the sholl shear zone west pilbara evidence for a domain boundary structure from integrated tectonostratigraphic analyses shrimp upb dating and isotopic and geochemical data of granitoids abstract the pilbara block provides a record of archaean continental growth involving the tectonic accretion of outboard island arcs and collisions with other continental scale fragments this record of continental growth is balanced by breakup and strike slip dismemberment of the continent new shrimp u pb in zircon ages and sm nd data provide evidence in the west pilbara which demonstrates that subduction related and tectonic accretion processes at the western margin of that ancestral continent between 3 15 2 78 ga were coeval with and genetically related to crustal scale tectonics and basin formation inboard of that margin the tectonic division of the west pilbara is defined by integrated tectonic analyses geochronology geochemistry and isotopic analyses geochronological studies clearly indicate that the western pilbara comparises two domains with different recorded geohistories whereas geochemistry and isotopic systematics reflect the changing tectonic regimes through time in combination these studies allow the development of a reconstruction of the relative positions of the domains through time on the western margin of the pilbara block the supracrustal rocks of the northern roebourne lithotectonic complex domain 6 in a pilbarawide scheme were formed in an island arc setting facing an ocean to the north west prior to 3260 ma the time of emplacement of voluminous granitoids into the complex in contrast the supracrustal rocks of the southern sholl belt pilbara domain 5 were formed in a back arc setting behind a north west facing arc between 3125 and 3112 ma with more or less synchronous granite emplacement at about 3115 ma the two domains were tectonically juxtaposed between 2991 and 2925 ma by the sholl shear zone a largely sinistral shear zone with subsequent volcanism in both domains to about 2925 ma the roebourne lithotectonic complex domain 6 is interpreted to be an allochthonous terrane which formed north east relative to its present position but indigenous to the pilbara block rather than an exotic terrane the east pilbara is interpreted to have acted as a cratonic hinterland during the convergent margin tectonics that affected the two west pilbara domains

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

epigenetic nature of the bif hosted gold mineralization at ajjanahalli southern india evidence from ore petrography and fluid inclusion studies abstract gold mineralization at ajjanahalli is bif hosted and structurally controlled ore petrographic studies revealed that gold was introduced as a consequence of focussed fluid flow and sulfidation of an oxide facies bif horizon ubiquitous syn kinematic quartz veins within the bif as well as in the associated metasedimentary units lay testimony to the high fluid flux during deformation fluid inclusion studies involving microthermometry and laser raman microspectroscopy on inclusions from quartz veins from different litho units were carried out the primary fluid preserved as aqueous carbonic inclusions within quartz veins in carbonaceous phyllites is found to be h2o co2 ch4 nacl type near isothermal decompression led to splitting of this fluid at greenschist facies conditions 0 8 kb 263 c to 1 15 kb 309 c and resulted in a low saline aqueous and a carbonic ch4 fluid the auriferous aqueous part reacted with the oxidic bif leading to epigenetic replacement of oxides to pyrite and concomitant precipitation of gold

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

the basement of the deccan traps and its madagascar connection constraints from xenoliths paleogeographic reconstructions of india and madagascar before their late cretaceous rifting juxtapose the antongil block of madagascar against the deccan traps of india indicating that the western dharwar craton extends below the deccan lavas some recent studies have suggested that the south maharashtra shear zone along the northern konkan coast of india limits the northern extent of the western dharwar craton implying that the craton does not extend below the deccan traps raising a question mark on paleogeographic reconstructions of india and madagascar the continuity of the western dharwar craton north of the south maharashtra shear zone below the deccan traps or its lack thereof is critical for validating tectonic models correlating madagascar with india in this study zircons in tonalitic basement xenoliths hosted in deccan trap dykes were dated in situ using the u pb isotope system the data furnish u pb ages that define three populations at 2527 6 2456 6 and 2379 9 ma the 2527 6 ma ages correspond to the igneous crystallization of the tonalites whereas the 2456 6 and 2379 9 ma ages date metamorphic overprints the results help to establish for the first time that the basement is a part of the neoarchean granitoid suite of the western dharwar craton which extends northward up to at least talvade in central and kihim beach in the western deccan by implication the south maharashtra shear zone cannot be the northern limit of the western dharwar craton the granitoids are correlated with the neoarchean felsic intrusions 2 57 2 49 of the masaola suite in the antongil block of madagascar supporting the existence of a neoarchean greater dharwar craton comprising the western dharwar craton and the antongil masora block

relating arctic ambient noise to thermally induced fracturing of the ice pack thermally induced stresses within pack ice floes are simulated using a viscoelastic model that accounts for the time variation of the atmospheric forcing parameters and the temporal and spatial variations of snow cover to relate the model predicted stresses to the observed 500 hz under ice noise variations a fracturing paradigm was developed the fracturing paradigm includes factors related to ice tensile strength given its salinity and temperature and stress relief as a result of fracturing a fracture count time history was produced using the thermal stress model and the fracturing paradigm for the fall of 1988 in the eastern arctic ocean the results were compared with the observed 500 hz noise the fracture count shows increases for 11 of 13 under ice noise episodes most of the observed under ice noise episodes were of the type identified in previous studies i e those associated with nightly cooling in addition this study identifies under ice noise episodes related to longer term occurrences a

sources of continental magmatism adjacent to the late archean kolar suture zone south india distinct isotopic and elemental signatures of two late archean magmatic series conspicuous nd sr and pb isotopic differences exist between the archean gneiss terranes adjoining the suture at the kolar schist belt south india these gneisses which are the deformed equivalents of plutonic and volcanic rocks have known or inferred igneous ages of 2630 to 2530 ma initial isotopic ratios of nd sr and pb suggest that metaplutonic gneisses west of the kolar schist belt were emplaced into and variably contaminated by an evolved continental crust that formed prior to 3200 ma felsic metaigneous gneisses that occur as slivers on the western margin of the schist belt have an isotopic character similar to that of the metaplutonic rocks on the same side of the kolar schist belt on the east side of the kolar schist belt the isotopic evidence suggests that the 2530 ma granitic gneisses were not derived from or contaminated by an older continental crust their source probably evolved with a nd isotopic composition similar to that of typical archean mantle but became light rare earth element enriched after 2900 to 2700 ma the inferred tectonic setting for the west side of the kolar schist belt is an andean continental magmatic arc for the east side of the kolar schist belt a possible phanerozoic analog is an evolved island arc such as japan

formation of archean 3600 2500 ma continental crust in the dharwar craton southern india abstract the generation preservation and destruction of continental crust on earth is of wide interest in understanding the formation of continents cratons and supercontinents as well as related mineral deposits in this contribution we integrate the available field petrographic geochronologic elemental nd hf pb isotope data for greenstones ttg gneisses sanukitoids and anatectic granites from the dharwar craton southern india this review allows us to evaluate the accretionary processes of juvenile crust mechanisms of continental growth and secular evolution of geodynamic processes through the 3600 2500 ma window hence providing important insights into building of continents in the early earth the dharwar craton formed by assembly of micro blocks with independent thermal records and accretionary histories the craton can be divided into three crustal blocks western central and eastern separated by major shear zones the western block contains some of the oldest basement rocks with two generations of volcano sedimentary greenstone sequences and discrete potassic plutons whereas the central block consist of older migmatitic ttgs abundant younger transitional ttgs remnants of ancient high grade supracrustal rocks linear volcanic dominated greenstone belts voluminous calc alkaline granitoids of sanukitoid affinity and anatectic granites in contrast the eastern block comprises younger transitional ttgs abundant diatexites thin volcanic sedimentary greenstone belts and calc alkaline plutons published geochronologic data show five major periods of felsic crust formation at ca 3450 3300 ma 3230 3150 ma 3000 2960 ma 2700 2600 ma and 2560 2520 ma which are sub contemporaneous with the episodes of greenstone volcanism u pb ages of inherited zircons in ttgs as well as detrital zircons together with nd pb hf isotope data reveal continental records of 3800 3600 ma the u pb zircon data suggest at least four major reworking events during ca 3200 ma 3000 ma 2620 2600 ma and 2530 2500 ma corresponding to lower crustal melting and spatially linked high grade metamorphic events the ttgs are sub divided into the older 3450 3000 ma ttgs and the younger 2700 2600 ma transitional ttgs the older ttgs can be further sub divided into low al and high al groups elemental and isotopic data suggest that the low al type formed by melting of oceanic island arc crust within plagioclase stability field in contrast the elemental and isotopic features for the high al group suggest derivation of their magmatic precursor by melting of oceanic arc crust at deeper levels 55 65 km with variable garnet and ilmenite in residue the transitional ttgs likely formed by melting of composite sources involving both enriched oceanic arc crust and sub arc mantle with minor contamination of ancient crustal components the geochemical and isotopic compositions of granitoids with sanukitoid affinity suggest derivation from enriched mantle reservoirs finally anatectic granites were produced by reworking of crustal sources with different histories in the light of the data reviewed in this contribution we propose the following scenario for the tectonic evolution of the dharwar craton during 3450 3000 ma ttgs sources oceanic arc crust formed by melting of down going slabs and subsequent melting of such newly formed crust at different depths resulted in ttg magmas on the contrary by 2700 ma the depth of slab melting increased melting of slab at greater depth alongside the detritus results in enriched melts partly modified the overlying mantle wedge subsequent melting of such newly formed enriched oceanic arc crust and surrounding arc mantle generated the magmatic precursor to transitional ttgs finally at ca 2600 2560 ma eventual breakoff of down going slab caused mantle upwelling which induced low degree 10 15 melting of overlying enriched mantle at different depths thereby generating the sanukitoid magmas which upon emplacement into the crust caused high temperature metamorphism reworking and final cratonization the crustal accretion patterns in the dharwar craton share similarities with those in other archean cratons such as the bundelkhand craton in central india pilbara yilgarn craton in western australia southern africa swaziland and limpopo belt north china craton tanzania craton antongil craton ne madagascar

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

geochronological constraints on pre syn and postmineralization events at the world class cleo gold deposit eastern goldfields province western australia recent geological and geochronological studies have led to a reevaluation of whether or not the majority of the lode gold deposits of the yilgarn craton formed nearly synchronously during an archean craton wide hydrothermal event as previously proposed the majority of reliable data indicate that gold mineralization took place at ca 2640 to 2625 ma however recent work in the far north of the eastern goldfields province provides structural evidence for instances of earlier gold mineralization and geochronological data for interpreted postore rocks that are considerably older than 2640 ma the documentation of xenotime and monazite in association with ore minerals at the cleo deposit provides a valuable means of determining the absolute age of gold mineralization at the cleo deposit high grade western lodes veins crosscut the sunrise shear zone at a high angle with only centimeter scale offset given that the stratigraphic sequence indicates at least several hundred meters offset across the shear zone the minimal offset of the western lodes veins indicates that gold mineralization was late in the history of movement in the shear zone the absolute ages of pre syn and postmineralization elements in the mine stratigraphic succession have been determined using a variety of geochronological techniques fuchsite micas formed in the sunrise shear zone yield an 40ar 39ar isochron age of 2667 19 ma rhyodacite porphyry dikes postdate the formation of the shear zone but predate the main phase of gold mineralization dike intrusion is constrained to 2674 3 ma by shrimp ii u pb analysis of zircons from these dikes rhenium os analysis of molybdenite from quartz chalcopyrite molybdenite veins that crosscut the porphyry dikes gives an age of formation at 2663 11 ma consistent with their relationship to the porphyry dikes both the dikes and these veins are crosscut by gold bearing western lodes veins mutually crosscutting relationships with ore minerals indicate that microscopic xenotime and monazite grains were deposited during ore formation in the western lodes ore zones shrimp ii u pb analyses of xenotime and monazite grains indicate that western lodes mineralization took place at 2654 8 ma at least 9 m y after the intrusion of the porphyry dikes 95 confidence evidence for substantial temporal separation between intrusive activity and gold mineralization is recorded by other researchers for other deposits associated with granitic intrusions in the leonora laverton area phlogopitic micas from a lamprophyre that crosscuts the whole succession including the sunrise shear zone yield an ar ar plateau age of 2080 4 ma this study constrains the timing of multiple events at cleo including the main phase of gold mineralization during a relatively short span of time however in the context of the larger debate about the existence of a dominant 2640 to 2625 ma period of gold mineralization and the extent of a proposed widespread pre 2660 ma mineralization event in the yilgarn craton the data from cleo are equivocal the 2654 8 ma age for mineralization being significantly older than 2640 ma but not significantly pre 2660 ma even if the maximum possible age is accepted

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

geochemistry petrogenesis and tectonic setting of metavolcanics and their implications for gold mineralisation in gadag gold field southern india gold bearing metavolcanics of gadag gold field ggf are represented by mafic metabasalt metabasaltic andesite intermediate metaandesite and felsic metadacite metarhyolite rocks mafic metavolcanic rocks are low k fe rich tholeiites and were derived by partial melting of the upper mantle sources with high fe mg ratios and low m values intermediate and felsic metavolcanics were formed by remelting of these tholeiites mainly in crustal regimes although a complete sequence of metavolcanic rocks from mafic to intermediate to felsic fractions occurs these products were not the result of differentiation from a single magma crustal contamination was involved in the formation of intermediate and felsic rocks a clear gap in the chemical composition as well as index of differentiation among the mafic intermediate and felsic fractions indicate that these metavolcanics constitute a typical bimodal character it is suggested that these metavolcanics were emplaced in an active continental margin or a continental island arc setting the petrogenetic processes of formation of fe rich tholeiites that evolved in an active continental margin or a continental island arc setting could have provided a favourable geochemical environment for gold mineralisation under the conditions of deformation and metamorphism

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

ice ocean acoustic energy transfer ambient noise in the ice edge region abstract the ambient noise produced by large areas of ice floes such as the odden and the marginal ice zone was recorded and analysed a model based on the collision compression and shearing of floes driven by ocean swell was constructed this model allows the level and variance of the ambient noise to be predicted provided the conditions of the ice and the sea state are known the frequency spectrum of the ambient noise in the range 100 hz to 100 khz is explained in terms of acoustic propagation and absorption by sea water the probability distribution of ambient noise intensity obtained from a statistical analysis of collision frequencies is a form of k distribution and agrees with observations

age of younger tonalitic magmatism and granulitic metamorphism in the south indian transition zone krishnagiri area comparison with older peninsular gneisses from the gorur hassan area abstract a major episode of continental crust formation associated with granulite facies metamorphism occurred at 2 55 2 51 ga and was related to accretional processes of juvenile crust dating of tonalitic trondhjemitic granitic gneisses and charnockites from the krishnagiri area of south india indicates that magmatic protoliths are 2550 2530 5 ma as shown by both u pb and 207pb 206pb single zircon methods monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 10 ma these data provide precise time constraints and sr nd isotopes confirm the existence of late tonalitic granodioritic juvenile gneisses at 2550 ma pb single zircon ages from the older peninsular gneisses gorur hassan area are in agreement with some previous sr ages and range between 3200 20 and 3328 10 ma these gneisses were derived from a 3 3 3 5 ga mantle source as indicated from nd isotopes they did not participate significantly in the genesis of the 2 55 ga juvenile magmas all these data together with previous work suggest that the 2 51 ga granulite facies metamorphism occurred near the contact of the ancient peninsular gneisses and the 2 55 2 52 ga juvenile tonalitic trondhjemitic terranes during synaccretional processes subduction mantle plume rb sr biotite ages between 2060 and 2340 ma indicate late cooling probably related to the dextral major east west shearing which displaced the 2 5 ga juvenile terranes toward the west

origin and evolution of granitoids associated with the kadiri greenstone belt eastern dharwar craton a history of orogenic to anorogenic magmatism abstract different tectonic models have been proposed to explain the neoarchaean evolution of the eastern dharwar craton edc here we present whole rock elemental and nd isotopic compositions coupled with zircon u pb age and trace elemental data on granitoids associated with the neoarchaean kadiri greenstone belt edc to track the tectonic evolutionary history of the terrain earlier works on volcanic rocks of the kadiri belt tholeiitic oceanic plateau basalt high mg andesite and potassic adakite envisaged continental margin arc oceanic plateau collision in this work the oldest recognized granitoid is a 2 56 ga syntectonic sanukitoid like hornblende granodiorite enriched in large ion lithophile as well as ferromagnesian elements it was probably derived from a metasomatized mantle wedge above a subduction zone this is followed by syntectonic emplacement of 2 54 ga transitional ttgs and potassic adakitic volcanic rocks intrusion of 2 51 2 52 ga crustally derived granites mostly k rich nd 0 9 to 3 7 marks the end of orogenesis in the area a convergent orogenic setting characterized by assembly of diverse terranes repeated slab break off and crustal reworking is proposed to interpret the association of various granitoids and volcanic rocks finally intrusion of crustally derived highly silicic k rich a type granite nd 0 6 to 4 8 and mantle derived syenite occurred in a post subduction non compressive environment during palaeoproterozoic lithospheric delamination and attendant mantle melting and crustal reworking are suggested as possible mechanism for generation of these rocks the negative nd values and mesoarchaean nd depleted mantle model ages of some neoarchaean to palaeoproterozoic kadiri granitoids attest to the presence of older crust in the eastern part of the edc the final cratonization of the edc 2 52 2 50 ga occurred later than in most of the other cratons

carbon isotope studies of auriferous quartz carbonate veins from two orogenic gold deposits from the neoarchean chitradurga schist belt dharwar craton india evidence for mantle magmatic source of auriferous fluid abstract carbon and oxygen isotopic compositions of carbonates from auriferous quartz carbonate veins qcvs of two orogenic gold deposits ajjanahalli and guddadarangavvana halli g r halli from the neoarchean chitradurga schist belt of the dharwar craton southern india are examined to understand the origin of the mineralizing fluids the average carbonate carbon 13cpdb and oxygen 18osmow isotope compositions of qcvs of ajjanahalli are 5 5 1 3 and 14 1 2 7 respectively the same ratios for the qcvs of g r halli are 6 2 1 9 and 14 1 0 5 the corresponding average fluid 13c and 18o compositions are 5 81 1 14 13 78 5 1 for ajjanahalli and 4 64 0 7 6 50 0 6 for g r halli the 13cpdb of syn sedimentary carbonates of bif of ajjanahalli 1 8 0 1 carbonated metabasalts of ajjanahalli 1 4 and g r halli 1 3 fall in the compositional range of marine carbonates 0 2 as dissolution decarbonation reactions during metamorphism of pre existing carbonate carbonated rocks produce co2 with 13c values similar to or more enriched than parent rock the carbonate or fluid 13c ratios of the qcvs which fall in the compositional range of mantle magmatic derived co2 or carbonates obtained in this work cannot be the result of metamorphism it is proposed that gold mineralizing fluids were derived from juvenile magmatic melts and were channeled through crustal scale shear zones to give rise to the gold deposits

environmental correlates of arctic ice edge noise temporal variations of low frequency broadband ambient noise measured in early summer under drifting ice floes of the marginal ice zone miz are cross correlated with local environmental forces and ice field descriptors surface gravity wave forcing is the primary correlate of the noise its interaction with ice floes generates sound most likely via flexural floe failure and unloading motion within a few kilometers of the ice edge ice concentration is also well correlated with the noise most likely parametrically that is as ice concentration increases so does the density of potential sound sources in the ice field ice stress ice moment and wind stress magnitude while highly correlated with low frequency noise in the fully ice covered arctic are poor correlates in the miz lateral melt rate as a surrogate for thermally induced ice stress is also poorly correlated with low frequency miz noise when surface wave forcing is weak about 1 2 the time in this experiment episodes of high noise a

matched field localization for multiple sources in an uncertain environment with application to arctic ambient noise this paper describes a general method to localize multiple broadband sources in a range and azimuth dependent environment when properties of the environment are not well known both the source locations and the unknown environmental parameters are included as variables in a simulated annealing inversion that searches for the best match between the measured and modeled fields the example used to illustrate the method involves localizing an unknown number of ice ridge building events in an arctic environment where the bathymetry is poorly known a number of aspects of this problem are examined in a synthetic study including the effect of errors in the bathymetry on localization the importance of simultaneous localization and determining the number of sources present the inversion accurately localizes multiple synthetic sources particularly in cases where the sources are spatially separated and the spectral shape is known the inversion is also applied to a set of ambient noise measurements recorded i

neoarchean greenstone volcanism and continental growth dharwar craton southern india constraints from sims u pb zircon geochronology and nd isotopes abstract we present sims u pb zircon ages and nd isotope data for the felsic volcanic rocks from seven neoarchean greenstone belts of the eastern dharwar craton edc and from the chitradurga greenstone belt in the western dharwar craton wdc zircon ages show bimodal age distribution of felsic volcanism the ca 2 70 2 65 ga felsic volcanic event is contemporaneous with 2 7 ga mafic greenstone volcanism and emplacement of juvenile tonalitic to granodioritic crust while 2 58 2 54 ga felsic volcanics are coeval and spatially and probably genetically in some cases linked to the major episode calc alkaline magmatic accretion in the edc the chitradurga and veligallu greenstone belts host felsic volcanics of the first generation the latter showing inheritance at ca 2 95 ga four of the five greenstone belts hosting the second generation of felsic volcanics chitradurga kolar kadiri hutti show crustal inheritance at ca 2 6 2 7 2 9 3 0 3 1 and 3 3 ga ndt indicate derivation of the felsic volcanics from juvenile sources or short lived crustal sources with minor influence of older crust the new and existing data are consistent with two stage growth of the dharwar craton in the neoarchean first stage accretion led to the growth of a 2 7 2 6 ga juvenile crustal province of mafic volcanics and felsic plutons along the eastern margin of the wdc second stage accretion 2 58 2 52 ga led to the emplacement of ttg and calc alkaline plutons and felsic volcanics throughout the eastern dharwar craton an active margin context could apply for the two stage accretion scenario considering a west dipping subducting slab beneath the craton in the framework of long lived ultra hot accretionary orogen but the great width of influence of magmatic accretion and or reworking particularly during the second stage would suggest large scale mantle flow reorganization that would have generated large plume s

the role of a transcrustal shear zone in orogenic gold mineralization at the ajjanahalli mine dharwar craton south india the ajjanahalli gold mine is spatially associated with a late archean craton scale shear zone in the eastern chitradurga greenstone belt of the dharwar craton india gold mineralization is hosted by an 100 m wide antiform in a banded iron formation original magnetite and siderite are replaced by a peak metamorphic alteration assemblage of chlorite stilpnomelane minnesotaite sericite ankerite arsenopyrite pyrite pyrrhotite and gold at ca 300 to 350 c elements enriched in the banded iron formation include ca mg c s au as bi cu sb zn pb se ag and te whereas in the wall rocks as cu zn bi ag and au are only slightly enriched strontium correlates with cao mgo co2 and as which indicates cogenetic formation of arsenopyrite and mg ca carbonates the greater extent of alteration in the fe rich banded iron formation layers than in the wall rock reflects the greater reactivity of the banded iron formation layers the ore fluids as interpreted from their isotopic composition 18o 6 5 8 5 initial 87sr 86sr 0 7068 0 7078 formed by metamorphic devolatilization of deeper levels of the chitradurga greenstone belt arsenopyrite chalcopyrite and pyrrhotite have 34s values within a narrow range between 2 1 and 2 7 per mil consistent with a sulfur source in chitradurga greenstone belt lithologies based on spatial and temporal relationships between mineralization local structure development and sinistral strike slip deformation in the shear zone at the eastern contact of the chitradurga greenstone belt we suggest that the ajjanahalli gold mineralization formed by fluid infiltration into a low strain area within the first order structure the ore fluids were transported along this shear zone into relatively shallow crustal levels during lateral terrane accretion and a change from thrust to transcurrent tectonics based on this model of fluid flow exploration should focus on similar low strain areas or potentially connected higher order splays of the first order shear zone

pb pb baddeleyite ages of mafic dykes from the western dharwar craton southern india a window into 2 21 2 18 ga global mafic magmatism abstract paleoproterozoic mafic magmatism in the dharwar craton dc is evident by ubiquitous dykes of varied orientations and emplacement ages eight distinct mafic dyke events have been identified in the eastern dharwar craton edc while studies in the western dharwar craton wdc were restricted to a previously dated dandeli dyke precise baddeleyite te tims pb pb ages for five mafic dykes across the wdc reveal emplacement ages of 2212 3 0 8 ma 2212 9 0 4 ma 2208 1 0 8 ma 2205 2 0 8 ma for nw se to nnw sse trending dykes and 1957 3 0 7 ma for ene wsw dyke the ca 1 96 ga mundargi dyke establishes a new episode of mafic magmatism in the dc whereas ca 2 21 ga mafic dykes suggests widespread mafic magmatism across the dc the collective trend of ca 2 21 2 20 ga radiating dykes vary from n100 e in edc to n165 e in wdc converging towards the north western margin of dc fanning angle 65 where a mantle plume appears to be located it is postulated that ca 2 18 ga mahbubnagar dandeli dyke swarm of the dc also originated from a similar plume source but as a distinct pulse within a time span of 35 40 myr present study depicts barcode of the wdc and represents an epitome of correlations with the edc superior slave and greenland cratons widespread ca 2 21 2 18 ga lip event indicates that these cratons were located at proximity and suggests breakup of supercraton sclavia superia the single 1 96 ga mundargi dyke is coeval with the dykes and sills of karelia north china and north atlantic cratons

paleostress and statistical analysis using quartz veins from mineralized and non mineralized zones application for exploration targeting abstract quartz vein orientations thickness and spacing data are analyzed from 10 transects each of mineralized and non mineralized zones of gadag region which is a gold bearing terrain within the dharwar craton southern india paleostress analysis reveals that the veins emplaced when fluid pressure pf varied from pf 2 intermediate principle stress to pf 2 to pf

shrimp zircon and titanite u pb ages lu hf isotope signatures and geochemical constraints for 2 56 ga granitic magmatism in western dharwar craton southern india evidence for short lived neoarchean episodic crustal growth abstract shrimp u pb zircon and titanite age data for granites adjacent to the gadag greenstone belt ggb in the western dharwar craton wdc india establish the occurrence of voluminous 2 56 ga granitic magmatism and indicate the development and evolution of the ggb over a time span of 70 m y these new data show that the ggb has an evolutionary history distinct from the chitradurga greenstone belt cgb to the south which was previously considered to be linked to that of the ggb based on the continuity of lithological associations whole rock geochemical data indicate granites marginal to the ggb are potassic were derived from intracrustal melting of an arc crust without the involvement of mantle and demonstrates the prevalence of horizontal tectonics in the evolution of dharwar craton lu hf zircon studies indicate that these rocks were derived from a heterogeneous arc crust with distinct crustal histories on either side of the ggb the voluminous 2 56 ga granitic magmatism surrounding the ggb in the wdc in addition to the known 2 61 and 2 56 2 52 ga felsic magmatism supports a number of short lived episodic crustal growth events within the dharwar craton during the neoarchean over a period of 100 m y short lived episodic crustal growth during the archean typically within the time scale of few million years marks the distinction with long lived modern style subduction processes these variations are often attributed to hotter archean mantle and prevailing geodynamic regime

alteration mineralogy fluid inclusions and stable isotope studies from chigargunta and bisanatham gold deposits south kolar greenstone belt dharwar craton india implications on genesis of gold mineralization abstract there are many neoarchaean orogenic gold hosting greenstone belts in the dharwar craton dc india among them the southern extension of the kolar greenstone belt skgb contain promising gold deposits such as chigargunta and bisanatham auriferous laminated quartz calcite veins are hosted mainly within the sheared and altered champion gneiss and amphibolite in chigargunta and bisanatham respectively the hydrothermal alteration mineral assemblages show three main alteration zones at chigargunta however only two alteration zones were identified at bisanatham the hydrothermal mineral assemblage at chigargunta and bisanatham indicate a similar mineralogy that consists of tourmaline muscovite biotite chlorite epidote sericite calcite and quartz along with sulfide and gold sulfides associated with ore are predominantly pyrrhotite pyrite arsenopyrite chalcopyrite molybdenite in both the places native gold is mainly associated with pyrrhotite in the inner and proximal zone of both the deposits fluid inclusion petrography and microthermometry from quartz calcite veins within the mineralized zone at chigargunta reveal a low to medium salinity 0 5 to 13 3 wt nacl equiv h2o nacl co2 ch4 n2 bearing fluid however at bisanatham only h2o nacl fluid with low salinity 1 56 to 6 44 wt nacl equiv was observed p t conditions of the gold mineralization at the chigargunta and bisanatham deposits range between 1 7 and 3 5 kbar pressure 285 c to 378 c temperature and 0 8 to 1 2 kbar pressure 365 c to 405 c temperature respectively alteration mineralogy ore mineral assemblages fluid compositions and p t conditions from the skgb confirm that the near neutral to slightly alkaline mineralizing fluid transported gold as au hs 2 complexes with low o2 conditions the observed drastic pressure and temperature fluctuation suggests that fault valve mechanism pressure cycling occurred at the time of gold precipitation carbon 13cco2 isotopic composition of ore fluid deduced from calcite varies from 2 4 to 3 3 average 0 3 and 2 1 to 1 4 average 0 8 for chigargunta and bisanatham respectively oxygen isotopic 18oh2o values for both the deposits vary from 7 2 to 14 6 average 9 3 and 7 3 to 8 9 average 8 3 respectively ore forming fluid could have possibly been derived from the decarbonation of marine carbonates during the metamorphic devolatilization of greenstones the slight intra deposit variation of the carbon isotopic value implies the rayleigh fractionation with progressive consumption of the co2 as a result of interaction between hydrothermal fluid and wall rock as well as immiscibility of ch4 and co2 fluid ore fluid sulfur 34sh2s isotopic composition derived from 34s values of sulfides varies from 0 42 to 2 43 and 0 25 to 2 34 for chigargunta and bisanatham respectively both the sulfur and carbon isotopic values from these deposits are comparable with other orogenic gold deposits in the dc and elsewhere in the world although carbon isotopic values possibly indicate the metamorphic fluid source the sulfur isotopes signify average crustal sulfur composition that is from crustally derived modified fluid based on the field geological mineralogical fluid and isotopic compositions the possible fluid source could be from the metamorphic devolatilization process of the thick greenstone sequence in the skgb however a magmatic source could be further tested with geochronological studies on intrusives in the area

origin of iron oxide spherules in the banded iron formation of the bababudan group dharwar craton southern india the banded iron formation of the bababudan group western dharwar craton india is composed of millimetric to centimetric alternating quartz and grey to red fe oxide bands major phases are quartz and martite hematized magnetite with minor fe sulfides and ca mg fe carbonates micrometric fe oxide spherules fill cavities in discontinuous micrometric layers of fe oxides that occur in the massive quartz layers and at the interface of massive fe oxide and quartz layers the spherules are composed of micrometric radial plates of hematite intergrown with nanometric magnetite these spherules contain carbonaceous matter cm with nanometric fe particles and have low n contents 900 ppm cm1 the spherule formation is attributed to a low temperature hydrothermal process 150 200 c at around 2 52 ga possibly favored by the presence of cm these hydrothermal fluids dissolved diagenetic interstitial sulfides or carbonates creating cavities which provided space for the spherule precipitation carbonaceous matter of semi anthracite maturity is encapsulated in quartz grains adjacent to the fe oxide spherules cm2 and it is thus concluded that cm1 and cm2 are most likely contemporaneous and of the same origin either incorporated at the time of bif formation or during the hydrothermal event at 2 52 ga from the underlying phyllitised black shales carbonaceous matter cm3 was also found around the fe oxide spherules and the martite grains cm3 has much higher n contents 5000 ppm is of a lower maturity than cm1 and cm2 and is related to weathering which is also indicated by the presence of goethite and kaolinite the 13c of all cms varies from 19 4 to 24 7 similar to values measured in the underlying phyllitised black shales and likely reflect denitrifying microbial activity

geochemistry and petrogenesis of amphibolites kolar schist belt south india evidence for komatiitic magma derived by low percentages of melting of the mantle the kolar schist belt of the dharwar craton of south india is an archean greenstone belt dominated by metavolcanic rocks the mafic metavolcanic rocks occur as komatiitic and tholeiitic amphibolites the komatiitic amphibolites occur along the margins of the n s trending synformal belt they are much less abundant than the tholeiitic amphibolites and have 14 to 21 3 wt per cent mgo the komatiitic amphibolites from the west central part of the belt have two distinctive ree patterns 1 those enriched in the middle to light ree but depleted in ce relative to nd and 2 those with patterns that are convex up i e depleted in both light and heavy ree although more depleted in the light ree associated tholeiites have light ree depleted to flat ree patterns komatiitic and tholeiitic amphibolites from the eastern part of the belt have enriched light ree patterns the tholeiitic amphibolites from the kolar schist belt are similar to the th i and th ii types of archean tholeiites of condie 1981 the komatiitic amphibolites are similar to komatiites and komatiitic basalts of barberton mountainland but have higher feo and tio 2 abundances and lower yb gd ratios the petrogenetic interpretations for these rocks are based primarily on a modification of the mgo feo diagram of hanson langmuir 1978 and modelling of zr ni and ree all of the rocks have undergone some fractionation while the modelling does not give accurate temperatures pressures compositions and extents of melting of the mantle sources for the various amphibolites it does present an approach which can be used for estimating these parameters for example the komatiitic amphibolites appear to be derived from melts generated by 10 to 25 per cent melting of the mantle over a range of depths and temperatures greater than 80 km and 1575 c the variation in the p t conditions of magma generation is possibly due to adiabatic melting in mantle diapirs with a range of feo mgo ratios if the tholeiitic amphibolites are derived from similar mantle sources it is not clear that they are their parent melts may have been generated by similar extents of melting but at depths of less than 80 km the komatiitic amphibolites from the west central part of the belt were generated from light ree depleted mantle whereas those from the eastern part of the belt appear to have been generated from light ree enriched mantle the sources for the komatiitic amphibolites in both areas were significantly enriched in feo relative to pyrolite thus a light ree depleted and a light ree enriched source appear to have provided mafic volcanics with similar major element chemistry to this belt during its evolution

isotope c and o composition of auriferous quartz carbonate veins central lode system gadag gold field dharwar craton india implications to source of ore fluids abstract carbon 13 c pdb and oxygen 18 o smow isotopic compositions of auriferous quartz carbonate veins qcvs of gold deposits from sangli kabuliyatkatti nagavi nabapur and mysore mining areas developed on the central lode system of the gadag gold field ggf in the neoarchaean gadag schist belt of the dharwar craton southern india have been examined for the first time to understand the origin of the mineralising fluids in majority of the samples 46 out of 49 13 c pdb of carbonates of the qcvs fall in the range from 2 2 to 9 7 and the 18 o values range from 12 0 to 30 5 smow the calculated fluid 13 c c compositions for these deposits range from 2 1 to 9 6 and 18 o h 2 o from 6 8 to 25 9 respectively carbonate 13 c and fluid 13 c c compositions of the carbonates of the qcvs of the ggf are not only distinct from the carbon isotope range of marine carbonates or meta sedimentary carbonates of the chitradurga schist belt but are consistent with c isotope values of magmatic 5 3 burrows et al 1986 and or mantle 6 2 ohmoto 1986 carbonates as dissolution decarbonation reactions during metamorphism of pre existing carbonate carbonated rocks produce co 2 with 13 c values similar to or more enriched than parent rock the carbonate or fluid 13 c ratios of the qcvs which fall in the compositional range of mantle magmatic derived co 2 or carbonates obtained in this work cannot be the result of metamorphism the present study corroborates our previous reports from ajjanahalli and g r halli gold deposits sarangi et al 2012 occurring in the vicinity of the southern extension of the same crustal scale shear zone on which all the ggf deposits are located the age of gold mineralisation in this area has been reported to be 2522 6 ma by sarma et al 2011 chardon et al 2011 have proposed large scale remobilization of the older gneissic basement as well as emplacement of juvenile granites between 2559 ma and 2507 ma close to the crustal scale shear zone along the eastern margin of the chitradurga schist belt based on these observations and our isotope studies it is proposed that gold mineralising fluids were derived from mantle juvenile magmatic melts and were channelled through crustal scale shear zones to give rise to the gold deposits in the ggf

neoarchean magmatism in shimoga greenstone belt india evidence for subduction accretion processes in the evolution of the western dharwar stratigraphy abstract new geochemical and geochronological data are presented for a suite of metavolcanic rocks from shimoga greenstone belt to understand their petrogenesis and to constrain the timing of final mafic magmatic activity in western dharwar craton we report an sm nd isochron age of 2638 66 ma for metabasaltic and metaandesitic rocks of the stratigraphically uppermost medur formation of dharwar supergroup exposed in the belt which is the youngest age obtained yet for neoarchean mafic magmatism in western dharwar craton the emplacement of the basalt andesite dacite rhyolite suite of rocks took place in a magmatic arc culminating in the deposition of vast greywacke argillites in a continental margin type setting the magmatic precursors of the metavolcanic rocks were generated by partial melting of depleted mantle by slab derived fluids with variable input from the subducting slab assimilation fractional crystallization processes may also have contributed to the magmas to a significant extent the younger mafic lithologies of the dharwar stratigraphy represent accretionary orogens and the date obtained in this study points towards the final stages for the assembly of western dharwar craton by subduction processes during neoarchean the thickness and extent of the greywacke argillite suite in sgb could be attributed to the protracted deposition during amalgamation of western dharwar craton with a northern archean continental crust that might underlie deccan volcanic rocks

the world class wallaby gold deposit laverton western australia an orogenic style overprint on a magmatic hydrothermal magnetite calcite alteration pipe gold mineralisation at the wallaby gold deposit is hosted by a 1 200 m thick mafic conglomerate the conglomerate is intruded by an apparently comagmatic alkaline dyke suite displaying increasing fractionation through mafic monzonite monzonite syenite syenite porphyry to late stage carbonatite in the mine area a pipe shaped zone of actinolite magnetite epidote calcite amec alteration overprints the conglomerate gold mineralisation associated with dolomite albite quartz pyrite alteration is hosted in a series of sub horizontal structurally controlled zones that are largely confined within the magnetite rich pipe the deposit has a current ore reserve of 2 0 moz au and a total resource of 7 1 moz au

the sound generated by propagating cracks in sea ice observations of the sound generated by ice cracking events in the amundsen gulf provide an opportunity for studying the mechanisms of the generation and propagation of acoustic signals beneath the arctic ice cover contributions from waves traveling in the ice longitudinal waves are identified and shown to be small compared to the direct acoustic wave both high frequency 5 khz and base band 200 300 hz components occur and are interpreted in terms of a moving source model modulated by the slip stick process considered in earthquake mechanics it is shown that the high and low frequency components can be related to the vertical and horizontal length scales of the crack

geochemistry of adakites from neoarchaean active continental margin of shimoga schist belt western dharwar craton india implications for the genesis of ttg abstract adakites are identified for the first time from a neoarchaean active continental margin acm preserved in the eastern part of the shimoga schist belt near ranibennur western dharwar craton india these adakites consists mainly of quartz plagioclase and minor amphiboles and are interlayered with the turbidites they exhibit typical adakitic low y 8 35 14 70 ppm and yb 0 85 1 47 ppm may be classified as high sio2 and low mg ni and cr type adakites other constituents such as na2o k2o nb zr lree and hree and their ratios such as na2o k2o zr sm nb la la yb n gd yb n and th u also closely resemble with the high silica adakites hsa of martin et al martin h smithies r h rapp r moyen j f champion d 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 24 sr y values are moderate generally lower than the high sr low silica cenozoic adakites but within the range of archaean adakites the negative nb ta and ti anomalies characteristic of cenozoic adakites are found in ranibennur adakites also zr zr and hf hf ratios are also similar to hsa these adakites further depict compositional similarities with meso and neoarchaean tonalite trondhjemite granodiorites ttgs such types of adakites have been experimentally generated by the partial melting of hydrous basalts that are similar to adakitic glass veins low mgo cao cr and ni found in peridotite xenoliths of kamchatka arc xiong x l xia b xu j f niu h c xiao w s 2006 na depletion in modern adakites via melt rock reaction within the sub arc mantle chem geol 229 273 292 since archaean had higher geothermal regime subduction of the basaltic slab was most probably shallow and significant mantle wedge was not available to react with adakitic melt hence archaean adakites in general and particularly from ranibennur are relatively depleted in the peridotiitic mantle wedge components their identification from an archaean acm and their compositional similarities with the paleo meso archaean ttgs suggest the possibility of identical origin i e the melting of a shallow hydrous basaltic slab this mechanism during archaean was widespread and rapid resulted in faster continental crustal growth during the early history of the earth identification of archaean adakites in association with island arc turbidites from a greenstone belt sequence supports the rapid subduction mechanism for the crustal growth that coincides with 2 7 ga peak episode of almost all geological processes

precise ages for lode gold mineralization in the xiaoqinling gold field southern margin of the north china craton new constraints from in situ u pb dating of hydrothermal monazite and rutile

integrating ams data with structural studies from granitoid rocks of the eastern dharwar craton south india implications on successive fabric development and regional tectonics abstract five successive deformation phases have been identified from structural analysis of granitoid and gneissic rocks of the study area the magnetic foliation data of the studied granitoids show two distinct nw and ne trending sets the nw trending magnetic foliation is found to be essentially parallel to the mesoscale planar structures of regional d1 d2 deformations and is thus correlated with the d1 d2 phases on the contrary the ne trending magnetic foliation appears to be kinematically compatible with the nw se compression direction deciphered from conjugate ductile shear zone patterns of the d3 event combining ams field and microstructural data the granitoid outcrop have been separated into older d1 d2 and younger d3 domains the published geochronological data from the area when compared with the established d1 d2 and d3 domains shows a good correlation pointing tentatively the timing of the d1 d2 deformation phase to be 2550 ma while the d3 phase to be 2524 ma hence the study highlights the importance of combining ams data with field and microstructural studies to constraint the timing of regional successive deformation imprints it also helps to identify the chronologically separated emplacement phases of the granitoid massif in question which would not have been possible otherwise

episodic ages of greenstones a key to mantle dynamics two greenstone age minima at 2 5 2 2 ga and 1 65 1 35 ga may correlate with ages of supercontinents when a greater proportion of greenstones were recycled into the mantle by subduction from the age distributions of greenstones and juvenile continental crust earth history can be divided into three stages i 2 8 ga when greenstones and microcontinents formed and collided continuously although probably not forming a supercontinent until about 3 ga ii 2 8 1 3 ga where a clear episodisity is apparent in greenstone eruption and collision ages and where two greenstone age minima coincide in age with supercontinents and iii 1 3 ga when greenstones and continents appear to have formed and collided continuously perhaps in response to significant overlap in the times of dispersal and assembly of supercontinents greenstone age peaks 2 7 1 9 and 1 3 ga may reflect catastrophic overturn of the mantle initiated by episodes of sinking of cold lithospheric slabs through the 660 km discontinuity to the d layer initiating an episode of major mantle plume activity continuous production and preservation of greenstones in the last 1 3 gy is consistent with seismic tomography data which suggest a steady sinking of slabs into the lower mantle

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

geological and shrimp ii u pb constraints on the age and origin of the breves cu au w bi sn deposit carajas brazil the breves deposit in the carajas copper gold belt brazil a member of the cu au w bi sn group of deposits contains about 50 mt of 1 22 cu 0 75 g t au 2 4 g t ag 1 200 g t w 70 g t sn 175 g t mo and 75 g t bi it is hosted by sandstones and siltstones of the aguas claras formation minimum age of 2 681 5 ma in the roof zone of a complex highly altered granite intrusion the mineralisation is disseminated in a greisenized zone resulting from alteration of probable monzogranites and syenogranites the ore bearing greisen contains abundant xenomorphic quartz in association with fe chlorite and muscovite the gangue assemblage also includes fluorite tourmaline and minor amounts of monazite xenotime chlorapatite thorite zircon calcite siderite and bastnaesite copper mineralisation is dominated by chalcopyrite associated with pyrite arsenopyrite pyrrhotite and molybdenite gold particles in equilibrium with native bismuth are common as inclusions in chalcopyrite the greisen contains sub economic concentrations of tungsten and niobium that are related to the presence of ferberite qitianlingite and nb rutile shrimp ii zircon dating of the host granites gives 207pb 206pb ages of 1 878 8 and 1 880 9 ma for two phases and a combined age of 1 879 6 ma shrimp ii dating of monazite and xenotime grains in late to post mineralisation veins gives a combined 207pb 206pb age of 1 872 7 ma indistinguishable from the ages of the granites this provides a genetic connection between the breves deposit and the ca 1 88 ga a type granite magmatism that typifies the carajas belt as part of a much larger intracratonic magmatic province that extends over much of the amazonian craton the recognition of this association has exploration implications not only for the geophysical signature of the granite roof zones but also for likely geochemical dispersion around the deposits of this type

fabric analysis in rocks of the gadag region southern india implications for time relationship between regional deformation and gold mineralization abstract in this paper the fabric in archaean age massive metabasalts and granite from the gadag region western dharwar craton southern india are identified using the anisotropy of magnetic susceptibility ams the magnetic foliation in the rocks is dominantly nw se striking this is inferred to have developed during regional d1 d2 deformation known to have affected the region which was on account of ne sw shortening the plunge of the magnetic lineation varies from nw to se this doubly plunging orientation is inferred to be a consequence of superimposition of nw se shortening d3 deformation over the earlier structure that is known to have resulted in dome basin geometry in the surrounding metapelitic rocks of the region thus it is concluded that the massive rocks preserve evidence of superposed deformation that affected the region which can be deciphered from the orientation of magnetic fabric data the gadag region comprises lode gold deposit 2500 ma in age and field investigations show that the quartz veins are dilational with dominantly nw se strike orientation the veins are inferred to have formed by dilation under nw se shortening during d3 it is envisaged that the nw se oriented fabric that formed in the metabasalt during d1 d2 channelized fluid flow thus leading to vein emplacement during d3 it is concluded that gold deposition in the veins took place during d3 which is related to the final stages of cratonization of the dharwar craton at 2500 ma

the nature of the crust in southern india implications for precambrian crustal evolution 1 we present crustal thickness and poisson s ratio determinations from receiver function analyzes at 32 sites on the archaean and proterozoic terrains of south india the crustal thickness in the late archaean 2 5 ga eastern dharwar craton varies from 34 39 km similar crustal thickness is observed beneath the deccan volcanic province and the cuddapah basin the most unexpected result is the anomalous present day crustal thickness of 42 51 km beneath the mid archaean 3 4 3 0 ga segment of the western dharwar craton since the amphibolite grade metamorphic mineral assemblages 5 7 kbar paleopressures in this part of western dharwar craton equilibrated at the depths of 15 20 km our observations suggest the existence of an exceptionally thick 57 70 km crust 3 0 ga ago beneath the exhumed granulite terrain in southernmost india the crustal thickness varies between 42 60 km the poisson s ratio ranges between 0 24 0 28 beneath the precambrian terrains indicating the presence of intermediate rock type in the lower crust these observations of thickened crust suggest significant crustal shortening in south india during the archaean

scaling the 3 d mohr circle and quantification of paleostress during fluid pressure fluctuation application to understand gold mineralization in quartz veins of gadag southern india abstract in this study orientations of 157 quartz veins occurring in metabasalts of the gadag region dharwar craton southern india are used to plot the 3 d mohr stress circle which provides information about relative stress fluid pressure pf conditions as well as stress state during pf fluctuation to scale the 3 d mohr circle vein orientation data are integrated with a available estimates from fluid inclusions of highest recorded pf 390 mpa and lowest recorded pf 50 mpa and b intrinsic rupture criterion that empirically quantify rock properties based on the scaled 3 d mohr circle the absolute magnitudes of the three principal stresses are quantified for high and low pf of 157 veins investigated here 14 veins are identified as having favourable orientation for dilation at high as well as low pf these 14 veins have a mean strike of 150 which is similar to the orientation of the gold bearing quartz lodes reported in the region the effective normal stress n n pf prevalent during dilation of fracture fabric anisotropy with 150 strike is calculated to be 11 5 mpa at high pf and 1 0 mpa at low pf thus it is interpreted that in the gadag region a change in n of 10 5 mpa prevailed during pf fluctuation and associated separation of gold from the fluid

u pb ages for zircon and titanite from the ramagiri area southern india evidence for accretionary origin of the eastern dharwar craton during the late archean abstract the north south trending ramagiri schist belt is trident shaped with three prongs separating three granitic terranes whereas the western and eastern prongs contain metatholeiites and banded ferruginous quartzite the central prong includes felsic metavolcanics and volcanogenic metasediments the u pb zircon age for the pyroclastics in the central prong of the ramagiri schist belt is documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 2707 pm 18 end document ma considered as the time of emplacement of the felsic volcan

2 61 ga potassic granites and crustal reworking in the western dharwar craton southern india tectonic geochronologic and geochemical constraints abstract we present combined field structural data and spot image analysis petrographic data u pb zircon ages nd isotopes and whole rock geochemical data for the late archaean granite plutons from arsikere banavara ab suite and chitradurga jampalnaikankote hosdurga cjh suite in the western dharwar craton wdc ab plutons are syn kinematic with respect to the development of dome and basin patterns resulting from partial crustal diapiric overturn d1 whilst cjh suite plutons show deformation patterns resulting from the superimposition of large scale regional strike slip shearing d2 onto d1 syn emplacement fabrics sims u pb zircon ages of 2614 10 ma for the chitradurga pluton cjh and of 2617 3 ma for the arsikere pluton ab define the minimum age of the d1 strain pattern the cjh and ab suites are mainly high k granites whose origin is to be sought in partial melting of an old 3 0 ga depleted lower crust of intermediate composition the cjh and ab suites correspond mainly to the ca1 and ca2 types of archaean calc alkaline granites and some facies to the transitional ttg their heterogeneous chemical compositions indicate involvement of various sources and also significantly different depths of partial melting to produce these granites these results allow documenting regional partial reworking of the wdc 2615 ma ago requiring an ht event to have taken place before 2 61 ga and thus different from the 2 55 to 2 51 ga granulite facies metamorphic episode associated to regional scale strike slip shearing d2 that affected the entire craton partial reworking of the older lower crust at 2 61 ga and associated deformation would result from heat advection and or crustal thickening that may relate to voluminous mafic greenstone volcanism and moderate shortening at that time

rb sr ages of late archean metavolcanics and granites dharwar craton south india and evidence for early proterozoic thermotectonic event s abstract the archean greenstone sequence of the dharwar craton is intruded by late and post kinematic granites like the chitradurga and toranagallu closepet granites with whole rock rb sr isochron ages of 2603 28 ma 2 and 2452 50 ma respectively the whole rock rb sr ages of their host metavolcanic rocks show a wide variation bababudan 2728 102 ma shimoga 2520 62 ma ingaldhal 2240 50 ma mardihalli 2160 174 ma gadag 2456 76 ma and sandur 2377 646 ma while the ages of the bababudan and the shimoga volcanic suites can be related to the time of the pervasive regional folding and metamorphic events 2 7 2 6 ga those of the other volcanic suites from the chitradurga and sandur supracrustal belts indicate effects of post archean thermal event s the internal isochron ages of 2 0 0 1 ga for the chitradurga and toranagallu granites suggest isotopic resetting on a mineral scale about 500 ma after their emplacement and support the evidence from the metavolcanic suites for an early proterozoic thermal event at 2 0 ga in the dharwar greenstone granite gneiss terrain

timing of skarn gold deposition in the giant beiya polymetallic gold deposit southwest china constraints from in situ monazite sims u th pb geochronology abstract robust dating of hydrothermal deposits is critical to understand ore genesis which depends on judicious selection of mineralization related chronometers especially for complex polymetallic mineral systems the beiya deposit in southwest china is a world class polymetallic gold system 11 9 moz au at a grade of 2 52 g t and the main mineralization is considered as skarn type however available dating results for the gold mineralization give a wide range of ages which in some cases could be inaccurate due to the use of non matrix matched external isotope standards or an equivocal relationship between gold and the dated minerals in this contribution hydrothermal monazite coexisting with gold bearing ore minerals magnetite pyrite and chalcopyrite from high grade massive magnetite sulfide gold ores at beiya was dated in situ using secondary ion mass spectrometry sims the u th pb analyses of hydrothermal monazite yielded a 208pb 232th age of 36 0 0 5 ma 2 mswd 2 0 n 27 which is considered a robust age for the beiya skarn gold mineralization our study supports a genetic link between the skarn gold mineralization and the post collisional quartz monzonite magmatism at beiya based on a new zircon u pb age of 37 2 0 4 ma compared with the previously reported molybdenite re os and sphalerite rb sr isochron ages our monazite u th pb result suggests that the gold deposition was broadly coeval with the formation of molybdenite but probably prior to the local pb zn ag mineralization at beiya

uniformity in sulfur isotope composition in the orogenic gold deposits from the dharwar craton southern india the sulfur isotope composition of sulfides mainly pyrite and arsenopyrite from gold deposits prospects of the dharwar craton such as hutti hira buddini uti kolar chigargunta ajjanahalli and jonnagiri has a narrow range 34s 1 1 to 7 1 such craton scale uniformity of the above gold camps is noteworthy in spite of the wide diversity in host rock compositions and their metamorphic conditions and suggests a magmatic or average crustal source of sulfur for all deposits studied in addition our study points towards gold precipitation from reduced ore fluids with near homogeneous sulfur isotope compositions

the relationship between magnetic anisotropy rock strength anisotropy and vein emplacement in gold bearing metabasalts of gadag south india abstract in this study the importance of rock strength and its anisotropy in controlling vein emplacement is evaluated by integrating anisotropy of magnetic susceptibility ams with rock mechanics data from massive visibly isotropic metabasalts of gadag region dharwar craton south india orientation of magnetic foliation mf is first recognized from ams subsequently rock mechanics tests viz ultrasonic p wave velocity vp uniaxial compressive strength ucs and point load strength is 50 are done in cores extracted parallel and perpendicular to mf vp is found to be higher in direction parallel to mf than perpendicular to it in contrast rock strength ucs and is 50 is greater in direction perpendicular to mf than parallel to it this proves that rocks from the gold mineralized belt of gadag have rock strength anisotropy orientation of mf in gadag region is nw se which is also the mean orientation of quartz veins previous studies indicate that emplacement of veins in the region took place during regional d3 nw se shortening based on the present study it is concluded that vein emplacement took place in nw se orientation because the rocks have strength anisotropy and are weaker in this direction orientation of mf which dilated to accommodate fluid flow in addition vein intensities are measured along three traverses and found to be variable it is argued that since mineralization is favoured when the system gets saturated with fluid variation in fluid flow could not have been responsible for variation in vein intensities in the study area since the rock strength of the different blocks investigated here is not uniform it is envisaged that variation in rock strength played an important role in controlling the vein intensities it is concluded that rock strength variation controlled strain partitioning and channelized fluid flow thus influencing vein emplacement and mineralization and formation of lodes

the application of u pb geochronology to mafic ultramafic and alkaline rocks an evaluation of three mineral standards abstract a key to proving the accuracy of u pb age determinations by the various techniques available is routine analysis of an internal well characterized matrix matched mineral standard of known age in general there are very few well studied natural mineral standards available to the geological community in this study the nature geochemistry and u pb systematics of baddeleyite titanite and perovskite from three potential mineral standards are evaluated a compilation of concordant to near concordant analyses of a single phalaborwa baddeleyite crystal yields a precise u pb age of 2059 60 0 35 ma a large variation in uranium 51 2124 ppm thorium 0 5 73 ppm and lead 19 782 ppm concentrations and the presence of some u bearing mineral inclusions zircon monazite and baddeleyite zircon intergrowths require some caution when using aliquots of this phalaborwa baddeleyite crystal as a mineral standard the khan titanite crystal investigated here overall has a relatively uniform chemical composition e g u 584 95 ppm th 473 73 ppm pb 57 8 ppm th u 0 81 0 05 but parts of the crystal contain abundant mineral inclusions the model 1 upper intercept age of 522 2 2 2 ma for nine khan titanite analyses is considered the current best estimate for its crystallization age some variability in the u pb systematics with 207pb 206pb ages that vary between 508 and 560 ma was observed and could be related to the presence of tiny u bearing mineral inclusions such as rutile or allanite so careful selection of material from this titanite crystal is also required a single perovskite crystal from the ice river alkaline intrusion has a relatively uniform chemical composition uranium 141 25 ppm thorium 1954 270 ppm lead 41 5 ppm and th u 14 5 1 3 the u pb results for eight perovskite fractions yield a weighted average 206pb 238u age of 356 5 1 0 ma although there are always some limitations to using natural mineral crystals as u pb standards such as the presence of mineral inclusions alteration and geochemical heterogeneity the three crystals investigated here are considered suitable to be used as internal u pb standards for any of the currently used u pb dating techniques

stress enhanced tensile fractures in elliptical clast in conglomerate abstract mode i fractures within the elliptical clasts from archaean age chitradurga schist belt dharwar craton south india are analyzed to decipher the paleostress condition these fractures are characterized as systematic steeply dipping and parallel to each other the analysis is performed in three steps 1 fixing and transforming of the coordinate systems to describe the elliptic domain under far field stress 2 proposing suitable complex potentials for inclusion and matrix followed by the plane strain solution for elliptical domain to fractured clasts 3 obtaining the state of stresses readily inside the inclusion we conclude that the mode i fractures in elliptical clasts are developed due to far field stress enhancement inside the rigid clasts at shallow depth the deduced paleostress conditions also fit very well with the orientation and sense of previously revealed regional tectonics we infer that the amplification of stresses in the rigid inclusions is function of elastic moduli of the clast and matrix ellipticity of the clasts and remote stresses we interpret that the enhanced intra clast differential stresses are more for the clast with higher aspect ratio thus is more susceptible for tensile fracturing than the lower one for a constant applied far field stresses we also estimate the minimum breakage loads as well as minimum depths required for the clasts of different aspect ratio to develop tensile fractures in them we imply that fractured elliptical clasts can be regarded as an important paleostress indicator

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

archean granite greenstone tectonics at kolar south india interplay of diapirism and bulk inhomogeneous contraction during juvenile magmatic accretion 1 the structural study of the kolar greenstone belt and surrounding granite gneiss terrains combined with u pb dating reveals that the middle and lower crustal tectonoplutonic pattern of the eastern dharwar craton developed during a major magmatic accretion event between 2550 and 2530 ma the granite greenstone pattern resulted from the interference of gravity driven sagging of the greenstones i e diapirism e w bulk inhomogeneous shortening combined with horizontal n s stretching and syntectonic juvenile pluton emplacement bulk inhomogeneous contraction is accommodated by the synchronous development of a pervasive n s trending vertical foliation shallow stretching lineation and conjugate strike slip shear zone pattern within and outside the greenstone belt resulting in regional horizontal pure shear deformation the plutons around the greenstone belt record regional contraction by developing one set of strike slip c s fabrics of the shear zone pattern the development of the granite greenstone pattern was coeval and compatible with deformation during juvenile magmatic accretion melting and granulite metamorphism in the lower crust the kolar example points to a specific crustal rheology that allowed sagduction of the greenstones and regional distributed bulk inhomogeneous strain due to mechanical homogeneity and low viscosity provided by large scale melting during the accretion event this example further suggests specific boundary conditions to the craton that allowed e w inhomogeneous shortening to be accommodated by n s stretching and spreading of the crust without significant tectonic thickening such tectonoplutonic pattern is specific to the archean and may develop as a consequence of mantle plume activity in intracontinental settings

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

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

detrital mineral age radiogenic isotopic stratigraphy and tectonic significance of the cuddapah basin india abstract the cuddapah basin is one of a series of proterozoic basins that overlie the cratons of india that due to limited geochronological and provenance constraints have remained subject to speculation as to their time of deposition sediment source locations and tectonic geodynamic significance here we present 21 new stratigraphically constrained u pb detrital zircon samples from all the main depositional units within the cuddapah basin these data are supported by hf isotopic data from 12 of these samples that also encompass the stratigraphic range and detrital muscovite 40ar 39ar data from a sample of the srisailam formation taken together the data demonstrate that the papaghni and lower chitravati groups were sourced from the dharwar craton in what is interpreted to be a rift basin that evolved into a passive margin the nallamalai group is here constrained to be deposited between 1659 22 ma and 1590 ma it was sourced from the coeval krishna orogen to the east and was deposited in its foreland basin nallamalai group detrital zircon u pb and hf isotope values directly overlap with similar data from the ongole domain metasedimentary rocks depositional age constraints on the srisailam formation are permissive with it being coeval with the nallamalai group and it was possibly deposited within the same basin the kurnool group saw a return to dharwar craton derived provenance and is constrained to being neoproterozoic it may represent deposition in a long wavelength basin forelandward of the tonian eastern ghats orogeny detrital zircons from the gandikota formation which is traditionally considered a part of the chitravati group constrain it to being deposited after 1181 29 ma more than 700 ma after the lower chitravati group it is possible that the gandikota formation is correlative with the kurnool group the new data suggest that the nallamalai group correlates temporally and tectonically with the somanpalli group of the pranhita godavari valley basin which is tightly constrained to being deposited at 1620 ma these syn orogenic foreland basin deposits firmly link the se india proterozoic basins to their orogenic hinterland with their discovery filling a missing link in the tectonic development of the region

zirconolite and xenotime u pb age constraints on the emplacement of the golden mile dolerite sill and gold mineralization at the mt charlotte mine eastern goldfields province yilgarn craton western australia in situ shrimp u pb dating of magmatic zirconolite cazrti2o7 in the golden mile dolerite from the mt charlotte gold deposit yilgarn craton australia has yielded the first robust emplacement age 2 680 9 ma for the principle host rock of gold mineralization in the kalgoorlie district in contrast co magmatic zircon gave ages from 2 68 ga to 2 17 ga reflecting isotopic resetting of high u and th crystals in situ shrimp analysis of hydrothermal xenotime ypo4 which co exists with gold in alteration pyrite provided a pb pb isochron age of 2 655 13 ma this date indicates that the youngest deposit in the kalgoorlie district mt charlotte formed at 2 65 ga and provides a new minimum age for the structurally older golden mile deposit our results indicate that gold mineralization at mt charlotte is 50 million years older than indicated by recent 40ar 39ar dating and places new constraints on the timing of late stage regional faulting d4 in the province

petrography chemistry and isotopic ages of peninsular gneiss dharwar acid volcanic rocks and the chitradurga granite with special reference to the late archean evolution of the karnataka craton southern india abstract new petrographic major and trace element and isotopic age data are presented for four suites of rocks from the low to medium grade part of the south indian archaean craton in central karnataka two suites are from the peninsular gneiss basement to the late archaean dharwar supergroup the third is a suite of dharwar potassic rhyolites and the fourth is from the chitradurga granite which has intrusive contacts with the dharwar supergroup the chikmagalur granite granodiorite which with its host gneisses forms part of the basement peninsular gneiss to the dharwar supergroup in the bababudan basin yields rb sr and pb pb whole rock isochron dates of 3080 110 ma and 3175 45 ma respectively an initial 87 sr 86 sr ratio of 0 7013 0 0009 and a 1 value 238 u 204 pb of 7 98 host tonalitic gneisses to the chikmagalur granite give an rb sr whole rock isochron date of 3060 160 ma with an initial 87 sr 86 sr ratio of 0 7015 0 0004 and a pb pb whole rock isochron date of 3190 100 ma with a model 1 value of 8 00 a suite of granites and granodioritic tonalitic gneisses chitradurga granitic rocks and gneisses forming part of the peninsular gneiss basement to the dharwar supergroup west of the chitradurga belt yields rb sr and pb pb whole rock isochron dates of 2970 100 ma and 3028 28 ma respectively with an initial 87 sr 86 sr ratio of 0 7035 0 0013 and a model 1 value of 7 62 the suite of dharwar acid volcanic rocks from north of honnali defines a poorly fitted 207 pb 206 pb linear array mswd 15 8 which gives an apparent age of 2565 28 ma and a model 1 value of 7 46 the poor fit of this isochron shows that a closed system was not maintained a conclusion supported by high k abundances which are attributed to a syn or post magmatic ion exchange process the chitradurga granite which intrudes the older rocks of the dharwar supergroup in the chitradurga belt yields a well fitted pb pb whole rock isochron date of 2605 18 ma with a model 1 value of 7 68 the u pb relations in these suites from karnataka are in marked contrast with the commonly severe u depletion and consequently unradiogenic pb isotopic compositions observed in deeply eroded high grade archaean gneiss terranes such as those bordering the north atlantic this contrast indicates that the cratonic rocks in central karnataka represent a relatively high level in the original archaean continental crust

late archaean crust mantle interactions geochemistry of lree enriched mantle derived magmas example of the closepet batholith southern india the closepet batholith in south india is generally considered as a typical crustal granite emplaced 2 5 ga ago and derived through partial melting of the surrounding peninsular gneisses 3 3 to 3 0 ga in the field it appears as a composite batholith made up of at least two groups of intrusions a an early sio2 poor group clinopyroxene quartz monzonite and porphyritic phyritic monzogranite is located in the central part of the batholith these rocks display a narrow range in both initial 87sr 86sr 0 7017 0 7035 and nd 0 9to 4 1 b a later sio2 rich group equigranular grey and pink granites is located along the interface between the sio2 poor group and the peninsular gneisses they progressively grade into migmatised peninsular gneisses thus indicating their anatectic derivation their isotopic characteristics vary over a wide range 87sr 86sr ratios 0 7028 0 7336 and nd values from 2 7 to 8 3 at 2 52 ga field and geochronological evidence shows that the two groups are broadly contemporaneous 2 518 2 513 ga and mechanically mixed this observation is supported by the chemical data that display well defined mixing trends in the sr vs nd and elemental variation diagrams the continuous chemical variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end members derived from different source regions enriched peridotitic mantle and peninsular gneisses it is proposed that the intrusion of mantle derived magmas into mid crustal levels occurred along a transcurrent shear zone these magmas supplied additional heat and fluids that initiated anatexis of the surrounding crust during this event large scale mixing occurred between mantle and crustal melts thus generating the composite closepet batholith the mantle derived magmatism is clearly associated with granulite facies metamorphism 2 51 0 01 ga ago both are interpreted as resulting from a major crustal accretion event possibly related to mantle plume activity

geochronological constraints on meso and neoarchean regional metamorphism and magmatism in the dharwar craton southern india this contribution addresses the time framework of the regional metamorphism in the three crustal provinces making the archean dharwar craton we present results of texturally controlled in situ epma chemical dating of monazites chemical dating of monazite separates as well as sm nd garnet whole rock isochrons and shrimp u pb zircon ages for pelites amphibolites and granitoids over target areas typical of the various crustal levels of the provinces the western dharwar craton has undergone a major thermal pulse at 2 52 ga followed by slow cooling to ca 2 4 ga and recorded earlier thermal events around 3 0 ga and 3 1 ga the central dharwar craton records a major high grade thermal imprint at ca 2 55 2 51 ga followed by cooling up to 2 45 ga and earlier thermal events at ca 2 62 and 3 20 ga in the eastern dharwar craton the widespread thermal pulse between 2 55 and 2 52 ga is best recorded from 2 52 ga on the entire craton ultimately and contemporaneously undergoes the main event of regional ht lp metamorphism the contrasted thermal records of the three provinces reflect their accretion age s and their degree of involvement in a wide latest archean hot orogen which sets the capacity of these lithospheric segments to be impact by deformation and mantle fluxes the tectonic setting of latest archean hot orogeny is compatible with active plate margin processes having interacted with mantle instabilities i e plumes the tectonic setting of pre 2 5 ga thermal pulses is difficult to assess but considering their systematic links with documented magmatic pulses they may have been generated in contexts comparable that of latest archean hot orogeny where lateral constrictional flow of hot orogenic crust achieves gravity driven flow 3d mass redistribution of viscous lower crust submitted to convergence

deciphering relative timing of fabric development in granitoids with similar absolute ages based on ams study dharwar craton south india abstract anisotropy of magnetic susceptibility ams data are presented from the koppal granitoid dharwar craton south india that has u pb zircon age of 2528 9 ma the magnetic fabric is oriented in nne ssw direction this is parallel to the planar structures that developed during regional d 3 deformation but oblique to the nnw sse oriented magnetic foliation as well as field foliation d 1 d 2 deformation recorded in the country rock peninsular gneiss variation in the intensity of fabric within the granitoid is mapped it is inferred that the emplacement of koppal granitoid took place by ballooning and fabric development within the pluton was syntectonic with regional d 3 these results are compared with the time relationship between emplacement fabric development and regional deformation reported from the mulgund granite 2555 6 ma u pb zircon which is also located in the dharwar craton and is equivalent to the koppal granitoid in age this granite is known to have emplaced syntectonically with regional d 1 d 2 deformation and is thus not related to the same deformation event as the koppal granitoid despite their similar absolute ages it is argued that in the study area d 3 is 2537 ma while d 1 d 2 is 2549 ma in age thus this study highlights the use of ams in a deciphering the relative timing of regional deformation and emplacement of granitoids of equivalent age and b constraining the timing of regional superposed deformation events

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

land use change and erosional history in a lake catchment system on the canadian prairies abstract the introduction of agriculture in the late 1800s and early 1900s brought about many changes in the natural prairie landscape of western canada the objective of this research project is to evaluate landscape response to land use change by relating observed changes in the sedimentary record of a lake drainage basin to documented land use change that has occurred since the onset of settlement a 52 cm lake sediment core was sectioned into 1 cm layers the sediment was separated into allogenic and authigenic fractions using a wet chemical extraction technique close interval 210pb dating enabled the calculation of sediment and elemental influx rates prior to european settlement erosion rates were low

evolution of fabric in chitradurga granite south india a study based on microstructure anisotropy of magnetic susceptibility ams and vorticity analysis abstract in this paper the fabric in massive granite 2 6 ga from the chitradurga region western dharwar craton south india is analyzed using microstructure anisotropy of magnetic susceptibility ams study and kinematic vorticity analysis the microstructural investigation on the granite shows a progressive textural overprint from magmatic through high t to low t solid state deformation textures the mean magnetic foliation in the rocks of the region is dominantly nw se striking which have developed during regional d1 d2 deformation on account of ne sw shortening the plunge of the magnetic lineation varies from nw to vertical to se and interpreted to be a consequence of regional d3 deformation on account of nw se to e w shortening the vorticity analysis from magnetic fabric in the region reveals that the nw se oriented fabric formed under pure shear condition during d1 d2 regional deformation however some parts of the region particularly close to the adjacent chitradurga shear zone show that the magnetic fabrics are oblique to the foliation as well as shear zone orientation and inferred to be controlled by simple shearing during d3 regional deformation the shape preferred orientation spo analysis from oriented thin sections suggest that the shape of the recrystallized quartz grains define the magnetic fabric in chitradurga granite and the degree of the spo reduces away from the chitradurga shear zone it is interpreted that the change in magnetic fabrics in some parts of the granite in the region are dominantly controlled by the late stage sinistral shearing which occurred during the development of chitradurga shear zone

alteration geochemistry and fluid inclusion characteristics of the greenstone hosted gold deposit of hutti eastern dharwar craton india gold mineralization of the hutti mine southern india is situated in closely spaced laminated quartz veins and associated alteration haloes along steeply dipping shear zones within a sequence of rather uniform amphibolites intense shearing has resulted in large scale mylonitization of the wall rocks anastomosing shear zones with intervening lensoid bodies of unsheared amphibolites are characteristic features of the deposit the general pattern of symmetrical alteration comprises a distal zone of chlorite rich rock with a proximal biotite rich zone adjacent to laminated quartz veins arsenopyrite thermometry yielded a temperature range of 350 477 c for the biotite alteration zone which preceded the formation of the laminated quartz veins mass balance calculations on the alteration zones indicate a gradual mass and volume loss during alteration the alteration is accompanied by intense potash metasomatism and addition of sulfur which resulted in the formation of arsenopyrite pyrrhotite and pyrite results of fluid inclusion studies suggest that low salinity 3 9 13 5 wt nacl equivalent h2o co2 rich fluids were responsible for gold rich laminated quartz vein formation in the hutti deposit these fluids constituted a later counterpart of the protracted fluid activity that first formed the biotite alteration zone the estimated p t values range from 1 0 to 1 7 kbar at 280 320 c these data along with the alteration assemblages and the characteristic gold sulfide association both in the altered wall rock and laminated quartz veins suggest that gold transported as reduced bisulfide complexes was deposited in response to sulfidation reactions in the wall rocks comparison of p t conditions of formation of gold quartz veins at hutti with two other large gold deposits in the eastern dharwar craton namely kolar 1 8 kbar 280 c and western ramagiri 1 45 1 7 kbar 240 270 c indicates broadly similar lode gold forming conditions in the dharwar craton

broad synchroneity of three gold mineralization styles in the kalgoorlie gold field shrimp u pb and 40ar 39ar geochronological evidence there has been a long standing controversy regarding the timing and number of gold mineralization events at kalgoorlie uranium pb dating of zircons and hydrothermal monazite and xenotime as well as ar 40 ar 39 analysis of metasomatic fuchsite and white mica are used to date pre to synore dikes alteration and orebodies in order to resolve this issue the majority of gold mineralization at kalgoorlie including ductile brittle fimiston brittle ductile oroya and brittle mount charlotte style gold are different expressions of a complex mineralizing system that was active at broadly the same time at ca 2 64 ga gold mineralization was thus deposited in both ductile and brittle structures at approximately the same crustal level at broadly the same time limier similar pt conditions this giant ore system formed after ea 2 69 ga basic magmatism intrusion of the golden mile dolerite sill at 2680 9 ma and intrusion or calc alkaline feldspar quartz porphyry dikes at 2670 5 ma gold mineralization was broadly coeval with lamprophyre dike intrusion at 2642 6 ma and overlapped the waning stages of hornblende and albite bearing porphyry dike emplacement at 2650 6 ma and regional metamorphism subsequent brittle deformation in the kalgoorlie gold held way accompanied by hydrothermal activity that may have led to some late gold mineralization or remobilization in extensional quartz vein arrays in the golden mile between about 2 61 and 2 60 ca this late hydrothermal activity and associated brittle deformation marks the last event to significantly affect the rocks it kalgoorlie and may be related to uplift lift and final cooling of the terrane despite this late event the geometry of the kalgoorlie gold field and its contained lode systems has remained essentially the same since the time of gold mineralization

pb and nd isotope constraints on the origin of high mg and tholeiitic amphibolites kolar schist belt south india the late archean north south trending kolar schist belt in south india 4 km wide by 80 km long is thought to be a suture between two gneiss terranes krogstad et al 1989 within this volcanics dominated belt are recognized both tholeiitic and high mg komatiitic and picritic amphibolites which make up some 70 and 5 respectively of the exposed outcrops a massive tholeiitic amphibolite separates the belt into western and eastern parts a pb pb whole rock age of 2732 155 ma on samples from a single outcrop of massive tholeiite is a minimum age for this rock samples of this rock have nd values at 2700 ma that range between 3 8 and 6 8 1 initial 238u 204pb of 7 5 and k1 initial 232th 238u of about 4 two different types of high mg amphibolites are recognized from the western part of the belt a picritic or p type and a komatiitic or k type the p type have highly variable ce al ratios all greater than chondritic nd yb ratios greater than chondritic nd at 2700 ma of 1 5 to 8 and pb isotope compositions variable in 207pb 204pb with 1 of about 8 0 and k1 of about 4 the trace element data suggest that the light ree enrichment is a character of the mantle source and is not due to residual garnet the k type amphibolites have near chondritic ce al and nd yb ratios nd at 2700 ma of 1 5 to 8 and 1 of about 8 and k1 of about 4 although the p type is light ree enriched compared to the k type both types have similar ce nd ratios as well as initial pb and nd isotopes if the 2696 136 ma age for the sm nd isochron which includes both types of high mg amphibolite has any significance it dates the time of light ree enrichment of the mantle source for the p type komatiitic amphibolites the high mg amphibolites in the eastern part of the belt are light ree enriched have pb isotopic compositions that are variable in 207pb 204pb with a 1 about 8 5 and nd at 2700 ma of 1 8 to 4 5 hydrothermal fluids associated with metamorphism and shearing prior to about 2400 ma ago were responsible for the introduction of gold quartz carbonate veins into the kolar schist belt the pb isotope composition of galena in these veins suggests that these fluids may have also introduced extraneous pb from adjacent older granitoid gneisses into the amphibolites which could be responsible for the variability in the 207pb 204pb ratios of the samples this extraneous pb probably is not responsible for the distinct pb isotope character of each type of amphibolite

the geologic setting mineralogy geochemistry and genesis of gold deposits of the archean kolar schist belt india the kolar schist belt one of the auriferous archean schist belts in the dharwar craton includes two types of gold deposits 1 a stratiform sulfide type associated with amphibolites and banded ferruginous quartzite and 2 a gold quartz carbonate vein type associated with light rare earth element lree enriched komatiitic and tholeiitic amphibolites on the eastern part of the belt many lodes of the former are banded and show deformation similar to that of their host rocks the latter type consists of veins localized within shear zones both types have only a few inches of altered border rock characterized dominantly by biotite with minor diopside and garnet the sulfide lodes contain 5 to 30 vol percent sulfides which include dominantly pyrrhotite and arsenopyrite the lodes also contain variable amounts of magnetite and ilmenite no systematic variation in ore minerals is discernible among the lodes of the entire belt however in the kolar gold fields area a set of four parallel sulfide lodes exhibit certain textural and mineralogical zoning ore minerals show metamorphic equilibrium textures and the temperature of final equilibration of sulfides is 500 degrees c or higher sulfide lodes have low but variable abundances of base metals no correlation among total sulfide contents sulfide minerals and abundances of base metals and gold is observed nor is there any geochemical coherence between gold and arsenopyrite in the several lodes studied major oxides particularly al 2 o 3 tio 2 and k 2 o also show a large variation the ree abundances of the sulfide lodes vary widely but their patterns are similar with enrichment in both light ree and heavy ree normalized eu shows strong to negligible enrichment the ree patterns are similar to those of the banded ferruginous quartzite or iron formation samples with high ree abundances also have high base metals al 2 o 3 tio 2 and k 2 o abundances and low arsenopyrite contents based on these observations and limited nd and o isotope data it is suggested that the sulfide lodes were originally auriferous complex interflow sediments formed from submarine hydrothermal exhalations seawater and detrital sediments the vein type mineralization has only trace amounts of sulfides with galena as the dominant phase narayanaswami et al 1960 the veins consist dominantly of quartz with minor amounts of calcite and alkali feldspar and trace amounts of scheelite and tourmaline the gold content of the veins is generally 10 ppm the veins are also unusually enriched in cr and ni these observations and the fluid inclusion studies on vein quartz santosh 1986 suggest the presence of co 2 bearing ore fluids one sample of the ore vein has a strongly fractionated light ree enriched pattern with an epsilon nd value of 2 8 at 2 600 ma galena from the ore samples have pb isotope ratios reported by venkatasubramanian et al 1977 and chernyshev et al 1980 that are similar to those of the 2 600 ma intrusive granodioritic gneisses present on the west side of the schist belt krogstad et al 1989 the delta 18 o of vein quartz is 11 6 or 0 7 per mil golding 1982 these geochemical data suggest that the gold quartz veins were formed dominantly from magmatic fluids derived from the crystallization of granitic rocks a contribution from metamorphic fluids is also conceivable as indicated by the abundance of ni and cr in the veins

smnd ages of archaean metavolcanics of the dharwar craton south india abstract well constrained sm nd arrays of whole rocks are reported for the first time from three important stratigraphic units of the dharwar volcano sedimentary sequence of south india if interpreted as isochrons these arrays will correspond to the following ages and initial nd ratios relative to chur 2 911 0 049 ga and 0 20 0 40 for the kalasapura mafic basal metavolcanics of the bababudan greenstone belt 2 848 0 070 ga and 0 88 0 82 for the santaveri formation comprising mafic to felsic metavolcanics overlying the kalasapura rocks and 2 747 0 015 ga and 1 72 0 10 for the ingaldhal mafic felsic metavolcanic suite of the chitradurga greenstone belt the possibility that these sm nd arrays are mixing lines between unrelated components depleted and enriched respectively in light rare earth elements has been discussed leading to the conclusion that the ages closely correspond to their time of eruption the older suites from the bababudan belt from a mantle source of chur like composition the younger suite from the chitradurga belt from one with a prior history of lree depletion

paleostress field reconstruction in the western dharwar craton south india evidence from brittle faults and associated structures of younger granites abstract we present new fault slip data to decipher the tectonic stress regime in younger granites 2 61 ga of western dharwar craton wdc south india the study is based on paleostress reconstruction of the terrain using fault slip data from the small scale oblique slip normal faults of chitradurga granite paleostress analysis using right dihedron and rotational optimization methods reveal that these faults have developed under nne ssw directed extension similar results are also obtained when left lateral and right lateral oblique slip normal faults are treated as separate entities it is envisaged that all faults in chitradurga granite have developed under a single tectonic event and are considered to be extensional shear fractures we interpret that the oblique slip normal faults in granite oriented along nnw sse to n s p shear nw se r shear ne sw r shear and wnw ese t are defined as the respective shear components of the regional riedel shear system it developed due to sinistral movement along the chitradurga shear zone csz during late d3 deformation at shallow crustal level the fault planes act as primary or secondary shear planes depending on their proximity to the csz and kinematically fit well with regional far field compression the present study addresses the formation of oblique slip normal faults in the chitradurga granite and its implication in reconstructing the paleostress field in the precambrian south indian shield

geochemistry of banded sulphidic cherts of sandur greenstone belt dharwar craton india constraints on hydrothermal processes and gold mineralization abstract the banded iron formations which are hosted in the greenstone belts of dharwar craton are potential sources of iron and also major archives of gold mineralization at some places the gold bearing banded sulphidic cherts from the neoarchean sandur greenstone belt of western dharwar craton have been reported in this paper in order to understand their geochemical characteristics source of gold mineralization and depositional conditions these rocks are predominantly characterized as mixed oxide sulphide facies which are generally referred as sulphide facies iron formations consisting of alternate mesoscale bands of silica and iron with sulphides such as pyrite chalcopyrite galena arsenopyrite and sphalerite occurring as minor opaque phases mineral chemical analyses of pyrite having high fe s as co ni and pb as ratios suggests hydrothermal source native gold occurs as disseminations in these sulphides with its concentrations ranging from 0 05 to 1 48 ppm gold mineralization is sediment hosted variety epigenetic epithermal hydrothermal in origin where the sulphidic auriferous fluids generated at mid oceanic ridge mor hydrothermal system were transported and deposited in an oxygen deficient environment low cao mgo k2o and ree contents suggests minor detrital input while la th sc relationship indicate minor volcaniclastic source from metabasic rocks low k rb and th u illustrate negligible effects of weathering in these samples geochemical proxies such as superchondritic y ho ratios along with post archean australian shale paas and mud from queensland muq normalized ree y patterns with depleted lree and positive eu la and y anomalies suggest that these bifs were generated by the chemical precipitation of iron and silica in sea water at mor and continental margin setting which had suboxic to anoxic paleo redox conditions

improved calibration procedures and new standards for u pb and th pb dating of phanerozoic xenotime by ion microprobe abstract xenotime is a widely occurring mineral that is amenable to u pb and th pb dating but often is found as micrometre sized crystals that can only be dated by in situ microanalytical techniques determining accurate ages for phanerozoic samples and assessing concordance in older samples requires accurate determination of pb u and pb th however ion microprobe data for these ratios are affected by the highly variable trace element composition of xenotime we have identified calibration procedures including matrix corrections for the effects of the dominant trace elements u th and ree that provide an accuracy of 1 for pb u and

acoustic properties of ice edge noise in the greenland sea a towed array experiment was performed by nrl in 1982 in the vicinity of the ice edge in the greenland sea the array was towed normal and parallel to and at ranges between 4 50 km from the ice edge the received data were beamformed to study the horizontal directionality of the noise the results indicated that a few localized hot spots with dimensions 5 km in the vicinity of the ice edge were the dominant noise sources these sources persisted for nearly 1 day the nominal separation between the hot spots was about 50 km the ice edge noise spectrum measured at these ranges decreased 11 12 db per octave with frequency between 100 500 hz as compared with the 6 db per octave spectrum of ambient sea noise possible sources are hypothesized in the context of known eddy wind and wave effects in the vicinity of the ice edge

plate tectonics 2 5 billion years ago evidence at kolar south india the archean kolar schist belt south india is a suture zone where two gneiss terranes and at least two amphibolite terranes with distinct histories were accrted amphibolites from the eastern and western sides of the schist belt have distinct incompatible element and isotopic characteristics sugesting that their volcanic protoliths were derived from dint mantle sources the amphibolite and gneiss terranes were juxtaposed by horizontal compression and shearing between 2530 and 2420 million years ago ma along a zone marked by the kolar schist belt this history of accretion of discrete crustal terranes resembles those of phanerozoic convergent margins and thus suggests that plate tectonics operated on earth by 2500 ma

matrix effects and calibration limitations in ion probe u pb and th pb dating of monazite abstract monazite can be used for u pb and th pb dating of a wide variety of rocks and geological processes particularly metamorphism in many instances ion microprobe analysis is the preferred method for dating monazite combining good analytical precision with m scale spatial resolution however ion microprobe dates can be compromised by matrix effects on the ionisation efficiencies of different secondary ionic species which can bias monazite pb tho n and pb uo n data by as much as 40 and 30 respectively for known monazite compositions these effects appear to be entirely attributable to the sputtering process in which case similar effects exist in data from all ion probes the effects on data ratios are modified sometimes exacerbated or reversed when data are subjected to the correlation based pb th and pb u calibrations that are now commonly used one dimensional calibrations of 206 pb 270 uo 2 and 208 pb 264 tho 2 are recommended corrections derived from data for a suite of reference materials with diverse compositions can reduce biases in 208 pb 232 th ages to 206 pb 238 u ages to 204 pb can be addressed without iterative data reduction

testing ore deposit models using in situ u pb geochronology of hydrothermal monazite paleoproterozoic gold mineralization in northern australia the inability to establish absolute ages for gold deposition in the pine creek orogen of northern australia has led to conflicting ore deposit models ranging from intrusion related which predict that gold mineralization was synchronous with granite magmatism ca 1835 1820 ma to orogenic which place ore deposition nearly 100 m y later here we present ion microprobe u pb geochronology for a mineralized quartz reef from tom s gully mine mount bundey northern territory australia and nearby granitic rocks and associated contact aureoles isotopic dating of zircon and monazite indicates that intrusion and contact metamorphism occurred ca 1825 ma whereas hydrothermal monazite from the auriferous quartz reef gives a mean 207pb 206pb age of 1780 10 ma interpreted as the time of gold mineralization mineralization therefore postdated intrusion by 45 m y and preceded a postulated ca 1740 1730 ma cratonwide orogenic gold event by 50 m y hence neither the intrusion related model nor the recently proposed orogenic model is applicable combined with a reevaluation of age data from the nearby goodall gold deposit our data suggest that mineralization coincides with and may be related to an episode of regional low grade metamorphism deformation and fluid circulation shoobridge event our results demonstrate the importance of high precision in situ geochronology and detailed petrography for deciphering age relationships in ore deposits and of testing the veracity of models for ore formation

3 35ga komatiite volcanism in the western dharwar craton southern india constraints from nd isotopes and whole rock geochemistry abstract we present field petrographic sm nd whole rock isochron and whole rock geochemical data for komatiites from sargur group greenstone belts of the western dharwar craton field evidence such as pillow structure indicates their eruption in a marine environment petrographic data reveal that the igneous mineralogy has been altered during post magmatic hydrothermal alteration processes corresponding to greenschist to lower amphibolite facies conditions with rarely preserved primary olivine and orthopyroxene a 16 point sm nd whole rock isochron gives an age of 3352 110 ma for the timing of eruption of komatiite lavas about 60 of the studied komatiite samples show al depletion whilst the remaining are al undepleted the al depleted komatiites are characterised by high cao al2o3 ratios 1 01 1 34 and low al2o3 tio2 5 16 whereas al undepleted komatiites show lower cao al2o3 ratios 0 59 0 99 and higher al2o3 tio2 17 26 trace element distribution patterns of komatiites suggest that most of the primary geochemical and nd isotopic compositions are preserved with only minor influence of post magmatic alteration processes and negligible crustal contamination the chemical characteristics of al depleted komatiites such as high gd yb n together with lower hree y zr and hf imply their derivation from deeper upper mantle with garnet majorite involvement whereas lower gd yb n slightly higher hree y zr and hf suggest derivation from shallower upper mantle without garnet involvement the observed chemical characteristics cao al2o3 al2o3 tio2 mgo ni cr nb zr y hf ree indicate derivation of the komatiite magmas from different depths in a plume setting whereas sub contemporaneous felsic volcanism and ttg accretion can be attributed to an arc setting in order to explain the spatial association of komatiite volcanism with contemporaneous mafic felsic volcanism and ttg accretion we propose a combined plume arc setting nd isotope data of the studied komatiites indicate depleted mantle reservoirs which may have evolved by early 4 53 ga global differentiation of the silicate earth as suggested by boyet and carlson boyet m carlson r w 2005 142nd evidence for early 4 53 ga global differentiation of silicate earth science 309 577 581 or extraction of continental crust during the early archaean

tectonostratigraphy of the late archean dharwar supergroup dharwar craton india defining a tectonic history from spatially linked but temporally distinct intracontinental and arc related basins abstract the dharwar supergroup comprises the unconformity bound bababudan and chitradurga groups the bababudan group which is best preserved in the western dharwar craton records a divergent margin comprising a basalt dominant intracontinental rift sequence a shale bif drift sequence and a sandstone shale thermal subsidence sequence the rift stage evolved from 2765 ma to 2720 ma whereas the succession was folded uplifted and eroded during development of a convergent continental margin from 2680 ma ocean opening was to the east or southeast the chitradurga group records a two stage back arc basin behind an east facing continental arc stage i evolved between 2609 ma and 2582 ma and stage ii between 2582 ma and 2540 ma the two stages are separated by a subaerial unconformity stage i comprises siliciclastic fluvial shallow marine and deep marine sedimentary rocks and bimodal volcanic rocks stage i is best preserved in the western dharwar craton but there are equivalents in the eastern dharwar craton stage ii in the western dharwar craton comprises deep marine turbidites of siliciclastic and volcaniclastic provenance and basaltic and felsic volcanic rocks volcanic stage ii sequences in the eastern dharwar craton define an arc adjacent position tectonically driven cyclic uplift and erosion were responsible for the mismatch between preserved stratigraphic thickness and time particularly for stage ii sequences a remnant of a

geochemical characterisation of xenotime formation environments using u th abstract xenotime ypo4 is a trace component in many metasedimentary and some igneous rocks altered rocks and many hydrothermal ore assemblages where it forms in response to a range of different processes from igneous crystallisation to low temperature early diagenesis due to its wide range of formation temperatures its suitability and isotopic robustness for reliable u pb geochronology dissolution precipitation characteristics during overprint events and widespread occurrence it is one of the most valuable minerals for u pb geochronology leading to four dimensional 4d studies of many terranes the formation environment of xenotime can be deduced from careful petrography and reliable u pb in situ geochronology within a 4d framework further xenotime is a physically robust mineral during sediment transport and may carry distinctive geochemical fingerprints including age to secondary environments from published works we review the u th contents of well characterised examples of xenotime formation to provide chemical fingerprints which assist in identifying the xenotime formation environment although the u th characteristics of xenotime from all formation environments show considerable overlap we make the following observations which may be distinctive of some ore formation environments i hydrothermal xenotime formed from low salinity ore fluids e g iron ore and orogenic gold deposits trend to have the lowest u contents 10 iii diagenetic xenotime has the most variable and highest u th whereas iv xenotime from precambrian orogenic gold deposits has the least variable and lowest u th petrogenetic inferences from these observations for ore deposit research and exploration include i support for the homogenising effect of the crustal scale of the fluid system in precambrian orogenic gold deposits and ii the potential for distinctive u th geochemical fingerprints for u ores and some au ores these characteristics may be reflected in detrital xenotime grains recovered from routine exploration sampling programs