the direction of fluid flow during contact metamorphism of siliceous carbonate rocks new data for the monzoni and predazzo aureoles northern italy and a global review periclase formed in siliceous dolomitic marbles during contact metamorphism in the monzoni and predazzo aureoles the dolomites northern italy by infiltration of the carbonate rocks by chemically reactive h2o rich fluids at 500 bar and 565 710 c the spatial distribution of periclase and oxygen isotope compositions is consistent with reactive fluid flow that was primarily vertical and upward in both aureoles with time integrated flux 5 000 and 300 mol fluid cm2 rock in the monzoni and predazzo aureoles respectively the new results for monzoni and predazzo are considered along with published studies of 13 other aureoles to draw general conclusions about the direction amount and controls on the geometry of reactive fluid flow during contact metamorphism of siliceous carbonate rocks flow in 12 aureoles was primarily vertically upward with and without a horizontal component directed away from the pluton fluid flow in two of the other three was primarily horizontal directed from the pluton into the aureole the direction of flow in the remaining aureole is uncertain earlier suggestions that fluid flow is often horizontal directed toward the pluton are likely explained by an erroneous assumption that widespread coexisting mineral reactants and products represent arrested prograde decarbonation reactions with the exception of three samples from one aureole time integrated fluid flux was in the range 102 104 mol cm2 both the amount and direction of fluid flow are consistent with hydrodynamic models of contact metamorphism the orientation of bedding and lithologic contacts appears to be the principal control over whether fluid flow was either primarily vertical or horizontal other pre metamorphic structures including dikes faults fold hinges and fracture zones served to channel fluid flow as well

quartz vein fabrics coupled to elevated fluid pressures in the stawell gold deposit south eastern australia shear and extensional veins formed during the reactivation of the magdala shear system at stawell in western victoria australia contribute to the formation of the auriferous central and basalt contact lodes within this shear system is a range of fault rocks accompanied by steep dipping 65 quartz rich laminated shear veins and relatively flat lying extensional veins both vein sets appear to have been a primary source for the host rock permeability during fluid flow in a regime of significant deviatoric stresses the macro and microstructures suggest that the dilatancy that produced mineralized veins formed under conditions of overpressure generated by fluid infiltration late in a tectonic regime a new microfabric analysis technique is used to investigate the quartz rich veins which allows rapid integration of the microstructure with the crystallographic preferred orientations cpos both the shear and extensional quartz veins have a random cpo with 120 dihedral angles between the quartz quartz grains which is typical of a metamorphic equilibrium microfabric the microstructures indicate that the quartz has undergone extensive grain adjustment in the solid state with grain shape and size affected by interfacial solution pressure solution effects these features are consistent with inferences from experimental rock deformation studies where grain boundary migration is enhanced in a water rich environment the onset of solution transfer processes pressure solution developed as the quartz microfabric stabilized and continued to modify the cpo and microstructure significantly it is concluded that grain growth and pressure solution are coupled diffusive mass transfer processes related to fluctuations in pore fluid pressures in a region undergoing deformation at near lithostatic pressures

quaternary history of the antarctic circumpolar current evidence from the scotia sea within the scotia sea the axis of the antarctic circumpolar current acc is geographically confined and sediments therefore contain a record of palaeo flow speed uncomplicated by acc axis migration we outline holocene and last glacial maximum lgm current controlled sedimentation using data from 3 5 khz profiles cores and current meter moorings geophysical surveys show areas of erosion and deposition controlled by neogene basement topography deposition occurs in mounded sediment drifts or flatter areas where 500 1000 m of sediment overlies acoustic basement 3 5 khz profiles show parallel continuous sub bottom reflectors with highest sedimentation rates in the centre of the drifts and reflectors converging towards marginal zones of non deposition locally on the flanks of continental blocks e g south georgia downslope processes are dominant the absence of mudwaves on the sediment drifts may result from the unsteadiness of acc flow a core transect from the acc axis south to the boundary with the weddell gyre shows a southward decrease in biogenic content controlled by the polar front and the spring sea ice edge both these features lay farther north at lgm the cores have been dated by relative abundance of the radiolarian cycladophora davisiana and by changes in the biogenic ba content a palaeoproductivity indicator sedimentation rates range from 3 to 17 cm ka the grain size of holocene sediments shows a coarsening trend from south to north consistent with strongest bottom current flow near the acc axis though interpretation is complicated by the presence of biogenic grains year long current meter records indicate mean speeds from 7 cm s in the south to 12 cm s in the north with benthic storm frequency increasing northwards lgm sediments are predominantly terrigenous and show a clearer northward coarsening trend with well sorted silts in the northern scotia sea assuming a constant terrigenous source this implies stronger acc flow at the lgm contrasting with weaker weddell gyre flow deduced from earlier work

mixing properties of phengitic micas and revised garnet phengite thermobarometers mixing properties for muscovite celadonite ferroceladonite solid solutions are derived from combining available experimental phase equilibrium data with tabulated thermodynamic data for mineral end members when a partially ordered solution model is assumed the enthalpy of mixing among the end members muscovite celadonite ferroceladonite is nearly ideal although the gibbs energies of muscovite celadonite and muscovite ferroceladonite solutions are asymmetric due to an asymmetry in the entropy of mixing thermodynamic consistency is achieved for data on phengite compositions inassemblages with a pyrope kyanite quartz coesite b almandine kyanite quartz coesite c talc kyanite quartz coesite and d garnet phengite pairs equilibrated both experimentally at high temperatures and natural pairs from low grade schists the muscovite paragonite solvus has been reanalysed using the asymmetric van laar model and the effects of the phengite substitution into muscovite have been quantitatively addressed in order to complete the simple thermodynamic mixing model for the solid solution among the mica end members results are applied to a natural pyrope coesite phengite talc rock from the western alps and to investigate the conditions under which biotite bearing mica schists transform to whiteschist like biotite absent assemblages for average pelite bulk compositions

late quaternary changes in antarctic bottom water velocity inferred from sediment grain size in the northern weddell sea abstract newly formed bottom water 0 7 c in the northern weddell sea flows e or ne at speeds up to 10 15 cm s with velocity decreasing towards the centre of the weddell gyre preliminary results from long term current meter moorings upper quaternary sediments from this area contain a fine grained terrigenous component from the nepheloid layer plus biogenic silica mostly diatoms with a small amount of ice rafted debris in cores from between 61 and 66 s and from 3300 m to 4700 m water depth the proportion of biogenic silica increases northwards corresponding to increasing seasonal extent of open water vs sea ice cover and the proportion of silt and well sorted fine sand in surface sediments increases with average current velocity downcore diatom rich and diatom poor sediments alternate on a scale of 1 3 m and intervals with more diatoms contain a higher proportion of silt to clay preliminary stratigraphy suggests the cyclicity in composition and texture is related to glacial interglacial cyclicity during warm periods indirectly correlated with isotope stages 1 3 5 etc biogenic silica production takes place during several months of each year and silt and fine sand are transported by bottom currents during glacial periods with greater sea ice cover than at present biogenic productivity was suppressed and no silica was preserved in the sediments in addition a lower proportion of terrigenous silt implies that bottom currents were weaker at sites with a present day average velocity of 10 cm s a last glacial maximum average velocity of 1 cm s or less is inferred from grain size measurements

tectonothermal history of the western lachlan fold belt australia insights from white mica studies detailed b lattice parameter and illite crystallinity ic studies of k white micas in slates from the stawell and ballarat bendigo zones sz bbz in the western lachlan fold belt of victoria australia reveal a metamorphic pattern characterized by regional metamorphism associated with crustal thickening and younger contact metamorphism accompanied by deformation the ic data indicate that rocks regionally metamorphosed prior to the intrusion of the early and late devonian granitoids vary in grade from epizonal greenschist facies to diagenetic zeolite facies and that most are of epizonal to anchizonal prehnite pumpellyite facies grade in the bbz a decrease in grade from west to east occurs across fault zones ic values show little change indicating that limited vertical displacement has occurred this is in accord with the thin skinned deformation model proposed for the western lachlan fold belt the b lattice parameters x 9 022 a n 137 s n 0 009 indicate baric conditions intermediate between those of new hampshire p al 2 sio 5 triple point and otago intermediate p thus a moderately low geother mal gradient existed 450 430 ma ago when these rocks were deformed k d fe mg actinolite fe mg chlorite values 0 52 0 70 obtained from coexisting actinolite and chlorite in metabasites from fault zones support the moderately high p c 4 kbar metamorphism suggested by the b cell parameter values the metamorphic conditions indicated by these data are contrary to the low p high t conditions pro posed by previous authors who inferred an intimate association between deformation granitoid intrusion and gold mineralization the b lattice parameter of white micas in slates adjacent to early devonian c 400 ma granitoids with schist bearing aureoles in the north eastern part of the bbz x 9 002 a n 27 s n 0 007 indicate pressures in the order of c 2 5 kbar which are in accord with those obtained from andalusite cordierite and zoisite garnet bearing assemblages observed in the higher grade metapelitic and calcareous rocks this contrasts with the higher pressure c 4 kbar existing during regional metamor phism and implies that c 6 5 8 km of metasedimentary rocks in the bbz were removed before the emplacement of the early devonian granitoids metamorphic assemblages in hornfelses associated with late devonian granitoids indicate a further 5 6 km of metasediment were removed in the next 40 ma prior to their emplacement this study shows the value of white mica studies in elucidating the tectono thermal history of a low grade metamorphic terrane dominated by metapelitic rocks

the magdala lode system stawell southeastern australia structural style and relationship to gold mineralization across the western lachlan fold belt gold mineralization in the magdala deposit at stawell is hosted within deformed turbidites and the underlying mafic volcanic successions this setting is atypical when compared to the slate hosted gold deposits elsewhere in the lachlan orogen the deposit is located at a regional plunge reversal culmination and gold lodes have developed on the western flank of a large doubly plunging basalt dome the magdala antiform during northeast southwest and east west shortening the irregular shape of the dome and the contacts between different stratigraphic packages are the key controls on the location of the gold lodes early deformation is defined by bedding parallel mica fabrics s 1 that are folded by a series of upright fold closures f 2 these upright folds are syn to postpeak regional metamorphism and are overprinted by another folding event associated with a shallow dipping crenulation cleavage s 3 gold mineralization postdates all of the ductile deformation and occurred after the formation of faults associated with large laminated and massive quartz veins that are barren of gold in the turbidites the mineralized structures are defined by laminated quartz veins with arsenopyrite and pyrite within the volcanic units gold lodes are sulfide rich mainly pyrite pyrrhotite and arsenopyrite reflecting the primary sulfide rich stratigraphy later gold lodes at stawell developed as a result of the reactivation of the system during sinistral oblique slip field and geochronological data suggest deformation initiated in the stawell region and migrated to the east resulting in dramatically different structural evolution and or complexity in the stawell and bendigo ballarat zones despite this the absolute age of major gold mineralization is the same in both zones

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

episodic gold mineralisation correlated with discrete structural events at ballarat east southeast australia abstract new 40 ar 39 ar geochronological data suggest orogenic gold mineralisation at the ballarat east deposit southeast australia occurred in three main episodes at ca 445 435 ma ca 420 415 ma and ca 380 370 ma the gold mineralisation is localised in muscovite bearing quartz and quartz carbonate veins hosted in the steep faults 70 90 on limbs of tight and isoclinal folds in an ordovician turbidite sequence and within west dipping 45 faults historically known as leather jacket lodes initiation of the 45 faults that are confined to fold culminations begins at ca 445 ma with peak metamorphic conditions at 440 ma the earliest vein sets v 1 were emplaced on limb thrusts at ca 445 435 ma and are characterised by arsenopyrite dominated quartz veins these v 1 veins parallel arsenopyrite rich shale units historically referred to as indicator beds both the steep and 45 faults were reactivated during fold amplification with deposition of the v 2 auriferous veins at ca 420 415 ma a later set of auriferous veins v 3 v 4 with ages of 380 370 ma dominated by pyrite sphalerite galena white mica quartz v 3 or carbonate rich v 4 veins are predominantly associated with reactivation of the 45 west dipping faults this new geochronological data constrains the local kinematic history of the ballarat east deposit and has regional implications the v 1 v 2 vein development appears to be synchronous across the entire western section of the lachlan orogen where previous studies have suggested that initial gold mineralisation was linked to orogenesis at 440 ma as a result of metamorphic devolatilisation reactions in the lower crust in contrast a close spatial and temporal relationship exists between the felsic dykes and the mineralisation recognised in the v 3 v 4 veins the deformation that accompanies v 3 v 4 vein development is attributed to small localised events during east west shortening utilising pre existing fold and fault structures the origin of the fluids producing the v 3 v 4 veins may be metamorphic devolatilisation associated with widespread felsic magmatism that occurred at this time across central victoria

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

ice shelf history from petrographic and foraminiferal evidence northeast antarctic peninsula a detailed record of late pleistocene deglaciation followed by mid holocene ice shelf breakup and late holocene re growth is contained in continental shelf sediments in the northern larsen area northeast antarctic peninsula the zero age of core tops is confirmed by new and published 210pb profiles and 70 accelerator mass spectrometer ams 14c dates on bulk organic carbon define sedimentation rates of 7 6 92 cm ka the varied geology in the local ice drainage basins facilitates the use of ice rafted debris ird provenance in determining the presence or absence of ice shelves all inshore cores contain an interval of non local ird in the post glacial section demonstrating widespread ice shelf breakup in the mid holocene both breakup and re growth may have taken centuries and there are no widespread debris layers associated with breakup cores beyond and up to 30 km inside the historical ice shelf limit exhibit a varied ird provenance throughout the holocene suggesting the maximum ice shelf limit may date only from the little ice age benthic foraminiferal assemblages are related to water masses and position on the continental shelf and have been modified by taphonomic processes nevertheless we discern a deglaciation signal in prince gustav channel of a calcareous spike in predominantly agglutinated assemblages and this is repeated at the time of mid holocene ice shelf breakup the inferred mid holocene warm period occurred later in the northern larsen area than on the west coast of the antarctic peninsula

fluid wall rock interaction in an archean hydrothermal gold deposit a thermodynamic model for the hunt mine kambalda gold mineralization at the hunt mine kambalda is situated in a 20 m thick steeply dipping shear zone within a sequence of rather uniform metabasalts zonal alteration of the host rocks occurred around fractures which acted as conduits during gold mineralization limited major element mobility ensured assemblages of low variance over most of the alteration zone providing a suitable environment for the application of thermodynamics in conjunction with detailed petrographic geochemical electron microprobe analysis of mineral compositions and fluid inclusion studies microthermometry equilibrium equations involving fluid components and alteration minerals are used to determine conditions during the alteration event and in particular activity gradients of components profiles across the schist zone show increasing f sub h 2 s around auriferous quartz veins log f sub h 2 s 0 6 but constant f sub o 2 log f sub o 2 29 7 these results are near the magnetite pyrrhotite pyrite triple point for the inferred temperature of 350 degrees c and explain the pyrite pyrrhotite magnetite zonation sequence outward from veins the profile of x sub co 2 is very close to the value of 0 25 determined independently from fluid inclusions the k super h super profile shows increases adjacent to auriferous veins corresponding to the zone of biotite stability using a value for fluid salinity of 2 equiv wt percent nacl ph was calculated to be around 6 9 the solubility of gold in solution was estimated for the calculated conditions for the complex au hs super 2 the maximum solubility is around 0 5 mg kg super 1 i e 0 5 ppm for au cl super 2 it is close to 10 super 6 mg kg super 1 indicating the efficacy of sulfide transport of gold compared to chloride transport under these conditions the calculated decrease in f sub h 2 s away from the auriferous quartz veins results in an 80 percent or greater decrease in gold solubility suggesting hat the sulfidation of fe rich wall rocks is a geologically reasonable and very efficient mechanism for precipitating gold from solution at elevated temperatures

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

using holo hilbert spectral analysis to quantify the modulation of dansgaard oeschger events by obliquity abstract astronomical forcing obliquity and precession has been thought to modulate dansgaard oeschger do events yet the detailed quantification of such modulations has not been examined in this study we apply the novel holo hilbert spectral analysis hhsa to five polar ice core records quantifying astronomical forcing s time varying amplitude modulation of do events and identifying the preferred obliquity phases for large amplitude modulations the unique advantages of hhsa over the widely used windowed fourier spectral analysis for quantifying astronomical forcing s nonlinear modulations of do events is first demonstrated with a synthetic data that closely resembles do events recorded in greenland ice cores ngrip grip and gisp2 cores on gicc05 modelext timescale the analysis of paleoclimatic proxies show that statistically significantly more frequent do events with larger amplitude modulation in the greenland region tend to occur in the decreasing phase of obliquity especially from its mean value to its minimum value in the eastern antarctic although statistically significantly more do events tend to occur in the decreasing obliquity phase in general the preferred phase of obliquity for large amplitude modulation on do events is a segment of the increasing phase near the maximum obliquity implying that the physical mechanisms of do events may be different for the two polar regions additionally by using cross spectrum and magnitude squared analyses greenland do mode at a timescale of about 1400 years leads the antarctic do mode at the same timescale by about 1000 years

wall rock petrology and geochemistry in alteration halos associated with mesothermal gold mineralization central victoria australia integrated petrological and geochemical examination of metasedimentary assemblages associated with mesothermal lode gold mineralization in five major centers of past and present production ballarat percydale tarnagulla maldon fosterville in central victoria australia reveal that hydrothermal alteration in the hosting cambro ordovician turbidites is much more pronounced and extensive than has previously been recognized characteristically the auriferous reef structures are surrounded by bleached zones up to several tens of meters wide sericitization carbonatization and carbonate spotting and pyrite and arsenopyrite porphyroblasts within and beyond zones of visible bleaching are the most obvious features of alteration quantitative xrd analysis illustrates that these marked bleached halos are due to the replacement of albitized plagioclase clasts by white mica and the breakdown of metamorphic chlorite to form secondary carbonates and to a lesser extent kaolin the negative correlation between metamorphic chlorite and hydrothermal carbonate phases provides a reliable mineralogical alteration index useful to exploration geochemical reconnaissance profiles also indicate the development of systematic trends in response to hydrothermal alteration involving mainly sio2 k2o na2o co2 as au and s at constant bulk rock volume calculation of a geochemical alteration index given by the ratio k2o co2 k2o co2 na2o al2o3 suggests that this can be a reliable indicator of alteration in particular where concentrations of individual invariable oxides such as al2o3 vary too much in response to lithological variations saturation indices that are based on ratios of 3k al and co2 mg fe ca are less reliable because muscovite and carbonate are not always the dominant alteration mineral species of relevance the alteration assemblages as well as mineralogical and geochemical trends at the five ore systems investigated herein are common to many slate belt hosted gold deposits the results of this investigation show that although the detection of such systematic spatial variations relative to mineralization can be complicated for example by postore modification of the wall rocks due to contact metamorphism e g at maldon or where mineralization is not associated with massive to laminated quartz veins e g at fosterville recognition of extensive though not always conspicuous alteration halos around mesothermal lode gold mineralization represents a potentially powerful tool for exploration in slate belts

a mineral equilibria study of the hydrothermal alteration in mafic greenschist facies rocks at kalgoorlie western australia the influx of a h2o co2 dominated fluid into actinolite bearing metabasic rocks during greenschist facies metamorphism in the kalgoorlie area of western australia resulted in a zoned alteration halo around inferred fluid conduits that contain gold mineralisation the alteration halo is divided into two outer zones the chlorite zone and the carbonate zone and an inner pyrite zone adjacent to the inferred fluid conduits reaction between the fluid and the protolith resulted in the breakdown of actinolite and the development of chlorite dolomite calcite and siderite in addition rocks in the pyrite zone developed muscovite bearing assemblages as a consequence of the introduction of potassium by the fluid mineral equilibria calculations undertaken using the computer software thermocalc in the model system na2o cao k2o feo mgo al2o3 sio2 h2o co2 show that mineral assemblages in the outer zones of the alteration halo are consistent with equilibrium of the protoliths with a fluid of composition xco2 co2 co2 h2o 0 1 0 25 for temperatures of 315 320 c the inner zone of the alteration halo reflect equilibrium with a fluid of composition xco2 0 25 fluid rock buffering calculations show that the alteration halo is consistent with interaction with a single fluid composition and that the zoned structure of the halo reflects the volume of this fluid with which the rocks reacted this fluid is likely to have also been the one responsible for the gold mineralisation at kalgoorlie

evidence for externally derived vein forming and mineralising fluids an example from the magdala gold mine stawell victoria australia abstract mineralogical and microfabric studies of the magdala gold deposit together with isotopic and fluid inclusion studies have shown that the mineralising fluid is externally derived the host rocks to the quartz veins do not possess any significant amounts of gold whereas the quartz veins can contain 7 g t gold the walls to the quartz veins possess alteration haloes in the form of stilpnomelane porphyroblasts attesting to the fact that the fluid in the veins was out of equilibrium with the host rocks the distribution of alteration haloes that occur parallel to the quartz vein margins necessitates that a subvertical fluid gradient be more or less constant during the deposition process in order to produce the observed architecture of the alteration haloes quartz lodes possess 18 o values that range from 15 8 to 17 3 per mil while associated host rocks range between 9 7 to 12 per mil in the metabasalts and 10 2 to 14 1 per mil in the volcanogenic and metasedimentary rocks the dissimilarity of 18 o values between host rocks and quartz veins suggests that there has been a high fluid influx in the veins implying external buffering four types of fluid inclusions were distinguished on the basis of spatial arrangement and composition using the laser raman spectroscopy method the salinity of the fluid inclusions range from 0 7 to 8 4 wt nacl equivalent suggesting that gold was deposited due to the influx of different external fluids the association of pyrite arsenopyrite pyrrhotite with gold and with pges suggests that these fluids have a lower crustal origin and may have originated from the upper mantle and mixed with fluids derived from deep seated metasediments

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

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

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

determination of time integrated metamorphic fluid fluxes from the reaction progress of multivariant assemblages an expression is derived for the calculation of time integrated metamorphic fluid fluxes in two or more dimensions in rocks undergoing multivariant reactions under conditions of varying pressure temperature and angle of flow this calcuation requires knowledge of mineral assemblages modes and compositions which are obtained from isobaric t xco2 pseudosections constructed using the program thermocalc and compared with those observed in east central vermont thermocalc is capable of reproducing peak mineral assemblages modes compositions and the observed reaction sequences within the system kcanafmasch for two kyanite grade pelitic carbonate rocks from a barrovian style regional metamorphic terrain in east central vermont u s a calculation of fluid fluxes for decarbonation reactions under conditions of horizontal layer parallel flow produces time integrated fluid flux figures of the order of 108 moles m 2 allowance for possible cross layer flow from adjacent dehydrating pelites reduces this figure significantly with episodic cross layer fluxes of the order of 105 moles m 2 being capable of driving the observed decarbonation chlorite bearing carbonate protoliths would have initially dehydrated with increasing temperature a process requiring down temperature fluid flow to produce the assemblages currently observed

a sediment record of cyclic bottom current variability from the northwest weddell sea cores and 3 5 khz sub bottom profiles collected from the continental slope and rise near 64 s 52 w off the tip of the antarctic peninsula reveal transport and depositional processes in a high energy environment a large channel runs downslope from the shelf break the remaining slope area is smooth and shows no acoustic penetration except locally in the lee of topography sediments on the slope and rise were mainly deposited by suspension from a 150 to 300 m thick nepheloid layer periodic resuspension and winnowing by bottom currents has produced coarse lags of ice rafted debris throughout these deposits and has also produced an elongated field of mudwaves running parallel to the base of the slope these low amplitude asymmetric features were formed by currents flowing from west to east parallel to the local slope short gravity cores show a compositional and textural cyclicity developed between two distinct lithologies intervals of poorly sorted sediment barren of fauna and with mud dominated size distributions are interpreted to result from a cold climate and low bottom current strength production of weddell sea bottom water probably lessened during these times and there was a greater duration of sea ice cover these sediments are interbedded with winnowed and better sorted deposits comprising fine sands and silts that contain abundant planktonic foraminifera interpreted as implying warmer climatic conditions and stronger current activity a varied calcareous benthic foraminiferal assemblage also indicates that these intervals lay above the ccd during deposition renewed and invigorated production of bottom water is implied contemporaneous with a shorter duration of sea ice cover petrographic evidence indicates an antarctic peninsula source throughout the cycles are considered to be the result of climatic fluctuations occurring over the timespan of glacial interstadials although confirmation of this timescale awaits acquisition of an absolute chronology

chronology and tectonic framework of turbidite hosted gold deposits in the western lachlan fold belt victoria results abstract 40 ar 39 ar data constrain the history of metamorphism deformation and mineralization in the western subprovince of the lachlan fold belt rocks in the mount stavely volcanic complex and the bushy creek pluton give 40 ar 39 ar ages of about 500 ma for hornblende and biotite the dates from the mount stavely volcanic complex limit the age of mineralization in the metavolcanic rocks of the stavely belt whereas the date for the bushy creek pluton extends the area of cambrian ordovician plutons of the delamerian orogeny to east of the grampians mountains and helps define the surface expression of the western border of the paleozoic lachlan fold belt 40 ar 39 ar dating of metamorphic mica growth phengite in cleaved slates and phyllites within the landsborough fault avoca fault zone heathcote fault zone and mount wellington fault zone gives ages of 453 2 440 2 426 4 and 410 390 ma respectively these data indicate a progression of deformation from west to east starting in late ordovician early silurian time in the stawell and bendigo ballarat structural zones the major phase of deformation in the western subprovince of the lachlan fold belt is therefore silurian and not devonian sericites from major gold deposits in the stawell zone and bendigo ballarat zone give 40 ar 39 ar dates of about 440 ma that are coincident with folding and thrusting a second episode of mineralization and fault reactivation at about 420 to 410 ma is found in some deposits in these zones based on data from a previous study in the melbourne zone including the woods point dike swarm and locally in the bendigo ballarat zone some gold deposits give sericite dates of about 380 360 ma these ages are consistent with the close association of many melbourne zone gold deposits with devonian dikes and plutons the results indicate that gold mineralization in the stawell zone bendigo ballarat zone and melbourne zone was episodic and both temporally and spatially associated with crustal heating linked to regional metamorphism and plutonism initial stages of mineralization in the province comprising many of the large deposits in western and central victoria e g bendigo ballarat stawell wattle gully tarnagulla accompanied metamorphism folding and thrusting subsequent mineralization is widespread but produced mainly smaller deposits

the bronzewing lode gold deposit western australia p t x evidence for fluid immiscibility caused by cyclic decompression in gold bearing quartz veins abstract the bronzewing lode gold deposit is located in the yandal greenstone belt in the yilgarn craton of western australia gold mineralization is hosted in tholeiitic basalt that is metamorphosed to greenschist facies individual ore bodies are controlled by a complex gold bearing quartz vein system that comprises shear and extension veins formed during a progressive d2 deformation event gold is localized in quartz along fractures and deformed grain boundaries and is interpreted to have formed late in the formation of the veins detailed petrography and microthermometry on primary pseudosecondary and secondary fluid inclusions trapped in gold bearing shear and extension veins revealed five types of fluid inclusions 1 co2 ch4 h2o nacl inclusions of variable salinity 0 1 to 17 5 eq wt nacl containing between 10 and 99 mol co2 and molar volumes that range from 22 to 76 cm3 2 h2o nacl inclusions of variable salinity 0 4 to 22 1 eq wt nacl 3 co2 ch4 inclusions with up to 58 mol ch4 and molar volumes between 54 and 73 cm3 4 ch4 h2o inclusions with ch4 ranging from 80 to 90 mol and 5 ch4 inclusions with low molar volumes of 19 to 23 cm3 types 1 2 and 3 constitute a fluid inclusion assemblage that occurs consistently in primary pseudosecondary and secondary trails and clusters within the gold bearing quartz vein system these co existing fluids are interpreted to have formed by fluid immiscibility of a low salinity homogeneous parent fluid at about 300 c and 1500 bars locally type 2 fluid inclusions exhibit total homogenization temperatures that are on average 100 c less than the co genetically trapped type 1 aqueous carbonic inclusions this discrepancy is interpreted to have involved co2 effervescence in response to fluid pressure fluctuations types 4 and 5 fluid inclusions are rare and are only present locally in secondary trails and clusters in extension veins significant pressure fluctuations but relatively constant homogenization temperatures for types 1 2 and 3 fluid inclusions suggest that cyclic decompression of the hydrothermal fluids due to seismic activity along the shear zones that host the gold bearing veins triggered fluid immiscibility the process of fluid immiscibility and subsequent lowering of gold solubility is interpreted to be the most efficient precipitation mechanism for gold in the d2 shear zone hosted vein system at bronzewing

submillennial variations in ocean conditions during deglaciation based on diatom assemblages from the southwest atlantic we present a high resolution paleoceanographic record of deglaciation based on diatom assemblages from a core located just south of the polar front in the southwest atlantic core kc073 is from a sediment drift at the mouth of the falkland trough and contains sediments from the last glacial maximum lgm to present dated using radiocarbon dates on bulk organic matter and radiolarian stratigraphy the site lies along the path of the antarctic circumpolar current acc and immediately downstream of where north atlantic deep water nadw is entrained into the acc significant variations in ocean conditions are reflected in high amplitude changes in diatom concentrations and assemblage composition the diatom assemblage at the lgm indicates that winter sea ice extent was at least 5 farther north than present until at least 19 0 ka calendar years and summer sea ice may have occasionally extended over the site but for the most part it lay to the south during deglaciation chaetoceros resting spores crs dominate the diatom assemblage with valve concentrations in excess of 500 106 valves per gram submillennial scale variations in the numbers of crs and thalassiosira antarctica occur throughout the late deglacial and dominate the changes in diatom concentration we propose that the influx of crs is controlled by the flow of nadw over the falkland plateau as such our data provide unique evidence that nadw impacted on this sector of the southern ocean during deglaciation during the holocene the sedimentation rate dramatically reduced we suggest that the acc flow increased over the site and inhibited settling and winnowed the surface sediments

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

calibration of a point counting technique for estimation of biogenic silica in marine sediments the simple technique of point counting is very quick and is suitable for shipboard use or in any study where high accuracy is not necessary point counting smear slides yields results different from measurements of weight percent silica if there are systematic differences in shape and density of biogenic and terrigenous grains in this note i present calibration curves for synthetic biogenic terrigenous mixtures to show how the accuracy of point counting may be improved

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

costerfield antimony gold deposit southeast australia coupling between brittle deformation and dissolution precipitation reactions in the melbourne zone abstract the antimony gold deposits at costerfield are a swarm of dyke like antimony gold lodes within a brittle transpressional fault system they are hosted in the lower silurian sedimentary rocks of the melbourne zone of the lachlan orogen individual lodes are dominated by stibnite or stibnite gold and widths vary from 1 cm to 200 cm averaging 30 cm and lengths can exceed 1 km lodes occur as fault parallel layers or lenses mantled by deformed quartz rich margins or overgrowths of hydrothermal quartz and carbonate inclusions in the stibnite rich lodes include vuggy randomly oriented prismatic crystals crack sealed and stretched quartz with sugary mosaics of equant quartz grains and quartz breccias randomly oriented quartz and stibnite textures common in the centre of the lodes indicate free growth from a fluid throughout the mineralized system the hydrothermal stibnite has replaced and overgrown the earlier quartz deformed lode margins preserve sub horizontal lineations and evidence of oblique crack seal re opening that reflect emplacement associated with late sinistral strike slip movements in bounding faults crystallographic orientation data collected from the marginal quartz using neutron diffraction and supported by microstructural observations reveal that lode emplacement also involved dissolution precipitation creep and pressure solution both the marginal quartz and the lodes are overprinted by later microfractures within which there has been precipitation of stibnite pyrite arsenopyrite sphalerite and minor gold 3 d neutron tomography revealed individual gold crystals with gold voxel sizes ranging from 140 m to 580 m were located in late arsenopyrite gold domains that overprint the stibnite the mechanical evolution of the lodes involved short lived brittle processes in combination with dissolution precipitation crystallization in active fault zones these ore bodies provide insights about the dynamics of fluid flow and flow velocities when faults breach over pressured reservoirs of hydrothermal fluid our results also suggest that the costerfield deposit was formed directly from metal rich metamorphic fluids produced during tabberabberan 375 ma prograde metamorphism of sedimentary rocks near the base of the lower paleozoic melbourne zone which lie above the proterozoic selwyn block from a reinterpretation of seismic reflection data the deformation in the melbourne zone sedimentary wedge is characterized by thrusts isolating relatively undeformed packages of sedimentary units which were moved in a piggyback manner up west dipping faults the ore bearing fluids migrated upwards at least in part along these footwall thrusts from the base of the heathcote fault zone in the upper levels of the sedimentary package out of sequence strike slip sub vertical splays from these second order faults carried the mineralizing fluids to the orebodies east west compression during the tabberabberan orogeny preceded a northwest southeast oriented stress field which reflects the collision of vandieland against the gondwanan margin this produced the sinistral strike slip faults which in turn controlled the overall permeability of the hydrothermal system

gold mineralization under cover in southeast australia a review of an exploration initiative for stawell type deposits abstract exploration for orogenic stawell type deposits in the northern extension of the stawell zone under the tertiary murray basin sediments in southeastern australia has produced the discovery of at least two new deposits at wildwood and kewell previous research on the stawell gold deposit has highlighted the importance of litho facies structural evolution and hydrothermal alteration in the localization of fluid flow at the time of mineralization application of geophysical tools including magnetic and gravity surveys at both regional and more detailed scales delineated a number of significant coincident magnetic and gravity anomalies which upon further investigation through aircore drilling proved to be basalt subsequent drilling on the margins of the wildwood and kewell basaltic domes delineated areas of prospective litho facies hydrothermal alteration quartz veins and mineralization akin to that found in the stawell deposit analysis of multi element geochemistry revealed significant gains in fe in the transition from unaltered to altered unmineralized 1 ppm au altered host rock magnetic remanence properties of the various lithologies within the stawell zone showed that high fluid flow can result in the precipitation of sulfides with fixed stable remanent magnetization the orientation of which may not be parallel to the present day magnetic field whereas zones of low fluid flow have soft magnetization that can be easily modified the implication of variation in remanence orientation for exploration is that regional aeromagnetic anomalies which have opposite polarities may coincide with zones of high fluid flow and are potential target areas for mineralization

isograds in internally buffered systems without solid solutions principles and examples internal buffering is recognised as an important end member process when considering the factors governing the composition of a fluid phase present in a rock during metamorphism in such processes the major isograd forming reactions take place at intersections in t xco2 space in p t space the trace of such intersections with changing pressure forms a univariant reaction along which the composition of the coexisting fluid phase changes in this paper it is demonstrated that the stoichiometry of these univariant reactions changes at singular points these singularities are controlled by divariant reactions which liberate co2 and or h2o and thus form degenerate univariant reactions in the complete chemical system a series of phase diagrams illustrating these points is presented for ultrabasic margarite kyanite clinozoisite and siliceous dolomite assemblages these diagrams provide a complete description of potential isograd forming reactions within each chemical system p t pseudosections are constructed for particular bulk compositions to illustrate the continuum between externally controlled and internally buffered conditions and the effects of changes in fluid and solid phases

the magdala lode system stawell southeastern australia structural style and relationship to gold mineralization to gold mineralization across the western lachlan fold belt

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

pliocene holocene contourite deposition under the antarctic circumpolar current western falkland trough south atlantic ocean the eastward flowing antarctic circumpolar current acc has influenced sedimentation on the slope and floor of the western falkland trough where the axis of the current is topographically constrained deep water flow below 3000 m has produced a symmetrical sediment drift on the trough floor with non depositional margins indicating higher current velocities at the base of slope to the southeast of the falkland islands there is a gap in the north scotia ridge north of which the floor of the trough is swept clean of sediment by the acc both echo character mapping and gloria side scan data indicate that currents follow the bathymetric contours along the slope redistributing sediment and locally eroding furrows from six cores on the drift and on the northern slope two styles of contourite deposition have been identified on the drift holocene biogenic sandy contourites overlie last glacial maximum muddy contourites and fine grained diatomaceous hemipelagites sedimentation rates here average 3 4 cm ka 1 the sandy contourites present in four of the cores from the sediment drift are sharply underlain by the finer grained diatomaceous hemipelagites the lack of a coarsening upward sequence commonly associated with an increase in current velocity may be indicative of high current activity eroding away the finer negative sequence pliocene and mid pleistocene glaucony rich sandy contourites containing radiolaria characterise the falkland plateau and the floor of the trough near the gap in the north scotia ridge we suggest that the glaucony is derived from a combination of authigenic formation and erosion of locally outcropping cretaceous and tertiary strata this is supported by dinoflagellate analysis sedimentation rates in these current swept areas average 1 cm ka 1

pore pressure during metamorphism of carbonate rock effect of volumetric properties of h 2 o co 2 mixtures the molar volume of mixtures of co2 and h2o is a strong function of the fluid composition both co2 and h2o participate in the metamorphism of carbonate rocks resulting in a change in the fluid composition during reaction one of the effects of the change in composition is the increase in pore fluid pressure with only small increases in extent of reaction pressure calculated from the volumetric properties of co2 h2o mixtures at 400 c increases greatly with small increases of but drops at greater values because of the increase in pore volume as a result of v solid the pore pressure increase at small values of though readily exceeds the reported tensile strength of carbonate rocks and the rock cannot sustain significant reaction without fracturing the result of a small amount of reaction is a fractured rock with increased permeability which promotes fluid transport

sedimentation on the continental rise west of the antarctic peninsula over the last three glacial cycles abstract the continental rise west of the antarctic peninsula includes a number of large sediment mounds interpreted as contourite drifts cores from six sediment drifts spanning some 650 km of the margin and 4 of latitude have been dated using chemical and isotopic tracers of palaeoproductivity and diatom biostratigraphy interglacial sedimentation rates range from 1 1 to 4 3 cm ka glacial sedimentation rates range from 1 8 to 13 5 cm ka and decrease from proximal to distal sites on each drift late quaternary sedimentation was cyclic with brown biogenic burrowed mud containing ice rafted debris ird in interglacials and grey barren laminated mud in glacials foraminiferal intervals occur in interglacial stages 5 and 7 but not in the holocene processes of terrigenous sediment supply during glacial stages differed meltwater plumes were more important in stages 2 4 turbidity currents and ice rafting in stage 6 the terrigenous component shows compositional changes along the margin more marked in glacials the major oxides al2o3 and k2o are higher in the southwest and cao and tio2 higher in the northeast there is more smectite among the clay minerals in the northeast magnetic susceptibility varies along and between drifts these changes reflect source variations along the margin interglacial sediments show less clear trends and their ird was derived from a wider area downslope processes were dominant in glacials but alongslope processes may have attained equal importance in interglacials the area contrasts with the east antarctic continental slope in the se weddell sea where ice rafting is the dominant process and where interglacial sedimentation rates are much higher than glacial the differences in glacial setting and margin physiography can account for these contrasts

phase equilibria in the system k 2 o feo mgo al 2 o 3 sio 2 h 2 o co 2 and the stability limit of stilpnomelane in metamorphosed precambrian iron formations the phase relations of al and fe bearing silicates in the system k2o feo mgo al2o3 sio2 h2o co2 in the presence of quartz and magnetite are discussed on the basis of mineralogic and petrologic data from precambrian iron formations and blueschist facies meta ironstone from the franciscan formation california these relations allow an estimation of the physiochemical conditions during low grade metamorphism of iron formations petrologic data together with available experimental and predicted thermodynamic data on the associated minerals place the upper stability limit of stilpnomelane in iron formations at about 430 470 c and 5 6 kilobars fe end member stilpnomelane can persist to a maximum temperature of 500 c and pressures up to 6 7 kilobars although it is unlikely to occur in metamorphosed iron formations in iron formation occurrences the stilpnomelane stability field is bordered by four equilibrium reactions with the assemblages stilpnomelane zussmanite chlorite minnesotaite stilpnomelane zussmanite chlorite grunerite stilpnomelane biotite chlorite grunerite and stilpnomelane biotite almandine grunerite the stability field is reduced by increasing x co2 and xmgstil and is also a function of a k a h in the metamorphic fluid if the value of a k a h is smaller than that defined by the above assemblages stilpnomelane decomposes to chlorite but if larger it is replaced by biotite at pressures less than 4 kilobars the zussmanite field is restricted to a very high value of a k a h 5 0 in log units at 1 0 kilobar where iron formation assemblages are not stable

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