cyclic fluid infiltration in structurally controlled ag pb cu occurrences schladming eastern alps abstract polysulphide occurrences in the schladming crystalline complex eastern alps are associated with the intersection of moderately n dipping shear zones type i structures and nw se and ne sw trending steep faults type ii structures geochemical and mineralogical profiling across the type i structures shows progressive mineral alteration feldspar breakdown silicification sericitisation from the margin to the center these observations are best interpreted by an external acidic fluid infiltrating these structures such alteration halos in deep seated structures are excellent leaching sites and transport channels for available metals however precipitation of ore took place at the contact with carbonates where metal bearing complexes became unstable as a result of fluid neutralisation the origin of this carbonate is linked with type ii structures showing distinct carbonatisation but no wall rock alteration it is therefore suggested that the ore occurrences in type i and type ii structures were formed during the cyclic infiltration of an acidic and a co 2 bearing fluid fluid inclusions from quartz carbonate ore veins are consistent with this interpretation pt estimates and the orientation of shear zones and faults are consistent with the geodynamic setting of cretaceous cooling and exhumation of the austroalpine realm east of the tauern window the exhumation of the schladming crystalline complex is therefore interpreted as the setting for a cyclic process of increased fluid pressure vein opening fluid infiltration ore precipitation and vein closure

u pb evidence for abitibi gold mineralization postdating greenstone magmatism and metamorphism two widely held theories for the origin of archaean lode gold deposits the magmatic1 and metamorphic replacement2 models predict that gold emplacement should have occurred near the end of late syn kinematic greenstone magmatism here we report that at the camflo mine in the abitibi subprovince of canada u pb ages for rutile and titanite in auriferous alteration 2 625 7 myr are much younger than the age of the host quartz syenite 2 680 4 myr and ages for regional magmatism 2 670 myr 3 and metamorphism 2 680 myr 4 mineral associations and pb pb systematics indicate that the hydrothermal minerals record crystallization ages that are probably coeval with the gold mineralization these ages and others from the abitibi subprovince4 6 rule out a direct connection between gold and magmatism or metamorphism associated with regional folding of the greenstone belt they are broadly contemporaneous with metamorphism at deep crustal levels7 and may record late devolatilization of the lower crust

role of plutonism in low pressure metamorphic belt formation wickham and oxburgh1 recently proposed that low pressure high temperature low p high t metamorphism in the eastern pyrenees and possibly all low p high t metamorphic belts resulted from anomalously high mantle heat flow brought about by rifting their model is largely constrained by the presence of nearby synmetamorphic rift related sedimentary rocks and the interpretation that the migmatites and granites are the product of in situ melting in the presence of an anomalously steep geotherm here we present an alternative model in which low p high t metamorphism pro grade reactions at pressures near or below the al2sio5 triple point results from contact effects near sill like igneous intrusions at intermediate crustal levels low p high t conditions can be achieved through this process in regions of continent continent collision with normal mantle heat flux as well as in zones of extension our model is based on studies of the low p high t metamorphic terrane in the new england appalachians

lode gold mineralization in the tanami region northern australia the tanami region of northern australia has emerged over the last two decades as the largest gold producing region in the northern territory gold is hosted by epigenetic quartz veins in sedimentary and mafic rocks and by sulfide rich replacement zones within iron formation although limited geochronological data suggest that most mineralization occurred at about 1 805 1 790 ma during a period of extensive granite intrusion although structural relationships suggest that some deposits predate this period there are three main goldfields in the tanami region the dead bullock soak goldfield which hosts the world class callie deposit the granites goldfield and the tanami goldfield in the dead bullock soak goldfield deposits are hosted by carbonaceous siltstone and iron formation where a late d5 structural corridor intersects an early f1 anticlinorium in the granites goldfield deposits are hosted by highly sheared iron formation and are interpreted to predate d5 the tanami goldfield consists of a large number of small mostly basalt hosted deposits that probably formed at a high structural level during d5 the d5 structures that host most deposits formed in a convergent structural regime with 1 oriented between e w and ene wsw structures active during d5 include ne trending oblique thrust dextral faults and ese trending sinistral faults that curve into n to nnw trending reverse faults localized in supracrustal belts between and around granite complexes granite intrusions also locally perturbed the stress field possibly localizing structures and deposits forward modeling and preliminary interpretations of reflection seismic data indicate that all faults extend into the mid crust in areas characterized by the n to nw trending faults orebodies also tend to be n to nw trending localized in dilational jogs or in fractured competent rock units in areas characterized by ese trending faults the orebodies and veins tend to strike broadly east at an angle consistent with tensional fractures opened during e w to ene wsw directed transpression many of these deposits are hosted by reactive rock units such as carbonaceous siltstone and iron formation ore deposition occurred at depths ranging from 1 5 to 11 km from generally low to moderate salinity carbonic fluids with temperatures from 200 to 430 c similar to lode gold fluids elsewhere in the world these fluids are interpreted as the product of metamorphic dewatering caused by enhanced heat flow although it is also possible that the fluids were derived from coeval granites lead isotope data suggest that lead in the ore fluids had multiple sources hydrogen and oxygen isotope data are consistent with both metamorphic and magmatic origins for ore fluids gold deposition is interpreted to be caused by fluid unmixing and sulfidation of host rocks fluid unmixing is caused by three different processes 1 co2 unmixing caused by interaction of ore fluids with carbonaceous siltstone 2 depressurization caused by pressure cycling in shear zones and 3 boiling as ore fluids move to shallow levels deposits in the tanami region may illustrate the continuum model of lode gold deposition suggested by groves mineralium deposita 28 366 374 1993 for archean districts

petrogenetic character and provenance of metabasalts in the aspiring and torlesse terranes south island new zealand implications for the gold endowment of the otago schist we report quantitative x ray diffraction whole rock geochemical and sm nd isotope data for metabasalts from the aspiring and torlesse terranes in the south island of new zealand these rocks underlie the mesozoic metasedimentary otago schist which is anomalously enriched in gold and host to at least one world class orogenic gold deposit at macraes 125 t au geochemical and nd isotopic similarities between the samples point to a common history of the two terranes rather than the amalgamation of one or two allochthonous plate fragments furthermore geochemical and nd data suggest the metabasalts within both terranes formed in an oceanic but essentially non subduction related setting the origin of the aspiring and torlesse basalts can be linked to the formation of an oceanic plateau that had resulted from a permian mantle plume initiation event proximal to a mid oceanic rise or triple junction given the intrinsically gold enriched nature of certain oceanic character mafic rocks the anomalous gold endowment of the otago schist may have been enhanced via the accretion and subduction of a gold enriched oceanic plateau fragment the metabasalts are generally enriched in gold up to 13 ppb compared to their enclosing metasedimentary rocks typically ca 1 ppb with sulphide rich metabasaltic rocks having up to 550 ppb au however the relatively small volume of metabasalts in the otago schist precludes these rocks as the principal source for otago schist orogenic gold with a primarily metasedimentary source of the gold potentially having a limiting effect on the overall endowment of the otago schist this approach that employs petrogenetic fingerprinting of potentially fertile source rocks for the assessment of gold endowment might prove useful in the conceptual exploration targeting of relatively immature and poorly exposed terrains

exhumation driven devolatilization as a fluid source for orogenic gold mineralization at the damang deposit ghana gold mineralization at the damang deposit is unique among currently known orogenic gold deposits in ghana comprising gold hosted within metasediments of the tarkwaian system and contained in a subhorizontal extensional quartz vein array that formed during regional compression the damang region has an extended paragenesis involving numerous structural metamorphic igneous and metasomatic events orogenic gold mineralization occurred late in the geologic paragenesis at damang postdating regional metamorphism and an earlier episode of hydrothermal alteration locally termed pink hematite alteration associated with the intrusion of mafic sills and dikes this earlier pink hematite alteration event involves extensive silicification that changed the rheology of the altered rocks and promoted later fracturing following peak regional metamorphism at around 2005 ma the damang region underwent a short period of rapid exhumation as constrained through numerical thermal modeling of existing pressure temperature time data this exhumation triggered the generation of a subhorizontal fracture array that was fed by fluids released through decompression driven metamorphic devolatilization the interaction between these fluids and the host rock resulted in precipitation of gold in association with sulfide carbonate potassic alteration halos around quartz veins such postpeak metamorphic exhumation driven devolatilization is unlikely to be a singular occurrence and represents a potentially important source of fluid for orogenic gold deposits elsewhere in ghana west africa and globally

paragenesis and geochemistry of ore minerals in the epizonal gold deposits of the yangshan gold belt west qinling china six epizonal gold deposits in the 30 km long yangshan gold belt gansu province are estimated to contain more than 300 t of gold at an average grade of 4 76 g t and thus define one of china s largest gold resources detailed paragenetic studies have recognized five stages of sulfide mineral precipitation in the deposits of the belt syngenetic diagenetic pyrite py0 has a framboidal or colloform texture and is disseminated in the metasedimentary host rocks early hydrothermal pyrite py1 in quartz veins is disseminated in metasedimentary rocks and dikes and also occurs as semi massive pyrite aggregates or bedding parallel pyrite bands in phyllite the main ore stage pyrite py2 commonly overgrows py1 and is typically associated with main ore stage arsenopyrite apy2 late ore stage pyrite py3 arsenopyrite apy3 and stibnite occur in quartz calcite veins or are disseminated in country rocks post ore stage pyrite py4 occurs in quartz calcite veins that cut all earlier formed mineralization electron probe microanalyses and laser ablation inductively coupled plasma mass spectrometry analyses reveal that different generations of sulfides have characteristic of major and trace element patterns which can be used as a proxy for the distinct hydrothermal events syngenetic diagenetic pyrite has high concentrations of as au bi co cu mn ni pb sb and zn the py0 also retains a sedimentary co ni ratio which is distinct from hydrothermal ore related pyrite early hydrothermal py1 has high contents of ag as au bi cu fe sb and v and it reflects elevated levels of these elements in the earliest mineralizing metamorphic fluids the main ore stage py2 has a very high content of as median value of 2 96 wt and au median value of 47 5 ppm and slightly elevated cu but relatively low values for other trace elements arsenic in the main ore stage py2 occurs in solid solution late ore stage py3 formed coevally with stibnite contains relatively high as median value of 1 44 wt au fe mn mo sb and zn and low bi co ni and pb the main ore stage apy2 compared to late ore stage arsenopyrite is relatively enriched in as whereas the later apy3 has high concentrations of s fe and sb which is consistent with element patterns in associated main and late ore stage pyrite generations compared with pyrite from other stages the post ore stage py4 has relatively low concentrations of fe and s whereas as remains elevated 2 05 3 20 wt which could be interpreted by the substitution of as for s in the pyrite structure these results suggest that syngenetic diagenetic pyrite is the main metal source for the yangshan gold deposits where such pyrite was metamorphosed at depth below presently exposed levels the ore forming elements were concentrated into the hydrothermal fluids during metamorphic devolatilization and subsequently during extensive fluid rock interaction at shallower levels these elements were precipitated via widespread sulfidation during the main ore stage

integrated model for meguma group lode gold deposits nova scotia canada meguma group lode gold deposits represent concentrations of a variety of auriferous quartz veins in lower paleozoic metaturbidites of the meguma terrane of southern nova scotia our studies incorporating extensive field regional and detailed mapping and laboratory petrography fluid inclusions geochemistry isotopes studies of 30 of these deposits in the eastern meguma terrane suggest that mineralization occurred at ca 370 ma 40 ar 39 ar dating of hydrothermal vein minerals during widespread incursion of mafic and felsic magmatism into the crust concurrent with subvertical shear zone development thus postdating regional acadian deformation by some 30 40 m y isotopic data s c o sr indicate that vein forming fluids were not solely magmatic and were in part exotic to the meguma group thus earlier magmatic and metamorphogenic lateral secretion models that considered either late stage magmatic fluids or the enveloping meguma group metasedimentary rocks as the sole source of vein components are invalidated

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

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

ore controls in the charters towers goldfield ne australia constraints from geological geophysical and numerical analyses the approach taken in this paper namely synthesising a wealth of previous information with new data and a genetic model in combination with integrated numerical analyses led to new insights into the geological controls on the localisation of auriferous veins and residual prospectivity of the charters towers goldfield ne australia the method also has implications for the assessment of other mature goldfields worldwide despite a number of different ore controls having operated within the charters towers goldfield the controlling factors can be linked to a single genetic model for orogenic granitoid hosted lode gold mineralisation in a brittle deformation regime d4 of ne sw to nne ssw shortening under conditions of supralithostatic fluid pressure and low stress difference spatial autocorrelation results suggest district scale alignment of the auriferous veins parallel to and overlapping with the ese wnw to e w striking charters towers ravenswood lineament a major crustal boundary in the basement to the ravenswood batholith at the camp scale auriferous veins have abundance and proximity relationships with nw se nnw sse ne sw and ene wsw oriented lineaments suggesting that structures that controlled gold deposition in one camp did not necessarily control mineralisation in other camps fractal dimensions obtained with the box counting method range from 1 02 to 1 10 whereas veins in the charters towers city camp are characterised by a significantly higher fractal dimension of 1 28 this discrepancy may be taken to imply that most or all outcropping and near surface deposits within the charters towers city camp have been found and that new discoveries are more likely to occur at greater levels of depth or outside the boundaries of this camp the new understanding has implications for the assessment of the residual prospectivity of the charters towers goldfield where large areas of prospective rock types and structures e g approximately 40 of the charters towers ravenswood lineament are hidden under cover this parameter space was inaccessible to the historic prospectors and has received relatively little attention from recent explorers the following steps are suggested for the development of a targeting strategy for lode gold exploration in areas of the goldfield under cover 1 identify from geological and geophysical data the ene wsw 15 and nnw sse 15 striking structures and geological boundaries within a 20 km wide corridor parallel to and centred upon the charters towers ravenswood lineament the potential control on gold deposit distribution at the regional to district scale 2 interpret from geological and geophysical data the distribution of pre middle devonian granitoids within these areas that are the preferred host rocks of the payable gold deposits 3 deduce from geophysical data the ene wsw 15 and nnw sse 15 striking structures that cut or bound the intrusions identified in step 2 4 locate segments along the structures identified in step 3 that deviate most from the geometry of a straight line e g potential bends or splays and or intersect other structures or geological contacts or both and 5 define and rank potential targets within the prospective areas identified in step 4 and systematically test the best ones

mayum an orogenic gold deposit in tibet china abstract located in western tibet china the mayum orogenic gold deposit was discovered in 2002 with estimated resources of 80 tonnes gold the gold mineralization is hosted by neoproterozoic cambrian schists and is controlled by nearly parallel e w trending bedding concordant fracture zones the au orebodies are composed of auriferous quartz veins and altered rocks with the au grades ranging from 2 23 g t to 69 56 g t and containing the 34s values of 18 separates of sulfides from auriferous quartz veins show a large variation from 0 2 to 16 8 the lead isotope compositions of sulfides from the gold ore are characterized by highly radiogenic values and very wide ranges of ratios 18 324 to 20 819 for 206pb 204pb 15 679 to 15 856 for 207pb 204pb and 38 401 to 41 204 for 208pb 204pb these data indicate that lead and sulfur in the quartz veins have multiple sources and were derived from the different wall rocks as the fluid flowed through them fluid inclusions indicate that the ore fluid was co2 rich with salinities mainly between 1 and 6 wt nacl equiv and homogenization temperatures predominantly ranging from 260 to 280 c the 18o values for quartz from auriferous veins range from 13 7 to 16 3 and the calculated 18oh2o values in equilibrium with quartz vary from 5 54 to 9 48 it suggests that the ore fluid may be derived from deep metamorphic deep crustal gold transporting fluids although a contribution from magmatic source cannot be ruled out 40ar 39ar age dating on the sericite from the alteration associated with the auriferous quartz veins in the mayum gold deposit gives a plateau like age of 59 34 0 62 ma later than the onset of the indo asian collision it is believed the gold mineralization was related to the indo asian collision and was formed during the early stage of orogenesis

gold occurrences of the archean north atlantic craton southwestern greenland a comprehensive genetic model abstract the north atlantic craton of southwestern greenland hosts several orogenic gold occurrences although to date none is in production four gold provinces are distinguished and include godthabsfjord tasiusarsuaq paamiut and tartoq in the godthabsfjord gold province the hypozonal gold occurrences are aligned along the major ca 2660 2600 ma ivinnguit fault orogenic gold mineralization correlates temporally with and is related to ductile deformation along this first order structure the northern part of the tasiusarsuaq gold province is characterized by small hypozonal gold occurrences that are controlled by 2670 2610 ma folds and shear zones auriferous fluids were focused into the structures in both gold provinces during west directed accretion of the kapisilik terrane 2650 2580 ma to the already amalgamated terranes of the north atlantic craton in the southern part of the tasiusarsuaq gold province hypozonal gold mineralization is hosted in back thrusts sermilik prospect and thrusts bjornesund prospect that formed at 2740 ma and 2860 2830 ma respectively the deformation is related to the ca 2850 ma accretion of the sioraq block and the tasiusarsuaq terrane and the 2800 2700 ma accretion of the tasiusarsuaq terrane and the faeringehavn and tre brodre terranes mesozonal orogenic gold mineralization is hosted in an accretionary complex in the paamiut and tartoq gold provinces gold occurrences cluster over a strike extent of approx 40 km in thrusts and complex strike slip settings in lateral ramps the timing of the e vergent terrane accretion in both areas is unknown and could either be at ca 2850 ma or 2740 ma in the eastern part of the paamiut gold province quartz veins and associated alteration zones were overprinted by granulite facies metamorphism and show evidence for partial melting these outermost parts of the accretionary complex were involved in burial exhumation tectonics during crustal accretion mainly three different orogenic stages related to gold mineralization are distinguished in the north atlantic craton between ca 2850 ma and 2610 ma these are generally accretionary tectonic episodes and gold mineralization is hosted either in reactivated fault systems between terranes or accretionary complex structures along the deformed cratonic margin the larger orogenic gold occurrences formed at ca 2740 2600 ma that appears to be a period of orogenic gold mineralization globally although significant gold resources in the north atlantic craton have yet to be identified

a model for low pressure facies metamorphism during crustal thickening low pressure metamorphic facies i e high t p ratios are widespread in a wide range of tectonic settings explanations offered for the occurrence of these facies include extensional and or magmatic models however these fail to explain that the low p facies metamorphism is commonly coeval with a phase of pervasive crustal thickening with t p ratios increasing during or slightly lagging behind the thickening we propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion thinning of the mantle lithosphere

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

link between ridge subduction and gold mineralization in southern alaska 40 ar 39 ar geochronology reveals that turbidite hosted gold deposits in the southern alaska accretionary prism are the same age as nearby near trench plutons these early tertiary plutons and gold lodes formed above a slab window during subduction of an oceanic spreading center ridge subduction is a previously unrecognized tectonic process for the generation of lode gold

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

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

lithostratigraphy geochronology and gold metallogeny in the northern guiana shield south america a review abstract with a surface area of nearly 900 000 km 2 the guiana shield represents the northern segment of the amazonian craton in south america lying for the most part between the amazon and orinoco river basins most of the guiana shield formed during protracted periods of intense magmatism metamorphism and deformation culminating with the trans amazonian tectono thermal event bracketed between 2 1 and 1 9 ga the guiana shield is among the least known precambrian terranes because it is relatively inaccessible lacks bedrock exposure due to intense weathering and is poorly documented in the international geological literature this situation has significantly improved during the last 20 years when shallow in situ gold occurrences attracted exploration and mining companies to initiate geological programs aimed at better understanding the geology and the mineral deposits of the shield the only archean terrane ca 3400 ma known to date in the guiana shield is the imataca complex in venezuela the paleoproterozoic low grade volcano sedimentary greenstone sequences and associated granitoid intrusions have yielded ages between 2 25 and 2 08 ga recent u pb age determinations of the granitoid greenstone belts suggest protracted magmatic cycles from pre to post peak regional metamorphism the younger terranes comprise anorogenic sedimentary sequences of the roraima formation as well as felsic volcanic rocks and associated intrusions of the uatuma formation mafic dikes of the avanavero suite and rapakivi type and alkali intrusions several large scale ductile shear zones have been documented in the guiana shield in northcentral venezuela the most outstanding structure documented to date the ne sw trending guri fault juxtaposes the archean imataca complex against paleoproterozoic terranes the central guiana shear zone cgsz extends from french guiana westerly towards central suriname and further west towards northcentral guyana where it matches with the makapa kuribrong shear zone mksz in french guiana the north guiana trough ngt is interpreted as a sinistral strike slip formed during the trans amazonian orogeny most gold deposits and occurrences discovered to date in the guiana shield are sited in close proximity to major structures in addition they are linked with low to medium metamorphic grade granitoid greenstone belts similar to other better explored precambrian terranes at a local scale the gold deposits are hosted within or in close proximity to quartz veins that are syn to late tectonic and to a lesser extent in stockworks breccias and lenses they are commonly located in units that behaved in a more brittle manner than the country rocks available information suggests that gold deposits are mainly epigenetic although some are associated with specific lithostratigraphic units pyrite pyrrhotite chalcopyrite galena sphalerite scheelite molybdenite and tellurides are the main metallic minerals associated with gold non metallic minerals are mainly quartz and carbonates ankerite calcite siderite associated with minor chlorite epidote albite muscovite and fuchsite silica carbonate propylitic and potassic alteration is common high erosion rates expected after the creation of an orogenic belt did not occur in the northern guiana shield shallow level deposits preserved in many settings suggest that the granitoid greenstone belts represent first order exploration targets for large tonnage low grade gold deposits

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

tectonic and economic implications of trace element 40ar 39ar and sm nd data from mafic dykes associated with orogenic gold mineralisation in central victoria australia abstract mafic to intermediate dykes are spatially and temporally closely associated with major post tectonic granitic complexes in the western lachlan orogen of se australia these dykes which range petrographically from basaltic to andesitic are concentrated within several north to northwest trending zones and were emplaced during two broad intervals of extensive magmatic activity during the silurian devonian period geochemical and sm nd isotopic characteristics of these mafic intrusives are consistent with their formation in a complex subduction related tectonic setting interaction between mantle wedge material sinking oceanic crust and input from the overlying continental crust resulted in the petrological and geochemical variations displayed by these and more felsic dykes throughout the study region field evidence and 40 ar 39 ar data show that in the eastern part of the stawell zone and in the northwest portion of the bendigo zone mafic dyke were intruded between 410 and 400 ma late silurian early devonian further emplacement in the bendigo zone and the eastern part of the melbourne zone took place at between 375 and 365 ma middle to late devonian episodic mantle derived magmatism was possibly related to step wise rollback slab detachment or changes in the angle and rate of westward subduction in response to periodically occurring accretionary pulses a close spatial and temporal relationship also exists between the dykes and orogenic gold mineralisation in the central victorian gold province mafic to intermediate dykes both crosscut and are host to mineralisation in a number of goldfields although there is little evidence for a direct genetic association the two processes are linked by the common utilisation of translithospheric structures which facilitated the rapid ascent into shallow crustal levels of both mantle derived magma and crustal scale ore forming fluid systems previous studies have suggested that transfer of heat into the crust via ascending mafic mantle magmas can provide a thermal engine which triggers and sustains extensive crustal melting thus explaining the commonly observed close association of mafic to intermediate and felsic intrusive suites this study supports the viability of this mechanism and in addition indicates that a causal link exists between the formation of mantle magmas in collisional zones and the generation of orogenic gold deposits

tectonic constraints on the timing relationships of metamorphism fluid production and gold bearing quartz vein emplacement abstract there is an ongoing discussion about the genetic significance of the postmetamorphic emplacement of many gold mineralizations as a contribution to this discussion this paper reviews the timing relationships between metamorphic devolatilization and metamorphism at different crustal levels for different tectonic processes for example denudation accompanying or following thickening causes metamorphism to occur later at depth than at shallow levels thus many internally heated terrains e g barrovian terrains are of deep later type that is peak metamorphism and devolatilization at deeper levels occurs at later times metamorphic processes of the deep later type may produce and release metamorphic fluids during prograde metamorphism at depth and precipitate them in the form of postmetamorphic quartz veins at shallow levels magmatic heating from below on the other hand is a deep earlier process causing inverse timing relationships in many metamorphic terrains deep later and deep earlier processes may compete for example if denudation at the surface accompanies heating from below if denudation and heating from below are of similar duration deep later characteristics prevail and a strongly bi modal age distribution of peak metamorphism in different crustal levels may arise the upper crust will experience metamorphism early but largely contemporaneously and the lower crust will experience metamorphism much later this process is particularly favourable for substantial simultaneous devolatilization of large sections of the lower crust to postdate metamorphism at shallow levels thus such processes may result in late infiltration events in cooling rocks of low metamorphic grade the common observation of postmetamorphic fluid infiltration and mineralization may therefore indicate but does not require that the mineralising fluids are related to the same metamorphic event

metallogenic relationships to tectonic evolution the lachlan orogen australia abstract placing ore formation within the overall tectonic framework of an evolving orogenic system provides important constraints for the development of plate tectonic models distinct metallogenic associations across the palaeozoic lachlan orogen in se australia are interpreted to be the manifestation of interactions between several microplates and three accretionary complexes in an oceanic back arc setting in the ordovician significant orogenic gold deposits formed within a developing accretionary wedge along the pacific margin of gondwana at the same time major porphyry cu au systems formed in an oceanic island arc outboard of an evolved magmatic arc that in turn gave rise to granite related sn w deposits in the early silurian during the ongoing evolution of the orogen in the late silurian to early devonian sediment hosted cu au and pb zn deposits formed in short lived intra arc basins whereas a developing fore arc system provided the conditions for the formation of several volcanogenic massive sulphide deposits inversion of these basins and accretion to the australian continental margin triggered another pulse of orogenic gold mineralisation during the final consolidation of the orogenic belt in the middle to late devonian

a holistic model for the origin of orogenic gold deposits and its implications for exploration the term orogenic gold deposits has been widely accepted but there has been continuing debate on their genesis early syn sedimentary or syn volcanic models and hydrothermal meteoric fluid models are now invalid magmatic hydrothermal models fail because of the lack of consistent spatially associated granitic intrusions and inconsistent temporal relationships the most plausible models involve metamorphic fluids but the source of these fluids is equivocal intra basin sources within deeper segments of the hosting supracrustal successions the underlying continental crust subducted oceanic lithosphere with its overlying sediment wedge and metasomatized lithosphere are all potential sources several features of precambrian orogenic gold deposits are inconsistent with derivation from a continental metamorphic fluid source these include the presence of hypozonal deposits in amphibolite facies domains their anomalous multiple sulfur isotopic compositions and problems of derivation of gold related elements from devolatilization of dominant basalts in the sequences the phanerozoic deposits are largely described as hosted in greenschist facies domains consistent with supracrustal devolatilization models a notable exception is the jiaodong gold deposits of china where ca 120 ma gold deposits are hosted in precambrian crust that was metamorphosed over 2000 million years prior to gold mineralization other deposits in china are comparable to those in the massif central and elsewhere in france in that they are hosted in amphibolite facies domains or clearly post date regional metamorphic events imposed on hosting supracrustal sequences if all orogenic gold deposits have a common genesis the only realistic source of fluid and gold is from devolatilization of a subducted oceanic slab with its overlying gold bearing sulfide rich sedimentary package or the associated metasomatized mantle wedge with co2 released during decarbonation and s and ore related elements released from transformation of pyrite to pyrrhotite at about 500 c although this model satisfies all geological geochronological isotopic and geochemical constraints and is consistent with limited computer based modeling of fluid release from subduction zones the precise mechanisms of fluid flux are model driven and remain uncertain from an exploration viewpoint the model re emphasizes the ubiquitous occurrence of orogenic gold deposits in subduction related orogenic belts and importance of continental scale lithosphere tapping fault and shear zones to focus large volumes of auriferous fluid it confirms the importance of the consistent spacing between world class deposits broadly equivalent to the depth of the moho as derived from empirical observations

link between gold provinces low salinity high temperature 200 degrees c reducing fluids are poorly represented in ore forming processes yet appear to account for 90 percent of the primary gold mined to date such fluids have been implicated in the formation of many archcan greenstone witwatersrand slate belt and epithermal gold deposits these low salinity auriferous fluids occur in variable time space lithological sequence and tectonic settings and are most easily explained by metamorphic devolatilization of mafic and or graywacke successions metamorphism of chlorite calcite albite quartz assemblages e g mafic or graywacke at the greenschist amphibolite facies boundary occurs around 480 degrees or 20 degrees c for 3 5 kbars and produces large volumes of low salinity h 2 o co 2 fluid similar in composition to those recorded in many gold deposits widespread pyrite in the above assemblage typically leads to elevated levels of dissolved sulfur such conditions are ideal for gold transport as a molecular au s complex and can lead to deposits rich in gold relative to silver and base metals deposition of gold occurs in the temperature range of 250 degrees to 400 degrees c and can be facilitated by lower temperature lower sulfur activity and changes in oxygen activity lower sulfur activity is most readily achieved by fluid interaction with fe rich host rocks with the formation of pyrite or pyrrhotite whereas lower oxygen activity is readily achieved by interaction with carbonaceous metasediments low salinity fluids produced by metamorphic devolatilization provide a common theme embracing the formation of many archcan greenstone slate belt witwatersrand and potentially epithermal gold deposits it involves the common features of high geothermal gradient low salinity fluid high au ag high au base metals and broad synchroneity with metamorphism it also accounts for the considerable variability in hosting structures detailed timing relations host rocks alteration assemblages and geologic age of different deposits interestingly the similarities are explained by deeper processes the differences by processes at or near the depositional sites

metamorphism in archaean greenstone belts calculated fluid compositions and implications for gold mineralization an inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions for mafic and ultramafic sequences the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits xco2 0 2 0 3 this result follows from the calculation of mineral equilibria in the model system cao mgo feo al2o3 sio2 h2o co2 using a new expanded internally consistent dataset greenstone metamorphism cannot have involved much crustal over thickening because very shallow levels of greenstone belts are preserved such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere in this case the observed metamorphism which was contemporaneous with deformation is of the low p high t type for this type of metamorphism the metamorphic peak should have occurred earlier at deeper levels in the crust i e the piezothermal array should be of the deeper earlier type however at shallow crustal levels the piezothermal array is likely to have been of deeper later type as a consequence of erosion thus while the lower crust reached maximum temperatures and partially melted to produce the observed granites mid crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde we propose that these fluids are responsible for the gold mineralization thus the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution and does not mean that the mineralization has to be a consequence of igneous processes upward migration of metamorphic fluid via appropriate structurally controlled pathways will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower xco2 these assemblages are therefore grossly out of equilibrium with the fluid in the case of infiltrated metabasic rocks intense carbonation and sulphidation is predicted if as seems reasonable gold is mobilized by the fluid generated by devolatilization then the combination of processes proposed most of which are an inevitable consequence of the metamorphism leads to the formation of greenstone gold deposits predominantly from metamorphic fluids

intrusion hosted mineralization in the charters towers goldfield north queensland new isotopic and fluid inclusion constraints on the timing and origin of the auriferous veins auriferous quartz sulfide veins of the charters towers goldfield are mainly hosted in oxidized i type granitoids of the ravenswood batholith k ar and ar ar isotope age data of hydrothermal muscovite from alteration envelopes of veins at charters towers and the hadleigh castle mine 40 km east of charters towers are indistinguishable within error suggesting broadly synchronous gold deposition between 410 and 404 ma and across a significant segment of the ravenswood batholith published geochronological data indicate that several granitoid bodies were emplaced into the ravenswood batholith at the time of gold mineralization despite their association in space and time with igneous rocks the gold deposits lack an obvious causative intrusion moreover published lead isotope studies indicate that the lead in the ore was not acquired from any of the exposed intrusions a distal origin of the ore forming fluids also may be inferred from nitrogen isotope values of hydrothermal sericite suggesting wall rock interaction with metamorphic fluids or fluids that were in equilibrium with metamorphic rocks during the paragenetic stage of pyrite and arsenopyrite deposition stage ii veins of the charters towers goldfield contain three different types of fluid inclusions which are distinguishable by petrography and microthermometry this study is the first to report saline fluid inclusions in sphalerite 18 9 28 3 wt nacl equiv and vein quartz 20 9 24 7 wt nacl equiv trapped during the stage of gold deposition stage iii the range of temperatures and salinities particularly of the saline inclusions in sphalerite could indicate mixing between hotter more saline fluids e g deep seated magmatic and cooler more dilute solutions e g modified meteoric as the cause of gold deposition the geological and geochemical data are not compatible with derivation of fluids metals and ligands from individual plutons similarity of host rocks ore element associations alteration assemblages structural controls and tectonic settings strongly suggest that the auriferous veins of the charters towers goldfield belong to a group of granitoid hosted lode gold deposits that are generally classified as orogenic

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

bimodal distribution of gold in pyrite and arsenopyrite examples from the archean boorara and bardoc shear systems yilgarn craton western australia this study investigates the microstructures geochemistry and hydrothermal evolution of gold bearing pyrite and arsenopyrite from six orogenic gold deposits in the archean eastern goldfields province western australia scanning electron microscope sem electron microprobe emp and laser ablation inductively coupled plasma mass spectroscopy la icp ms analyses show that the gold bearing minerals possess a number of similar textural features including the occurrence of invisible gold within initial phases of growth and later stage visible gold associated with alteration rims the alteration rims are characterized by a higher than average atomic mass mainly owing to arsenic enrichment and are preferentially located along fractures and grain boundaries in the pyrite and arsenopyrite these observations suggest that visible gold formation is associated with hydrothermal alteration of preexisting pyrite and arsenopyrite textural observations and la icp ms data suggest that some invisible gold was remobilized from early formed pyrite and arsenopyrite to form visible gold during development of these alteration rims gold may also have been added by hydrothermal fluids during a later stage of mineralization in situ geochemistry and phase relationships of alteration rims are used to further constrain the hydrothermal process responsible for formation of alteration rims and visible gold in fractures based on sulfide stability relations our data indicate that development of arsenopyrite alteration rims associated with late stage visible gold formation was related to an increase in temperature maximum increase from 310 to 415 c and up to of sic orders of magnitude increase in sulfur fugacity whereas changes in oxygen fugacity were less important la icp ms analyses show that the relative and absolute variations in selected trace element au ag sb bi ba te pb co and mo concentrations can also be used to distinguish between unaltered and altered pyrite and arsenopyrite in general trace elements within pyrite and arsenopyrite have a relatively uniform distribution whereas later stage alteration rims have more variable trace element distributions although the observed textures are typical of prograde metamorphic coronae we suggest that they are the consequence of variations in fluid conditions and chemistry and that mineralization occurred in response to syn and or postpeak metamorphic fluid infiltration

orogenic gold mineralization at the chah bagh deposit muteh gold district iran abstract the chah bagh gold deposit in the muteh gold district is located in the central part of the sanandaj sirjan zone ssz iran gold mineralization at chah bagh is hosted by a paleozoic sequence of rocks that is dominated by deformed schist metarhyolite and amphibolite that exhibits a greenschist to lower amphibolite facies metamorphism three deformation events are recognized in the chah bagh area d1 d2 and d3 the major nw trending n280 n290 dextral strike slip shear zone in the area was formed during d2 ductile events gold mineralization at chah bagh occurred over a prolonged deformation history but is closely related to alteration retrograde greenschist facies assemblages and ductile and brittle deformation during d2 and d3 the geometry of the au bearing quartz veins indicates that they are temporally related to the s2 foliation and therefore to the d2 flattening and shearing some veins however are spatially and temporally related to d3 brittle normal faults and are brecciated and boudinaged during the associated shear movement the presence of deformed au bearing quartz veins and their concordant and discordant relation with respect to the main mylonitic foliation and the shear zone indicates continuous mineralization during the d2 and d3 episodes the au hosting shear zones are characterized by extensive development of heterogeneous mylonitic rocks that enhanced the permeability within the shear zones this gave rise to further extensive dilatancy within major dilational jogs and produced a suitable structural regime for vein hosted au mineralization the epigenetic au mineralization resulted from metamorphic hydrothermal fluids circulating through major shear zones and associated structures during the late stages of orogeny our investigation shows that granitic intrusions have no genetic link with gold mineralization and we propose an orogenic gold model for chah bagh deposit similar to qolqoleh and kervian in the northwestern part of the ssz this model is consistent with a dextral transpressional deformation kinematic model along the ssz and refutes the previous intrusion related model suggested for the muteh gold deposits metamorphic devolatilization and fluid flow within a ca 30 m y long period in late cretaceous and early tertiary is suggested for the genesis of the gold occurrences in both ductile and post transpressional brittle structures

a two stage model for the formation of the granite greenstone terrains of the kalgoorlie norseman area western australia abstract accurate dating of granites and greenstones from the kalgoorlie norseman area shows that there is considerable overlap in time between the formation of granites and greenstones however within any given area the oldest granites are always younger than the oldest greenstones it is suggested that the long lived 100 ma thermal anomaly in the uppermost asthenosphere that gave rise to the greenstones is also responsible for regional metamorphism and widespread crustal melting leading to the production of granites the time lag between greenstone and granite formation reflects the time taken to conduct heat from the mantle to the lower crust anatexis of the lower crust produces a layer of light granitic magma that may temporarily stop the ascent of basaltic magma where this happens the basaltic magma ponds at the base of the crust supplying additional heat and accelerating the melting process eventually the granitic melt layer becomes unstable and rises within the crust areas intruded by granite become topographic highs with topographic lows forming above areas where light melt has been withdrawn from the lower crust when the topographic highs protrude above sea level they are eroded to shed sediments into the topographic lows which contain the basaltic greenstones thus the change from a basalt dominated lower greenstone succession to a sediment and felsic volcanic dominated upper whitestone sequence seen in many granite greenstone terrains results from the intrusion of granitic magma into the upper crust the crust of the kalgoorlie norseman area formed in two distinct stages separated by a time break of up to 700 ma the first stage involved the formation of primitive crust probably above subduction zones this primitive crust was characterized by low rb sr and high sm nd ratios the second stage resulted from a major change in mantle convection that produced thermal upwellings in the mantle below the continents this led to the eruption of basaltic magmas and to metamorphism and anatexis of the lower crust to produce granitic magmas melting reset the rb sr sm nd and u pb systems giving the impression that the second event which occurred at 2 7 ga was a major crust forming event it was in fact a crustal reworking process although some basalt was added to the crust at this time although the two stage history of the kalgoorlie norseman area is clearly seen in the lead isotopes and in zircons it is not readily discernible from the rb sr and sm nd systematics due to mantle like ratios of those elements in the primitive crust

geology geochronology and sulphur isotope geochemistry of the black schist hosted haoyaoerhudong gold deposit of inner mongolia china implications for ore genesis abstract the haoyaoerhudong gold deposit is located in the urad middle banner area of inner mongolia in china and is hosted by black schist of the bilute formation in the mesoproterozoic bayan obo group the gold mineralization is within a 4500 m long and 100 m wide zone associated with sulphide quartz veins following the schistosity and crosscutting fractures in the host rocks locally as disseminated sulphide in altered black schist the sulphide mineral assemblage is pyrrhotite pyrite arsenopyrite chalcopyrite sphalerite and is associated with a weak hydrothermal alteration characterized by a silica halo with variable amounts of biotite sericite and carbonate the bilute formation was intruded by multi stage dykes these include metamorphosed porphyritic diorite that pre dates mineralization unmetamorphosed amphibole olivine gabbro that pre dates mineralization and tourmaline bearing pegmatite that post dates mineralization in order to constrain the timing of the gold mineralization and host rocks zircons from the porphyritic diorite amphibole olivine gabbro and tourmaline bearing pegmatite were extracted for u pb dating gold bearing pyrite samples were analysed for 34 s to help determine the possible source of s in the mineral zones a weighted mean 206 pb 238 u age date of 288 3 ma mswd 2 4 was obtained for the porphyritic diorite a weighted mean 206 pb 238 u age date of 282 1 ma mswd 0 58 was obtained from the amphibole olivine gabbro and a weighted mean 206 pb 238 u age date of 182 1 ma mswd 1 04 was obtained from the tourmaline bearing pegmatite these dates constrain the timing of gold mineralization to between 282 1 and 182 1 ma and are consistent with the hydrothermal alteration ages of ca 267 246 ma reported from the literature these new zircon dates indicate that the regional metamorphism of the bayan obo group ceased by the time the haoyaoerhudong gold mineralization was deposited and the genesis of mineralization is not related to the regional metamorphic event the gold bearing pyrites gave positive 34 s v cdt values between 6 8 and 13 4 and are different from the 34 s widely distributed values for the typical black rocks indicating that the sulphur is not derived from the black bilute formation but sourced from some other relatively uniform reservoir widespread contemporaneous granitic magmatism is developed in the haoyaoerhudong area and we suggest that the sulphur forming the gold bearing sulphides is probably sourced from a carbonate contaminated granitic magma reservoir the new zircon dates for the dykes and the 34 s data for gold bearing sulphides indicate that the gold mineralization did not form from metamorphosed black schist in the bayan obo group but is associated with magmatic hydrothermal activity

tellurium magmatic fluids and orogenic gold an early magmatic fluid pulse at cononish gold deposit scotland significant tellurium enrichment occurs in many orogenic gold deposits but the factors causing this are little understood some authors suggest this demands a magmatic input whereas others suggest it need not fractionation of te from se and s could offer insight into source pathway processes of auriferous fluids the metasedimentary hosted cononish vein gold deposit scotland is unusually te rich compared to many orogenic gold deposits with te au 2 4 whereas most orogenic deposits have te au 1 here ag in au ag alloy increases from 10 to 90 wt through the paragenesis correlating with decreasing hessite ag2te abundance this suggests the au ag alloy composition was controlled by the fluid te activity and that this decreased through time this is coupled to an increase in pyrite 34s from 2 0 to 11 4 through the paragenesis thus the deposit formed from a primary fluid with a low 34s and high te au ag that evolved to a high 34s low te pb cu bearing fluid the high 34s of the later fluid suggests it can only be sourced from specific nearby metamorphosed sedex horizons the early fluid that deposited most of the gold could be sourced from other metasedimentary units in the stratigraphy or be magmatic in origin we argue that two observations taken together suggest it is most likely that this fluid was magmatic the age of the mineralisation is identical to the last stage of crystallization of nearby granite batholiths and the fluid has a s isotope signature consistent with a magmatic source gold deposits in orogenic belts are almost certainly polygenetic and this study demonstrates evidence for te rich orogenic deposits having a significant magmatic component

mechanical consequences of granite emplacement during high t low p metamorphism and the origin of anticlockwise pt paths high temperature t low pressure p metamorphic belts preserve evidence of metamorphism under extremely perturbed and hence transient thermal regimes pt paths are commonly anticlockwise that is the maximumt was attained prior to or at the same time as the maximump with cooling at near constant or increasingp and in many terrains peak metamorphic temperatures prevailed during convergent deformation using a simple coupled thermal mechanical model that assumes a strongly temperature dependent rheology for the continental lithosphere we show that the coincidence of metamorphism with convergent deformation in high t low p terrains may reflect the thermal weakening attendant with rapid advective movement of heat within the lithosphere via granite magma ascent the model shows that the magnitude of the thermal weakening effect is sensitive to the critical temperature for granite segregation tm crit the depth of emplacement of the magmas and most importantly the granite forming mechanism during the prograde cycle material points attain maximum temperatures during a transient 3 ma high strain rate pulse withe up to 10 14 s 1 at levels near the site of granite emplacement the attainment of maximum pressures follows peak temperatures with rapid cooling paths that are close to isobaric or involve slight compression

origin and classification of the late triassic huaishuping gold deposit in the eastern part of the qinling dabie orogen china implications for gold metallogeny the huaishuping gold deposit is located in the xiong ershan mountains of the qinling dabie orogen of central china the mineralisation is structurally controlled and hosted by faulted rhyolite quartz andesite volcaniclastic rocks and volcanic breccia assigned to the jidanping formation towards the top the palaeoproterozoic xiong er group the deposit has a resource of around 32 t with an average grade of 5 5 g t au alteration at the deposit progressed from an early k feldspar quartz pyrite assemblage through quartz pyrite gold quartz base metal sulfides to a late stage assemblage of quartz carbonate the 34s v cdt values for pyrite in the ore range from 13 3 to 1 6 the calcite has c isotopes ranging from 6 1 to 2 5 v pdb and o isotopes from 10 6 to 15 8 v smow the 18o quartz ranges from 10 5 to 15 1 and the d values for fluid inclusions in quartz range from 93 to 76 the 56fe value for the mineralisation varies between 0 1 and 0 5 with corresponding 57fe values between 0 2 and 0 7 the isotope systematics indicates that the hydrothermal fluids were derived from metamorphic fluid but the source of gold remains uncertain re os dating of molybdenite yields a date of 202 8 ma interpreted as the age of the gold mineralisation this age is consistent with the triassic onset of extensional tectonics following the collision between north and south china

lithospheric controls on the formation of provinces hosting giant orogenic gold deposits ages of giant gold systems 500 t gold cluster within well defined periods of lithospheric growth at continental margins and it is the orogen scale processes during these mainly late archaean palaeoproterozoic and phanerozoic times that ultimately determine gold endowment of a province in an orogen a critical factor for giant orogenic gold provinces appears to be thickness of the subcontinental lithospheric mantle sclm beneath a province at the time of gold mineralisation as giant gold deposits are much more likely to develop in orogens with subducted oceanic or thin continental lithosphere a proxy for the latter is a short pre mineralisation crustal history such that thick sclm was not developed before gold deposition in constrast orogens with protracted pre mineralisation crustal histories are more likely to be characterised by a thick sclm that is difficult to delaminate and hence such provinces will normally be poorly endowed the nature of the lithosphere also influences the intrinsic gold concentrations of potential source rocks with back arc basalts transitional basalts and basanites enriched in gold relative to other rock sequences thus segments of orogens with thin lithosphere may enjoy the conjunction of giant scale fluid flux through gold enriched sequences although the nature of the lithosphere plays the crucial role in dictating which orogenic gold provinces will contain one or more giant deposits the precise siting of those giants depends on the critical conjunction of a number of province scale factors such features control plumbing systems traps and seals in tectonically and lithospherically suitable terranes within orogens

gold mineralization as a consequence of continental collision an example from the southern alps new zealand abstract formation of gold quartz veins within the alpine schist near the alpine fault was synchronous with uplift the temperature of vein formation was greater than 320 c at a depth of 4 5 km vein precipitation was due to dilution and cooling of metamorphic metal bearing fluids by cooler less saline meteoric fluids numerical modeling of the topographic and thermal processes associated with continental collision has identified the time and space scales necessary for the generation of the geochemical conditions which resulted in vein genesis a predictable topographic pattern of 1500 m high ridges normal to the plate boundary leading to the main divide is established within the first 500 ka of uplift producing the head driven flow patterns of meteoric circulation the high thermal inertia of a rapidly uplifted region provides the thermal driving force leading to buoyancy driven convection of metamorphic fluids when the geothermal gradient is in excess of 50 c km using boundary conditions derived from the southern alps elevation of temperatures to 300 c at 5 km occurs after 1 8 ma of uplift at which time gold formation may occur if the intersection of head and buoyancy driven fluids lies within greenschist facies rocks further uplift causes erosion of the greenschist facies rocks together with the gold deposits adjacent to the fault and migration of the loci of mineralization further into the uplifting block the results of the examination of the southern alps gold regions are extended to other phanerozoic and archean deposits which are spatially related to major fault zones the characteristic topographic and thermal behavior of major collisional zones provides the driving forces necessary for gold mineralization to occur in the absence of any igneous heat source

on the timing relationship between fluid production and metamorphism in metamorphic piles some implications for the origin of post metamorphic gold mineralisation abstract the relative timing of metamorphism at different crustal levels is quantified for a range of metamorphic environments associated with convergent deformation we characterise different metamorphic environments by making simple assumptions about the nature of heat input and the nature of heat withdrawal from the terrane if internal heat production is the major heating agent during crustal thickening then deep crustal levels reach their metamorphic temperature peak later than shallow levels in terranes heated from below such relationships apply only if erosion commenced early in the thermal history or if erosion rates are fast quantitative knowledge of these relations can be used together with simple assumptions about prograde fluid release to predict the likely timing of fluid passage through rocks higher in the column in terranes with deep later characteristics the evidence for fluid penetration may be preserved in the form of mineralised quartz veins or late stage alteration within our simple parameterisation of crustal heat sources we can use the grade of a metamorphic rock together with field evidence on the timing of metamorphism igneous intrusion and deformation in order to constrain the time scale of the underlying thermal perturbation and erosion process for a given terrane we demonstrate that time intervals up to several tens of million years may separate metamorphism and fluid emplacement in model evolutions that are scaled to describe greenschist facies metamorphic terranes that formed at low pressure such timing relationships are in fact recorded in many greenschist facies metamorphic terranes that host late stage mesothermal gold deposits we suggest therefore that there is no conflict between the observation that the emplacement of many gold deposits occurred demonstrably after peak metamorphism of their host rocks and the interpretation that metamorphic fluids are responsible for the mineralisation

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

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

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

origin of the intrusion related lang vai gold antimony district northeastern vietnam constraints from fluid inclusions study and c o s pb isotope systematics abstract the lang vai ore district located in the central part of the lo gam domain in the northeast vietnam contains at least three deposits khoun puc lang vai and lung luong the general paragenetic sequences for these deposits may be represented by the following mineral associations from early to late i arsenopyrite pyrite native gold sphalerite ii bi rich pb sb sulfosalts iii bi poor pb sb sulfosalts iv sb sulfosalts v sb rich minerals native antimony and vi pb rich sb sulfosalts as a distal mineral association three major types were recognized within the primary fluid inclusions based on the phases present at room temperature including v type vapor rich inclusions l type liquid rich inclusions and c type co2 rich inclusions the early au as stage quartz crystals contain the v and l type fluid inclusions the results indicate that the initial ore forming fluid belonged to a co2 ch4 h2o nacl system characterized by high temperatures 290 352 c and variable to moderate salinities 3 16 13 89 wt nacl which are analogous to reduced intrusion related ore forming fluids that exhibit high homogenization temperatures and moderate salinities the quartz crystals from the bi pb sb mineral paragenesis occurred only at the khoun puc deposit contain both v and l type fluid inclusions and characterized by high temperatures mainly 304 340 c and moderate salinities 6 28 8 22 wt nacl equiv the quartz crystals from the middle hydrothermal stage contain the v and l type fluid inclusions that yield moderate homogenization temperatures 240 290 c and variable salinities 3 3 8 2 wt nacl equiv these features indicate that the fluid evolved into an co2 ch4 h2o nacl system at moderate temperatures lower ch4 concentrations and variable salinities thus indicating changing physicochemical conditions in the last sb stage the lang vai deposit and pb sb stage the lung luong deposit the ore forming fluid belongs to co2 h2o nacl system which is characterized by low temperature 180 240 c enrichment of co2 and medium low salinity 1 12 9 5 wt nacl equiv that corresponded to typical fluids which formed the sb and distal ag pb zn veins of the irgs deposits in the tintina gold province of alaska usa and yukon canada the fluid inclusion studies and laser raman spectroscopy suggest that the early fluids were dominated by ch4 whereas the late fluids by co2 our study suggests that the fluid boiling and accompanying decrease in the solubility of gold bisulfide complex played an important role in gold deposition at the khoun puc and lang vai deposits the trapping pressures estimated from the boiling fluid inclusions were 123 343 mpa during the ore forming process this suggests an alternating lithostatic hydrostatic fluid system controlled by a fault valve activity at a depth of 12 9 15 2 km the characteristics of the s isotopes and fluid inclusions suggest that the ore forming fluids were of magmatic origin during mineralization process the fluid rock interaction between the fluids and host rocks resulted in the increase of 34s values of the fluids as reflected in the range of sulfur isotope from the different deposits on a plumbotectonic diagram the sulfides show correlations between 206pb 204pb and both 207pb 204pb and 208pb 204pb which suggest a two component mixing between a high radiogenic end member and another low radiogenic end member the 13cpdb vs 18osmow study clear show that c and o in the khoun puc ore forming fluids were most likely to be derived from the magma with limited contribution from the carbonate host rocks predominantly magmatic origin of hydrothermal fluids wherein the calcite separates from the lang vai and lung luong deposits have 18osmow values higher than those of the khoun puc suggesting that the carbonate host rocks may have provided more c and o for ore forming fluids in these deposits the results combined with existing data on the regional geology ore geology ore geochemistry age of mineralization and tectonic setting indicate that the lang vai ore district is an intrusion related gold system