pb isotope fingerprinting of mesothermal gold deposits from central victoria australia implications for ore genesis new pb isotope data from three major mesothermal lode gold deposits ballarat west tarnagulla maldon in central victoria support a model whereby the metals derived from a large reservoir with a long residence time in the crust below the palaeozoic lachlan fold belt the pb isotopic ratios of least radiogenic samples from these deposits are in close agreement with published pb signatures for turbidite hosted gold deposits and for devonian granites elsewhere in the lachlan fold belt despite their spatial distribution and variations in the geological setting the pb signatures point to the extraction and transport of metals from a crustal source area by long lasting large scale hydrothermal systems resulting in the prominent homogenisation of pb isotopic ratios the enduring interaction between large hydrothermal systems and an extensive crustal source reservoir were a vital pre requisite in the formation of the victorian gold province in this regard the prospectivity of victoria is analogous to world class ore provinces elsewhere such as the archaean yilgarn block in western australia

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

tectonic setting of synorogenic gold deposits of the pacific rim abstract more than 420 million oz of gold were concentrated in circum pacific synorogenic quartz lodes mainly during two periods of continental growth one along the gondwanan margin in the palaeozoic and the other in the northern pacific basin between 170 and 50 ma these ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary rock dominant terranes the auriferous veins contain only a few percent sulphide minerals have gold silver ratios typically greater than 1 1 show a distinct association with medium grade metamorphic rocks and may be associated with large scale fault zones ore fluids are consistently of low salinity and are co2 rich in the early and middle palaeozoic in the southern pacific basin a single immense turbidite sequence was added to the eastern margin of gondwanaland deformation of these rocks in southeastern australia was accompanied by deposition of at least 80 million oz of gold in the victorian sector of the lachlan fold belt mainly during the middle and late devonian lesser devonian gold accumulations characterized the more northerly parts of the gondwanan margin within the hodgkinson broken river and thomson fold belts additional lodes were emplaced in this flyschoid sequence in devonian or earlier palaeozoic times in what is now the buller terrane westland new zealand minor post devonian growth of gondwanaland included terrane collision and formation of gold bearing veins in the permian in australia s new england fold belt and in the jurassic early cretaceous in new zealand s otago schists collision and accretion of dozens of terranes for a 100 m y long period against the western margin of north america and eastern margin of eurasia led to widespread latest jurassic to eocene gold veining in the northern pacific basin in the former location late jurassic and early cretaceous veins and related placer deposits along the western margin of the sierra nevada batholith have yielded more than 100 million oz of gold additional significant ore forming events during the development of north america s cordilleran orogen included those in the klamath mountains region california in the late jurassic and early cretaceous the klondike district yukon by the early cretaceous the nome and fairbanks districts alaska and the bridge river district british columbia in the middle cretaceous and the juneau gold belt alaska in the eocene gold bearing veins deposited during the late jurassic and early cretaceous terrane collision that formed the present day russian far east have been the source for more than 130 million oz of placer gold the abundance of gold bearing quartz carbonate veins throughout the gondwanan north american and eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata the common temporal association between gold veining and magmatism around the pacific rim reflects these thermal episodes melting of the lower thickened crust during arc formation slab rollback and extensional tectonism and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initiation of the ore forming processes

reconstructing mid late holocene cyclone variability in the central pacific using sedimentary records from tahaa french polynesia abstract we lack an understanding of the geographic and temporal controls on south pacific cyclone activity overwash records from backbarrier salt marshes and coastal ponds have been used to reconstruct tropical cyclone strikes in the north atlantic basin however these specific backbarrier environments are scarce in the south pacific with cyclone records limited primarily to the period of modern observation this instrumental record suggests a correlation with the el nino southern oscillation enso but longer records are necessary to test this relationship over geologic timescales and explore other potential climate drivers of tropical cyclone variability deep lagoons behind coral reefs are widespread in the pacific and provide an alternative setting for developing long term sedimentary reconstructions of tropical cyclone occurrence coarse grained event deposits within the sediments of a back reef lagoon surrounding tahaa reveal a 5000 year record of cyclone occurrences timing of recent high energy deposits matches well with observed tropical cyclone strikes and indicates coarse deposits are storm derived longer records show tropical cyclone activity was higher from 5000 to 3800 and 2900 to 500 yrs bp comparison to records from the north pacific out of phase and north atlantic in phase suggests a coordinated pattern of storm activity across tropical cyclone basins over the mid late holocene the changes in tropical cyclone activity we observe in the south pacific and across other basins may be related to enso as well as precession driven changes in ocean atmosphere thermal gradients

deformation volume and cleavage development in metasedimentary rocks from the ballarat slate belt psammites from the ballarat slate belt in se australia exhibit well developed differentiated or spaced cleavage defined by alternating phyllosilicate and quartz rich domains termed p and q domains respectively strain estimates derived from independent microstructural and chemical observations suggest that the p q fabrics developed in response to plane strain deformation dominated by solution transfer with the principal finite shortening in the p domains approximately twice that in the adjacent q domains significant finite extensions are indicated by ubiquitous quartz albite chlorite overgrowths in both p and q domains while pressure shadow development around syntectonic pyrite porphyroblasts suggest finite extension of at least 100 this estimate is comparable with the extension predicted for constant volume deformation and consequently there appears to have been no significant bulk material loss or gain on the hand specimen scale these observations are consistent with solute transfer scales as little as a few centimetres via diffusion in a stationary fluid and do not require although do not necessarily preclude large scale advective fluid transport through the slate belt during cleavage formation as suggested in previous studies

lithologically controlled invisible gold yukon canada the newly discovered cretaceous coffee orogenic gold deposit 4 moz resource consists of an extensive oxidised zone developed on primary sulphidic rock the primary mineralised rock is characterised by invisible gold in arsenian pyrite that has replaced biotite in selected host rocks the deposit has a cryptic surface expression and is an example of an extremely subtle exploration target hydrothermal emplacement was controlled by extensional fractures with breccias but most mineralisation was focused on biotite bearing granitic gneiss metasedimentary gneisses and younger biotite granite fine grained 0 1 mm arsenian pyrite replaced biotite along mineral cleavage planes and followed biotite rich metamorphic and post metamorphic structural fabrics arsenian pyrite also formed overgrowths on earlier coarse grained up to 2 mm barren hydrothermal pyrite arsenian pyrite is concentrically zoned on the 1 10 m scale with respect to as sb and au contents and typically contains 5 wt as 500 mg kg sb and 500 mg kg au in solid solution biotite replacement was accompanied by sericitisation silicification and ankerite impregnation hydrothermal alteration involved dilution and localised depletion of k na and al in silicified host rocks but most ca mg and fe concentrations remained broadly constant magnesium rich ultramafic host rocks were only weakly mineralised with auriferous arsenian pyrite and have fuchsite and magnesite alteration near surface oxidation has liberated nanoparticulate and microparticulate supergene gold which remains essentially invisible varying degrees of oxidation extend as deep as 250 m below the present subdued topographic surface well beyond the present vadose zone and this deep oxidation may have occurred during post mineralisation uplift and erosion in the cretaceous oxidation has leached some as from the surficial mineralised rocks decreasing the geochemical signal which is also obscured by the localised presence of high background as up to 100 mg kg in metasedimentary quartzites in the region antimony provides more reliable soil anomalies than as but most sb anomalies are 100 mg kg the persistence of invisible gold into the extensive supergene zone with little gold particle size enhancement has ensured that no placer deposits have formed in nearby streams further restricting the surface footprint and au dispersal halo of this subtle exploration target

carlin gold deposits nevada origin in a deep zone of mixing between normally pressured and overpressured fluids gold mineralization at carlin is clearly younger than hydrocarbon maturation pre cretaceous and felsic dike intrusion cretaceous and older than deep oxidation late tertiary within the episode of gold mineralization the main gold ore mgo stage and late gold ore ego stage are distinguished paragenetically with a variety of vein and mineralization types in each mgo stage fluids contained 5 to 10 mole percent co 2 appreciable h 2 s and 3 or 1 wt percent nacl equiv at least portions of mgo stage mineralization were characterized by two phase boiling co 2 exsolution at 215 degrees or 30 degrees c and 800 or 400 bars in contrast ego stage fluids were gas poor with salinities 18 o sub h 2 o values of 5 to 9 per mil whereas ego stage fluids resembled unevolved meteoric water with delta 18 o sub h 2 o values 18 o values shifted from near whole rock values of 12 or 3 per mil to around 0 or 1 per mil as ego stage fluids flooded the system jasperoids also record a large range 9 22 in delta 18 o sub h 2 o values these data indicate the involvement of two very different fluids in ore deposition because mgo and ego stage features are closely associated spatially with each other and with au as sb hg and other ore elements both fluids are believed to have both been present during most stages of ore deposition at pressures of 80 to 85 percent lithostatic depths of 3 8 or 1 9 km are required to accommodate the 800 or 400 bars of pressure recorded in mgo stage fluid inclusions carlin therefore is not an epithermal or hot spring deposit carbon dioxide in gas rich mgo stage fluids may have originated either directly from buried intrusions or their contact aureoles or from low grade regional metamorphism at depth the water may have been originally meteoric and au may be magmatic or derived from leaching of deep metamorphic or sedimentary rocks ore deposition appears to have occurred in zones of throttling at a pressure seal between normally pressured and overpressured regimes where fluids experienced a change from near lithostatic to hydrostatic conditions such pressure seals are common in deep sedimentary basins and may be a key to highly localized gold deposition mixing of two fluids and interaction with host rocks along thin permeable bioclastic horizons are believed to have been the major factors in depositing ore

sulphur isotopic composition of sulphides from the beaver dam and other meguma group hosted gold deposits nova scotia implications for genetic models sulphur isotopic compositions were determined for sulphide mineral phases in meguma group hosted gold deposits nova scotia in order to resolve i potential source regions for sulphur in these de

supergene ore and hypogene nonore mineralization at the nagambie sediment hosted gold deposit victoria australia the nagambie gold deposit central victoria australia consists of approximately 7 million metric tons of ore averaging 1 2 g t gold mineralization is confined largely to east west and n 50 degrees e oriented shears and faults contained in a doubly plunging anticlinal dome of undifferentiated silurian and early devonian siltstones and shales ore is associated with an east west trending anticlinal axis and is particularly concentrated in two east northeast trending structural zones the central and 303 shear zones five stages of protore vein genesis have been identified the first two a b are barren crenulated massive quartz primary gold mineralization occurs in stibnite bearing quartz stockwork stringers which belong to the third c and fourth d quartz veining stages whereas the fifth consists of unmineralized small veins e fluid inclusion studies on the two gold mineralization stages c and d yield trapping temperatures and pressures of 280 degrees or 25 degrees c and 1 000 to 1 200 bars and 200 degrees or 30 degrees c and 700 to 1 000 bars respectively the mineralization fluids contain h 2 o co 2 0 69 mole ch 4 0 21 mole and trace n 2 with wt percent nacl equiv values in the range of 0 to 10 2 wt percent calculated delta 18 o values 15 0 6 8 and measured delta d data 88 to 99 of the hypogene fluids indicate a meteoric origin these fluids underwent extreme 18 o enrichment by reacting with country rocks metamorphic or igneous genesis of the ore as opposed to the protore is as secondary supergene enrichment accompanied by varying degrees of surficial limonite alteration ore mineralization occurs almost exclusively in limonitized fractures up to 60 m below the surface in and adjacent to major shears based on features such as the spatial distribution high purity 999 26 fine and the form of the gold and its close association with limonite it is proposed that the gold is mobilized by surficial leaching and concentrated near the water table vertical movement of the water table perhaps due to seasonal changes as well as erosion produces rapid dissolution of hypogene gold and or aurostibite from above or at the water table

a review of the mesothermal gold deposits of the meguma group nova scotia canada abstract the gold province of the meguma terrane in nova scotia contains several classic examples of turbidite hosted mesothermal gold deposits there are over 300 gold occurrences and deposits hosted by the cambro ordovician meguma group rocks in southern nova scotia canada the meguma group consists of two formations 1 a lower sand dominated flysch goldenville formation and 2 an upper shaley flysch halifax formation subdivision of the goldenville formation into several map units is leading to a better understanding of the stratigraphic constraints on the gold mineralization although detailed stratigraphic investigations in all of the major gold producing areas is still ongoing it is evident that the steve s road beaverbank and the mt uniacke stratigraphic intervals contain most of the major past gold producers in the province the meguma group rocks are variably deformed into upright gently to moderately doubly plunging folds with multiple cleavages the strata are metamorphosed from greenschist to amphibolite facies and are subsequently intruded by ca 370 ma peraluminous granites and minor mafic intrusions the gold deposits can be divided into three main types 1 high grade approximately 15 g t au narrow gold bearing quartz veins 2 low grade 0 5 4 g t au slate argillite hosted and 3 low grade meta sandstone hosted a combination of two or more of the above can also occur almost all of the historic production 47 million g au has come from the high grade veins within 200 m of surface a variety of vein types occur although most of the gold produced came from bedding concordant stockwork and fissure veins isotopic investigations and dating of the vein minerals indicate that the mineralization was related to migration of metamorphic fluids from multiple reservoirs in the waning stages of the acadian orogeny disseminated low grade gold with minor electrum and associated intermetallic compounds and metal alloys have recently been recognized in host rocks at several of the former gold mining areas in nova scotia the presence of disseminated gold in combination with high grade vein mineralization greatly enhances the economic viability of gold deposits in the meguma group the association of disseminated gold and narrow vein hosted deposits has important exploration implications for similar mesothermal gold districts elsewhere

neocrystallization or cooling 40ar 39ar ages of white micas from low grade mylonites abstract 40 ar 39 ar data for white micas from quartzite mylonites from a thrust system are evaluated with respect to whether the minerals record cooling ages or alternatively ages of deformation induced neocrystallization to address these interpretations three types of information are needed 1 the effective diffusion length scale controlling volume diffusion of argon in white micas 2 an estimate of the diffusion parameters for volume diffusion of argon and 3 the thermal history completely recrystallized quartzites exhibit smooth grain size distributions for white micas with good exponential or power law fits suggesting that the grains grew under similar conditions the microstructures indicate that the grains grew over a narrow range in time near the end of the deformation history measured grain size distributions provide a maximum limit for the diffusion length scale thermal histories are estimated by 40 ar 39 ar diffusion experiments on k feldspars using the multi diffusion domain method of lovera lovera o m richter f m harrison t m 1989 the 40 ar 39 ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes j geophys res 94 17917 17935 these results are consistent with the expected temperature time path for the thrusting environment and predict maximum temperatures of 250 350 c during the time span recorded by the white micas experimentally and empirically calibrated diffusion parameters for white micas are applied to a cylindrical diffusion geometry to explore the question of cooling ages versus neocrystallization ages forward modeling of the passage of the white micas through the closure window for argon diffusion using the thermal histories derived from the k feldspars indicates that the white micas record neocrystallization rather than cooling and supports the idea that the white micas record the timing of when ductile deformation ceased

timing of gold mineralisation in western and central victoria australia new constraints from shrimp ii analysis of zircon grains from felsic intrusive rocks abstract age constraints provided by felsic intrusive rocks indicate that gold mineralisation in western and central victoria australia occurred over a prolonged period of time ranging from prior to the late silurian to the late devonian an early phase of gold mineralisation associated with regional deformation pre dated the emplacement of late silurian felsic porphyry dykes at 413 3 ma in the central lode system of the magdala deposit stawell gold mineralisation associated with the final stages of the main regional deformation also pre dated the intrusion of early ca 400 ma to late devonian ca 370 ma plutons at stawell wonga deposit mt bute tarnagulla tallangalook maldon and heathcote in both the stawell pyrenees and ballarat metallogenic provinces consistent with preliminary dating studies of hydrothermal micas from major gold deposits later phases of gold mineralisation throughout western and central victoria were either temporally associated with or post dated the intrusion of late silurian to late devonian felsic dykes examples of this style of mineralisation include the wonga deposit at stawell mafeking fosterville maryborough minor occurrences near linton and the woods point region preliminary constraints from ballarat west indicate that late devonian dykes intruded at ca 386 ma were pervasively altered by interaction with an auriferous fluid similar to that responsible for gold deposition within the guiding star lode but that this was probably related to remobilisation during emplacement of the dykes into a pre existing mineralised structure with the exception of the woods point region and ballarat west these later phases often have a magmatic affiliation indicated by an association with cu mo bi sb and w these complex timing relationships reflect the diachronous nature of deformation metamorphism and magmatism in the western lachlan fold belt of southeast australia and suggest at least two discrete periods of gold mineralisation in the region

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

sedimentary facies of bora bora darwin s type barrier reef society islands south pacific the unexpected occurrence of non skeletal grains abstract barrier reefs when encircling small islands have been comparatively little noticed by voyagers but they well deserve attention in their structure they are little less marvelous than atolls and they give a singular and most picturesque character to the scenery of the islands they surround darwin 1842 p 2 sediment sampling revealed six modern sedimentary facies in the barrier reef system around the volcanic island of bora bora including mixed skeletal packstone peloidal packstone halimeda rich packstone to grainstone coralgal grainstone wackestone and mudstone whereas sediments rich in corals and coralline algae are abundant on reefs and carbonate shoals mud rich sediments with mollusks and foraminifera occur in deep lagoons 15 40 m carbonate mud is composed largely of aragonite needles probably derived from codiaceans like halimeda and coccoliths in moderate abundances despite the proximity to a volcanic island the content of siliciclastics in the sediments is low 10 on average which is a consequence of the absence of permanent water courses in addition to skeletal grains non skeletal grains cemented fecal pellets aggregates occur in high abundance 30 50 in shallow lagoons 5 m on the eastern side of bora bora this observation opposes the widely held concept that non skeletal carbonate grains are abundant on platforms from the atlantic ocean but virtually missing in recent indo pacific reefs and carbonate platforms in bora bora non skeletal grains and aggregates apparently form in warm shallow 5 m moderately agitated areas with relatively low sedimentation rates in addition early cementation is probably favored by elevated alkalinity and ph in surface waters in this part of the south pacific in the same area some 900 km to the southwest of the study area ooids and peloids have recently been discovered in the shallow lagoon of aitutaki cook islands more occurrences of abundant non skeletal grains probably exist in the indo pacific region however too few locations have been systematically studied a comparison of the new bora bora data with other modern reefs and platforms shows that sediment occurrence and distribution exhibits basic patterns in that marginal areas are dominated by coarse grained coralgal sediment whereas lagoons and platform interiors are characterized by mollusk foram halimeda and non skeletal grains and fines still grain abundances may vary significantly among locations and again suggest the need for more studies in order to fully understand the factors controlling occurrence and distribution of modern sediments

cyclone pumping sediment partitioning and the development of the great barrier reef shelf system a review abstract the modern great barrier reef gbr is part of the world s largest and best known mixed terrigenous carbonate continental margin the gbr shelf contains three shore parallel sedimentary belts an inner shelf zone of terrigenous sedimentation at depths of 0 22 m a middle shelf zone of sediment starvation at depths of 22 40 m and an outer shelf reef tract with its inner edge at ca 40 m depth these zones are controlled by the dynamics of northward fair weather along shelf drift driven by southeasterly trade winds and by the regular passage of tropical cyclones cyclones cause wind driven north directed middle shelf flows in excess of 130 cm s which erode the seabed concentrate the sparse mobile sediment into sand ribbons and advect suspended load onto the outer part of the nearshore terrigenous sediment prism and into inter reef depocentres within the outer shelf reef tract cyclones largely control the input of new sediment into the gbr system via river flooding seabed erosion or reef breakage they also help to control the partitioning and dispersion of the three main shore parallel belts of sediment and hence stratigraphic accumulation acting as a sediment pump especially during interglacial highstands cyclones have exerted great control on the development of the modern gbr province and its sediments by maintaining a broad shelf parallel zone of episodically mobilised sediment and scoured seabed upon which coral reefs have been unable to form cyclones may also have partly controlled the timing of initiation of the first gbr at 0 6 mybp contrary to current models gbr storm beds are most likely to be preserved intact close to the shoreline and become coarser grained away from the shoreline for the central gbr highstand shedding only applies to carbonate sediment at the scale of local reefs system wide oceanographic controls cause high rates of carbonate sedimentation on the slope during both sea level rise and highstand concomitantly terrigenous sediment accumulates fastest on the slope during sea level rise and slowest during sea level lowstand and highstand

40ar 39ar dating of white micas from an alpine high pressure metamorphic belt on naxos greece the resetting of the argon isotopic system overprinting of white micas from high pressure low to medium temperature m1 metamorphic assemblages in pelitic schists on naxos during subsequent thermal dome m2 metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome the original m1 minerals are phengites si4 6 7 7 0 and the overprinting minerals are muscovites si4 6 0 6 45 40ar 39ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between these upward convex age spectra are shown to be caused by mixing of two generations of micas each of which has a different age spectrum and argon release pattern seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages further the upward convex age spectra have been used to trace the isotopic signature of phengites toward increasing m2 metamorphic grade and suggest that as long as phengites can be observed in the rocks upward convex age spectra occur on naxos crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas however there is also good evidence for argon loss by volume diffusion from phengites simple diffusion calculations suggest that the m2 metamorphism was caused by a shortlived heat source

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

the p t t history of blocks in serpentinite matrix melange west central baja california thermochronologic petrologic and geochemical analyses of epidote amphibolite amphilbolite eclogite and blueschist facies blocks in serpentinite matrix melanges from east san benito island and cedros island west central baja california provide constraints on the p t t history of this disrupted terrane results of sup 40 ar sup 39 ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different p t t histories mid jurassic 160 170 ma epidote amphibolite and amphibolite facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during limitation of subduction coarse grained blueschist blocks were likely metamorphosed during continued subduction in late early cretaceous time that is 115 100 ma some epidote amphibolite blocks are partially overprinted by blueschist facies mineral assemblages and apparently record both metamorphic events fission track analyses of apatites indicate that the blocks underwent significantly different post metamorphic cooling histories epidote amphilbolite and amphibolite blocks cooled below approximately 100 degrees c form mid jurassic to paleocene time blueschist blocks cooled below approximately 100 degrees c in oligocene miocene time in general mafic blocks have trace element concentrations and ree patterns characteristic of ocean flood basalts this study demonstrates that samples with very different p t t histories can evidently occur over relatively small length scales 1 km in serpentinite matrix melanges 67 refs 16 figs 4 tabs

the record of pliocene sea level change at enewetak atoll abstract detailed seismic stratigraphy lithostratigraphy and chemostratigraphy indicate that atoll wide subaerial exposure surfaces major disconformities developed during major sea level lowstands form prominent seismic reflectors and are coincident with biostratigraphic breaks in the plio pleistocene on enewetak atoll sea level models based on the stratigraphic position and age of major disconformities suggest a maximum sea level highstand elevation of 36 m above present sea level and a maximum sea level lowstand elevation of 63 m below present sea level for the pliocene

geochronology of blueschists from west central baja california and the timing of uplift in subduction complexes a geochronologic study of blueschists from west central baja california has provided constraints on the timing of high pressure low temperature metamorphism and the subsequent uplift of this subduction complex mineral separates from coherent blueschists and blueschist blocks enclosed in serpentinite melange were analyzed by 40 ar 39 ar fission track and rb sr methods 40 ar 39 ar age spectra for white mica separated from four blueschist blocks and an overprinted eclogite block in serpentinite matrix melange show slow cooling gradients between 115 95 ma rb sr apparent ages for one block are concordant with these results 40 ar 39 ar ages of blue amphibole are primarily the result of the degassing of white mica intergrowths as confirmed by microprobe analyses apatite fission track ages indicate a blueschist block and an eclogite block cooled to a temperature below 100 c at 22 ma and 32 ma respectively subduction related metamorphism of the coherent blueschists occurred in la

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

the effect of the instability of muscovite during in vacuo heating on 40ar 39ar step heating spectra abstract the goal of this study was to ascertain the nature of the ar release mechanism s for a single well characterized muscovite heated incrementally in vacuo 1 at temperatures and time intervals typical of 40ar 39ar step heating experiments and 2 isothermally at 500 c and 700 c for time intervals much longer than typical 40ar 39ar experiments the rate of delamination was monitored by sem and the rate of dehydroxylation was determined by xrd analyses the rate of in vacuo ar loss was compared to that for muscovite heated at 700 c and 2 kbar h2o pressure for all experiments the rate of ar loss was found to be consistent with the rates of dehydroxylation and delamination at 500 c ar loss behavior for a 3000 m diameter muscovite grain was approximated by volume diffusion but the ar release pattern became increasingly non fickian with decreasing grain size we propose that the cause of the non fickian behavior is delamination the portions of the isothermal runs exhibiting diffusion like profiles also yielded discordant 40ar 39ar ages and low atmospheric ar contamination we attribute this phenomenon to the preferential concentration of atmospheric ar in interlayer extended defects and of 40ar in interlayer vacancies whereas 39ar is uniformly distributed throughout the muscovite structure by recoil the 40ar 39ar age spectrum of the muscovite heated under hydrothermal conditions has a staircase shape comparable to that usually interpreted as due to 40ar diffusive loss whereas 40ar 39ar laser spot analyses did not allow this interpretation the in vacuo ar release rate for this sample however indicates that it had been altered during the experiment therefore we propose that this 40ar 39ar age spectrum reflects the differential release of ar from the alteration phases vs muscovite for our sample dehydroxylation and delamination control the in vacuo ar release at temperatures less than 900 c whereas ar diffusion from thin dehydroxylated slabs may dominate the ar release mechanism for temperatures greater than 900 c this model would predict that the recovery of fossil 40ar concentration gradients from muscovite by typical incremental heating schedules is not possible the rates of delamination in vacuo and the rate of ar diffusion for muscovite in nature however both depend upon its the chemical composition structural integrity and grain size consequently for samples with either a naturally occurring broad range of grain sizes such as detrital muscovite and authigenic clay from sedimentary rocks or a compositionally or structurally heterogeneous population of grains such as muscovite from shear zones 40ar 39ar incremental heating may be able to detect intergranular variations in the k ar age

postglacial growth history of a french polynesian barrier reef tract tahiti central pacific bthe internal structure and growth pattern of tahiti reefs over the last 14 ka is reconstructed using sedimentological morphological and palaeobiological data coupled with radiometric dates in drill cores through the modern barrier reef flooding of the volcaniclastic deposits or the karst surface of a pleistocene reef started at 14 ka bp and coral growth began shortly after inundation the sequence in the tahiti barrier reef edge has formed predominantly through long term keep up growth controlled by stable environmental conditions while the adjacent backreef deposits did not start to accumulate before sea level stabilization around 6 ka the dominance of porites communities and the coeval occurrence of branching gracile lithophyllum in the lowermost part of the postglacial reef sequence 14 11 ka suggest the prevalence of uniformly moderate to low energy conditions and or growth in slightly deeper waters all over the drflled area during the early reef stages during the last 11 ka the reef frameworks developed in a high energy environment at maximum water depths of 5 6 m and were dominated by an acropora robusta danaihydrolithon onkodes association the local interlayering of other coralgal assemblages dominated by tabular acropora or domal porites reflects distinct diversification stages resulting either from the palaeotopographic control of the substrate or from slight and episodic environmental changes

the role of carbonaceous indicator slates in the genesis of lode gold mineralization in the western lachlan orogen victoria southeastern australia indicator slates have long been considered to represent a useful exploration guide for turbidite hosted mesothermal lode gold mineralization in central victoria this assumption has been based on an apparent close spatial relationship between high gold grades and the proximity of thin commonly pyritic and carbon rich marker units detailed studies in a number of gold deposits throughout central victoria however reveal that highest gold grades do not necessarily coincide with the presence of carbonaceous units in many places where gold mineralization is associated with carbon rich matter the high c accumulations are the result of epigenetic remobilization during hydrothermal alteration and ore genesis petrographic geochemical and stable isotope c o s investigations into the origin and nature of the carbonaceous matter mainly amorphous bitumens and pyrobitumen of organic origin with biological fragments and rare graphite of both detrital and metamorphic origin demonstrate that black shales within the cambro ordovician succession in central victoria lacked the vital constituents to provide 1 a primary sink for precious metals and 2 whereas the presence of carbonaceous matter was likely to affect the evolution of epigenetic hydrothermal fluids and locally may have facilitated gold enrichment carbon rich sedimentary rocks were not crucial for ore genesis on the deposit scale instead the size of the hydrothermal cell physicochemical conditions of the ascending fluids and dynamic fault valve behavior played far more significant roles in controlling gold precipitation the importance of these processes has implications for exploration targeting sediment hosted lode and disseminated style gold mineralization in the western lachlan orogen and in slate belt provinces elsewhere

tibetan tectonics from 40ar 39ar analysis of a single k feldspar sample ar 39ar data on an alkali feldspar sample from the quxu pluton gangdese batholith southern tibet allow a detailed assessment of unroofing and uplift history between 35 and 18 ma the 39ar arrhenius plot for this sample shows departures from a linear relationship between the effective diffusion parameter log d r2 and reciprocal temperature which we interpret to be the result of a distribution of distinct diffusion domain sizes we use an alternative way of plotting the arrhenius data that exhibits domain size versus cumulative 39ar released during step heating the 4 ar 39ar age spectrum of the sample has features such as local age plateaus that are most easily explained in terms of the distinctive closure age of particular domains the fact that the same distribution of diffusion domain sizes explains both the arrhenius data and the age spectrum is an indication that the diffusion properties operating in the laboratory are those of the sample while it was in its natural environment modelling of the age spectrum with a distribution of domain sizes results in the recovery of a continuous cooling history segment rather than a single time temperature datum we demonstrate the robustness of the cooling curve determination by showing the large misfits to the age spectrum that arise from relatively small changes in the cooling history the best fit cooling curve for the quxu sample shows a decreasing rate of cooling in the time interval 35 18 ma followed by a very rapid cooling beginning at about 18 ma we have used a thermal model for the conductive cooling of an unroofing pluton to estimate the rate of unroofing required to explain the quxu cooling curve and find that in the 35 20 ma time interval the primary control of the thermal evolution is the conductive loss of magmatic heat with little or no unroofing unroofing rates of approximately 0 05 mm yr followed by a brief period 5 ma of very rapid unroofing with rates of order 2 mm yr

gold mineralization in shear zones within a turbidite terrane examples from central victoria s e australia abstract central victoria in s e australia is regarded as the classical area for gold mineralization associated with saddle reefs and with reverse faults within a turbidite terrane however there is also an important contribution by steep brittle shear zones to the mineralization and two such areas fosterville and heathcote are the subject of this article the shear zones hosting mineralization are sinistral transpressional structures that while accommodating regional shortening do it by both crustal thickening and lateral expulsion the mineralization occurs at dilational jogs or bends within the zones the shearing and mineralization postdate the main phase of folding and thrusting but predate widespread post tectonic granitoid intrusion it is postulated that the shears are a part of the initial phases of orogenic collapse which culminated in the emplacement of the granites as well as sedimentation and acid to bimodal vulcanicity furthermore it is argued that mineralization within the saddle reefs and reverse faults that characterize central victoria are also related to the initial phase of orogenic collapse a final postulate is that orogenic collapse provides a common tectonic thread to some turbidite hosted epithermal and archean lode types of gold mineralization

geology and geochemistry of the vantage gold deposits alligator ridge bald mountain mining district nevada geologic relationships major element data and isotopic geochemistry of a group of carlintype au deposits in the alligator ridge bald mountain district of east central nevada were investigated to help constrain the origin and relative timing of au mineralization and associated alteration the vantage gold deposits were the largest of 18 known sediment hosted disseminated gold deposits and prospects that are distributed over a strike length of 40 km the district consists predominantly of paleozoic carbonate and siliciclastic sedimentary rocks minor amounts of tertiary volcanic and volcanic related sedimentary rocks and a small granitic stock in the northern end of the district are also present the intrusion and its surrounding aureole also host gold mineralization but most gold deposits in the district are not spatially associated with intrusive rocks the vantage au deposits formed at the contact between the devonian devils gate limestone and the devonian mississippian pilot shale adjacent to a major fault zone herein referred to as the vantage fault gold mineralization was accompanied by silver antimony and arsenic and showed a strong preference for the pilot shale alteration related in time and space to au mineralization was the replacement of host rock carbonates by quartz replacement in the pilot shale closely followed the ore waste boundary whereas replacement in the devils gate limestone occurred as a stratiform jasperold not confined to the ore zones quartz kaolinite stibnite veins were common within the central high grade portion of each orebody peripheral to silicification diagenetic dolomite in the pilot shale was replaced by hydrothermal calcite and calcite veins constituted as much as 10 percent of the rocks two oxidation events that destroyed organic matter and sulfides occurred after au deposition and silicification the early oxidation event was accompanied by the destruction of detrital illite in the pilot shale and deposition of alunite or barite veins this event was barren of au but did redistribute metals introduced during silicification the spatial distribution of this alteration was similar to that of the silicification k ar dating of this alunite indicates an age of 11 ma for its formation following a period of erosion weathering related oxidation overprinted much of the earlier alteration and deposited jarosite and goethite but it did not affect metal distribution patterns or the matrix of the pilot shale whole rock chemical analyses indicate that silica and sulfur were the only major components introduced during silicification and that very little material other than iron and organic matter was removed during the intense oxidation oxygen isotope compositions determined for jasperoid and vein quartz varied from 15 9 to 18 2 per mil and from 16 1 to 20 1 per mil respectively carbonate from whole rock samples peripheral to silicification showed an increase in their carbon isotope values of up to 3 per mil to 1 4ppm and a decrease in their oxygen isotope values of up to 12 per mil to 12 7ppm in comparison with unaltered samples vein calcite had even more extreme values of up to 3 1 per mil for delta 13 c and 3 8 per mil for delta 18 o sulfur isotope measurements on stibnite and orpiment varied from 5 5 to 10 per mil and from 2 5 to 12 3 per mil respectively vein alunite and barite associated with intense oxidation had sulfur isotope compositions that generally varied from 3 8 to 13 4 per mil and from 21 0 to 29 2 per mil respectively barite that appeared to be sedimentary in origin yielded values of 25 9 to 47 1 per mil hydrothermal fluids that formed the deposits ascended from depth along the vantage fault near the level of the deposits fluids were displaced into minor faults in the footwall limestone at the contact between the devils gate limestone and the pilot shale fluids spread laterally and deposited au ag sb and as and quartz following au deposition hypogene oxidizing fluids using the same conduits destroyed reduced components in the pilot shale and produced major textural modifications the distribution of the elements within the deposits appears to be related to 1 proximity to hydrothermal conduits 2 composition of the host lithologies and 3 extent of postmineralization oxidation oxygen isotope data of jasperoid and vein quartz indicate that the ore fluids were evolved meteoric waters isotopic data for epigenetic sulfides indicate that sulfur was derived from sedimentary sulfides local stratigraphic considerations suggest that cambrian siliciclastic rocks were the source the deposits were geochemically similar to other sediment hosted gold deposits in the great basin e g carlin but they were uncomplicated by extraneous geologic features such as paleozoic thrusts or premineralization intrusions thus the vantage deposits may be more easily understood examples of this class of ore deposits

contradicting barrier reef relationships for darwin s evolution of reef types the darwinian progressive subsidence model for the evolution of fringing reefs barrier reefs and atolls has been generally accepted following the indisputable proof of subsidence provided by drilling results in the pacific nonetheless there are data that do not fit the expectations of the model such as the similar lagoon depths of barrier reefs and atolls as opposed to the subsidence theory s implicit prediction that atolls should have significantly greater depths in contrast a great deal of evidence supports the influence of meteoric solution on barrier reef morphology for example the maximum lagoon depth of 56 modern barrier reefs is statistically correlated with the lagoon catchment area for modern annual rainfall these modern rainfall patterns would seem to be a reasonable proxy for relative geographic differences in glacial lowstand rainfall even though the absolute amounts of such rainfall are unknown the correlation therefore suggests the importance of pleistocene subaerial solution in contributing to barrier reef morphology further support for antecedent influence occurs in the form of barrier reef passes in which the depth of the reef pass is correlated with onshore drainage volumes on a larger scale the cook island of mangaia provides evidence that solution can produce barrier reef morphology independent of reef development in contrast there are no examples of the subsidence predicted lagoon transition of fringing reefs to barrier reefs to atolls moreover the common occurrence of fringing reefs within barrier reefs negates subsidence as a causal factor in their presumed progressive evolutionary development consequently the evidence to date suggests that a solution morphology template has been accentuated by reef construction to produce the diagnostic barrier reef morphology we see today the importance of subsidence would seem to be in accounting for the overall thickness of the resulting carbonate caps of oceanic examples and in contributing to lagoon depth variation among the larger continental entities

40ar 39ar analyses of artificially mixed micas and the treatment of complex age spectra from samples with multiple mica populations abstract artificial mixtures of two age populations of muscovite and of biotite were prepared at relative weight percents of 3 1 1 1 and 1 3 and analyzed by the 40ar 39ar vacuum furnace step heating method the original micas are late jurassic and late cretaceous samples that yield flat age spectra with plateau ages defined by 97 100 of the gas release and no evidence for excess argon the age spectra from mica mixtures yield patterns that systematically decrease with decreasing jurassic mica content and increasing cretaceous component however the mixed muscovite spectra are relatively flat consistent with the two original samples showing similar degassing patterns during vacuum furnace heating in contrast the mixed biotite spectra are highly discordant with an overall decrease in age during the step heat analysis this is consistent with the degassing patterns of the original mica samples which indicate the jurassic biotite high fe mg outgases at lower temperatures and the cretaceous biotite low fe mg remains retentive until the higher temperature steps the individual degassing patterns and compositions show consistencies with known compositional controls on argon diffusivity indicating that micas may maintain argon retention characteristics rooted in crystal chemistry during vacuum furnace heating all but one of the mixed samples failed to yield ages reflecting the age of the original micas these results imply that complex age spectra obtained for multiply deformed rocks with multiple mica populations may define geologically meaningless ages as a result of simultaneous degassing i e mixing of sample reservoirs during heating comparison of laboratory degassing rates and crystal chemistry indicates compositional controls on argon retention are likely preserved during vacuum furnace step heating and micas may degas retain argon in a predictable manner these conclusions support assertions that recovery of fossil age gradients in slowly cooled samples may be possible using furnace step heating

gold mineralization without quartz veins in a ductile brittle shear zone macraes mine otago schist new zealand greenschist facies schist which hosts the macraes mine in east otago new zealand has been pervasively altered by post metamorphic lower greenschist facies fluids over a 120 m thick section perpendicular to foliation metamorphic titanite has been replaced by rutile and epidote has been replaced by a variety of metamorphic minerals including siderite chlorite muscovite and calcite the early stages of this alteration occurred during development of a ductile cleavage associated with kilometre scale recumbent folding the cleavage was widely overprinted by a subparallel set of spaced mm scale microshears which are locally enriched in rutile and hydrothermal graphite strain was then concentrated into narrow m scale zones where more intensely deformed portions of the rock are crossed and highly disrupted by closely spaced 100 m scale microshears the highly strained rocks show a combination of mylonitic and cataclastic microstructures including crystal plastic grain size reduction and recrystallization of micas to form a new foliation muscovite has grown at the expense of albite in the mylonitic cataclasites hydrothermal alteration was accompanied by addition of pyrite arsenopyrite and gold without development of quartz veins gold precipitated with sulphides during reduction of the fluid by hydrothermal graphite the whole altered rock sequence was later cut sporadically by mesothermal quartz veins which contain gold scheelite rutile pyrite and arsenopyrite this deposit displays a continuum of post metamorphic processes and hydrothermal fluid flow which occurred during uplift of the schist belt

mossman orogenic gold province in north queensland australia regional metallogenic controls and undiscovered gold endowment the hodgkinson and broken river provinces of the mossman orogen in north queensland host numerous orogenic gold deposits and still remain under explored this paper discusses regional metallogenic controls and results of a probabilistic quantitative assessment of undiscovered gold potential in the region significant orogenic gold deposits in the region occur only within relatively small well endowed metallogenic zones likely to be controlled by the eastern margin of the paleoproterozoic continental crust underlying the western mossman orogen three distinct styles of primary orogenic gold deposits are present in the area gold quartz veins refractory gold associated with quartz pyrite arsenopyrite veins and stockworks and stibnite quartz gold veins refractory gold deposits are estimated to have the highest potential for significant undiscovered resources in the region the hodgkinson province is estimated to host between one and ten significant undiscovered refractory gold ore fields with a 50 probability of at least 20 t of total contained gold and a 90 probability of at least 1 t the broken river province is estimated to host up to five significant undiscovered refractory gold ore fields with a 50 probability of at least 12 t of contained gold

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

late pleistocene evolution of the mixed siliciclastic and carbonate southwestern new caledonia continental shelf lagoon abstract the main island of new caledonia nc is surrounded by the second largest barrier reef in the world after the australian great barrier reef the upper pleistocene evolution of this barrier reef lagoon complex offers the opportunity to examine the response of a modern mixed tropical siliciclastic carbonate system to relative sea level changes and to better constrain the relative role of lagoon backfilling and paleo drainage system in sediment transfer across the shelf this paper mainly focuses on the results of chronostratigraphic and sedimentological interpretations of new seismic bathymetric and coring data collected in the sw nc lagoon several significant points are highlighted from this study and allow the construction of a refined model which includes the evolution of both the sw nc mixed shelf and lagoon new results show that the nc lagoon has not recorded a reciprocal sedimentation during glacial interglacial sea level cycles but a unique mixed regime strong contrasts are observed and these depend above all on the two lagoon domains the outer coral plateau and the inner lagoon depression corresponding to a semi closed proximal basin which is separated from the plateau by a basement ridge in the depression clastic sediments remain dominant from early flooding to highstand stages while carbonate sedimentation continued through time across the coral plateau this spatial contrast between terrigenous and carbonate deposits was maintained throughout various sea level cycles thus even if spatial segregation is also observed for other examples of mixed shelf environments the sw nc shelf appears quite unique in comparison to the other mixed systems when considering conceptual models of reciprocal sedimentation moreover the presence of a semi enclosed lagoon depression allows the preservation of larger volume of terrigenous deposits this semi filled geometry of the lagoon is assumed to be the result of several factors a significant subsidence that offers accommodation for aggrading deposits evacuation through channels of sediments mobilized during storm events the tidal currents and fairweather wave dynamics lastly the inherited morphologic and structures may explain the geomorphic evolution of the escarpment margin type of the sw nc rimmed shelf and its differentiation with the opposite se nc ramp type shelf

diagenetic alteration of a mesozoic fluvial gold placer deposit southern new zealand abstract gold paleoplacers become progressively more affected by diagenetic processes with age and burial mesozoic paleoplacer deposits in southern new zealand display intermediate stages of diagenetic transformation compared to little affected late cenozoic paleoplacers and strongly affected paleozoic and precambrian paleoplacers the mesozoic cretaceous diagenesis resulted in near pervasive alteration cementation and lithification of the paleoplacer lithic clasts and matrix have been extensively altered to illite ferrous iron bearing smectite vermiculite and kaolinite and the cement consists mainly of clays and calcite diagenetic pyrite marcasite vivianite and mn oxide also contributed to cementation alteration occurred under near surface

undiscovered orogenic gold endowment in northern victoria australia abstract the northern part of the victorian gold province australia is covered by cainozoic sediments and remains poorly explored this contiguous covered area of over 44 000 km 2 is highly prospective for orogenic gold deposits this paper discusses results of probabilistic quantitative assessments of undiscovered gold potential in the northern covered parts of the western lachlan orogen the stawell bendigo and melbourne zones for the bendigo and stawell zones quantitative estimates were only made for undiscovered mesozonal orogenic gold quartz vein deposits the predominant type of gold mineralisation in that region for the northern part of the bendigo zone under cover 10 000 km 2 the mean estimate is approximately 1000 t with a 90 probability of at least 290 t of gold in undiscovered mesozonal orogenic gold deposits for the northern covered area of the stawell zone 30 000 km 2 the mean estimate is 1200 t with a 90 probability of at least 200 t of undiscovered gold the northern part of the melbourne zone 4400 km 2 is considered prospective for epizonal orogenic gold deposits associated with quartz pyrite arsenopyrite veins and stockworks or with quartz stibnite gold veins the mean estimate for that area is 90 t with a 90 probability of at least 10 t of gold in undiscovered epizonal orogenic gold deposits

seismic architecture and sediment distribution within the holocene barrier reef lagoon complex of mayotte comoro archipelago sw indian ocean twenty gravity cores and a large set of high resolution seismic profiles from various lagoonal settings were studied to determine the holocene sediment distribution and sequence architecture within the mayotte barrier reef lagoon complex the holocene seismic sequence comprises a type 1 sequence with lowstand transgressive and highstand systems tracts the lowstand systems tract consists of a paleosoil horizon formed during subaerial exposure the transgressive systems tract is composed of four depositional systems 1 inner transgressive layer 2 proximal and distal incised valley fills 3 mid lagoonal layer and 4 keep up or catch up fringing and barrier reef sequence the highstand systems tract comprises three depositional systems 1 of a proximal terrigenous wedge 2 mid lagoonal and distal carbonate sands or muds and 3 reefal carbonates our studies show that the nature of the holocene sequence is controlled by the rate and amplitude of sea level rise and environmental changes which are expressed by changes in clastic sediment supply and carbonate production the pre holocene topography and water dynamics steer the vertical and spatial sediment thickness distribution of the holocene additional important parameters are the proximity to a source area carbonate or terrigenous and the width of the depositional area climate dynamics are also of great importance while they determine carbonate production and terrigenous runoff sedimentation rates in the subtidal settings always lacked behind sea level rates thus a steep relief was created keeping most lagoonal parts within the deep subtidal realm in which sediment production was not efficient enough to fill up accommodation space in addition wave and or current energy might prevent the fill up of the lagoon this ultimately resulted in a typical empty bucket morphology only a high amplitude sea level fall would allow the subtidal lagoon to build up to base level unfilled accommodation space therefore must be a very common feature in the geologic record

geology and structural evolution of the muruntau gold deposit kyzylkum desert uzbekistan abstract the muruntau gold deposit in the kyzylkum desert of uzbekistan is the largest single deposit 1100 tonnes of gold of the class of low sulfide syndeformation synigenous gold deposits formed in the brittle ductile transition zone of the crust within transpressional shear zones hosted by the cambrian to ordovician besopan suite the ores were deposited in pre existing thrust fault and metamorphism related permeabilities and in synmineralization dilational zones created in a large fault related fold the besopan suite is a 5 000 m thick sequence of turbiditic siltstones shales and sandstones the ore is primarily localized at the base of the besopan 3 unit which is a 2 000 m thick series of carbonaceous shales siltstones sandstones and cherts initial gold deposition took place within the sangruntau tamdytau shear zone which was developed along the stratigraphic contact between the besopan 3 and besopan 4 units during the mineralization process folding of the besopan suite and a left step adjustment in the sangruntau tamdytau shear zone were caused by two concurrent events 1 the activation of the left lateral muruntau daugyztau shear zone that developed at nearly a 90 angle to the preceding shear zone and 2 the intrusion of granitoid plutons these structural events also resulted in the refocusing of hydrothermal fluid flow into new zones of permeability

dominant influence of volcanic loading on vertical motions of the hawaiian islands abstract rates of island vertical motion above intra plate hotspots record the processes that arise from interactions between lithospheric plates and mantle plumes to assess the contribution of mantle and lithospheric processes to surface motion above the hawaiian hotspot we compare simple models of lithospheric deformation to vertical motion rates measured from dated paleoshorelines and tide gauge records in the hawaiian islands our analysis shows that observed uplift and subsidence rates mainly record the flexural response of the lithosphere to volcanic loads the effective elastic plate thickness that best fits the spatial distribution of subsidence and uplift rates is 40 km consistent with previous estimates based on total vertical deflection because volcanic loading dominates the vertical motion signal hawaiian islands appear to follow a predictable trajectory of vertical motion when they reside within one flexural half wavelength of the active volcanic center islands initially subside at rapid and decreasing rates in the first 1 myr following their construction uplift relatively slowly 1 2 5 myr following their construction and eventually subside again but at slow rates within 5 myr of their construction this observed pattern of uplift and subsidence is consistent with the pattern of vertical motion predicted to result from volcanic loading at the hawaiian hotspot lithospheric migration over the long wavelength topographic swell associated with the hawaiian hotspot has a comparatively minor influence on island uplift and subsidence its contribution to island vertical motion is not readily observed in our data with the possible exception of some uplift observed in the past 500 kyr on o ahu that might correspond to non steady state behavior of the hawaiian plume

when can muscovite 40ar 39ar dating constrain the timing of metamorphic exhumation abstract metamorphic cooling rates determined using muscovite 40 ar 39 ar ages underpin many tectonic models but are based on several simplifying approximations we demonstrate that a number of these approximations especially those relating to the assignment of a single closure temperature might not hold for certain metamorphic scenarios and particularly for metamorphism during rapid orogenic cycles using numerical diffusion models that include a recently reported significant pressure dependence of ar diffusion in muscovite we systematically interrogate the approximations associated with linking 40 ar 39 ar dates to pressure temperature histories the results of the simulations are presented in a simple graphic form allowing evaluation of the pressure temperature pt regions in which ar diffusion is efficient and hence those in which the muscovite may yield true 40 ar 39 ar cooling ages we suggest that a robust method for determining whether metamorphic muscovite 40 ar 39 ar ages relate to the timing of cooling involves 1 the determination of the pt conditions and relative timing of muscovite crystallisation during the metamorphic cycle 2 the collection of high precision and high spatial resolution 40 ar 39 ar data from muscovite grains and 3 the comparison between analytical data and numerical diffusion models which test different post crystallisation pressure temperature histories

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

gold nugget morphology and geochemical environments of nugget formation southern new zealand abstract gold nuggets centimetre scale have formed in a supergene alteration zone on hydrothermal gold deposits and occur intergrown with quartz and iron oxyhydroxide pseudomorphs after sulphide minerals and along fractures in quartz and host rocks the supergene alteration was driven by groundwater driven water rock interaction near to a regional unconformity beneath fluvial sediments and involved clay alteration and oxidation that extended up to 50 m below the unconformity oxidation of pyrite and arsenopyrite produced temporary thiosulphate ligands that mobilised microparticulate gold encapsulated in the sulphide minerals the nuggets have some crystalline form and internally they consist of anhedral grains elongated gold plates and intimate intergrowths of gold and iron oxyhydroxide nugget surfaces have further micron scale overgrowths of microparticulate gold gold plates and gold crystals nuggets were eroded and recycled into nearby proximal miocene quartz pebble conglomerates where they concentrated in placers near the basal unconformity later recycling transferred gold into pleistocene fluvial channels gold dissolution and redeposition as plates and crystals occurred on the exterior surfaces of placer gold particles with little change in mass all groundwater maintained high ph throughout the geological history because there was sufficient calcite in the basement rocks to neutralise any acid generated by pyrite oxidation hence gold mobility in sediments was driven by thiosulphate complexes as for the in situ nuggets albeit with lower dissolved sulphur concentrations despite aridification of the climate in the late cenozoic with resulting localised high dissolved chloride concentrations chloride complexation did not contribute to gold mobility

an amagmatic origin of carlin type gold deposits carlin type deposits are major sources of gold yet their origins are enigmatic suggested genetic models make connections to magmatism regional metamorphism or regional extension depositional mechanisms are uncertain as well we propose on the basis of geologic physical and chemical reasoning a genetic model in which meteoric fluids were circulated by heat released during crustal extension these fluids interacted at depth with the sedimentary rock pile and scavenged gold upon upwelling these fluids interacted with various lithologies and or other fluids and produced the characteristic alteration and metal suites of these deposits to test the viability of this amagmatic model we have investigated certain physical and chemical constraints implicit to the model heat balance calculations indicate that ample surface waters could be heated to appropriate temperatures and circulated during rapid crustal extension mass balance calculations indicate that solution transport efficiencies of 10 percent can account for several times the amount of known alteration and mineralization in contrast magma driven or metamorphic fluids appear to be less adequate sources a two step source to deposit approach was used to simulate fluid rock interaction and mass transport implied in the amagmatic model fluids with a fixed sigma cl were first equilibrated with various potential source rocks arkose graywacke and pelite each with various f o2 and f s2 buffer assemblages at 300 degrees c and 1 kbar to simulate reaction at depth perhaps near the brittle ductile transition these fluids were then reequilibrated under conditions indicated for deposit formation 225 degrees 150 degrees c 0 5 kbars to investigate processes including cooling wall rock reaction and mixing which could be important to ore deposition gold transport was favored for low chlorinity intermediate oxidation state below pyrrhotite pyrite magnetite slightly acid fluids from sulfide bearing sources i e equilibration with arkosic rocks these conditions are not conducive to base metal transport and explain the rarity of base metals in carlin type systems when reacted under deposit level conditions these calculated ore fluids leach carbonate and deposit quartz the volume ratio of carbonate dissolved to quartz precipitated is approximately 0 2 to 2 5 and increases when mixing with a second fluid is considered arkose equilibrated source fluids can precipitate gold by combinations of wall rock reaction with a decarbonated host mixing with various sulfide poor fluids and reaction with iron rich host rocks cooling alone is ineffective over this temperature interval although most deposits are hosted by decarbonated marls geochemical calculations suggest that other depositional environments are feasible consistent with observation although here applied to the amagmatic hypothesis these chemical calculations are not mechanism specific and might also be appropriate to metamorphic or magma driven hydrothermal systems these results demonstrate that carlin type deposits can result from the coincidence of an effective driving force rapid major crustal extension a source of dilute fluids and chemically appropriate sets of both source and host rocks the aqueous chemistry involved is unexceptional and shares many features with other types of moderate to low temperature gold bearing hydrothermal systems it is the geologic setting and perhaps the fluid driving force that is unusual an amagmatic model is consistent with both deposit scale and regional scale observations it rationalizes carlin type systems in the framework of southwestern north american metallogeny and it implies multiple possible mechanisms for gold mineralization detachment type gold systems may be the high oxidation state high salinity analogues of carlin type systems formed by major extension but from contrasting lithologic and fluid sources

orogenic gold mineral systems of the western lachlan orogen victoria and the hodgkinson province queensland crustal metal sources and cryptic zones of regional fluid flow abstract orogenic gold mineral systems in the western lachlan orogen victoria and the hodgkinson province queensland produced gold provinces characterised by vastly different scales of gold endowment and strongly uneven distribution of gold mineralisation within each province the volume of hydrous pyrite bearing rocks undergoing metamorphic devolatilisation during thermo tectonic events driving orogenic gold mineral systems represents a fundamental first order constraint on the total gold endowment and its broad spatial distribution both between and within the provinces most of the largest gold deposits in both regions occur in linear richly endowed metallogenic zones oblique to the dominant regional structures and related to deep crustal domain boundaries these boundaries with only subtle surface expressions were the major regional structural controls which promoted focused near vertical flow of mineralising metamorphic fluids above the outer margins of cratonic blocks in the lower crust recognised major faults represented only more local scale and often indirect controls on the focused fluid flow particularly effective above the deep cratonic block boundaries overlain by relatively thick crustal source rocks

seismic evidence of glacial age river incision into the tahaa barrier reef french polynesia rivers have long been recognized for their ability to shape reef bound volcanic islands on the time scale of glacial interglacial sea level cycles fluvial incision of exposed barrier reef lagoons may compete with constructional coral growth to shape the coastal geomorphology of ocean islands however overprinting of pleistocene landscapes by holocene erosion or sedimentation has largely obscured the role lowstand river incision may have played in developing the deep lagoons typical of modern barrier reefs here we use high resolution seismic imagery and core stratigraphy to examine how erosion and or deposition by upland drainage networks has shaped coastal morphology on tahaa a barrier reef bound island located along the society islands hotspot chain in french polynesia at tahaa we find that many channels incised into the lagoon floor during pleistocene sea level lowstands are located near the mouths of upstream terrestrial drainages steeper antecedent topography appears to have enhanced lowstand fluvial erosion along tahaa s southwestern coast and maintained a deep pass during highstands upland drainages appear to contribute little sediment to refilling accommodation space in the lagoon rather the flushing of fine carbonate sediment out of incised fluvial channels by storms and currents appears to have limited lagoonal infilling and further reinforced development of deep barrier reef lagoons during periods of highstand submersion

p t t estimation of deformation in low grade quartz feldspar bearing rocks using thermodynamic modelling and 40ar 39ar dating techniques example of the plan de phasy shear zone unit brianconnais zone western alps pressure temperature time p t t estimates of the syn kinematic strain at the peak pressure conditions reached during shallow underthrusting of the brianconnais zone in the alpine subduction zone was made by thermodynamic modelling and 40ar 39ar dating in the plan de phasy unit se of the pelvoux massif western alps the dated phengite minerals crystallized syn kinematically in a shear zone indicating top to the n motion by combining x ray mapping with multi equilibrium calculations we estimate the phengite crystallization conditions at 270 50 c and 8 1 2 kbar at an age of 45 9 1 1 ma combining this p t t estimate with data from the literature allows us to constrain the timing and geometry of alpine continental subduction we propose that the brianconnais units were scalped on top of the slab during ongoing continental subduction and exhumed continuously until collision

wallrock alteration in the bendigo gold ore field victoria australia uses in exploration the bendigo goldfield which has produced most of victoria s gold 22 million ounces 684 3 ton consists of numerous deposits located mainly along narrow parallel trending anticlinal domes separated by 100 to 400 m the individual domes are parts of a regional anticlinorium in lower ordovician sedimentary rocks highest gold ore values are generally located in the eastern limbs and particularly in the apical areas of the smaller individual anticlinal domes and in the central part of the more regional anticlinorium alteration present in metasandstone units in the nell gwynne anticline consists of phengitic sericite to 15 vol chlorite to 7 carbonates siderite sideroplesite ankerite and calcite to 10 and sulphides arsenopyrite pyrite chalcopyrite and sphalerite to 5 the alteration generally follows the trend of the structure extending upward and above mineralisation in anticlinal areas and below largely unmineralised synclinal areas phengitic sericite extends furthest to at least 150 m above mineralised saddle reef positions chlorite to 130 m sulphides to 80 m and carbonate to 50 m chlorite geothermometry suggests temperatures of 260 to 290 c for the alteration the sequence of carbonate precipitation generally is from siderite early sideroplesite ankerite and calcite the more fe rich carbonates tend to occur near the apical parts of the anticlinal domes arsenopyrite occurs within 10 m of the veins while hydrothermal pyrite chalcopyrite and sphalerite all extend to 50 m framboidal pyrite possibly greigite present in unaltered metasediments becomes progressively recrystallised and replaced by hydrothermal pyrite chalcopyrite and sphalerite as mineralisation is approached hydrothermal pyrite is generally euhedral and anomalous in having high as contents when compared to framboidal pyrite maximum au grades 205 ppm occur in the saddle reef position and below a thick shale unit lower grade mineralisation 0 5 6 0 ppm occurs above this in the overlying stratigraphy as and pb anomalies cross cut the alteration zoning and occur as plume like dispersion haloes above the mineralisation the au occurs in and near the as plume cu concentrations reach their maximum values adjacent to high au values measured co2 values confirm the observed carbonate distributions h2o and zn values in the section show relatively random distributions two separate stages of mineralisation are indicated the earlier localised within classic saddle reef structures and containing the highest au cu and some as and a later cross cutting stage of mineralisation associated with high as the alteration is mainly associated with the former

principles of mobilization dissolution of metals in mafic and ultramafic rocks the role of immiscible magmatic sulphides in the generation of hydrothermal gold and volcanogenic massive sulphide deposits abstract immiscible magmatic sulphides exert a major control over both the absolute precious and base metal contents of magmas and on the availability of these metals from rocks during ore formation it is concluded that only those mafic magmas that attained s saturation at a late stage are not depleted in the chalcophile elements and are hence suitable source rocks for metals in hydrothermal ore deposits immiscible magmatic sulphides are not only strongly enriched in the ore forming metals but they are also much more reactive than silicates the ore forming metals may be liberated from the sulphides at the sea floor alteration stage and deposited in interflow chemical sediments au deposits or volcanogenic massive sulphide deposits subsequent mobilization remobilization of au from the interflow sediments may concentrate au within the interflow sediments to form economic au deposits the mobilized remobilized au may also be transported to form epigenetic style gold deposits in archaean greenstone belts and in palaeozoic slate belts

dehydration of dioctahedral aluminous phyllosilicates thermodynamic modelling and implications for thermobarometric estimates we propose a solid solution model for dioctahedral aluminous phyllosilicates accounting for the main compositional variations including hydration observed in natural smectites interlayered illite smectite illites and phengites from diagenetic to high grade metamorphic conditions the suggested formalism involves dehydrated micas and hydrated pyrophyllite like thermodynamic end members with these end members the equilibrium conditions of quartz water k bearing mica like phyllosilicates of fixed 2 1 composition are represented by a line in p t space along which the interlayer water content varies the relevant thermodynamic properties required for the calculation of equilibrium conditions were derived using a set of 250 natural data of known maximal temperature and pressure conditions which covers a range between 25 c and few mpa to 800 c and 5 gpa the temperatures calculated at fixed pressure with our model are in fair agreement with those reported in the literature for the 250 natural data at low temperature and pressure the amount of interlayer water in k deficient phengite and illite is predicted to reach 100 of the apparent vacancies which is consistent with previous values reported in the literature although the amount of interlayer water is predicted to decrease with pressure and temperature it is calculated to be significant in k deficient phengite from lt hp pelites metamorphosed at about 350 c 10 kbar the presence of molecular water in the interlayer site of such phengites has been confirmed by ftir mapping its implications for p t estimates are discussed

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

a 300 000 yr coral reef record of sea level changes mururoa atoll tuamotu archipelago french polynesia abstract the accurate dating of fossil coral reefs is of prime importance in determining the timing of deglaciation events and thus understanding the mechanisms driving glacial interglacial cycles so far the most useful coral reef records of past sea level changes are those related to the last deglaciation and the last interglacial period u th ages for older isotopic stages are more limited due to the scarcity of datable material reflecting diagenetic alteration most data are from emergent parts of reefs and reef terraces in active subduction zones where relative sea level records may be biased by variations in rates of tectonic uplift new constraints on sea level changes over the past 300 000 yr are based on high precision u series age measurements of successive reef units on mururoa these have been cored in four continuous 300 m long drill holes with seaward inclinations of 30 to 45 on the northeastern rim of the atoll past sea level positions have been calculated from the radiometric ages of corals by correcting the present depth of subsurface horizons both for thermal subsidence and for depositional palaeodepth the location of this atoll at a considerable distance from former ice sheets far field minimises the influence of glacio isostatic rebound prominent units formed during four periods of relative sea level highstands including the holocene and stages 5 125 ka 7 212 ka and 9 332 ka these are primarily composed of coralgal frameworks that grew in very shallow water three periods of relative low stand correspond to stages 2 17 23 ka 4 60 ka and 8 270 ka during which small reefs developed in association with large bioclastic accumulations good agreement with the timing of sea level changes based on oxygen isotope measurements in deep sea cores is noted for most of the dated reef units we report here the first accurate coral record of the last glacial maximum in the pacific 135 143 m below the present sea surface suggesting that sea level may have been lower than expected during this period

the oil bearing carlin type gold deposits of yankee basin alligator ridge district nevada the carlin type gold orebodies of yankee basin in the alligator ridge district of northeastern nevada are unique for this deposit type in that they contain abundant oil this liquid hydrocarbon occurs as 1 primary and secondary fluid inclusions in calcite or realgar vein networks encircling the orebodies in variously fresh appearing to strongly decalcified silicified and collapse brecciated mississippian to devonian pilot shale and 2 vug and fracture filling free oil proximal to the orebodies in scattered residual pods and lenses of unoxidized basal pilot limestone the fluid inclusion and free oils are geochemically equivalent and have similar thermal maturities early to peak oil generation stage timing of entrapment of the fluid inclusion oils is paragenetically constrained as dominantly premineral and synmineral associated free oil could have arrived at any time prior to during or after mineralization but before late oil barren spelean calcite vein mineralization and subsequent supergene oxidation biomarker fingerprints and carbon isotope signatures indicate that the oils were self sourced from the pilot shale their concentration in the organically lean basal limestone suggests derivation from carbonaceous siltstones higher in the formation but in structurally lower configurations the yankee fluid inclusion oils were clearly involved in the gold mineralizing hydrothermal system but were not thermally degraded to pyrobitumen the analogous solid hydrocarbon characteristic of carlin type gold deposits this relationship suggests that the yankee system was cooler than the 175 degrees to 250 degrees c widely cited as typical for such mineralization a contention supported by independent geothermometers the oil bearing fluid inclusions all have homogenization temperatures lower than 150 degrees c with most less than 120 degrees c temperature sensitive biomarker transformation ratios of the oils expressed as equivalent vitrinite reflectance r o 0 75 0 95 suggest peak paleotemperatures no higher than about 145 degrees c these implied low system temperatures are consistent with the lack of evidence for a contemporaneous igneous heat source the fossil yankee basin hydrothermal system is similar in many ways such as paleotemperature host rocks hydrocarbons hydrothermal alteration and geochemistry to nearby active moderate temperature 120 degrees 130 degrees c but gold poor systems which encompass producing oil reservoirs numerous such warm systems have likely existed in the past in this region we suggest that many of them formed small carlin type gold deposits and or oil reservoirs that still await discovery

origin and deposition of a graphitic schist hosted metamorphogenic au w deposit macraes east otago new zealand the macraes gold tungsten deposit occurs in a low angle thrust system in biotite grade otago schist native gold scheelite pyrite and arsenopyrite are found in and adjacent to quartz veins and silicified schist of lenticular reef zones where the thrust system cuts through graphitic pelitic schist mineralization is confined to a shear zone up to 80 m thick which is closely sub parallel to the regional schistosity chemical alteration is dominated by silicification with some addition of cr and depletion of sr and ba alteration extends only about 5 m from major veins oxygen becomes isotopically heavier away from veins due to temperature decrease as hot fluids penetrated into cooler 250 c rock graphite within the shear zone rocks has reflectance of 6 7 in oil similar to graphite in medium high grade otago schist and is presumed to be metamorphic in origin this graphite has acted as a reducing agent to cause precipitation of gold where the thrust system acting as a conduit for metamorphic fluids intersects the graphitic schist the metals were derived from the underlying schist pile which may include an over thrust oceanic assemblage containing metal enriched horizons

modelling the effect of arbitrary p t t histories on argon diffusion in minerals using the macargon program for the apple macintosh argon diffusion in mineral grains has been numerically modelled using p t t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the earth s crust for long durations the macargon program1 generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion it can be run on an apple macintosh computer with arbitrary p t t histories used as input finite difference equations are used in the calculation of 40ar concentration profiles across individual diffusion domains the associated macspectrometer generates model spectra after a p t t history has been specified the form of model 40ar39ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40ar39ar apparent age spectra particularly in cases involving metamorphic rocks where complex p t t histories might apply although modelled spectra cannot be directly compared to experimentally determined 40ar39ar age spectra especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion they do provide insight into theoretically expected age spectra for samples that have experienced complex p t t histories

orogenic disseminated gold in phanerozoic fold belts examples from victoria australia and elsewhere abstract orogenic gold mineralisation in the lachlan orogen of southeastern australia and in other phanerozoic fold belts has generally been assumed to be confined entirely to quartz veins however an improved understanding of the tectonic structural and geochronological framework in which these occurrences form and recent advances towards the extent of wallrock alteration around these veins cast doubt on the validity of this deeply entrenched lode gold paradigm ore grade orogenic disseminated gold mineralisation has been recognised in a number of fold belts that are known to host significant orogenic lode gold deposits lachlan orogen australia buller terrane and otago schist new zealand meguma terrane canada tien shan mountains in uzbekistan and kyrgyzstan yana kolyma province baikal verhoyansk and allakh yun fold belts in north central mongolia and far east russia disseminated mineralisation in these collisional belts is invariably hosted by pervasively altered greenschist to subgreenschist facies rocks these occurrences also share a number of characteristics with some carlin type gold deposits in the great basin of western north america e g getchell regarding structural relationships fluid composition mineral paragenesis and a common association of carbonaceous matter with mineralisation features of alteration physico chemical characteristics of the ore bearing fluids and strong structural control in all of the studied occurrences point to a close genetic association with lode gold mineralisation the two styles are considered to represent end members of a crustal continuum of orogenic gold emplacement in phanerozoic fold belts with disseminated mineralisation more likely to develop at shallower levels and within more permeable units during the waning stages of uplift and exhumation these conditions facilitate ground preparation via brittle fracturing and the development of intricate stockwork systems efficient gold segregation from bisulphide complexes is largely controlled by fluid mixing of an evolved metamorphic fluid with either more oxidised ascending fluids or meteoric fluids and accelerated by the presence of carbonaceous matter at the site of deposition

hydrothermal gold mineralization in the witwatersrand basin the origin of the gold mineralization in the witwaterstrand basin of south africa the largest known gold province has been controversial for decades with arguments favouring detrital1 2 placer modified placer3 4 and hydrothermal5 6 origins here we present the results of an extensive geological study of witwatersrand rocks which show that the gold and associated uranium mineralization is hydrothermal in origin and postdates a regional high temperature alteration event alteration processes identified on a small scale can be mapped out regionally as roughly strata bound zones of acid metasomatism extending far into the basin the fluid flow responsible for this alteration was concentrated in small scale structures localized along lithological boundaries we find that the gold precipitated as a consequence of interactions of the fluid with shale derived hydrocarbons present within the basin

reply to terry j and goff j comment on late cenozoic sea level and the rise of modern rimmed atolls by toomey et al 2016 palaeogeography palaeoclimatology palaeoecology 4 51 73 83 this paper is not subject to u s copyright the definitive version was published in palaeogeography palaeoclimatology palaeoecology 469 2017 159 160 doi 10 1016 j palaeo 2016 11 028

a fluid inclusion study of the fosterville mine a turbidite hosted gold field in the western lachlan fold belt victoria australia abstract gold mineralisation at fosterville occurs in association with disseminated arsenopyrite and pyrite adjacent to a complex series of quartz carbonate veins along two nnw trending fault zones quartz carbonate veins which have formed in dilational fractures within the sulphide ore zone have 18 o values between 16 0 and 17 6 and contain the following types of fluid inclusions type ia two phase liquid rich inclusions with less than 10 vol vapour and no detectable gases type ib two phase inclusions with approximately 10 vol co 2 rich vapour type ii co 2 bearing inclusions with 30 vol vapour and type iii liquid h 2 o and liquid plus vapour co 2 inclusions with variable h 2 o co 2 ratios raman microprobe analysis showed that types ib ii and iii contain co 2 with variable amounts of n 2 and ch 4 in some inclusions type 1a aqueous inclusions from stage 4 of the paragenesis have salinities ranging from 5 to 7 wt nacl equivalent whereas the co 2 bearing inclusions in the same stage have average salinities around 3 5 4 0 wt nacl equivalent the co 2 bearing inclusions exhibited two different types of homogenisation behaviour vapour rich type ii and iii inclusions either homogenised to vapour or showed near critical behaviour by gradual fading of the meniscus with homogenisation temperatures from 234 to 384 c and a mode at 270 c liquid rich co 2 bearing inclusions homogenised to the liquid phase with a bimodal distribution the majority of inclusions homogenised between 243 c and 314 c with a mode around 270 c but a secondary mode occurs around 180 c this mode corresponds fairly closely to the mode observed for the aqueous type 1a inclusions at 170 c the variable salinities coexisting liquid and vapour rich inclusions and the overlap of the homogenisation temperatures suggest that phase separation has occurred during vein formation and the resulting depth of mineralisation is estimated to be between 2 6 and 5 7 km gold precipitation resulted from the partitioning of h 2 s into the vapour phase during fluid unmixing which was enhanced both by the decreasing pressure as fluid flowed from the faults and veins into the host rocks and by the addition of n 2 and ch 4 comparisons with available fluid inclusion data for the western lachlan fold belt indicates that the fosterville gold field formed at lower temperatures and at a higher crustal level than the quartz reef style of gold deposits in the bendigo ballarat zone the fluids at fosterville also contain greater amounts of n 2 and ch 4 suggesting that the mineralising fluids penetrated further into the host rocks and reacted either directly with carbonaceous matter or with reduced fluids contained within the host rocks

geochemistry of the la erma and qiongmo au se deposits in the western qinling mountains china abstract the microdisseminated gold deposits at la erma and qiongmo are typically hosted in cambrian chert formation in western qinling mountains china both in the host rocks and the gold ores selenium is present in high abundance as selenide minerals or isomorphous admixtures in sulfides with independent selenium orebodies can be delineated in some localities intergrowth of selenides and native gold is commonly observed selenium is also noticed in considerably high content in fluid inclusions in quartz and barite which are closely related to gold mineralization studies show that gold and selenium may have been transported together in ore solutions as au s se complex in view of the wide difference in electrode potential between selenium and sulfur the two elements would be separated under oxidizing conditions selenium would mostly occur as isomorphic mixtures in sulfides during the early stage of mineralization when the environment is more reducing but would precipitate as selenides during the late oxidizing stage simultaneous enrichment of gold and selenium would be favored by boiling of the ore solution or mixing of ore solution with oxygen rich water at shallow depths

wallrock alteration associated with turbidite hosted gold deposits examples from the palaeozoic lachlan fold belt in central victoria australia abstract the study of wallrock alteration phenomena around slate belt hosted gold deposits can provide valuable information regarding the physico chemical conditions of the ore bearing fluids and the timing relationships between deformation peak metamorphism granitoid emplacement and mineralisation thus studying the processes involved in the interaction of the hydrothermal fluids with the host rock is critical in order to constrain genetic models and ultimately as a tool for exploration largely due to the characteristics of the host rocks turbidite hosted mesothermal gold deposits were historically believed to lack such diagnostic alteration haloes and their study has long been neglected however recent studies in the palaeozoic lachlan fold belt in southeastern australia and in similar tectonic settings elsewhere have demonstrated that alteration around many of these deposits is more widespread and pervasive than previously thought a review of existing information and new data strongly support the viability of studying hydrothermal alteration as a guide to turbidite hosted mineralisation and as an indicator of the character of the solutions associated with ore deposition characteristically the auriferous reef structures are surrounded by broad bleached zones while disseminations of carbonate spots pyrite and arsenopyrite porphyroblasts are the most obvious features of alteration depending on parameters such as fault geometry fluid pressure and reactivity and permeability of the host rocks these disseminations can occur up to a kilometre away from the auriferous structures sericitisation and carbonatisation are commonly discernible within 3 to 10 m wide haloes wallrock alteration chemistry of turbidite hosted gold deposits in the lachlan fold belt shows consistent enrichments of k2o co2 s and as and the anomalous values of these elements also provide the most useful dispersion haloes for turbidite hosted gold exploration due to both their regularity and extent

overthrust terranes in the lachlan fold belt southeastern australia the paleozoic history of the lachlan fold belt involves terrane translation and accretion of discrete allochthonous fragments four major terranes developed adjacent to the gondwana continental margin and were overthrust in the middle devonian to form a partly composite crust for southeastern australia

geochemistry and mineralogy of contrasting supergene gold alteration zones southern new zealand abstract supergene alteration of gold deposits has had important geochemical effects that changed the mineralogy and textures of near surface exposures and can affect the economics of gold mines by enhancing gold liberation and increasing gold grades this study links observed mineralogy with 655 water analyses from environments relevant to supergene processes in the otago schist goldfield of southern new zealand and these analyses are used as proxies for past supergene processes there are two contrasting supergene alteration styles in otherwise identical host rocks and mineralisation zones in this area long term groundwater alteration of sediments and underlying basement near to a regional unconformity developed progressively since the cretaceous this alteration has affected orogenic gold deposits exposed at the unconformity and has resulted in supergene enrichment of gold with localised formation of centimetre scale gold nuggets alteration processes occurred under circumneutral ph conditions ph 6 8 5 and was accompanied by extensive oxidation and clay alteration of basement rocks and sediments gold was mobilised as thiosulphate and bisulphide complexes and was reprecipitated by redox changes near the unconformity the chemical processes of supergene enrichment were primarily abiotic with minor bacterial facilitation of late stage gold mobility at the micron scale in contrast supergene alteration of eroding freshly exposed mountain outcrops has occurred in the late cenozoic without au enrichment apart from liberation and minor particle size enhancement the ambient ph of the associated waters was also circumneutral but localised and temporary acidification to ph 3 has occured on the metre scale or less in sulphidic outcrops this supergene alteration was dominated by oxidation but some low redox minerals senarmontite and secondary stibnite have formed during the early stages of oxidation or subsequent reduction sulphide decomposition with associated localised acidification and gold remobilisation was probably enhanced by bacterial processes in these exposed outcrops