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

timing of gold mineralization in the hutti gold deposit dharwar craton south india the hutti orogenic gold deposit in the eastern dharwar craton of south india is located in a cuspate greenstone belt which is tectonically juxtaposed against tonalite trondhjemite granodiorite ttg basement on its western margin the host rocks to gold mineralization are mafic and felsic volcanic rocks metamorphosed to the amphibolite facies mineralization is confined to nine broadly parallel quartz reefs within north south to north northwest south southeast trending shear zones that dip approximately 70 w six of these 2 to 10 m thick boudinaged veins have been mined to a depth of over 810 m free gold occurs in the veins and alteration haloes and is spatially and temporally associated with quartz and disseminated pyrite and arsenopyrite which in turn are closely associated with hydrothermal monazite and xenotime in the biotite plagioclase wall rock alteration around veins gold is considered to have been deposited during d 2 to d 3 deformation broadly coincident with the emplacement of syntectonic granitoids the history of volcanism granitic magmatism and gold mineralization is defined by u pb geochronology of magmatic zircons and hydrothermal monazite and xenotime the felsic volcanic host rocks have a u pb zircon age of 2587 7 ma about 40 m y older than the age of gold mineralization at 2547 10 ma determined from hydrothermal monazite the syntectonic kavital granitoid has a u pb zircon age of 2545 7 ma which overlaps with the timing of gold deposition and is consistent with structural interpretations the ramagiri and kolar gold deposits elsewhere in the dharwar craton are considered to have a similar structural timing and the hutti geochronology suggests that gold mineralization occurred throughout the dharwar craton some 80 to 120 m y later than the major peak of late archean world class orogenic gold mineralization in most other archean cratons

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

archaean gold mineralization synchronous with late cratonization of the western dharwar craton india 2 52 ga u pb ages of hydrothermal monazite and xenotime in gold deposits new zircon u pb ages for a felsic volcanic rock 2 588 10 ma and an intrusive granite 2 555 6 ma in the gadag greenstone belt in the western dharwar craton southern india are similar to dates for equivalent rocks in the eastern dharwar craton and indicates docking of the two cratons prior to this time the zircons in the intrusive granite are strongly overprinted and coexisting titanites yielded two different age populations the dominant group gives an age of 2 566 7 ma interpreted as the emplacement age whereas the minor group gives an age of 2 516 10 ma reflecting a hydrothermal overprint in situ u pb dating of monazite and xenotime in gold reefs of the gadag 2 522 6 ma and ajjanahalli 2 520 9 ma gold deposits reveal a previously undated episode of gold mineralization at 2 52 ga substantially younger than the 2 55 ga hutti deposit in the eastern dharwar craton the new dates confirm that both the younger greenstone belts and lode gold mineralization in the dharwar craton are about 100 120 my younger than in other well dated archaean cratons although gold mineralization across the craton postdates most of the magmatic activity and metamorphism at upper crustal levels widespread thermal reworking of the lower middle crust involving partial melting metamorphism and lower crustal granitoid intrusion occurred concurrently with gold mineralization it is likely that the large scale hydrothermal fluid flow that produced widespread gold deposition was also part of this tectono thermal event during the final stages of cratonization of the dharwar craton in southern india

geochemical characteristics of gold bearing boninites and banded iron formations from shimoga greenstone belt india implications for gold genesis and hydrothermal processes in diverse tectonic settings abstract this paper is the first documentation of boninites from the kudrekonda formation of shimoga greenstone belt sgb in western dharwar craton wdc india and the occurrence of native gold in the associated quartz veins we also investigate the gold mineralization hosted in the banded iron formations bifs of ganajur karajgi and palavanahalli areas of the sbg the kudrekonda metavolcanic rocks are composed of clinopyroxene orthopyroxene and plagioclase with secondary amphibole the geochemical attributes of these rocks based on mgo tio 2 ni cr and co contents al 2 o 3 tio 2 gd yb n and la sm n ratios classify them as boninites and their relative lile hfse abundance suggests magma generation in an intraoceanic arc setting the gd yb and dy yb ratios suggest parent magma origin through partial melting of a spinel lherzolite mantle source geochemical signatures of the boninites suggest high temperature shallow level melting of refractory mantle wedge metasomatized by fluids derived through dehydration of oceanic slab during subduction initiation quartz veins present in the kudrekonda boninites contain gold grains showing affinity towards quartz carbonate vein type greenstone hosted occurrences epma data record 88 5 90 3 wt au content with 7 7 8 4 wt fe 0 05 2 1 wt s and negligible ag the au is hosted in fe oxides bulk chemical analyses show 4 1 684 4 ppb au and 139 4 2186 4 ppb ag in the studied samples the gold mineralization of kudrekonda boninites is syngenetic mesothermal convergent margin orogenic type decarbonation desulphidization and metamorphism of subducted oceanic lithosphere and devolatilization of metasomatized enriched mantle wedge in intraoceanic arc setting are the viable processes that contributed to gold genesis in the kudrekonda boninites devolatilization during prograde metamorphism hydrothermal fluid generation syntectonic gold remobilization and migration of gold enriched fluids through shear zones account for au transportation and precipitation in quartz veins emplaced within the boninites the bifs of ganajur karajgi and palavanahalli areas of shimoga greenstone belt dominantly comprise alternating si rich layers of quartz and fe rich hematite and magnetite layers geochemical characteristics of the bif samples including their depleted ree positive eu anomalies negative to negligible ce anomalies and superchondritic y ho ratios suggest that the bifs were i derived by chemical sedimentation from paleo ocean enriched in dissolved iron and silica with marked contributions from hydrothermal fluids of volcanic origin and ii deposited in an off shelf marine environment proximal to a mid oceanic spreading centre where hydrothermal activity induced release of eu and y enriched hydrothermal solutions under low oxygen fugacity conditions native gold occurs in banded strata bound disseminated sulphide lenses compositionally characterized as pyrite arsenopyrite chalcopyrite and galena mineral chemical analyses reveal 73 100 au content with variable concentrations of ag 0 02 15 2 wt geochemical compositions of bifs are characterized by pronounced enrichment in au 706 15 831 ppb and ag 630 2435 ppb gold mineralization in shimoga bifs is epigenetic epithermal type akin to homestake type iron formation hosted vein and disseminated lode gold deposit gold genesis in these bifs is attributed to migration of oxygen depleted reducing sulphidic auriferous fluids of hydrothermal origin to the site of bif deposition that served as chemical traps for the auriferous fluids the kudrekonda boninites represent accretion and preservation of a slice of older oceanic crust onto a relatively younger active continental margin through arc continent collision during neoarchean time

late archean convergent margin tectonics and gold mineralization a new look at the norseman wiluna belt western australia on a world scale the most important hydrothermal gold deposits occur either in late archean ca 2700 ma greenstone belts or at late paleozoic to quaternary convergent plate boundaries in the latter environments epithermal and porphyry hosted deposits form during subduction related magmatism in volcanic arcs and deep level meso thermal deposits form during deformation in continental margin orogenic belts the mesothermal deposits are similar to archean gold deposits in greenstone belts such as the norseman wiluna belt in western australia we here suggest that this similarity exists because the norseman wiluna belt is an orogenic belt with a tectonic history similar to that of younger mineralized convergent margins such as the north american cordillera it is most likely that the norseman wiluna belt resulted from the interaction of lithospheric plates during the major period of continental growth and stabilization that occurred in the late archean the distribution of gold mineralization being controlled by convergent margin tectonics

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

the hydrothermal fluid of archaean lode gold deposits at different metamorphic grades compositional constraints from ore and wallrock alteration assemblages a low salinity mixed aqueous carbonic fluid is common to all archaean lode gold deposits throughout the range of mineralising conditions from sub greenschist to lower granulite facies temperatures alteration assemblages and fluid inclusion data give constraints on the fluid composition fluid xco2 is 0 1 0 3 in typical greenschist facies mesothermal deposits at higher temperatures the assemblages are consistent with formation from a fluid of similar composition but slightly higher or lower xco2 cannot be ruled out and fluid inclusion data indicate that ch4 may be an important component in ore fluids at these temperatures fluid ph is neutral or weakly alkaline at all conditions a range of relative oxidation states of four orders of magnitude fo2 is indicated at any temperature with deposits more oxidising relative to qfm at lower temperature sulphur contents of the fluids vary from 10 to 10 3 5m s with a trend towards lower sulphur contents at lower temperatures the relative concentrations of major cations in solution are similar at all conditions with na k ca although ca may be less abundant at low temperatures the broad similarities in ore fluid composition at all temperatures give support to crustal continuum models in which archaean lode gold mineralisation involved either a single fluid moving through the middle and upper crust or derivation of ore fluids by similar processes at different crustal levels many of the compositional differences between high and lowtemperature ore fluids may be attributed to evolution of deep sourced hydrothermal solutions as they rise along structurally controlled conduits the constancy of major ore fluid component concentration e g co2 cl k suggests fluid buffering and high fluid rock ratios along fluid pathways fluid buffered conditions can also explain the ore fluid fo2 temperature relations with equilibria between oxidised and reduced aqueous carbon or sulphur species controlling the oxidation state in contrast the concentrations of components present in lesser abundance in archaean gold ore fluids e g s ca h were probably controlled either by saturation of one or more mineral phases brought on by decreasing temperature or were rock buffered through fluid rock reactions extrapolation to high temperatures of the k na and ca contents of the gold bearing fluids indicates that their composition is consistent with derivation from or final equilibration with rocks of intermediate granitic composition thus giving support to isotopic and geological arguments for ore fluid source regions external to the greenstone belts the fluid oxidation states are characteristic of a wide range of potential source rocks including mantle derived igneous rocks calc alkaline granitoids and magmas and seaflooraltered metabasalts strongly oxidised magmatic sources or unusually oxidising source processes e g co2 streaming during granulitisation of the lower crust are therefore not required in the genesis of arch an lode gold deposits

the nature and dimensions of regional and local gold related hydrothermal alteration in tholeiitic metabasalts in the norseman goldfields the missing link in a crustal continuum of gold deposits the archaean lode gold deposits at norseman western australia consist of auriferous quartz veins in dextral reverse ductile brittle shear zones within tholeiitic metabasalts of upper greenschist to amphibolite facies metamorphic grade three types of deposits northern central southern are delineated on the basis of their spatial distribution veining style alteration mineraloty and metamorphic grade of host rocks northern deposits hosted in upper greenschist to lower amphibolite facies rocks comprise massive to laminated quartz veins with selvedges of quartz chlorite calcite biotite plagioclase assemblages central deposits hosted in lower amphibolite facies rocks consist of laminated to massive quartz veins with selvedges of quartz actinolite biotite plagioclasecalcite assemblages southern deposits hosted in middleamphibolite facies metabasalts consist of banded quartz diopside calcite microcline zoisite veins all deposits exhibit variable ductile deformation of veins and contiguous alteration haloes consistent with a syn deformational genesis at high temperatures from northern to southern deposits the alteration assemblages are indicative of higher temperatures of formation and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals these observations imply that a thermal variation of gold related hydrothermal alteration exists within the norseman terrane over a distance of 40 km with tnorthern tcentral tsouthern this thermal zonation is corroborated by t xco 2 phase relations between vein selvedge assemblages which signify formation temperatures of approximately 420 475 c 470 495 c and 500 c for northern central and southern deposits respectively the sum of structural petrographic and mineral chemistry data indicates that the alteration assemblages formed in high temperature open hydrothermal systems and have not been subsequently metamorphosed the thermal differences between the deposit groups may reflect 1 a temperature gradient at relatively constant p corresponding to the proximity of the deposits to regional granitoid complexes or 2 formation of the deposits at progressively deeper crustal levels from north to south in either case the deposits represent a continuum of gold deposition from upper greenschist to amphibolite facies now exposed in an oblique section through the archaean crust at norseman

lode gold deposits and archean mantle plume island arc interaction abitibi subprovince canada abstract in combination with seismic interpretations and geochronological constraints the association of juvenile arc type low ti tholeiitic basalts with komatiites in the southeastern abitibi subprovince canada supports a history of subduction step back following late archean mantle plume island arc interaction the resulting paired collision zones preserved abundant komatiites and numerous massive sulphide deposits and established the critical metallogenic features to concentrate the majority of canada s precambrian gold resources in a small area of the southern abitibi subprovince

extraterrestrial 3 he as a tracer of marine sediment transport and accumulation the deposition rate of deep sea sediments and their focused redeposition by deep sea currents can be evaluated from analyses of sedimentary 230th with a temporal resolution limited only by bioturbation6 7 10 11 230th is produced uniformly throughout the ocean by radioactive decay of dissolved 234u and is removed sufficiently fast by sorption onto sinking particles to act as a constant flux tracer of sedimentation rates but the half life of 230th 75 kyr limits its use for this purpose to the past 200 250 kyr here we explore the use of extraterrestrial 3he from interplanetary dust particles1 4 idps as a constant flux proxy that is free from this limitation a comparison of 3he with 230th in two cores from the equatorial pacific ocean indicates that the variability in the mean flux of idps over the past 200 kyr is less than 75 but in contrast to this relatively constant rate of supply of 3he to the deep sea the local burial rates of 3he and 230th have varied by a factor of five over the past 450 and 200 kyr respect ively we interpret this variability as reflecting sediment focusing with a temporal pattern that suggests regular cycles of climate driven reorganization of near bottom currents in the deep pacific ocean

hydrothermal zircon and baddeleyite in val d or archean mesothermal gold deposits characteristics compositions and fluid inclusion properties with implications fortiming of primary gold mineralization zircon and baddeleyite occur within quartz tourmaline veins at four gold deposits in the val d or district of the archean abitibi southern volcanic zone host rocks have experienced intense metasomatic enrichment of zr hf y and rare earth elements the zircons contain primary inclusions of quartz tourmaline pyrite albite k mica scheelite and gold and gold occurs in primary fluid inclusions in zircons magmatic zircons in host rocks do not have this suite of inclusions consequently a wall rock inheritance model for the vein zircons is implausible compositionally the zircons feature pronounced interzone and intergrain variations of hf y yb th and u and sporadic anomalous ce contents of 1100 ppm distinct from magmatic counterparts two principal types of primary fluid inclusion occur in the vein zircons type 1 h2o co2 inclusions have low salinities variable quantities of co2 and homogenization temperatures of 260 380 c and type 2 co2 rich inclusions contain minor h2o and ch4 the vei

late archaean 2550 2520 ma juvenile magmatism in the eastern dharwar craton southern india constraints from geochronology nd sr isotopes and whole rock geochemistry the results of field geochronologic geochemical and isotopic studies are presented for the granitoids that occur east of the closepet batholith up to the kolar schist belt ksb field data such as common foliation strong shear deformation occasionally leading to mylonitization together with petrographic data including reduction in grain size with corroded borders show characteristics of the syn kinematic emplacement of the granitoids single zircon evaporation ages define a minimum age of 3127 ma for the tonalitic trondhjemitic granodioritic ttg basement and 2552 2534 ma plateau ages for the emplacement of the granitoids which slightly predate 20 30 ma the emplacement of the 2518 ma closepet batholith major and trace element data together with isotopic data suggest at least four magmatic suites from closepet batholith to the east which have independent magmatic evolution histories the observed data are compatible with magma mixing for the closepet batholith melting of ttg and assimilation fractional crystallization processes for bangalore granites either melting of heterogeneous source or different degree of melting of the same source for the granitoids of hoskote kolar and fractional crystallization for the western margin of the ksb isotopic nd sr and geochemical data lree and lil elements suggest highly enriched mantle and ancient ttg crust for the closepet batholith enriched mantle and ttg crust for the bangalore granites c a chondritic mantle source for the granitoids of hoskote kolar and the quartz monzonites of the western margin of the ksb and slightly depleted mantle for granodiorites of the eastern margin of the ksb we interpret all these geochronologic geochemical and isotopic characteristics of granitoids from the closepet batholith to the east up to the ksb in terms of a plume model the centre of the plume would be an enriched hot spot in the mantle that lies below the present exposure level of the closepet batholith melting of such an enriched mantle hot spot produces high temperature magmas closepet that penetrate overlying ancient crust where they strongly interact and induce partial melting of the surrounding crust these magmas cool very slowly as the hot spot maintains high temperatures for a long time thus they appear younger 2518 ma on the contrary to the east the plume induces melting of c a chondritic or slightly depleted mantle that produces relatively colder and less enriched magmas which show less or no interactions with the surrounding crust and cool rapidly and appear slightly older 2552 2534 ma this plume model can also account for late archaean geodynamic evolution including juvenile magmatism heat source for reworking inverse diapirism and granulite metamorphism in the dharwar craton

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

gold mineralisation and alteration of penakacherla schist belt india constraints on archaean subduction and fluid processes abstract in the dharwar craton southern india gold deposits are found mostly along the six arcuate shear zones passing through late archaean greenstone belts 2 7 ga one such shear zone complex extends for about 400 km within and along the ramagiri hungund schist belt the penakacherla sector of this shear zone is excellently exposed enabling a detailed investigation of synorogenic gold mineralisation and its relationship to associated hydrothermal alteration metamorphism and deformation under ne sw compression associated with archaean subduction processes converted mafic volcanic rocks into amphibolites and intermediate to felsic volcanic rocks into quartz mica schists continued compression generated a 50 100 m wide shear zone complex consisting of mafic phyllonites advection of hydrothermal fluids through this shear zone and reaction between fluids and the mafic phyllonites resulted in a silicified k metasomatic assemblage mainly consisting of chlorite amphibole k mica plagioclase ankerite quartz fe oxides pyrite chalcopyrite and arsenopyrite networks of quartz and carbonate veinlets a few millimeters to a few centimeters thick formed along the foliation planes giving rise to microscopic alteration envelope in which individual veinlet systems are merged into one another to form a composite alteration system gold is found within these quartz veinlets mafic phyllonites and at their mutual contacts hydrothermal fluids have modified the primary major minor trace and lree compositions of host rocks such that their mutual behaviour became non systematic some hfse and hree also show minor mobility but the overall ree pattern generally resembles that of the precursor mafic volcanic rocks mass and volume loss gain by si and ca has made significant impact on al ti and zr abundances which are generally immobile during hydrothermal alteration however element pairs such as zr hf v sc and nb ta maintain primary inter element ratios although their absolute abundances are drastically diluted similarly ree in highly silicified and carbonatised samples are reduced but patterns remain similar to those of relatively least altered mafic phyllonites with la n yb n between 1 and 3 in some samples lree enrichment is observed elevating in la n yb n from 3 to 11 pathfinder elements and base metals such as as cd cu pb zn and sb have been added along with the au and ag 13 c of carbon varies from 16 to 21 suggesting a biogenic origin whereas coexisting pyrite 34 s ranges from 1 to 3 pointing towards the involvement of magmatic or average crustal sulphur overall concentrations of k rb sr ba nb ta ti cs cr co v y and sc and many of the ratios such as k rb la sc la yb indicate that metamorphism devolatilisation and dehydration of an oceanic subducting slab might have partially contributed the mineralising fluids and generated the alteration assemblage observed in the host rocks fluid sources were mantle and greenstone belt dehydration and devolatilisation generating observed compositional and alteration diversity

petrogenesis and source characteristics of metatholeiites from the archean ramagiri schist belt eastern part of dharwar craton india the n s trending 2 4 km wide ramagiri schist belt is made up of three blocks dominated by metavolcanic rocks separated and surrounded by granitic rocks of distinct characteristics the metavolcanic rocks are tholeiitic in composition and are very similar in their major element composition as well as in their abundances of some trace elements however the rare earth elements ree require distinct sources the rocks of the amphibolite facies eastern block have lree depleted ree patterns ce yb 0 7 0 9 requiring derivation from depleted mantle like sources the greenschist facies metatholeiitic rocks of the central block have lree enriched ree patterns ce yb 3 6 reflecting the nature of their source s the nd isotopic data require that the lree enriched nature could not have been attained significantly prior to its melting the fine grained upper greenschist facies metatholeiites of the western block have flat to slightly lree depleted patterns ce yb 0 8 0 95 minor fractional crystallization of rock forming minerals may relate a few samples to each other among samples from each of the three blocks different extents of partial melting of distinct mantle sources have played a dominant role in the generation of the parent magmas to the central versus eastern and western block metatholeiites

thermochronology of the camflo gold deposit malartic quebec implications for magmatic underplating and the formation of gold bearing quartz veins the dominant ore host at camflo is a small monzonite pluton cut by main stage gold bearing quartz veins containing hydrothermal titanite and k feldspar main stage veins are spatially and temporally related to brittle ductile faults emplaced into most faults are younger transitional stage quartz pyrite lodes with sericite rutile alteration halos titanite in main stage gold quartz veins is replaced by rutile calcite quartz where overprinted by transitional stage sericitic alteration a u pb zircon age from the camflo monzonite indicates that the stock was emplaced at 2685 or 10 ma a u pb age of 2621 or 4 ma for hydrothermal titanite and a pb pb isochron age of 2621 or 7 ma defined by seven titanite and two k feldspar samples indicate that the gold quartz veins formed approximately 60 m y later which precludes a direct genetic link between these two events rutile analyses plot below the titanite feldspar pb pb isochron and so support field and petrographic evidence suggesting rutile did not precipitate from the same fluid that formed the titanite and k feldspar incremental heating and total fusion 40 ar 39 ar ages of hydrothermal micas related to gold quartz veins and quartz pyrite lodes are 70 to 170 m y younger than the hydrothermal u pb ages in addition 40 ar 39 ar ages of micas on the 1650 level are approximately 10 to 15 m y older than like micas on the 3475 level the younger 40 ar 39 ar ages are interpreted as cooling and not recrystallization or reset ages 40 ar 39 ar ages indicate cooling rates of approximately 1 degrees c m y for 2600 to 2550 ma rutile vein muscovite approximately 0 6 degrees c m y for 2550 to 2511 ma vein muscovite wall rock muscovite and approximately 0 3 degrees c m y for 2507 to 2452 ma large biotite small biotite an apparent uplift rate of approximately 0 04 km m y for 2520 to 2478 ma is calculated by comparing biotite and white mica ages from the 1650 and 3475 levels the u pb and 40 ar 39 ar thermochronology is consistent with an accretionary tectonic setting for camflo during the terminal stages of tectonic construction of the superior province a critical feature of this setting is large volumes of mantle derived basalt underplating the lower crust during peak granulite formation at approximately 2648 to 2640 ma co 2 rich fluids liberated by solidification of ponded mafic magma after peak granulite formation interacted with other fluids magmas and rock either at their source or during migration in shear zones to upper crustal levels where they deposited significant quantities of gold at camflo at approximately 2621 ma slow cooling and slow apparent uplift at camflo ensued for 150 m y after magmatic under plating resulting in the young 40 ar 39 ar cooling ages recorded by hydrothermal minerals this model is consistent with ages for granulite facies metamorphism in the kapuskasing structural zone postpeak metamorphic ages for co 2 h 2 o fluid inclusions in regional granulite terranes and geochronological data from other gold deposits and may have widespread implications for the formation of gold deposits in the southern superior province

rare earth element systematics in hydrothermal fluids abstract rare earth element concentrations have been measured in hydrothermal solutions from geothermal fields in italy dominica valles caldera salton sea and the mid atlantic ridge the measured abundances show that hydrothermal activity is not expected to affect the ree balance of either continental or oceanic rocks the ree enrichment of the solutions increases when the ph decreases high temperature solutions 230 c percolating through different rock types may show similar ree patterns

plumes and hotspots as sources of greenstone belts abstract greenstone belts that are older than 0 18 ga formed under circumstances that generate a buoyant garnet poor root because they do not generate enough depletion in the lithospheric mantle juvenile arcs involving average temperature early precambrian mantle were not a significant source of early precambrian greenstone belts instead early precambrian greenstone belts originated from mantle plumes and hotspots we focus on the origin of highly deformed morb like greenstone belts that are underlain by tonalitic to trondhjemitic plutons and surrounded by metamorphosed accretionary wedge like sediments these greenstone belts could form as the consequence of subducting a ridge segment within 500 km of a hotspot located beneath the continent as has happened on the taitao peninsula in chile the tonalitic to trondhjemitic plutons result from partial melting of oceanic crust thickened by extra melt derived from the hotspot the morb like rocks result from decompressive partial melting within the slab window produced by the subducting ridge the stratigraphy and tectonic setting of the recently emplaced taitao ophiolite are a possible phanerozoic analog of some morb like archean and early proterozoic greenstone belts

continental lithospheric evolution constraints from the geochemistry of felsic volcanic rocks in the dharwar craton india abstract felsic magmatism associated with ocean ocean and ocean continent subduction processes provide important evidence for distinct episodes of crust generation and continental lithospheric evolution rhyolites constitute an integral component of the tholeiitic to calc alkaline basalt andesite dacite rhyolite badr association and contribute to crustal growth processes at convergent plate margins the evolution of the dharwar craton of southern peninsular india during meso to neoarchean times was marked by extensive development of greenstone belts these granite greenstone terranes have distinct volcano sedimentary associations consistent with their geodynamic setting the present study deals with geochemistry of rhyolites from the chitradurga shimoga greenstone belts of western wdc and the gadwal kadiri greenstone belts of eastern edc sectors of dharwar craton to compare and evaluate their petrogenesis and geodynamic setting and their control on the continental lithospheric evolution of the dharwar craton at a similar range of sio2 al2o3 fe2o3 the rhyolites of wdc are more potassic whereas the edc rhyolites are more sodic and less magnesian with slight increase in tio2 minor increase in mgo content of wdc rhyolites reflects their ferromagnesian trace elements which are comparatively lower in the rhyolites of edc the relative enrichment in lile k rb and depletion in hfse nb ta zr hf marked by negative nb ta zr hf and ti anomalies endorse the convergent margin processes for the generation of rhyolites of both the sectors of dharwar craton the high silica potassic rhyolites of shimoga and chitradurga greenstone belts of wdc showing prominent negative eu and ti anomalies flat hree patterns correspond to type 3 rhyolites and clearly point towards their generation and emplacement in an active continental margin environment the geochemical characteristics of gadwal and kadiri rhyolites from eastern dharwar craton marked by aluminous compositions with low and fractionated hree patterns and minor negative eu anomalies are in conformity with type 1 rhyolites and suggest that they were erupted in an intraoceanic island arc system the overall geochemical systematics of the rhyolites from both the sectors of dharwar craton suggest a change in the geodynamic conditions from intraoceanic island arc of eastern dharwar craton and an active continental margin of western dharwar marked by ocean ocean subduction and migration of oceanic arc towards a continent followed by arc continent collision that contributed for the evolution of continental lithosphere in the dharwar craton

relative timing of deformation and two stage gold mineralization at the hutti mine dharwar craton india gold mineralization at hutti is confined to a series of nine parallel n s to nnw sse trending steeply dipping shear zones the host rocks are amphibolites and meta rhyolites metamorphosed at peak conditions of 660 40 c and 4 1 kbar they are weakly foliated s1 and contain barren quartz extension veins the auriferous shear zones reefs are typically characterized by four alteration assemblages and laminated quartz veins which in places occupy the entire reef width of 2 10 m and contain the bulk of gold mineralization a 1 5 m wide distal chlorite sericite biotite calcite plagioclase alteration zone can be distinguished from a 3 5 m wide proximal biotite plagioclase quartz muscovite calcite alteration zone gold is both spatially and temporally associated with disseminated arsenopyrite and pyrite mineralization an inner chlorite k feldspar quartz calcite scheelite tourmaline sphene epidote sericite alteration halo which rims the laminated quartz veins is characterized by a pyrrhotite chalcopyrite sphalerite ilmenite rutile and gold paragenesis the distal chlorite sericite and proximal biotite plagioclase alteration assemblages are developed in microlithons of the s2 s3 crenulation cleavage and are replaced along s3 by the inner chlorite k feldspar alteration indicating a two stage evolution for gold mineralization ductile d2 shearing alteration and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper greenschist to lower amphibolite facies conditions 560 60 c 2 1 kbar the reefs were reactivated in the d3 dextral strike slip to oblique slip environment by fault valve behavior at lower greenschist facies conditions ca 300 350 c which formed the auriferous laminated quartz veins later d4 crosscutting veins and d5 faults overprint the gold mineralization the alteration mineralogy and the structural control of the deposit clearly points to an orogenic style of gold mineralization which took place either during isobaric cooling or at different levels of the archean crust from overlaps in the tectono metamorphic history it is concluded that gold mineralization occurred during two tectonic events affecting the eastern dharwar craton in south india between ca 2550 2530 ma 1 the assemblage of various terranes of the eastern block and 2 a tectono magmatic event which caused late to posttectonic plutonism and a thermal perturbation it differs however from the pre peak metamorphic gold mineralization at kolar and the single stage mineralization at ramagiri notably greenschist facies gold mineralization occurred at hutti 35 90 million years later than in the western dharwar craton

oxygen isotope ratio of quartz veins from the auriferous ramagiri penakacherla schist belt and surrounding granitoids in the eastern dharwar craton a case for a possible link between gold mineralization and granite magmatism abstract in the eastern dharwar craton among the many shear zone hosted lode gold deposits those at ramagiri and penakacherla are located near the western margin of the craton mineralized quartz sulfide carbonate veins are hosted by the schistose metavolcanic and carbonaceous metasedimentary rocks in close spatial association with granitoids having quartz and quartzofeldspathic veins representing hydrothermal activities associated with them mineralized quartz veins from the ore zones in ramagiri and penakacherla regions and quartz or pegmatitic veins from the surrounding granitic terrane were chosen for 18 o analysis samples from the schistose and granitic domains show 18 o quartz values in the range of 10 4 14 9 and 9 3 10 9 respectively the ore zone fluids from the ramagiri and penakacherla regions give 18 o values of 7 9 1 5 and 5 1 0 8 calculated at pressure corrected temperatures obtained from fluid inclusion microthermometry the late magmatic fluid is relatively 18 o poor with 18 o values estimated at 4 5 0 7 and the value is closer to what is obtained for the ore zones based on the 18 o values reported and a possible magmatic contribution to ore fluid deciphered from fluid inclusion characteristics a genetic relationship between granitic magmatism and gold mineralization is surmised the observed increase in the 18 o 16 o ratio from the magmatic fluid to ore fluid in the shear zone is attributed to interaction of the magmatic fluid with host metasediments that agrees well with the variation in the co 2 ch 4 ratio of carbonic component in such fluids

temporal variation of 3he 4he ratio recorded in deep sea sediment cores he isotopic composition was measured for 72 deep sea sediments from 16 sites in the pacific and atlantic oceans and the japan sea 3he content ranges from less than 10 12 cm3 stp g to about 6 10 11 cm3 stp g and 3he 4he from about 10 8 to 1 67 10 4 the he isotopic data lie fairly well on a mixing curve in a 3he 4he 3he diagram where the end members of the mixing curve are extraterrestrial he in interplanetary dust particles idp s and terrestrial he in ocean sediments this confirms the view that there are significant amounts of extraterrestrial he in ocean sediments from global fallout of idp s into deep sea sediments 3he flux estimated for two long cores gives a value of about 1 5 1 0 10 15 cm3 stp cm2 yr over a time span of 40 m y which corresponds to an annual fallout rate of idp s size 10 m of about 400 t in one of the cores we observe a 3he spike which is associated with anomalously high ir content 1 86 0 56 ppb this may be due to meteorite debris

terrigenous fe input and biogenic sedimentation in the glacial and interglacial equatorial pacific ocean many ocean regions important to the global carbon budget including the equatorial pacific ocean have low chlorophyll concentrations despite high levels of conventional nutrients iron may instead be the limiting nutrient and elevated input of terrigenous fe during windy glacial episodes has been hypothesized to stimulate oceanic productivity through time and thus regulate the oceanic and atmospheric co2 balance to test whether particulate fe input is related to the accumulation of biogenic matter in one important low chlorophyll high nutrient area that is the equatorial pacific ocean we present results from a suite of sediment cores that collectively record biogenic deposition through the last six glacial interglacial cycles 600 000 years our data set includes new chemical data on total fe terrigenous and biogenic components in three cores as well as previously published mineralogic records of coljan input to the region chemical spectral and stratigraphic analysis indicates that 1 terrigenous input to the region shows no consistent pattern of either glacial or interglacial maxima 2 the accumulation of particulate fe is closely related to the accumulation of terrigenous matter linear r 2 0 81 0 98 3 there are no coherent spectral relationships between fe input and glacial periodicity i e 5180 in any of the orbital frequency bands 4 the linear and cross spectral correlations between fe or coljan input and caco3 concentration are most commonly the strongest observed relationships between fe and any biogenic component yet indicate a largely inverse pattern with higher fe being associated with low caco3 5 there is no consistent linear r 2 correlation or spectral coherence between the accumulation of fe and that of caco3 corg or opal thus in total there is no relationship between terrigenous fe input and sedimentary sequestering of carbon additionally although we cannot specifically address the potential for changes in solubility of the terrigenous fraction that may be driven by a terrigenous compositional change the fe ti ratio which monitors first order mineralogic changes records only slight variations that also are linearly and spectrally unrelated to glacial periodicity the bulk fe flux and the accumulation of any biogenic component finally we find that the paleoceanographic flux of fe is several order of magnitudes larger than modem observations of coljan fe input suggesting that the long term importance of fe input by dust storms which deliver fe on the order of the sedimentary burial may be underestimated the removal of particulate terrigenous fe from the recently discovered source within the equatorial undercurrent however remains unquanfified and may also prove significant

hydrothermal gold mineralization at the hira buddini gold mine india constraints on fluid evolution and fluid sources from boron isotopic compositions of tourmaline we determined the boron isotope and chemical compositions of tourmaline from the hira buddini gold deposit within the archean hutti maski greenstone belt in southern india to investigate the evolution of the hydrothermal system and to constrain its fluid sources tourmaline is a minor but widespread constituent in the inner and distal alteration zones of metabasaltic and metadacite host rocks associated with the hydrothermal gold mineralization the hira buddini tourmaline belongs to the dravite schorl series with variations in al fe fe mg ca ti and cr contents that can be related to their host lithology the total range of 11b values determined is extreme from 13 3 to 9 0 but 95 of the values are between 4 and 9 the boron isotope compositions of metabasalt hosted tourmaline show a bimodal distribution with peak 11b values at about 2 and 6 the wide range and bimodal distribution of boron isotope ratios in tourmaline require an origin from at least two isotopically distinct fluid sources which entered the hydrothermal system separately and were subsequently mixed the estimated 11b values of the hydrothermal fluids based on the peak tourmaline compositions and a mineralization temperature of 550 c are around 1 and 10 the isotopically lighter of the two fluids is consistent with boron released by metamorphic devolatilization reactions from the greenstone lithologies whereas the 11b rich fluid is attributed to degassing of i type granitic magmas that intruded the greenstone sequence providing heat and fluids to the hydrothermal system

subduction related tectonic evolution of the neoarchean eastern dharwar craton southern india new geochemical and isotopic constraints the neoarchean eastern dharwar craton edc is distinct from the mesoarchean western dharwar craton wdc in many aspects of its geology the important distinguishing features of the edc are the predominance of younger granitoids abundance of gold mineralization and the exposure at lower crustal depths no consensus exists on evolutionary models for edc mutually exclusive plume and subduction derived tectonic models have been proposed geochemical and radiogenic isotopic studies on the granitoids and volcanic rocks of three greenstone belts along a cross section in the northern part of edc are presented herein the evolved nd isotopic signatures radiogenic pb isotopic ratios and arc like geochemical signatures are suggestive of a subduction regime and the involvement of recycled older crust in the derivation of these rocks the proposed petrogenetic mechanism involves multi stage processes in a supra subduction regime involving slab dehydration formation of hydrous basaltic melts and re melting and interaction with sub arc basaltic crust at low pressures where amphibole plagioclase is the dominant residual phase there is a notable systematic decrease in the extent of older crustal involvement from west to east in the edc this is in concurrence with the younging of dharwar craton from west to east and eastward subduction the proposed petrogenetic model can efficiently explain the variations in older crustal involvement which is very common in other archean cratons c 2012 elsevier b v all rights reserved

comparison of metasomatic reactions between a common co 2 rich vein fluid and diverse wall rocks intensive variables mass transfers and au mineralization at alleghany california the gold deposits at alleghany california are typical of many epigenetic gold bearing hydrothermal vein systems in metamorphic terranes worldwide detailed analyses of alteration halos in serpentinite mafic amphibolite and granite wall rocks at alleghany indicate that widely contrasting deposit types ranging from fuchsite carbonate schists to pyrite albitites resulted when different wall rocks interacted with the same externally derived co 2 rich hydrothermal vein fluid patterns of element redistribution within halos and among lithologic units suggest a complex process involving fluid flow along vein fractures and diffusion or infiltration normal to the veins wall rocks locally controlled both the directions and magnitudes of chemical fluxes across vein walls vein fluids at constant temperature and pressure with relatively constant ratios of mobile species na super k super h super and co 2 h 2 s h 2 caused contrasting divergent reactions in proximal alteration zones because the wall rocks controlled the ratios of relatively immobile species al cr fe mg and fe mg and the component activities of hydrothermal mineral solid solutions thus pyrite and albite were stable with respect to the relatively pure siderite and k mica that occur sparingly in proximal granite zones but they were not stable with respect to the relatively impure siderite and k mica that characterize proximal serpentinite zones partly as a result of divergent reactions altered rocks exhibit contrasting mobile element concentration gradients e g ca ba sr rb na k c s and imply local sources and sinks and possibly short residence times for some of these components along discordant fluid flow paths calculated reaction coefficients for h super h 2 and h 2 s indicate that different wall rocks at different stages of alteration may have inhibited or promoted gold precipitation the distribution of gold and its calculated solubility are consistent with its having been precipitated from au hs 2 super complexes in response to pyritization of granite fluid desulfidation and reduction and possibly in part in response to dolomitization and sericitization of serpentinite fluid acidification strata bound disseminated orebodies may be expected in a variety of other host rocks with higher than average fe mg ratios where pyrite was stable with respect to intermediate magnesite siderite as in the granite it is proposed that the co 2 rich vein fluids approached partial equilibrium with alteration assemblages in the average or dominant rocks along their flow paths simpler but radically different proximal assemblages were then produced where the fluids encountered chemically extreme rock types fluid rock interaction equilibria can account for much of the diversity in the mineralogy and chemistry of gold deposits in heterogeneous metamorphic terranes while maintaining a simple genetic classification based on the distinctive chemistry of the hydrothermal fluids comparative studies of multicomponent phase equilibria and mass exchanges can provide both rationalizations and predictions of local wall rock effects on geochemical anomalies and the locations of ore deposits in lithologically heterogeneous terranes simultaneous consideration of contrasting high variance phase deficient assemblages along discordant veins is suggested as a general method for quantifying fluid intensive variables in metasomatic systems

geochemistry of the neoarchaean high mg basalts boninites and adakites from the kushtagi hungund greenstone belt of the eastern dharwar craton edc implications for the tectonic setting abstract volcanic rocks of the kushtagi hungund greenstone belt from the eastern dharwar craton edc show extreme diversity due to the presence of high mg high fe high mg fe basalts high mg dacites and andesites boninitic affinities and sodic plagioclase quartz rich felsic rocks adakites with na2o 4 6 wt the adakites drummond and defant 1990 martin 1999 and high mg dacites andesites are being reported for the first time from greenstone belts of the dharwar craton trace elements in these different types of volcanic rocks have distinct characteristics for example the basalts have 1 al2o3 tio2 16 pm value of 17 2 la yb n 1 3 3 low order negative anomalies of nb nb zr zr hf hf and ti ti 4 zr nb 4 10 and 5 ce nb 3 4 two samples of andesites having mg enrichment and al2o3 tio2 ratios of 31 35 are interpreted to have boninitic affinities other dacites and andesites are characterized by 1 high mg 8 10 wt 2 high al2o3 tio2 22 27 3 zr nb 8 14 4 la yb n 2 7 5 negative nb nb zr zr hf hf and ti ti anomalies and 6 sc lu 67 172 the adakites are characterized by 1 a narrow range of sio2 67 70 wt 2 enriched na2o 4 6 wt with very high na2o k2o 2 5 7 7 3 low mg 0 38 0 48 4 high la yb n 13 31 with fractionated patterns no eu anomaly and gd yb n 2 4 5 very high al2o3 tio2 50 80 and 6 enriched ni cr and co total 50 ppm these adakites resemble 3 0 ga tonalite trondhjemite gneisses of the dharwar craton which are interpreted as products of partial melting of the basaltic slab the available data suggests that partial melting of the different mantle and slab sources generated this collage of compositionally different rocks within an island arc setting

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

a model for the distribution of trace elements between calcite and dolomite abstract a crystal growth model is proposed which allows ions of a trace element to enter the ca and mg sites of dolomite in proportion to the size of the ions relative to that of ca and mg ions and which assigns equal portions of the trace element to the ca site of dolomite and the ca site of associated calcite the model produces calcite dolomite distribution coefficients of 0 79 for mn and 0 43 for fe which may be compared with 0 85 and 0 28 as observed in marble and a distribution coefficient of 2 0 for sr and ba which may be compared with observed values of 2 3 for sr and 1 8 for ba

geology geochemistry and genesis of bif of kushtagi schist belt archaean dharwar craton india the banded iron formation bif of the kushtagi schist belt dharwar craton is interbedded with metavolcanics the oxide fades cherty al2o3 2 bifs show large scale variations in their major and trace elements abundance cherty banded iron formation cbif is depleted in al2o3 tio2 zr hf and other trace elements like cr ni co rb sr v y and ree in comparison to shaley banded iron formation sbif depleted ree positive eu anomalies and the flat to hree enriched pattern of cbif indicate that fe and sio2 for these bifs were added to ambient ocean water by hydrothermal solutions at the amor vent sites it is inferred that the higher amount of hydrothermal fluid flux with a higher exit temperature provided enormous quantities of iron and silica fine grained sedimentation in the basin gave rise to the observed variability in the composition of bif during transgression a wave base was raised up consequently deposition of cbif became possible whereas during the regressive stage these chemical sediments were buried by and or mixed with the terrigenous sediments resulting in deposition of sbif and interbedded shales volcaniclastic activity within the basin appears to have contributed significantly to the composition of some sbif and shales the hydrothermal exhalative hypothesis combined with the archaean miniplate model explains most of the chemical features of the bifs of greenstone belts

geology and alteration in the golden mile kalgoorlie the giant kalgoorlie gold deposit 1 200 metric tons au recovered is surrounded by a wide wall rock alteration halo 1 x 3 km this alteration primarily represents the selectively pervasive formation of carbonate chlorite and localized intense pyrite and muscovite alteration which all formed synchronously with gold mineralization the carbonate alteration halo is interpreted as epigenetic because of evidence that carbonate chlorite assemblages overprint regional metamorphic fabrics furthermore the geometry of the alteration zone boundaries is controlled by ductile shear zones formed during regional deformation and the geochemistry of all the alteration zones suggests that one fluid was responsible for all assemblages there is no evidence of intense premetamorphic carbonation in the kalgoorlie area kalgoorlie is situated in the highly auriferous eastern goldfields province of the yilgarn block within a narrow corridor rift zone that contains several large gold deposits regional metamorphism has produced upper greenschist facies assemblages but has retained original rock textures at kalgoorlie the most important gold host is the golden mile dolerite 80 of gold produced a differentiated tholeiitic sill intruded between a lower mafic ultramafic pile including the paringa basalt the next most important gold host and overlying felsic rocks and metasediments the kalgoorlie syncline is the dominant tectonic structure in the main gold producing area of the golden mile adjacent to this infold of metasediments the golden mile dolerite and paringa basalt are the site of a number of ductile shear zones which formed synchronously with the syncline these zones and the many smaller shear zones between them have provided fluid channelways throughout the golden mile and are the site of most economic gold mineralization three alteration zones have been mapped in the golden mile dolerite on the basis of the mineralogic siting of fe i e the chlorite carbonate and pyrite zones the additional actinolite zone representing the unaltered regional metamorphic mafic assemblage has been converted to chlorite zone assemblages throughout the mined areas of the golden mile the carbonate zone bleached dolerite can be traced for 2 km through the main mineralized parts of the golden mile and to the full depth of lake view mine the boundary between the chlorite and carbonate alteration zones is locally sharp and controlled by ductile shear zones but is generally strata bound and preferentially developed in the more fe rich parts of the dolerite the pyrite zone which includes all economically gold mineralized areas is the site of the strongest muscovite alteration and is most intense and continuous in ductile shear zones the chlorite zone chlorite calcite or ankerite forms from mild carbonation of actinolite zone assemblages the carbonate zone ankerite siderite is a more local expression of this carbonation even after extensive carbonate alteration relative major element abundances are little changed and the main added components are limited to co 2 s k and rb the pyrite zone i e the gold lodes has ankerite muscovite quartz albite pyrite and minor siderite bearing assemblages it represents significant addition of sulfur the three alteration zones not only reflect differing degrees of alteration intensity but are also strongly influenced by host rock fe mg and this ratio largely determines the stability of chlorite favored by low fe mg and thus the distribution of the carbonate zone favored by high fe mg progressive deformation within the shear zones reworked existing mineralization kalgoorlie fits a metamorphic replacement model of gold formation by auriferous metamorphic fluids interacting with fe rich host rocks its great size is a function of a very well developed fluid channelway system via ductile shear zones and later fault zones and the favorable chemical nature of the host rocks especially the golden mile dolerite the full width of carbonation chlorite and carbonate zones reflects overlapping alteration envelopes around the closely spaced fluid pathways the processes responsible for the formation of most of the gold at kalgoorlie were essentially similar to those at several other archean gold deposits but on a larger scale

the source of lead in archaean lode gold deposits of the menzies kalgoorlie kambalda region yilgarn block western australia the archaean greenstone terrane between menzies and kambalda exhibits a coherent although deformed stratigraphic sequence intruded by granitoids and bounded by major nnw trending shear and or fault zones the greenstone terrane hosts a large number of lode gold prospects and deposits including the giant kalgoorlie deposits the initial pb isotope compositions of lode gold deposits as determined from ore related galena and pyrite vary systematically in a linear trend on a207pb 204pb versus206pb 204pb diagram which reflects crustal heterogeneity at the time of mineralisation deposits hosted within a 90 km section of the menzies boorara shear zone have a uniform radiogenic initial pb isotope composition irrespective of temperature of mineralisation and proximity to granitoid gneiss in plan view the pb in these deposits is considered to be derived largely from older felsic crust underlying the greenstone belt and was accessed via this major shear zone system deposits in a transect unrelated to a major shear zone show a systematic correlation between initial pb isotope compositions and proximity to granitoid gneiss and or to mineralisation temperature these compositions are less radiogenic than those within the menzies boorara shear zone but trend on a207pb 204pb versus606pb 204pb diagram between this isotope signature and the uniform pb isotope signature which characterises the 100 km greenstone transect from the mt pleasant area through kalgoorlie to kambalda these data are interpreted to reflect pb derivation from discrete crustal segments within and below the greenstones and require that mineralisation was related to crustal scale hydrothermal systems that accessed both sialic mid to lower crust and the greenstone succession

postcrystallization disturbance in the neodymium and lead isotope systems of metabasalts from the ramagiri schist belt southern india archean metabasaltic rocks from three separate blocks that constitute the north south trending ramagiri schist belt of the eastern dharwar craton have large variations in nd2750 even samples of the same rock type from one outcrop that are less than 50 cm apart from each other show a range of up to 5 5 nd2750 units the trends of the fields defined by the three suites in smnd vs nd2750 space intersect at smnd 0 1 to 0 2 and nd2750 8 to 12 suggesting that these rocks have lost lrees and have interacted with and exchanged at least nd with a common extraneous phase the isotopic composition of this phase suggest derivation of its neodymium from sources which had a long term history of lree enrichment this phase may have derived its isotopic composition from older continental crust one of the unaltered dikes cutting the three blocks in the belt and the surrounding gneisses gives a 147sm144nd 143nd144nd age of 2454 100 ma a minimum for the time of block assembly and alteration of rees including nd while a set of samples from the central block lie along an isochron with a pb pb age of 2746 64 ma the samples from the eastern and western blocks show large uncorrelated variations in 207pb204pb with 206pb204pb residues after acid leaching of sample powders have lead isotope ratios that are distinct for samples from each of the block and are compatible with their derivation from archean mantle the lead isotope composition of the acid leaches suggest disturbance of the metabasalts by an isotopically extraneous phase that had variable and high 207pb204pb and 208pb204pb ratios for a given 206ph204pb relative to expected archean mantle values both lead and neodymium isotope disturbances are best explained as a result of alteration of these rocks by an extraneous phase which had derived its isotopic composition from ca 3 5 ga continental crust

tectonic control and fluid evolution in the quartz hill california lode gold deposits at quartz hill in the klamath mountains of northern california gold bearing quartz carbonate pyrite chlorite sericite veins occupy reactivated moderately southeast dipping faults that originally formed synchronously with the transcrustal soap creek ridge fault geophysical evidence suggests that the soap creek ridge fault dips shallowly eastward to a depth of approximately 14 km where it intersects a major crustal suture zone that joins layered accreted oceanic crust to crystalline continental crust gold mineralization occurs in the footwall along the linear trace of this fault co 2 rich fluids reacted with wall rock to form alteration zones containing pyrite siderite sericite ankerite and chlorite calcite microthermometric studies reveal coexisting high salinity 11 16 wt salt aqueous and co 2 rich 13 51 mole co 2 inclusions that homogenize at temperatures greater than 285 degrees c these inclusions probably represent the unmixing of a parent fluid that was metamorphic in origin low salinity 0 6 wt salt inclusions with variable co 2 concentrations 0 9 mole co 2 homogenize at temperatures from 170 degrees to 270 degrees c these inclusions are likely to have formed from the mixing of hot saline co 2 rich solutions with cooler less saline fluids of meteoric origin k ar analyses of two size fractions of hydrothermal sericite from a quartz hill mine yielded concordant dates of 145 8 or 3 0 and 147 7 or 2 8 ma and probably represent the age of mineralization similarities in tectonic setting local structural control host rocks fluid inclusion properties and alteration and vein mineralogy suggest that the early cretaceous gold mineralization in the sierra nevada foothills was equivalent to the ore forming episode at quartz hill and throughout the klamath mountains a tectonic model for ore genesis consistent with thermochemical geophysical structural and geochronological data is that hydrothermal solutions were derived from subducting oceanic plates by devolatilization reactions associated with prograde metamorphism these co 2 rich fluids migrated upward through a crustal suture zone and were channeled along a transcrustal fault deposition occurred in response to pressure fluctuations within brittle fault zones important changes in farallon north american convergent plate motion occurred at 150 and 133 ma the synchronism of these plate motion changes with regional tectonic activity and gold mineralization suggests a genetic relationship these plate motion changes produced crustal scale tectonic responses which generated fluids and created pathways for ore fluid transport

the tianger bingdaban shear zone hosted gold deposit west tianshan nw china petrographic and geochemical characteristics abstract the wangfeng tianger saridala ore district consists of over 20 orebodies with total proven gold reserves exceeding 15 tonnes the district is located on the ew striking tianger bingdaban shear zone in west tianshan mountains northwest china the lensoid orebodies are distributed along the shear zone in a region about 25 km in length and 1 km in width enhanced fluid flow and fracturing controlled location and orientation of mineralized zones during deformation along narrow shear bands oriented oblique to the overall shear zone margins fabrics in fractures and strain shadow overgrowths generally plunge steeply parallel to the lineation in the surrounding cataclasite matrix consisting of new grains of quartz and minor amounts of micas native gold occurs in mylonitized granites or mylonitized sulfide mica quartz veins gold grade is closely related with the occurrence of sulfide quartz veins pyrite and pyrrhotite in gold bearing sulfide quartz veins contain inclusions of native gold chondrite normalized rare earth element ree distribution patterns for pyrite from the orebodies show light ree enrichments and negative eu anomalies relationships between gold contents and th u zr hf and la yb n values for pyrites imply that gold rich pyrites are different from gold poor pyrites thus indicate that the pyrites in the orebodies have a different origin compared with those in wall rocks a rb sr isochron for the mylonitized au bearing quartz veins indicates an age of 224 14 ma with an initial 87 sr 86 sr ratio of 0 7294 0 0089 mswd 1 1 this age is conformed by 40 ar 39 ar dating on muscovites 220 9 to 222 5 ma which is interpreted as the age of ore formation in the tianger deposit high initial 87 sr 86 sr ratios 0 7294 high 34 s values 11 2 to 16 5 low 18 o h2o values 1 67 to 3 07 and low d values 84 to 104 indicate that the ore forming fluid was unrelated to any magmatic process

structural pattern in the dharwar craton high grade and low grade supracrustal belts sargur and dharwar set in a matrix of gneisses dotted with granitic plutons of different ages make up the dharwar craton the overall structural pattern is one of subparallel linear belts and stringers whose regional trend changes from nw se in the northern part to n s in the south both the high grade and low grade belts show three principal episodes of folding however the correlation between the deformation episodes in the sargur and the dharwar rocks is controversial and the structural relations between them are not unequivocally established parallelism of the first phase structures in the sargur and the dharwar rocks can be interpreted either as synchronous development or rotation of the earlier sargur structures into parallelism with the younger dharwar structures during later deformation the extension of the supracrustal belts is parallel to the f 2 axial traces of the second deformation granite diapirism and basement uplift further modified

mobility of metals over the redox boundary in peru basin sediments abstract downcore variations and phase distribution of metals in the quaternary sediments from six areas of the peru basin were investigated the sediments are characterised by a 5 20 cm thick oxic layer at the water sediment interface formed under the influence of dissolved oxygen in the bottom water in deeper sediment sections suboxic conditions are found due to the degradation of organic matter variations in the downcore abundance of metals e g fe mn co ni cu zn v mo cr were determined by bulk chemical methods a significant accumulation of manganese is observed at the depth of the manganese redox boundary caused by diagenetic remobilisation under suboxic conditions and reprecipitation under oxic conditions to investigate their phase distributions partitioning of elements in selected samples over the sediment column was carried out in sequential leaching experiments a comparison between the surface layer and deeper sediment layers shows a distinct change in the element distribution between operationally defined host phases carbonates oxy hydroxides detrital components above the redox boundary at the sediment surface transition metals are enriched in the reducible mn oxide fraction when the horizontal position of the mn redox boundary moves upward influenced by naturally occurring enhancement of surface water productivity these elements are mobilised and available to the manganese nodule accretion process if the redox boundary reaches the level of growth of the nodules in contrast to the naturally induced mobilisation of metals a technical impact e g mining of manganese nodules would be a rapid event occurring under completely different redox conditions metals would be released into the bottom water an oxic environment where scavenging of dissolved metals and resedimentation takes place in relatively short periods of time presumably a long distance transport of contaminants will not happen

geodynamic setting of mesothermal gold deposits an association with accretionary tectonic regimes mesothermal gold provinces of phanerozoic age are characteristically associated with regional structures along which allochthonous terranes have been accreted onto continental margins or arcs a recurring sequence of transpressive deformation uplift late kinematic mineralization and shoshonitic magmatism is consistent with thermal reequilibration of tectonically thickened crust mesothermal gold camps in the superior province are spatially associated with large scale structures that have been interpreted as zones of transpressive accretion of individual subprovinces or allochthonous terranes these boundary structures are characterized by the sequence of significant horizontal shortening uplift late kinematic mineralization and shoshonitic lamprophyres and therefore may have the same geodynamic significance as phanerozoic counterparts in this model thermal re equilibration of underplated and subducted oceanic lithosphere and sediments in a transpressive regime over time scales of 10 to 40 m y is a necessary precursor to gold mineralization hydrothermal fluids are released along boundary faults and their splays during uplift the uniform temperature low salinity and mole co2 signify uniform source conditions whereas the variable o c sr and pb isotopic compositions of fluids reflect lithological complexity of the source regions and conduits ou the basis of this model it is suggested that mesothermal lode gold deposits are the product of subduction related crustal underplating and deep late metamorphism rather than magmatic or metamorphic events in the supracrustal rocks secular variations in the generation of archean proterozoic and phanerozoic mesothermal au provinces reflect the timing of collisional orogenies within terranes of these eras

dissecting through the metallogenic potentials of precambrian granitoids case studies from the bastar and eastern dharwar cratons india

a synmetamorphic lateral fluid flow model for gold mineralization in the archean southern kalgoorlie and norseman terranes western australia gold deposits are widely distributed in the archean southern kalgoorlie and norseman greenstone terranes eastern goldfields superterrane western australia which have collectively produced more than 1 850 metric tons t of gold gold deposits occur in a variety of host rocks and structural settings but all are associated with quartz veins and or shear zones that formed during the latest stages of regional compression available data indicate a similar h 2 o co 2 ch 4 ore fluid at each deposit alteration assemblages vary with metamorphic grade in a manner which indicates that late tectonic regional metamorphic gradients were still in place when gold was deposited from hydrothermal fluids these late tectonic thermal gradients were related to differential uplift of deep hot crust at the margin of the greenstone belt accompanied by intrusion of voluminous monzogranite magma at this time the western margin of the greenstone belt was a zone of high heat flow as were intrusive centers in the northern part of the southern kalgoorlie terrane these spatial and temporal relationships in combination with the apparent absence of a viable local source for the ore fluid at most deposits suggest that gold was deposited from a small number of large synmetamorphic hydrothermal systems published thermal models predict up temperature lateral fluid flow toward zones of high heat flow under these circumstances a model involving up temperature lateral flow of metamorphic fluids overcomes some of the problems associated with deeply sourced and conventional metamorphic models for the ore fluids and is consistent with most of the features of gold deposits in the southern kalgoorlie and norseman terranes potassium metasomatism and radiogenic isotope signatures associated with gold mineralization are ascribed to a metamorphic fluid that was modified by exchange with biotite bearing felsic rocks granitoids felsic volcanic and volcaniclastic rocks in the course of lateral flow toward zones of high heat flow

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt

the gold carbonate association source of co2 and co2 fixation reactions in archaean lode deposits abstract abundant carbonate is a ubiquitous feature of archaean lode gold deposits both in domains of alteration enveloping veins and in rocks enclosing auriferous sediments detailed studies of chemical mass balance in metabasalts progressively altered towards gold bearing quartz carbonate veins disposed within shear zones at yellowknife reveal massive additions of co2 k si and fe accompanying mineralisation with concomitant depletions of na coherent behaviour of al sc zr also v and nb provides a reference for constraining the volume relations during hydrothermal alteration in the peripheral regions of alteration depletions of ca and mg result in overall volume reduction but these elements are added to veins and their immediate alteration envelopes where there is a large positive volume change precious metals together with cr ni cu zn pb rb cd and ba have been added to the veins and altered wall rocks whereas quartz noble metals and other trace elements have been precipitated from hydrothermal fluids solution of ca mg and some fe indigenous to peripheral alteration regions combining with co2 from the mineralising reservoir appears to be the process for forming the abundant ferri dolomite gangue observations of many archaean au deposits reveals that carbonate chemistry reflects the nature of wall rocks with wall rocks donating the bivalent metal cations and hydrothermal fluids the co2 alteration reactions at yellowknife involved hydrolysis of albite accompanied by fixation of kaqueous to produce muscovite with na loss and hydrolysis of chlorite epidote with co2 fixation to form ferri dolomite studies of oxygen isotopes fe 2 fe 2 fe 3 and structure reveal that au quartz and carbonate were precipitated in the presence of fluids of probable metamorphic origin 18 o 8 9 at low redox potential and at ambient temperatures of 400 450 c during episodes of hydrofracturing the abundant co2 and k required for extensive carbonate muscovite replacement alteration could be supplied by fluids released during prograde metamorphism under greenschist or greenschist amphibolite facies conditions where the relative proportion of co 2 h 2 o is in the order of 0 2 to 0 5 and k na 1 given high co2 co in the hydrothermal reservoir these molecules may act as complexing agents for transport of au and other rare elements e g w pd ni cr