial deformation in their upper parts to non coaxial high strain deformation with incremental i between those expected for pure shear and simple shear i e 0 i 5 45 in their basal parts observed geometries and intensities reflect partitioning of strain and deformation mechanisms within individual thrust sheet volumes both during the initiation and propagation of subsurface detachment faults and with their emplacement to shallower crustal levels strain softening related to chevron folding and more intense cleavage development within a 500 m basal zone has controlled the emplacement of these turbidite dominated thrust sheets transport and internal deformation of the thrust sheets did not involve gravitationally induced spreading but required either a push from the rear or some form of underthrusting

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

high precision u pb zircon chronostratigraphy of the mount read volcanic belt in western tasmania australia implications for vhms deposit formation the middle cambrian mount read volcanics of western tasmania australia host several world class volcanic hosted massive sulfide vhms deposits representing a wide range of deposit styles although the deposits and their host sequences are variably deformed and locally preserve spectacular examples of primary textures and structures rapid lateral and vertical facies changes and faults with uncertain sense and magnitude of displacement have made it impossible to correlate stratigraphy across the belt previous dating studies in the area have yielded relatively imprecise crystallization ages we have employed the chemical abrasion isotope dilution thermal ionization mass spectroscopy id tims u pb zircon method to obtain highly precise crystallization ages for a total of 18 samples of volcanic and intrusive rock units from throughout the mount read volcanics and underlying mafic ultramafic complexes the new data permit detailed resolution of age relationships within the belt the study establishes an age of 516 0 0 9 ma for the mcivor hill gabbro which is part of a mafic ultramafic complex interpreted to underlie the mount read volcanics magmatism in the central mount read volcanics lasted at least 12 7 m y from 506 8 1 0 ma for a massive dacite unit in the lower part of the central volcanic complex to 496 0 0 9 ma for a welded ignimbrite in the lower tyndall group together with previous age constraints results of the study provide a precise chronostratigraphic framework for magmatism and vhms deposit formation within the mount read volcanics the precise age data indicate that north of the henty fault magmatism occurred in three discrete pulses at least two of which were separated by periods of sedimentation we demonstrate that vhms deposits in the mount lyell roseberry hercules and que hellyer districts comprising the majority of the known significant vhms deposits in the belt formed within a narrow time interval at 500 1 ma at a relatively late stage in the evolution of the belt some of the larger intrusions in the belt e g the bonds range porphyry 500 4 0 8 ma were emplaced contemporaneously with vhms deposit formation however other bodies such as the murchison granite 497 3 0 9 ma are younger than the deposits and are unlikely to have been involved in their genesis as has previously been suggested these conclusions will aid explorers in targeting vhms mineralization in the mount read volcanics

application of mineral equilibria modeling to constrain t and x co2 conditions during the evolution of the magdala gold deposit stawell victoria australia mineral equilibria modeling involving solid solution calculations has been combined with mineral assemblage information from the alteration zones associated with gold mineralization to determine the t and xco2 conditions for the formation of the magdala gold deposit at stawell victoria australia economic gold mineralization is primarily hosted within the stilpnomelane alteration zone of the stawell facies that is adjacent to the magdala basalt evolution of the magdala gold deposit involved at least three fluid infiltration events 1 a co2 bearing fluid during the d2 deformation event produced carbonate spots throughout the chlorite zone 2 a co2 s k bearing fluid accompanied the d3 4ab deformation and produced a muscovite zone and siderite rims on ankerite and 3 a co2 k s au bearing fluid during the d4c deformation event produced the stilpnomelane zone of the stawell facies the proximal and distal alteration zones within the magdala basalt and the main economic gold mineralization mineral equilibria modeling constrains the temperature of formation of the magdala deposit to t 345 390 c at 3kbar substantially lower than indicated by other previous classical thermobarometry methods furthermore this method has allowed the characterization of the mineralizing fluid and constrained its composition to xco2 0 08 at 3kbar the timing and composition of the mineralizing fluids are similar to that of metamorphic fluid generated from devolatilization of a greenstone pile with peak of metamorphism occurring earlier and at deeper levels in the crust

fluid infiltration into fault zones chemical isotopic and mechanical effects fluid infiltration into fault zones and their deeper level counterparts brittle ductile shear zones is examined in diverse tectonic environments in the 2 7 ga abitibi greenstone belt major tectonic discontinuities with lateral extents of hundreds of kilometres initiated as listric normal faults accommodating rift extension and acted as sites for komatiite extrusion and locally intense metasomatism during reverse motion on the structures accommodating shortening of the belt these transcrustal faults were utilised as a conduit for the ascent of trondhjemitic magmas from the base of the crust and of alkaline magmas from the asthenosphere and for the discharge of thousands of cubic kilometres of hydrothermal fluids such fluids were characterised by 18o 6 2 d 50 20 13c 4 4 and temperatures of 270 to 450 c probably derived from devolatilisation of crustal rocks undergoing prograde metamorphism hydrothermal fluids were more radiogenic 87sr 86sr 0 7010 to 0 7040 and possessed higher than did contemporaneous mantle komatiites or tholeiites and thus carried a contribution from older sialic basement a provinciality of87sr 86sr and 13c is evident signifying that fault plumbing sampled lower crust which was heterogeneous at the scale of tens of kilometres mineralised faults possess enrichments of large ion lithophile lil lil elements including k rb ba cs b and co2 and rare elements such as au ag as sb se te bi and w fluids were characterised by xco2 0 1 neutral to slightly acidic ph low salinity 3 wt k na 0 1 they carried minor ch4 co and n2 and they underwent transient effervescence of co2 during decompression clastic sediments occupy graben developed at fault flexures the40ar 39ar release spectra indicate that fault rocks experienced episodic disturbance on time scales of hundreds of millions of years

water mass and coastal current variability near barbados west indies moored current meter observations complimented by conductivity temperature depth ctd profiles and shipboard acoustic doppler current profiler measurements have revealed two distinctly different coastal circulation patterns south of the island of barbados west indies in the springs of 1990 and 1991 in the first year an abrupt increase in speed and change in direction of the currents south of the island was found to be associated with a large anticyclonic horizontally sheared circulation and a marked variation in temperature salinity characteristics the shift in currents was also accompanied by the incursion of a shallow pool of low salinity water a strong internal semidiurnal tidal signal and a change in the location of larval fish assemblages west of the island rapid variations in current speed and direction were also observed in 1991 though less persistent than in the previous year in this year the island was at one time completely surrounded by a large shallow pool of low salinity water a comparison was made of the 1990 hydrographic observations with ctd data from the national oceanographic data center archive for the region 0 20 n 40 60 w this suggests that the variation in hydrographic characteristics was due to the intrusion of a water mass of north brazil current origin which displaced the ambient north atlantic subtropical underwater around the island several recent studies have demonstrated that mesoscale anticyclonic eddies formed near the retroflection region of the north brazil current can be advected toward the eastern caribbean before being dissipated it is concluded that the most likely cause of the observed high energy event in 1990 was the passage of such an eddy past barbados

investigation of point source pollution by chlorinated solvents in two different geologies a multi layered carboniferous sandstone mudstone sequence and the chalk

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

structural constraints and numerical simulation of strain localization in the bendigo goldfield victoria australia strain localization and fluid flow patterns adjacent to the massive quartz hosted reefs of the bendigo gold field have been investigated using new structural observations three dimensional geometry of fold fault relationships and gold distribution quartz reef and vein formation in the bendigo goldfield is associated with an initial e w compression event that produced a penetrative upright cleavage and n s trending fold hinges in the immediate hanging wall of quartz reefs with high gold grades the s1 cleavage is locally deformed by an e w trending s2 cleavage that is attributed to a low strain n s oriented compressive event further evidence of this low strain event is provided by gently s plunging quartz carbonate slickenline lineations on s1 cleavage surfaces and the reef margin surfaces and visible gold within the outer portions of the reef the reefs are commonly situated on or immediately to the south of a left flex sinistral change in strike of the axial surface of the folds and the flexure occurs close to an anticlinal culmination defined by the double plunging n s striking folds the folds become deformed with meter to decimeter wavelengths which generally affect the stiff units in a disharmonic style accompanying this is a reactivation of reef margins by oblique slip and the formation of dilational sites and tensional vein arrays that provide the conduits for fluid flow and gold deposition numerical models that best simulate the observed distribution of strain based upon the location of quartz reefs involve nw se oriented contraction with a significant sinistral strike slip component this contraction event has an overall n s sense orientation and is therefore consistent with deformation features in the immediate hanging wall of quartz reefs the models also demonstrate a geometrical dependence on the localization of strain and thus the potential localization of fluid flow volume and shear strain increase with the flex angle and variations in the magnitude of strain are also associated with variations in the strike angle of the flexure this analysis highlights that the modification of preexisting fold structures and quartz veins by a low strain event may enhance the localization of dilation sites within individual quartz reefs where there are subtle changes in the orientation of the reef margins such numerical modeling can also improve ranking of exploration targets based on fold geometry and provide a better understanding of fluid flow in hydrothermal mineral systems

propagation of the seasonal upwelling in the eastern equatorial atlantic abstract several mechanisms have been proposed to explain the coastal and equatorial upwelling in the eastern atlantic guinea gulf the most controversial is the mechanism of remote wind forcing in the western equatorial atlantic suggested by moore et al 1978 most of the possible explanations for the upwelling and their relative importance are discussed in view of recent observations detailed analysis of daily sea surface temperature sst collected at 16 coastal stations along the northern coast of the guinea gulf reveals that the upwelling event propagates westward along this coast at a mean speed of 0 7 m s 1 similar analysis of historical monthly mean sst data shows that the coastal upwelling event propagates poleward from 1 s to at least 13 s at the same phase speed furthermore the northern hemisphere and southern hemisphere coastal upwelling signals seem to start at the same time from the equator the same kind of analysis applied to hydrographic data from a station situated 41 km off abid

crustal scale fluid pathways and source rocks in the victorian gold province australia insights from deep seismic reflection profiles deep seismic reflection data collected across the western lachlan orogen of southeast australia have provided important insights into crustal scale fluid pathways and possible source rocks in one of the richest orogenic gold provinces in the world the profiles span three of the most productive structural zones in victoria the stawell bendigo and melbourne zones zone scale variations in the age and style of gold deposits correspond with differences in crustal structure and composition the bilateral distribution of gold production in the stawell and bendigo zones is related to the v shaped crustal scale geometry of the two zones in cross section major first order faults like the east dipping moyston fault and a set of west dipping listric faults within the bendigo zone were probably major fluid conduits in the lower to middle crust during gold mineralization first order faults in the stawell and bendigo structural zones appear to have accommodated large scale thickening down to the lower crust the faults converge in a region beneath the western bendigo zone where a thick section of mafic volcanic rocks and lesser sedimentary rocks are identified as a likely common source of gold bearing fluid that was largely generated by late ordovician to early silurian metamorphism the areas with the greatest gold endowment lie above lower crustal regions that have preserved the thickest succession of fertile mafic igneous rocks up to about 25 km thick they also correspond to a region of thin or absent precambrian lithosphere mafic rocks in the stawell zone and far western bendigo zone were probably partly consumed by cambrian west dipping subduction similar rocks in the rest of the bendigo zone lay outside the influence of cambrian subduction accretion and were deformed later probably beginning in the late ordovician early silurian benambran orogeny during crustal scale imbrication this imbrication preserved much of the mafic rocks to form the lower to middle crust first order listric faults in the bendigo zone are interpreted as major controls on the locations of goldfields even though they are largely unmineralized near the present surface the shallow dipping segments of first order listric faults were favorably oriented for reactivation at the time of gold mineralization and acted as major fluid conduits in the lower to middle crust in contrast the upper steeply dipping segments of first order listric faults were unfavorably oriented for reactivation and were poor fluid conduits the seismic data show that the transition from predominantly shallow to steeply dipping fault segments occurs in the middle to upper crust near the boundary between thick imbricated metavolcanic rocks that lie immediately below 6 to 15 km of folded metasedimentary rocks this transition may have coincided with fluid escape zones that aided the transfer of permeability away from first order faults and into the overlying fold dominated turbidites this transfer of permeability was enhanced by the growth of subvertical fold related fault and fracture meshes in the upper crustal turbidites the fault and fracture meshes consisted of bedding parallel faults limb thrusts and tension vein arrays that developed along fold hinges in the bendigo zone individual fold hinges and regional fold culminations were important controls on the distribution of fluid flow fluid flow was partly syndeformational but overlapped into the immediate postdeformational period of the benambran orogeny later reactivation of first order faults in the late silurian to early devonian and again in the late devonian led to further although less important mineralizing events as fluids exploited the preexisting fault architecture deeply penetrating north dipping listric faults in the less gold rich melbourne zone cut into inferred proterozoic basement and may have been fluid conduits during a late devonian mineralizing event

re os dating of sulfides associated with gold mineralization in central victoria australia we report re os isotope ages that are the first for sulfides closely associated with gold mineralization at the bendigo and maldon goldfields in central victoria two arsenopyrite samples from bendigo contain 11 ppb re and 0 08 ppb highly radiogenic os llhr samples stein et al 2000 and have model ages of 444 7 and 447 7 ma with a mean model age of 446 5 ma a five point isochron based on these arsenopyrite analyses plus three pyrite analyses including one replicate gives an age of 438 6 ma molybdenite associated with au te cu and sb mineralization at maldon gives an re os age of 376 2 ma similar to the age of the nearby har court granite these results support previous 40 ar 39 ar data indicating that initial emplacement of gold mineralization in central victoria occurred in the late ordovician during the earliest stages of accretionary development of the southern lachlan fold belt the molybdenite data are also consistent with the mobilization and or introduction of gold during late devonian magmatism

metabasalts as sources of metals in orogenic gold deposits although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits the mobility of au and related elements as sb se and hg from these rocks during alteration and metamorphism is poorly constrained we investigate the effects of increasing metamorphic grade on the concentrations of au and related elements in a suite of metabasaltic rocks from the otago and alpine schists new zealand the metabasaltic rocks in the otago and alpine schists are of morb and wpb affinity and are interpreted to be fragments accreted from subducting oceanic crust gold concentrations are systematically lower in the higher metamorphic grade rocks average au concentrations vary little between sub greenschist 0 9 0 5 ppb and upper greenschist facies 1 0 0 5 ppb but decrease significantly in amphibolite facies samples 0 21 0 07 ppb the amount of au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in otago arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element the concentrations of sb and hg decrease between sub greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered morb protoliths and therefore unaltered oceanic crust cannot be a net source of sb and hg in a metamorphic environment the concentrations of au as sb and hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism we suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit

late structural control of mesothermal vein hosted gold deposits in central victoria australia mineralization mechanisms and exploration potential abstract comparison of the structural relationships between five gold deposits in ordovician rocks of the central victorian slate belt reveals a similar history and structural control of gold mineralization large quartz veins spatially associated with gold mineralization such as the famous saddle reefs of bendigo formed during the second deformation earlier than the gold mineralization itself where they were overprinted by the effects of the fourth deformation they provided structural sites for gold deposition on their boundaries with country rock where brecciation jostling and fracture were favoured access of fluids resulting from metamorphism and deformation to the brecciated fractured and jostled blocks of country rock created conditions suitable for the precipitation of gold through a sequence of alteration replacement and infill the dominant structural relationships that resulted in the generation of these sites were a the high angle of the axial plane of the fourth deformation to earlier structures and b the presence during mineralization of large pre existing quartz veins that acted as structural discontinuities on which brecciation could occur these factors allow a predictive capacity for exploration in the region

on the north brazilian eddy field abstract the region of large eddies occurring off the coast of brazil between approximately 3 n 10 n is revealed in some detail by repeated temperature observations on the along shore sea lane by ships of opportunity augmented by one airborne xbt expendable bathythermograph survey the general pattern of the circulation implied by the temperature structure is similar to that indicated by past hydrographic measurements and drifter tracks in general a large anticyclonic eddy to the north 7 n 10 n as well as a southern eddy like feature 3 n 6 n appear to be permanent patterns of the western north equatorial circulation this pattern somewhat resembles that found in the somali basin during the southwest monsoon

estimates of the zonal slope and seasonal transport of the atlantic north equatorial countercurrent we use data from six inverted echo sounder moorings and the geosat satellite altimeter to examine the seasonal variability of sea surface elevation monthly sea level maps are constructed using a contemporaneous subsurface temperature survey to provide a reference sea level field the maps are then used to describe the origin and structure of the western tropical atlantic north equatorial countercurrent necc during a 2 year period beginning in november 1987 the data reveal a zonal current which is confined between 3 n and 9 n with a typical width of 300 km the necc flows strongly eastward during november and december 1986 and may 1987 through january 1988 the reappearance of the current is then delayed until august but the current flows strongly from august until the end of our record in october 1988 volume transport is estimated for the 2 year period from surface elevation by approximating the vertical structure of the ocean as a two layer fluid we find that the necc has a maximum transport of 40 106 m3 s 1 at 38 w this water has its origin in the retroflection of the north brazil current eastward of 38 w the necc loses strength as water exits the current to join the currents further south

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event