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

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

redetermination and reevaluation of compositional variations in metamorphosed sediments of the littleton formation new hampshire in a study of metamorphosed pelites of the devonian aged littleton formation new hampshire shaw 1954 found little evidence for large scale compositional change other than devolatilization because his data are still used for such studies we have reanalyzed his original samples using more precise methods and for additional elements to provide a more useful data set since shaw s study remapping in new hampshire has shown that 21 samples from the original set of 67 appear to be from underlying silurian formations the new data for the remaining 46 littleton samples show statistically unambiguous differences between sample sets in average concentration ratios to al2o3 or tio2 for several elements on a loss on ignition free basis the data do not however show systematic compositional differences as a function of metamorphic grade variations in element concentrations within sample sets of low medium and high metamorphic grade are as large or larger than differences in average concentrations between pairs of sample sets this renders the method of comparing average compositions intrinsically insensitive even to substantial compositional change during metamorphism we find no compelling evidence in support of metamorphic gain or loss of the elements we studied but taken at face value the statistical uncertainties would allow considerable gain or loss of most elements during metamorphism of littleton pelites further detailed sampling would be required to determine whether the samples of each metamorphic grade are representative and to interpret the compositional variations and differences accurately in terms of metamorphic element mobility on the one hand and inheritance from the heterogeneous sedimentary protolith on the other

archean turbidite hosted orogenic gold mineralization in the gadag greenstone belt western dharwar craton peninsular india abstract turbidite hosted orogenic gold mineralization in the archean gadag greenstone belt of the western dharwar craton forms a major auriferous zone central auriferous zone extending over a strike length of about 12 km in the gadag duplex the turbidite sequence comprises thick inter bedded medium to coarse grained lithic graywacke and thin laminated layers of fine grained carbonaceous phyllite gold bearing quartz veins impregnate preferentially along the en echelon shear planes fractures and schistosity planes auriferous quartz veins are enveloped by the altered wall rocks mineralogy of the auriferous zone is dominated by gangue minerals like quartz ankerite chlorite sericite and carbonaceous matter with subordinate plagioclase monazite and xenotime are the important accessory minerals arsenopyrite and pyrite are the major sulfide minerals but pyrrhotite chalcopyrite sphalerite galena and scheelite are also present gold in native state occurs within quartz silicates and arsenopyrite notable distinctions in mineral assemblage texture and in chemical compositions of altered wall rocks compared to the precursor host rock in the study area implies that the metasomatism and wall rock alterations are the results of pervasive infiltration and intense interaction between hydrothermal fluids and the surrounding host rocks over a prolonged period sulfides carbonates carbonaceous matter k2o mgo cao cr ni cu pb zn as and higher values of gold 0 98 4 72 ppm are added into the altered wall rocks immediately enveloping the auriferous quartz vein bodies the chondrite normalized ree pattern of altered wall rocks exhibits enriched lree lan ybn av 9 54 with prominent negative eu anomaly the observed variation in geochemical characteristics and mineral assemblages in the alteration zones indicates differential response of the host rock and intensity of alteration depending on the composition of host rocks and hydrothermal fluids the auriferous hydrothermal fluids were of low salinity 2 0 to 6 6 wt nacl dominated by co2 h2o about 30 mol co2 with moderate densities 0 7 to 1 04 g cm3 and gold deposition occurred over a wide temperature range between 175 c and 325 c gold deposition was influenced by fluid mixing phase separation and redox reactions mixing between co2 h2o fluids and more reduced fluids which evolved during fluid reaction with adjacent carbonaceous wall rocks was the key factor causing gold deposition the formation of the gadag duplex deformation folds and reverse strike slip faults discontinuities was caused by the compression associated with subduction related tectonic processes during the initial period of intrusive magmatism 2 555 6 ma regional metamorphism occurred in the entire greenstone belt while during later period hydrothermal fluids responsible for gold mineralization probably were derived from metamorphic processes as well as from intrusive granites such fluids channeled through the thrust in host turbidite sequence carrying dissolved gold associated metals and sulfur ultimately were precipitated in a reducing environment in the splays to the thrust in the gadag duplex at about 2 522 6 ma resulting in retrograde alteration assemblages

further intensification of deep convection in the labrador sea in 2016 there has been a progressive deepening of winter convection in the labrador sea since 2012 with the individual profile maximum depth exceeding 1800 m since 2014 and reaching 2100 m in 2016 this increase during repeated positive phases of the winter north atlantic oscillation nao resembles that during the formation of the record depth 2500 m labrador sea water lsw class in 1987 1994 attributed to repeated positive nao forcing having provided critical preconditioning the 2012 2016 lsw class is one of the deepest and most persistent ever observed back to 1938 year round observations from profiling argo floats since 2002 complemented by annual surveys are providing novel information on the seasonal to decadal evolution of lsw such as its variable density the recent multiyear preconditioning and its 2016 density being the highest since the mid 1990s these findings should help international observation programs and numerical model studies investigating lsw influences on the subpolar north atlantic and atlantic meridional overturning circulation

local and regional mass transfer during thrusting veining and boudinage in the genesis of the giant shale hosted paracatu gold deposit minas gerais brazil multiscale structural and geochemical studies have been applied to understand the genesis of the paracatu deposit in brazil a shallow dipping bulk tonnage vein style low grade au ag pb zn orebody hosted in 1000 ma black phyllites of the paracatu formation associated with intense shearing accompanying thrusting during the 680 ma brasiliano orogeny massive to laminated quartz sulfide carbonate veins and associated alteration and au mineralization formed early in the deformation history and these veins were boudinaged during subsequent progressive increase in shear strain geochemical profiles across the ore allow recognition of a relatively homogeneous protolith at 100 m scales with respect to ti al zr v and rare earth elements including a footwall with protolith attributes similar to those of the ore oxygen hydrogen and sulfur isotopes do not reveal a distinctive external fluid source rather they reflect fluid rock equilibration with the host phyllites during greenschist facies regional metamorphism detailed geochemical sampling around a prominent population of smaller boudinaged veins shows that si ca and sr were removed and al k and ti were residually concentrated during formation of synboudinage dark selvages this process of mass transfer occurred at scales more local than the processes responsible for ore genesis despite the local processes of chemical equilibration and redistribution many of the main ore components appear externally derived at scales broader than the orebody gold as ag sb bi and to some extent pb and zn show a strong spatial and statistical correlation with boudinaged quartz sulfide carbonate veins and these are surrounded by a broader 10 50 m vertical scale halo of enriched k ba and volatiles loss on ignition with depleted na and sr the majority of gold is present as inclusions in pyrite and arsenopyrite within and immediately surrounding the veins and gold bearing pyrite shows mineral chemistry consistent with a hydrothermal origin the main population of large veins locally preserve preboudinage complex internal vein textures including sulfide rich wall rock laminae similar to laminated fault fill veins in many orogenic style vein deposits these larger veins contain most of the gold in the paracatu resource metals were most likely sourced distal to the paracatu formation and were precipitated in sheeted shear or tensile veins during the early stages of the brasiliano thrusting the oblate strain recorded by the boudinaged mineralized veins is anomalous and distinctive to paracatu at km scales even though the thrusting is regional although the orebody may have been physically dislocated by thrusting from the place where the early veins formed the anomalous strains recorded suggest a perturbation in the footwall to the regional thrust or perhaps a spatially restricted competency contrast within the stratigraphy which contributed to the localization of the present mineralization during the thrusting

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

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

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

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

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

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

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

the touquoy zone deposit an example of unusual orogenic gold mineralisation in the meguma terrane nova scotia canada the touquoy zone deposit is host to disseminated gold mineralisation in metasiltstones of lower palaeozoic age from the close correlation between ore grades and the intersection of favourable stratigraphy and bounding faults it is apparent that mineralisation is controlled by both structural and lithological influences within the ore zone disseminated gold arsenopyrite pyrite and rare base metal sulphides are associated with a network of widely spaced millimetre scale quartz carbonate veinlets quasi pervasive fluid flow and prolonged interaction with the host rocks resulted in a diffuse but pronounced halo of wall rock alteration that is characterized by the breakdown of detrital feldspar and metamorphic chlorite and the development of hydrothermal carbonate phases k mica and disseminated sulphides these mineralogical changes are accompanied by enrichment in co2 k au as and s and depletion in na across the ore zone vein formation occurred at between 250 and 350 c and pressures of less than 1 2 kbar 1 kbar 100 mpa corresponding to an estimated depth of between 2 8 and 6 km mineralisation resulted from the unmixing of an overpressured low salinity co2 rich fluid in response to decreasing pressure during its ascent and penetration into permeable host rocks geological and geochemical features of mineralisation in the touquoy zone deposit are seen as convincing evidence for a close genetic association between disseminated style and lode gold mineralisation in orogenic terrains with the resulting style of mineralisation largely controlled by the overall structural geometry of the mineralising site rheological properties permeability and chemical receptiveness of the host rock and structural level of emplacement

hydrothermal transport and depositional processes in archean lode gold systems a review abstract compelling evidence now exists for the formation of archean lode gold deposits over a substantial range in metamorphic conditions and crustal depth the commonality of ore fluid isotopic and geochemical characteristics the extreme fluid rock ratios within actual ore zones and the observed depth continuity within individual orebodies all suggest that these deposits formed within vertically extensive crustal scale hydrothermal systems calculations based on new thermodynamic data and estimated ore fluid conditions suggest that gold would have been transported as au hs 2 over most of this range gold transport in the ore fluids that formed some diopside and diopside k feldspar class deposits t 550 c would have been as aucl2 whereas the au hs complex may have been significant in some of the lower temperature 270 c less alkaline ore fluids the high solubility of gold in lode gold ore fluids at amphibolite facies conditions 10 s to 1000 s ppb suggests that fluids may have been undersaturated in gold within their source regions promoting the ore fluid s capacity to retain gold in solution over the large transport distances involved ore depositional mechanisms were also likely to vary between crustal levels fluid rock interactions and wallrock sulphidation in particular appear to be the most important type of precipitation mechanism however with decreasing depth of formation the scope of other depositional mechanisms notably phase separation and fluid mixing becomes greater

estimating vertical motion profile shape within tropical weather states over the oceans abstractthe vertical structure of tropical deep convection strongly influences interactions with larger scale circulations and climate this paper focuses on investigating this vertical structure and its relationship with mesoscale tropical weather states the authors test the hypothesis that latent heating plus turbulent flux convergence varies in space and time in association with weather state type the authors estimate mean state vertical motion profile shape and latent heating plus turbulent flux convergence for six weather states defined using cloud top pressure and optical depth properties from the international satellite cloud climatology project isccp dataset assuming two modes of vertical motion profile variability these modes are statistically extracted from reanalysis data using a principal component analysis using these modes and the relationship between vertical motion the dry static energy budget and mass continuity the authors estimate vertical motion profile shape in these estim

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

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

structural controls on hydrothermal alteration and gold antimony mineralisation in the hillgrove area nsw australia the hillgrove gold antimony deposit is hosted in late palaeozoic biotite grade metasedimentary rocks and permo carboniferous granitoid intrusions of the new england orogen mineralisation occurred at a range of structural levels during rapid uplift in the orogen at 255 245 ma hydrothermal fluids were controlled by extensional faults in a regional scale sinistral strike slip fault system principal faults in this system were developed in and possibly evolved from mylonite zones which were active during late permian tectonics earliest mineralisation formed scheelite bearing quartz veins and these were followed by auriferous arsenopyrite pyrite quartz carbonate veins with minor base metal sulphides this latter type was accompanied by sericitisation and carbonation of the host rock with addition of sulphur arsenic and gold in zones up to 20 m from veins quartz stibnite veins with electrum gold aurostibite and arsenopyrite form a prominent and economically important hydrothermal type with little wall rock alteration but extensive hydrothermal breccia formation and local open space filling textures below a mining depth of 300 500 m this type passes over a short distance downwards into stibnite poor gold bearing veins late stage carbonate stibnite veins with gold and silver sulphosalts cut all earlier veins and have open space filling textures aspects of the hillgrove deposit have similarities to many other orogenic gold deposits in the sw pacific which have been formed at different structural levels hillgrove is distinctive in having evidence for mineralisation at this wide range of structural levels in the one deposit formed progressively during syn orogenic uplift

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

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

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

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

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

a comparison of orogenic gold mineralisation in central victoria aus western south island nz and nova scotia can implications for variations in the endowment of palaeozoic metamorphic terrains abstract the palaeozoic western lachlan orogen in victoria se australia the buller terrane western south island new zealand and the meguma terrane nova scotia canada are remarkably similar in regard to their geological age and history structural make up and the setting and formation of orogenic lode and disseminated style gold deposits within these fold belts a comprehensive review of the principal characteristics of orogenic gold deposits in each of the three historically important gold provinces illustrates the many similarities but it also reveals several major geological structural tectonic geochemical and geochronological differences that may account for the disparity in contained gold and overall endowment between the three fold belts aspects that stand out as potentially playing a critical role in the generation of a phanerozoic world class orogenic gold province include 1 the presence of a hydrated oceanic character substrate that can provide a fertile source rock for both fluids and metals 2 asthenospheric thermal input to trigger and sustain crustal devolatilisation and melting 3 the existence of a number of near vertical deep seated faults 4 substantial transcurrent movement and 5 evolution of an accretionary subduction system that promotes development of an extensive fore arc system with prolonged fluid generation and circulation quantification of these controlling factors provide a significantly improved understanding of the processes that control the genesis of orogenic gold deposits and also have the potential to enable discrimination between more and less gold favourable orogens and parts of orogens

hydrothermal alteration zoning and gold concentration at the kerr addison archean lode gold deposit kirkland lake ontario kerr addison a major archcan lode gold deposit of the abitibi belt with a production of approximately 10 x 10 6 oz au approximately 320 metric tons up to 1983 is localized in a zone of talc carbonate and chlorite carbonate rocks immediately south of a regional structural break the kirkland lake larder lake fault zone relict volcanic textures taken in conjunction with the ratios of diag nostic relatively less mobile elements collectively indicate that most of the wall rocks of the mine were originally mafic and ultramafic rocks along with their clastic derivatives regional greenschist facies metamorphism is locally superposed by a distinct zonation of gold associated hydrothermal alteration characterized by massive introduction of co 2 coupled with variable enrichments of alkali metals and calcium which produced systematic trends of alteration mineral assemblages alteration types of the initial stage contain chlorite which is replaced by muscovite or albite in the advanced stage of alteration with a concomitant increase in the proportion of carbonate minerals numerous generations of crosscutting veins are enveloped by alteration borders in which the sequence of the al bearing minerals namely albite mus covite and chlorite develop from the vein to the wall rocks always in a systematic sequence the same sequence is observed at a larger scale from the ore zone to the periphery of the mine area the intensity of alteration can be monitored by saturation indices co 2 ca fe mg for carbonates and 3k na al 3k al and na al for muscovite albite muscovite and albite respectively molar ratios plots of the moving averages of these indices across the 3 850 ft level of the mine reveal a remarkable correspondence to variations of gold abundance suggesting a genetic relationship of gold enrichment to hydrothermal alteration on a broad scale gold is concentrated to the south of the kirkland lake larder lake fault zone and at a more detailed scale in the transition zone between carbonate muscovite and carbonate albite alteration types during advanced alteration fluid x sub co 2 was approximately 0 1 and na k asymp 10 volcanics of the kinojevis and larder lake groups are 18 o enriched relative to pristine tholeiites and possess distinct mineral delta 18 o values probably due to different conditions of marine water alteration during or shortly after extrusion in domains of advanced alteration these are all shifted down to a common narrow bracket here 18 o 16 o and d h data indicate fairly constant temperatures 270 degrees 300 degrees c and a common isotopic composition of hydro thermal fluids delta 18 o 5ppm delta d 30 to 50ppm across the alteration zoning these results are in common with many other archcan lode gold deposits fe dolomite has a uniform delta 13 c value of 2 8 or 0 42 per mil 1sigma throughout the footwall and ore zones but it is enriched by approximately 1 per mil in the graphite free hanging wall rocks fluid delta 13 c value was 4 or 1 per mil reflecting either a magmatic carbon reservoir or mixtures of marine carbonate carbonaceous and magmatic sources the enrichment of gold at one side of the kirkland lake fault zone is interpreted to be the result of development of a metasomatic zoning induced by gradients in oxidation state the precious metal being concentrated in the reducing side zonation of alteration types south of the kirkland lake larder lake fault zone represents various steps of a continuous process of incremental changes of the rocks to adapt to the composition of an externally generated fluid successive generations of fracture arrays and veins have the same alteration patterns and thus the composition of the incoming hydrothermal fluid seems to have remained essentially unchanged through time gold mineralization at the kerr addison mine and its contiguous alteration halo are interpreted to be the result of repeated cycles of hydraulic fracturing fluid penetration and local reaction with rocks fluid rock interaction produced lateral variations of fluid composition which led to gold concentration probably due to gradients in ph preferentially at the interface between the carbonate muscovite and carbonate albite alteration types which is also the site of pyrite concentration the alteration assemblages and specific site of gold pyrite concentration at kerr addison are common to several other described archcan lode gold deposits

geology mineralogy and geochemistry of the shangxu orogenic gold deposit central tibet china implications for mineral exploration abstract shangxu is an orogenic gold deposit within the bangong nujiang suture zone central tibet china it is hosted by turbidite sedimentary rocks of the jurassic mugagangri group the host rocks were metamorphosed to subgreenschist facies prior to being hydrothermally altered adjacent to mineralization hydrothermal minerals in wall rocks consist of muscovite carbonate sulfides and chlorite whole rock geochemistry indicates hydrothermal alteration is characterized by the introduction of k2o co2 and s and leaching of na2o increasing 3 k al and decreasing na al alteration indexes approaching the ore correlate to muscovitization co2 concentration reflects the degree of carbonation which in conjunction with molar mn fe fe ca mg mn and co2 cao can be used to distinguish siderite from ankerite in carbonate alteration mass transfer analysis shows gains in as au co2 w ca cd ni cr sr rb k and losses in na pb cu during hydrothermal alteration in the muscovite alteration zone muscovite within shear zones ms1 shows an increase in fe mg and na towards the ore whereas metamorphic muscovite along cleavage planes and in pressure shadows ms2 and hydrothermal muscovite ms3 change in composition from phengitic mg fe distal to mineralization to paragonitic na adjacent to gold in carbonate alteration the composition in carbonate phases changes from early magnesian siderite to ankerite and then to calcite and dolomite during the evolution of the hydrothermal system siderite spots in host rocks and ankerite in veins wall rock or its replacement of siderite are all enriched in fe mn and depleted in mg from proximal to distal alteration zones pyrite is a dominant mineral in sulfide alteration of the shangxu deposit the earliest framboidal pyrite py1 formed during diagenesis and overprinted by py2 which is recrystallized pyrite associated with burial metamorphism py3 rims and enlarges previous pyrite aggregates or recrystallizes into cubic grains with cracks and quartz pressure shadows during deformation py4 is ore related euhedral pyrite disseminated near gold and formed in hydrothermal stage vein pyrite py5 coexists with marcasite and fills cracks in quartz veins hydrothermal alteration minerals form overlapping alteration haloes surrounding the main auriferous lodes distal muscovitization carbonatization and proximal sulfidation constitute a progressive alteration front from gold the alteration envelope is tabular in shape following the strike and dip of the mineralization and tends to be the widest around the thickest parts of the ore such that the occurrence and thickness of the alteration envelope can be used as an approximate guide to the size of gold mineralization shangxu shares many diagnostic features with turbidite hosted orogenic gold deposits such as bendigo reefton and meguma districts in deposit geology alteration mineralogy and lithogeochemistry

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

geochemical signature of orogenic hydrothermal activity in an active tectonic intersection zone alpine fault new zealand a highly faulted and fractured rock mass has developed at the intersection of the alpine and hope faults two major active faults in the south island new zealand the alpine fault is an oblique dextral reverse fault at the late cenozoic recent pacific australian plate boundary the hope fault is a strike slip fault parallel to the plate convergence vector hydrothermal fluids driven by the active tectonic processes have passed through the fractured rock mass causing localised rock alteration and vein formation mylonites in the alpine fault zone are crosscut by cm scale veins of quartz and or ankerite with minor sulphides with cemented breccias in dilational jogs breccia clasts and immediate cm scale host rocks have been variably impregnated with carbonates and quartz this generation of veins breccias and altered rocks is post dated by cataclasite and fault gouge zones which have been cemented by calcite illite smectite and chamosite ankerite and calcite have 18o between 10 and 30 and 13c between 0 and 8 these minerals are inferred to have formed from water with variable components of both meteoric and crustally exchanged fluid rock alteration associated with ankerite quartz veins has added arsenic up to 200 ppm as strontium and some y to the rocks host rock mylonites 2 ppm as have been depleted in arsenic compared to their precursors 5 15 ppm as this depletion of arsenic in the middle crust provides the source for arsenic in shallower level vein systems

the use of lithogeochemistry in delineating hydrothermal fluid pathways and vectoring towards gold mineralization in the malartic district quebec abstract the world class oxidized intrusion related canadian malartic gold deposit with reserves estimated at 5 56 moz au grading 1 10 g t au and a total geological endowment of 16 3 moz au is one of the largest gold deposits in the archean superior province of canada the gold mineralization is hosted predominantly by pontiac group metasedimentary rocks piche group metavolcanic rocks and quartz monzodiorite to granodiorite porphyritic intrusions the ore takes the form of a low grade envelope of disseminated pyrite 0 35 to 1 g t au grading inwards into higher grade 1 g t au stockwork and breccia zones hydrothermal alteration in the metasedimentary rocks is zonally distributed around the fluid pathways proximal alteration is characterized by a microcline albite quartz replacement type assemblage with lesser phlogopite calcite fe dolomite pyrite and rutile the distal alteration assemblage comprises biotite microcline albite phengite quartz calcite pyrite and rutile in this study we assess the magnitude and distribution of fluid rock interaction in the metasedimentary rocks of the malartic district the metaturbidites are separated into four lithotypes based on grain size to reduce the effects of primary depositional processes on mass change calculations despite the variability in protolith composition the metasedimentary rocks define a geochemically consistent cogenetic sequence the results of the mass transfer calculations indicate progressive gains in co2 s k2o and loi as well as au te w ag as be sb bi mo and pb from background to distal and proximal alteration zones relative to the least altered samples molar element ratio analysis alkali aluminum indicates an increase in alkali metasomatism k and na adjacent to the main hydrothermal fluid pathways which is manifested by the progressive stabilization of microcline and albite at the expense of oligoclase biotite and white mica ore associated pathfinder elements delineate broad enrichment patterns around the deposit and are used to understand hydrothermal fluid circulation in the malartic district a statistical approach based on a comparison of the mass change results with the background composition provides robust constraints on the magnitude and extent of the lithogeochemical haloes generally the alteration forms envelopes that extend along the s2 fabric with the largest lithogeochemical anomalies e g au w te and ag reaching up to 10 km in length and 2 km in width the results of this study demonstrate that whole rock lithogeochemistry can provide a valuable tool with which to define vectors toward gold mineralization in a regional exploration context

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

the late orogenic timing of mineralisation in some slate belt gold deposits victoria australia gold mineralisation in classic australian slate belt gold deposits at ballarat bendigo st arnaud and inglewood occurred very late in the orogenic history of these rocks rather than during formation of the main slaty cleavage this has been revealed through the examination of microstructural relationships in gold bearing quartz veins and their host rocks from these deposits which has established a d1 to d4 deformation stage history and consistent timing for gold mineralisation over a wide area the gold was deposited synorogenically but during the fourth deformation stage d4 of the orogeny a relatively weak event occurring two deformations after the main slaty cleavage producing event d2 previously d2 had been regarded as both the source and control of gold mineralisation as most of the quartz veins that occur in these deposits formed before or during this deformation event however most gold is hosted in breccia veins that formed during d4 the wallrock clasts within these breccia veins contain a young rotated foliation and the breccia veins are spatially associated with a paragenetically consistent alteration of the host rocks in the deposits this alteration both crosscuts and preferentially mineralises wallrock s4 allowing the timing of the breccia veins alteration and gold deposition to be defined as syn d4 in age

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

regional scale metasomatism in the fortescue group volcanics hamersley basin western australia implications for hydrothermal ore systems mafic to intermediate volcanic rocks of the fortescue group form the lowermost stratigraphic unit of the 100 000 km2 hamersley basin on the southern margin of the archaean pilbara craton western australia these represent one of the oldest 2 8 2 7 ga known examples of a continental flood basalt sequence a regional burial metamorphic gradient extends across the basin from prehnite pumpellyite facies in the north to epidote actinolite greenschist facies in the south and west superimposed on this metamorphic gradient regional scale metasomatism has affected extensive areas of the fortescue group metasomatized mafic lavas are characterized by well developed assemblages dominated by pumpellyite quartz or epidote quartz associations the mineral associations of metasomatic domain types broadly match the distribution of metamorphic isograd indicator minerals with a southward and westward increase in the proportion of epidote a continuum exists between least altered rocks that preserve the regional metamorphic signature and the most intensely altered metasomatized rocks metasomatism is essentially continuous over a stratigraphic strike length of 100 km and across a strike width of 20 km regionally metasomatically altered volcanic rocks occur widely across the hamersley basin and its outliers over an area of some 450 km by 200 km metasomatic alteration is most conspicuous in the lower grade metamorphic zones because pumpellyite and epidote rich rocks are green to yellow green in outcrop whole rock geochemical data indicate that metasomatism is associated with strong depletions in alkalis na k li rb alkali earths mg sr be ba and heavy first transition series metals mn fe co ni cu zn with a significant enrichment in si calcium shows more variable mobility such geochemical trends particularly depletions in fe mn and base metals co ni cu zn together with the metasomatic mineral assemblages are comparable with those associated with the sub sea floor circulation of seawater particularly in relation to the metal depleted root zones of base metal deposits petrographic features indicate that the development of metasomatic mineral associations post dates the formation of regional metamorphic assemblages consequently it is interpreted that the hydrothermal fluid flowed through the buried pile after sufficient time to allow for the metamorphic mineral assemblages to approach equilibrium with ambient p t conditions the hydrothermal depletion in fe observed across the fortescue group mafic lavas is intriguing given the abundance of iron ore in the overlying hamersley group and a possible connection cannot be ignored furthermore the scale of fluid flow observed in the fortescue group which occurred through zones of inherent permeability such as vesicular and brecciated lava flow tops without the aid of major cross cutting structures has significant implications for the size of metasomatic systems in other mafic volcanic terranes with potential consequences for the exploration for hydrothermal mineral deposits

assessing metasomatic mass and volume changes using the bootstrap with application to deep crustal hydrothermal alteration of marble a statistically rigorous approach for determining likely errors on estimates of mass and volume change in metasomatic systems is presented and then used to assess mass transfer resulting from hydrothermal alteration of marble during regional metamorphism analysis of metasomatic effects using standard statistical methods designed for unconstrained univariant data often fails to provide useful results for several important reasons 1 the concentration of any constituent in a composition is constrained to be between 0 and 100 wt percent 2 the sum of the concentrations of all constituents must be 100 wt percent the closure constraint compositions are multivariate and provide information only about the relative proportions of constituents 3 the maximum possible mass or volume loss is 100 percent 100 corresponds to complete mass or volume loss 4 rock bulk density must be greater than 0 g cm super 3 5 the underlying probability distributions for mass and volume changes are commonly non normal to address these issues we use statistical procedures recently developed to treat the special properties of compositional data including closure and the bootstrap method to compute accurate confidence intervals for assessing how far in error best estimates of mass and volume change are likely to be the bootstrap deals effectively with non normality and constraints 1 3 and 4 we apply our approach to gain a better understanding of synmetamorphic acadian orogeny hydrothermal alteration of upper greenschist facies and amphibolite facies marble beds of the wepawaug schist connecticut the marbles lost significant amounts of volatiles dominantly co 2 and h 2 o si ti k rb sr and ba best estimates of total mass and volume change are 27 and 32 percent respectively the bulk of the lost mass was volatiles approximately 140 g kg super 1 sio 2 approximately 60 g kg super 1 and k 2 o approximately 15 g kg super 1 numbers are g lost per kg of parent rock volatile loss was probably regional in scope loss of si ti k rb sr and ba occurred over minimum length scales on the order of typical hand sample dimensions maximum length scales remain to be determined the fortran 77 computer code that performs the calculations described herein is available from the senior author

depletion and enrichment zones in the bendigo gold field a possible source of gold and implications for exploration a regional lithogeochemical survey in the bendigo region of central victoria australia covering an area of 3 750 km2 and including the giant bendigo gold field and other large regional gold fields such as maldon castlemaine and fosterville was completed in 2004 by ionex pty ltd a total of 142 rock samples were collected and analyzed for gold and associated elements extensive areas having an anomalously low gold content termed depletion zones were detected in the vicinity of known gold fields and constitute a large proportion of the surveyed area the results show that the gold content in the depletion zones is 70 percent below regional background levels the bendigo gold field itself is located within a positive gold anomaly enrichment zone with an average value of 14 ppb in all rock types and an extent of approx 100 km2 to the north is a depletion zone on the order of 700 to 800 km2 with an average gold content of less than 0 5 ppb in all rock types in total the gold enrichment and depletion zones occupy an area of more than 1 000 km2 against a regional background of 0 7 to 4 7 ppb au avg 1 29 ppb if this depletion zone extends to a depth of 2 km the volume of depleted rock would be at least 1 400 km3 and the extent of apparent depletion would be 3 000 to 4 000 tonnes t of gold the quantum of apparent gold depletion at bendigo is on the same order of magnitude as the gold enrichment in the bendigo gold field and associated dispersion haloes 3 000 t au substantial depletion zones are also present adjacent to the other important gold deposits within the survey area the presence of extensive gold depletion zones adjacent to the major gold deposits of central victoria raises the possibility that these rocks were a source of gold in the deposits these findings provide possible new criteria for the appraisal of prospective regions

geology geochronology and fluid characteristics of the pingqiu gold deposit southeastern guizhou province china abstract the junction of the southeastern guizhou the southwestern hunan and the northern guangxi regions is located within the southwestern jiangnan orogen and forms a ne trending 250 km gold belt containing more than 100 gold deposits and occurrences the pingqiu gold deposit is one of the numerous lode gold deposits in the southeastern guizhou district gold mineralization is hosted in neoproterozoic lower greenschist facies metamorphic rocks and controlled by fold related structures vein types present at pingqiu include bedding parallel and discordant types with saddle reefs and their down limb extensions dominating but with lesser discordant types the major sulfide minerals are arsenopyrite and pyrite with minor sphalerite galena chalcopyrite and rare pyrrhotite marcasite and tetrahedrite much of the gold is m to mm sized grains and occurs as fracture controlled isolated grains or filaments in quartz galena sphalerite pyrite and wallrock three types of fluid inclusions are distinguished in hydrothermal minerals type 1 aqueous inclusions have homogenization temperatures of 171 396 c and salinities of 1 4 9 8 wt nacl equiv type 2 aqueous carbonic inclusions yield final homogenization temperatures of 187 350 c with salinities of 0 2 7 7 wt nacl equiv type 3 inclusions are carbonic inclusions with variable relative content of co2 and ch4 and minor amounts of n2 and h2o the close association of co2 rich inclusions and h2o rich inclusions in groups and along the same trail suggests the presence of fluid immiscibility the calculated 18oh2o values range from 4 3 to 8 3 and dh2o values of fluid inclusions vary from 55 8 to 46 9 a metamorphic origin is preferred on the basis of geological background and analogies with other similar deposit types two ore related sericite samples yield well defined 40ar 39ar plateau ages of 425 7 1 7 ma and 425 2 1 3 ma respectively these data overlap the duration of the caledonian gold mineralization along the jiangnan orogen and suggest that gold mineralization was post peak regional metamorphism and occurred during the later stages of the caledonian orogeny overall the pingqiu gold deposit displays many of the principal characteristics of the bendigo gold mines in the western lachlan orogen se australia and the dufferin gold deposit in the meguma terrane nova scotia canada but also some important differences which may lead to the disparity in gold endowment however the structural make up at deposit scale and the shallow mining depth at present indicate that the pingqiu gold deposit may have considerable gold potential at depth

development of framboidal pyrite during diagenesis low grade regional metamorphism and hydrothermal alteration distribution patterns microstructural relationships and compositional variations for framboidal pyrite in the paleozoic sedimentary host rocks to gold deposits on the northern carlin trend nevada and at fosterville in central victoria collectively demonstrate that it is not all of synsedimentary early diagenetic origin framboidal pyrite also nucleated in hydrothermal veins formed at several kilometers depth and along dissolution surfaces stylolites and crenulation cleavages formed during metamorphism at up to anchizonal grade i e prehnitepumpellyite facies there are no obvious differences in either the size range or internal morphology of framboids formed in these various settings suggesting that a single relatively simple crystallization path may operate over a wide depth range in the upper crust at fosterville anomalously metal rich including au framboidal pyrite is most abundant adjacent to laminated shear veins that developed by slip along bedding surfaces during folding of the sedimentary host rocks microstructural relationships and pb isotope compositions for the framboidal pyrite and other similarly distributed early formed sulfides support development in response to the localized episodic influx of metal enriched h 2s bearing hydrothermal fluids during regional deformation between 440 and 400 ma these findings may have implications for other similarly deformed metasedimentary successions where the discovery of au bearing framboidal pyrite has recently been cited as evidence for significant au enrichment during sedimentation 2009 society of economic geologists inc