geochemical heterogeneities of metagraywackes from the sandur schist belt implications for active plate margin processes abstract metagraywackes found at four stratigraphic levels in different blocks of the sandur schist belt represent unmixed sediments exhibiting great diversity in petrography mineralogy major trace and rare earth element ree geochemistry most of these rocks have a low q f c ratio and compared to aucc average upper continental crust are depleted in incompatible elements like rb ba th u nb ta k na ca ce and sr and enriched in compatible elements such as mg cr ni and sc they are enriched in fe2o3 upto 25 and their mg k ratio varies from la sc th sc th yb and co th ratios along with several diagnostic geochemical twins such as u th hf zr and nb ta indicate that upper continental crust has contributed marginally to the composition of most of these graywackes in each block except one two types of ree patterns are found type i patterns have moderate ree la n yb n 3 and no significant eu anomalies on the other hand type ii patterns show lree enrichment resulting in significant increase in ree and la n yb n between 5 and 15 type ii patterns are common in the deogiri block type i patterns are similar to those found in modern deep sea forearc basins significant negative eu anomalies are found in a few cases of type ii graywackes where recycling of sediments such as micaceous quartzite is evident la th sc and co th th sc and la sc ratios show that most of these graywackes were supplied from a source that was mainly basic volcanic rocks banded iron formations and cherts however significant compositional variations are found from block to block and within the same block these are attributed to the proportional differences in their local sources the overall composition indicates that the graywackes of different blocks have been deposited in various environments of oceanic subduction complex and have not sampled much of the upper continental crust the compositional structural and metamorphic discontinuities observed in the belt are best explained by a small plate tectonic model involving accretion of diverse blocks to a pericontinental passive margin succession it is argued that even in the archaean volcanosedimentary processes at young undifferentiated arcs involving ocean floor ridge basalts aorb have generated turbidites which in many geochemical features resemble those found in modern forearc basins

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

coseismic displacements and holocene slip rates for two active thrust faults at the mountain front of the andean precordillera 33 s 1 during the last few hundred years several destructive earthquakes occurred along the eastern margin of the andean precordillera where gps data reveal a shortening rate of 4 5 mm a we use fault scarp profiles and age determinations of deformed terraces t1 t4 to infer coseismic displacements and quantify slip rates for the penas and cal thrust faults near mendoza city scarps on the lowest terrace level t1 reveal vertical offsets of 0 8 1 0 m for both faults which are interpreted as coseismic displacements during the last earthquake together with the fault dip these offsets indicate that both faults are capable of producing magnitude mw 6 9 earthquakes which is corroborated by a magnitude ms 7 0 event on the cal fault that destroyed mendoza in 1861 at the penas thrust fault terrace t2 has an age of 3 3 ka and is offset by 1 9 m whereas the 12 ka old terrace t3 is displaced by 11 m combined with the fault dip of 25 the age and offset of terrace t3 define a shortening rate of 2 0 mm a on the penas fault i e about half of the present day shortening at the eastern margin of the precordillera at the cal fault terraces t2 to t4 have ages of 0 8 ka osl 3 9 ka 14c and 12 ka 10be and are vertically offset by 2 6 3 6 and 7 0 m respectively which implies that slip on the fault has recently accelerated hence the cal fault poses a serious seismic hazard to the one million inhabitants of mendoza

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

basement cover relations between a granite gneiss body and its metasedimentary envelope a structural study from the early precambrian dharwar tectonic province southern india the sirankatte granite gneiss an oval shaped body within the metasedimentary rocks of the chitradurga group in the dharwar tectonic province of early precambrian age occupies the core of a slightly overturned antiform with axial culmination this antiform with steep and moderate ene dips of the western and eastern limbs respectively and with axes plunging gently nnw in the northern part and sse to ese in the southern part represents a second generation fold isoclinal folds on stratification in the cover rocks as well as on gneissic banding in the granite gneiss have been involved in this folding coaxially an associated axial planar cleavage dipping steeply towards ene has been superimposed on both the granitic and metasedimentary rocks although conglomerates with pebbles cobbles and boulders of quartz and granite gneiss at the contact of the granite at a number of places suggest the granite to be the basement synkinematic migmatization and ductile deformation during the first folding show that the original granitic basement must have been extensively remobilized during the first deformation coaxial second folding due to compression in an almost horizontal wsw ene direction resulted in the development of an antiform with granite core resembling a mantled gneiss dome late folds on ene wsw axial planes accentuated the culminations and depressions of the folds of the first and second phases

late quaternary slip rates across the central tien shan kyrgyzstan central asia 1 slip rates across active faults and folds show that late quaternary faulting is distributed across the central tien shan not concentrated at its margins nearly every intermontane basin contains neogene and quaternary syntectonic strata deformed by holocene north south shortening on thrust or reverse faults in a region that spans two thirds of the north south width of the central tien shan slip rates on eight faults in five basins range from 0 1 to 3 mm yr fault slip rates are derived from faulted and folded river terraces and from trenches radiocarbon optically stimulated luminescence and thermoluminescence ages limit ages of terraces and aid in their regional correlation monte carlo simulations that sample from normally distributed and discrete probability distributions for each variable in the slip rate calculations generate most likely slip rate values and 95 confidence limits faults in basins appear to merge at relatively shallow depths with crustal scale ramps that underlie mountain ranges composed of pre cenozoic rocks the sum and overall pattern of late quaternary rates of shortening are similar to current rates of north south shortening measured using global positioning system geodesy this similarity suggests that deformation is concentrated along major fault zones near range basin margins such faults separated by rigid blocks accommodate most of the shortening in the upper crust index terms 8107 tectonophysics continental neotectonics 9320 information related to geographic region asia 8010 structural geology fractures and faults

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

daily banding in corals implications for paleoclimatic reconstruction and skeletonization abstract corals have the potential to record several centuries of highly detailed environmental information in the form of chemical proxies such as trace elements and stable isotopes massive corals commonly lay down density bands that are used as annual markers allowing time series records to be produced in addition montastraea faveolata a common caribbean coral species used for proxy records apparently erects its skeleton in two phases with a second phase of skeletonization producing micro scale banding that may reflect daily cycles of carbonate secretion while this raises the possibility of producing proxy records of daily environmental variation it also introduces uncertainty as we seek to sample at progressively finer scales until we better understand the mechanisms and timing of skeletonization a scanning electron microscope sem and backscattered electron imaging bei study of montastraea faveolata reveals that it produces a complex pattern of microbands three orders of magnitude smaller than the density bands commonly used for proxy records etched polished sections of coral skeleton show that the microbands consist of shingled aragonite crystallites oriented at high angles to the main axis of colony growth the microbands are 1 to 10 m in width and suggest a second phase of skeletonization that occurs later than the main axis of corallite extension individual imaged crystallites show a stepped increase in crystal breadth that could either be real or an artifact of sample preparation related to differential solubility bei analysis reveals that the bands of crystallites contain cyclical alternations of elemental abundance high resolution electron microprobe traverses 1 m beam diameter 1 m step size reveal that the elemental alternations are controlled at least in part by variations in sr ca within individual crystallites sr concentration in the microbands ranges cyclically between 9 800 and 10 800 ppm the microbands appear to fill in the pore space between thecae corallite walls and dissepiments corallite floors this secondary skeletonization process appears to be responsible for the production of the larger scale annual high density bands via destruction of pore space the possibility exists that the crystallite microbanding and the chemical alternation within it is driven by external cyclicity such as diurnal temperature variation if this is the case then the direction and timing of this microscale skeletonization could interfere with the collection of proxy data within the high density band the secondary skeletonization would likely cause blurring of the chemical signal and be responsible for chemical noise in proxy patterns produced by analytical techniques such as ion microprobe analysis as the resolution of analytical techniques improves these problems would likely be exacerbated on the other hand the possibility exists for extremely high resolution sampling within the high density band to reveal daily environmental variation and short lived pulse events presently difficult to detect in coral records it is also possible that the chemistry and fabric of the microbanding patterns could be used as an indicator of the degree of diagenetic alteration an important consideration as fossil corals are used increasingly to study paleoclimatic conditions

seismicity within the actively deforming eastern sierras pampeanas argentina summary the eastern sierras pampeanas of central argentina have been interpreted by previous studies to represent the easternmost expression of continental deformation resulting from the flat slab subduction of the nazca plate beneath the south american plate yet the seismic activity and deformation of this area as well as its relationship to subduction more than 700 km to the west are poorly understood using seismic data recorded by the eastern sierras pampeanas broadband seismic array between 2008 august and 2009 august we identified and located 445 local crustal earthquakes most with magnitudes of 2 5 or less we employed a double difference location method to clarify the hypocentral distribution of local seismicity and to define zones of both clustered and diffuse seismicity the relocated events define active structures in the region including a horizontal zone of seismicity between 15 and 25 km depth a prominent west dipping structure beneath the central ranges and a vertical structure along the eastern margin of the study area overall seismicity in this region during the study period is constrained to the upper 25 km of the crust and is mostly concentrated along minor structures and not along the major range bounding faults focal mechanism solutions from the nine clearest events show mainly normal deformation with an oblique component and e w oriented p axes that likely result from shortening along structures with oblique orientations to the direction of plate convergence this is the first seismic study to document that the eastern sierras pampeanas are actively deforming in response to compression from the convergent margin

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

crustal deformation in the south central andes backarc terranes as viewed from regional broad band seismic waveform modelling summary the convergence between the nazca and south america tectonic plates generates a seismically active backarc region near 31 s earthquake locations define the subhorizontal subducted oceanic nazca plate at depths of 90 120 km another seismic region is located within the continental upper plate with events at depths 1 80 th 45 55 km than those of the eastern sierras pampeanas vp 6 0 6 2 km s 1 vp vs 1 70 th 27 35 km in addition we observed an apparent distribution of reverse crustal earthquakes along the suture that connects those terranes finally we estimated average p and t axes over the charge period the entire region showed p and t axis orientations of 275 and 90 plunging 6 and 84 respectively

sedimentary rocks and crustal evolution tectonic setting and secular trends a significant change in the composition of turbidites deposited at active tectonic settings is seen at the archean post archean transition in comparison to post archean active margin turbidites archean greenstone turbidites exhibit more uniform th sc ratios but with a similar average greater hree depletion gd n yb n 2 less eu depletion eu eu mostly 0 85 and an absence of low th u ratios th u 3 these data indicate that differing mantle sources and or conditions of mantle melting existed during the archean and that intracrustally differentiated rocks older cratons or young differentiated arcs were relatively unimportant as provenance components for archean turbidites in contrast some sedimentary rocks preserved in archean platformal sequences of high grade terranes display significant negative eu anomalies similar to the majority of post archean shales indicating that the general process of cratonization including intracrustal differentiation has taken place since at least 3

the effect of heterogeneous stress and strain on metamorphic fluid flow mary kathleen australia and a model for large scale fluid circulation in the proterozoic mary kathleen fold belt northern australia infiltration of large volumes of externally derived fluid occurred synchronously with regional amphibolite facies metamorphism and deformation this paper develops a model of structurally controlled fluid migration by comparing the distribution of fossil fluid pathways with the inferred stress and strain patterns during the deformation intense fluid flow was localized within strong relatively brittle meta intrusive bodies and in discrete veined brecciated and altered zones around their margins in metasediments folded in a ductile manner outside these areas fluid infiltration was negligible the direct correlation between structural styles and the magnitude of veining and metasomatism suggests control of permeability enhancement and hence fluid flow by deformation finite difference modelling of a strong body in a weaker matrix has been used to evaluate the variation of stresses during the deformation from which it is clear that stress and strain heterogeneities have systematically influenced the development and maintenance of metamorphic fluid pathways particular regions in which mean stress may be significantly lower than the average lithostatic pressures include the strain shadow zones adjacent to the strong bodies other dilatant zones around the bodies and the bodies themselves this geometry is favourable not only for localized brittle deformation under amphilobite facies conditions but also for focused fluid flow in the low mean stress regions as evidenced by the abundance of veins fluid access through these metamorphic aquifers occurred during tensile failure episodes with particularly large dilations and decimetre scale veining in areas of strain incompatibility it appears likely that fluid circulated many times through the fold belt with flow concentrated in the metamorphic aquifers a model is developed that explains both the structurally focused fluid flow and the postulated multi pass recirculation by dilatancy pumping the pump engines comprising the low mean stress zones

crustal structure of the south central andes cordillera and backarc region from regional waveform modelling summary we investigate the crustal structure in the andes cordillera and its backarc region using regional broadband waveforms from crustal earthquakes we consider seismic waveforms recorded at regional distances by the chile argentina geophysical experiment charge during 2000 2002 and utilize previous seismic moment tensor inversion results for each single station earthquake pair we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation cordillera and the accreted terranes in the backarc region our investigation indicates that synthetic seismograms for our earthquake station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters we performed a grid search around crustal thickness p wave seismic velocity vp and p t os wave seismic velocity ratio vp vs fixing mantle parameters we evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components we identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s our results indicate along strike variations in the crustal structure for the north south high cordillera with higher p wave velocity and thickness in the northern segment north of 33 s and persistently high vp vs ratio 1 85 in both segments this is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region in contrast the results for the current volcanic arc south of 33 s agree with a warmer crust consistent with partial melt related to quaternary volcanism presumably of an intermediate to mafic composition a distinctive feature in the backarc region is the marked contrast between the seismic properties of the cuyania and pampia terranes that correlates with their heterogeneous crustal composition the cuyania terrane composed of mafic ultramafic rocks exhibits high vp high vp vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern sierras pampeanas associated with low vp and low vp vs these differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper 30 km in this active compressional region in particular the seismic properties of the cuyania terrane which shows evidence for a high vp and high vp vs crust may be reflecting the complex tectonic evolution history of this terrane including accretion rifting and re accretion processes since the palaeozoic that promote a high level of crustal seismicity in the upper 30 km enhanced by the flat slab subduction in the segment between 31 s and 32 s another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the cuyania terrane between 30 s and 34 s

the midway sequence a timiskaming type pull apart basin deposit in the western wawa subprovince minnesota the midway sequence is an assemblage of subaerially deposited clastic and volcanic rocks that forms a narrow wedge within neoarchean greenstone of the western wawa subprovince of the superior province volcanic conglomerate in the midway sequence contains clasts of stratigraphically older greenstone together with clasts of a distinctive hornblende phyric trachyandesite that is not represented among the older greenstone flows the trachyandesite forms flows and pyroclastic units that are interbedded with lenticular deposits of volcanic conglomerate in a manner interpreted to indicate approximately coeval volcanism and alluvial fan fluvial sedimentation within a linear restricted and tectonically active depocentre the midway sequence unconformably overlies greenstone on one side and is bounded by a regional scale strike slip fault on the other structural analyses show that the midway sequence was deposited after an early precleavage folding event d1 in greenstone but before the regional metamorp

structural control of low sulfidation epithermal gold mineralization in the rosario bunawan district east mindanao ridge philippines the rosario bunawan district is situated about 200 km north of davao city the capital of the mindanao island southern philippines gold is produced from the co o mine containing about 2 034 000 t of ore at 10 9 g t au and in numerous small scale operations by local miners epithermal gold mineralization in the rosario bunawan district and the co o mine is confined to narrow 0 2 4 m low sulfidation quartz chalcedony calcite veins in volcanic and volcaniclastic wall rocks three major vein orientations are distinguished 1 the nnw sse trending set with a sinistral strike slip sense of deformation philippine fault trend 2 the ene wsw trending dextral strike slip set palawan trend and associated veins in the riedel geometry and 3 the wnw ese trending conjugate set co o trend three structural stages are defined 1 extensional shear or shear veins formed in the co o the philippine fault and palawan trends during regional nw se compression and near vertical vein opening d1 2 reactivation of veins in the philippine fault veins associated with the palawan and to a lesser extent the co o trends during e w compression and near horizontal n s oriented vein opening d2 new d2 extensional shear or shear veins formed in the philippine fault and structures associated with the palawan and associated riedel trends 3 the d3 stage block faulting subsequently displaced all of the auriferous veins the auriferous rosario bunawan district is situated between two splays of the philippine fault which acted as a lateral ramp system during the oblique convergence of the philippine sea plate and the eurasian plate the oblique convergence resulted in a change from a compressional d1 to a transpressional d2 regime which was a prerequisite for the two stage vein opening and hydrothermal mineralization leading to an economic gold enrichment d1 compressional tectonics may have caused an elevated geothermal gradient in shallow crustal levels forming the heat source for the fluid plumbing system which is at variance to typical epithermal deposits formed in extensional zones d2 thrusting of a limestone nappe together with syn tectonic diorite intrusions may have further increased the geothermal gradient maintaining the fluid plumbing system the limestone nappe may at the same time have represented an aquitard forcing the hydrothermal fluids into the volcanic and volcaniclastic wall rocks which is regarded as critical for the two stage gold mineralization in the rosario bunawan district

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

rare earth element geochemistry of some archean iron formations north of lake superior ontario rare earth element ree compositions of iron formation from two archean terrains in western ontario have been determined in order to assess the possible influence of hydrothermal activity on the ree patterns of chemical sediments one terrain is characterized by sulfide facies iron formation in association with volcanic flows and volcaniclastic sediments whereas the other is dominated by oxide facies iron formation intercalated within submarine fan clastic sediments mineral separates of chert magnetite and pyrite from the iron formations have low ree concentrations 20 30 ppm and display moderate to strong positive eu anomalies relative to archean shale the positive anomalies and lack of negative ce anomalies indicate that archean sea water from which iron formation locally precipitated was reduced although to varying degrees the ree patterns of mineral separates from a given locality are almost identical but the patterns for various localities differ in detail a number of iron formation

the geological setting of lode gold deposits in the central southern urals a review the late paleozoic uralides represent one of the largest lode gold metallogenic provinces in the world in the southern urals gold distribution is heterogeneous and is confined mainly to two tectonostratigraphic zones namely the main uralian fault and the east uralian zone the important lode gold districts within and in the immediate hangingwall of the first order crustal suture of the main uralian fault are characterized by a complex tectonic history of earlier compressional tectonics involving thrusting folding and reverse faulting and later transcurrent shearing gold mineralization is hosted by second and third order brittle to brittle ductile strike slip faults that developed late during the kinematic history of the main uralian fault strike slip reactivation of earlier compressional structures was related to the late stage docking of the passive margin of the east european platform with island arc complexes of the southern urals an event that is tentatively related to changes in plate motion during the final stages of terrane accretion during the upper permian and lower triassic gold mineralization was controlled by the permeability characteristics of the hydrothermal conduits as well as by competence contrasts and geochemistry of the mainly volcanic host rocks mineralization occurred at relatively shallow crustal levels 2 6 km and largely post dates peak metamorphism of the host rocks the large and very large up to 300 to au gold deposits of the east uralian zone are hosted by upper paleozoic granitoid massifs gold mineralization is temporally associated with the main phase of regional scale compressional tectonics and granite plutonism during the upper carboniferous and lower permian controlling structures have a dominantly east west strike and occur as hybrid shear tensional vein systems in competent granitoids subjected to east west directed regional shortening deformation textures and alteration mineral assemblages indicate lower amphibolite facies conditions of mineralization close to peak metamorphic conditions that are associated with the mid permian regional metamorphism and tectonism gold deposits in the southern urals are therefore polygenetic and are temporally and genetically distinct in each of the two major mineralized tectonostratigraphic zones of this well preserved collisional orogenic belt the different timing of ore fluid generation and fluid discharge is interpreted to be the result of the different tectonic metamorphic and magmatic evolution of terranes in the southern urals

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

exhumation and uplift of the sierras pampeanas preliminary implications from k ar fault gouge dating and low t thermochronology in the sierra de comechingones argentina the sierras pampeanas in central and north western argentina constitute a distinct morphotectonic feature between 27 s and 33 s the last stage of uplift and deformation in this area are interpreted to be closely related to the andean flat slab subduction of the nazca plate beneath the south american plate k ar fault gouge dating and low temperature thermochronology along two transects within the sierra de comechingones reveal a minimum age for the onset of brittle deformation about 340 ma very low exhumation rates since late paleozoic time as well as a total exhumation of about 2 3 km since the late cretaceous new ar ar ages 7 54 1 91 ma of volcanic rocks from the san luis volcanic belt support the eastward propagation of the flat slab magmatic front confirming the onset of flat slab related deformation in this region at 11 2 ma although low temperature thermochronology does not clearly constrain the signal of the andean uplift it is understood that the current structural relief related to the comechingones range has been achieved after the exhumation of both fault walls circa 80 70 ma

the stratigraphy and structure of the chitradurga region an illustration of cover basement interaction in the late archaean evolution of the karnataka craton southern india abstract the succession in the late archaean supracrustal belts in the chitradurga region begins with basal quartz pebble conglomerates and quartzites which were deposited on a basement of tonalitic granitic gneisses ca 3000 ma containing tracts and enclaves of older supracrustal rocks the basal beds are overlain by basic metalavas and amphibolites interbedded with cross bedded quartzites and siliceous phyllites this series passes up into siliceous phyllites locally containing a large lenticular mass of polymict conglomerate talya conglomerate overlain by banded iron formations and siliceous phyllites including sporadic chert pebble conglomerates greater subsidence in part of the basin accommodated greater thicknesses of basic volcanic and acid pyroclastic rocks the latter being more abundant at higher levels with progressive deepening of the whole basin fine grained graded greywackes and local polymict conglomerates aimangala conglomerate were deposited the turbidite sequence forms the youngest sedimentary rocks exposed estimated total stratigraphic thickness is at least 10 km in deeper parts of the basin deformation led to a series of upright anticlines and synclines and related ls fabrics with extreme variations in plunge of the coaxial fold axes and l fabrics within axial surfaces which maintain a steep orientation low grade metamorphism outlasted the deformation sedimentation and volcanism in the chitradurga supracrustal belt is linked to variable subsidence of the basement foundation probably as a result of rifting the structural evolution of the belt followed as a direct continuation of events that controlled the depositional and volcanic history variable displacement of the basement during these late archaean events in the karnataka craton took place on shear zones and related fractures that acted as channels for water and co 2 as indicated by vein minerals and alteration products uplift of basement segments and their distension by granitic diapirs led to deformation of the supracrustal cover made ductile by its high water content

crustal scale shear zones and their significance to archaean gold mineralization in western australia many large archaean epigenetic gold deposits show a broad spatial relationship to regional lineaments in greenstone belts although in detail they are sited in subsidiary brittle ductile fault structures fluids originating from a deep source follow a complex path and re equilibrate with different lithologies and with metamorphic fluid during migration to higher crustal levels temperature and pressure conditions at or below the amphibolite greenschist metamorphic boundary where most gold deposits are located favour the establishment of brittle ductile and brittle subsidiary structures the preferred structural setting of gold deposits physical gradients between the regional ductile structures and more brittle subsidiary structures ensure transient strongly localized fluid flow into the latter where lower temperatures and suitable host rocks with high fe fe mg ratios favour gold deposition the multi source origin and continuous re equilibration of the fluid with crustal rock which includes granitoid and greenstone belt lithologies of different ages is reflected in the diverse isotopic and geochemical signature of the gold deposits

the relations between mean rock stress and fluid flow in the crust with reference to vein and lode style gold deposits abstract fluid movement in the crust at the typical depths of formation of vein and lode style gold deposits will be dominated by upward flow focussing of upward flow into discrete channelways as required to form gold deposits is due to lateral gradients in fluid pressure fluid channelways should have lower fluid pressures than surrounding rock if lateral gradients are induced by variations in permeability this is however at variance with inferences from quartz vein structures of high relative fluid pressures during the formation of gold deposits focussing into zones of low mean rock stress will in contrast be associated with high relative fluid pressures in the zone of focussed fluid flow variations in mean rock stress are a direct consequence of a regional deviatoric stress acting on an inhomogeneous rock sequence analysis of stress fields shows a wide variety of potential sites of low mean stress dependent on the geometry of rock units and on patterns of faults fractures or shear zones a model of fluid focussing in the crust due to variations in mean stress is thus consistent with the large variety of structural setting of vein and lode gold deposits observed in nature use of the model through the technology of stress mapping has the potential to generate viable exploration targets

fault gouge analyses k ar illite dating clay mineralogy and tectonic significance a study from the sierras pampeanas argentina we introduce a method for the detailed interpretation of k ar illite fine fraction ages of fault gouges from non sedimentary host rocks ages are cross evaluated with several independent parameters e g illite crystallinity illite polytype quantification grain size mineralogical observations k ar muscovite and biotite host rock cooling ages as well as low temperature thermochronological data aft ahe zhe this interpretation approach is applied to a regional study in order to constrain the deformation path of the eastern sierras pampeanas in nw argentina in the course of this study a large number of gouge bearing fault zones were systematically sampled and analysed obtained k ar illite fine fraction ages range from devonian to cretaceous times documenting a long lasting brittle fault activity in this region ages 320 ma are synchronous with a period of intra carboniferous compressional tectonism whereas permo triassic ages are contemporaneous to a flat slab subduction episode of the farallon plate beneath the south american plate middle to late permian and early triassic ages as well as early jurassic to middle cretaceous ages correlate with extensional tectonics in this region additionally k ar illite ages reveal a propagation of brittle deformation from north to south in the sierras de cordoba and san luis data integrity and consistency with other chronometers and geological evidence show that the here suggested interpretation is valid and can provide a powerful tool to evaluate cooling and deformation histories despite of that we could show that the reliability of fault gouge data strongly depends on the regional cooling

reactivation inversion and basement faulting and thrusting in the sierras pampeanas of cordoba argentina during andean flat slab deformation the sierras pampeanas of cordoba are the easternmost uplifted blocks caused by andean foreland deformation over 700 km from the chile trench this deformation started at c 340 ma through basement faults thrusts and reactivation of normal faults of the cretaceous rift during the opening of the atlantic ocean other older faults major oblique lineaments were also reactivated thermochronological and geothermobarometric data indicate that some topographic relief could have been palaeozoic mesozoic relicts and not only produced by the andean orogeny faults are partially controlled by the early cambrian s 2 metamorphic foliation coincident with the curved fault traces at map scale during pliocene time two deformation phases post dating miocene pliocene magmatism are recognized shallow seismicity data c 25 km depth indicate that the sierras de cordoba accommodate quaternary displacement magnetotelluric studies detect the interface between the pampia terrane and the rio de la plata craton the role of the oblique lineaments in the nucleation and development of the tertiary faulting has been little considered they could be correlated with an old pan gondwanan trend during the cretaceous period these lineaments worked in a transtensive way producing the uplift of high grade rocks and segmentation of the mountain chain favouring the diachronous uplift along the ranges recently both the brittle ductile transition at c 23 km depth and the crustal thickness have been determined by seismicity analysis the oblique lineaments displace normally the mohorovicic discontinuity main basement thrusts were probably rooted in the suture between the pampia terrane and the rio de la plata craton

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

paragenesis and composition of ore minerals in the randalls bif hosted gold deposits yilgarn craton western australia implications for the timing of deposit formation and constraints on gold sources the randalls district comprises three individual gold deposits cock eyed bob maxwells and santa craze hosted in the same unit of banded iron formation bif in the southern eastern goldfields superterrane yilgarn craton western australia the iron formation is a silicate oxide facies unit with overprinting sulfides and has undergone metamorphism upper greenschist facies and deformation two generations of folds the gold deposits are hosted in two structural locations hinge zones of anticlinal folds e g santa craze and maxwells and overturned steeply dipping limbs of anticlinal folds e g cock eyed bob gold dominantly occurs as inclusions of native gold and or electrum within or around pyrrhotite magnetite and arsenopyrite the earliest mineral assemblage preserved in the banded iron formation at randalls is banded magnetite quartz magnetite in all forms has elevated mg al ti v mn cr zn and w but very low ni compared to other bifs globally pyrrhotite the most abundant sulfide is restricted to ore zones and occurs as laminae blebs and veinlets all of which have replaced magnetite some pyrrhotite in the hydrothermally altered bifs especially in near surface samples has been re sulfidized to pyrite trace elements associated with pyrrhotite include co ni ag sb and pb large coarse grained arsenopyrite overgrows the pyrrhotite defined foliation it is enriched in co ni sb and te but contains very little invisible gold 0 01 ppm au unlike arsenopyrite in many orogenic au systems globally commonly 0 1 ppm au petrographic studies of several mineralized bifs from randalls reveal that pyrrhotite and arsenopyrite are associated with fe ca amphibole which grew during contact metamorphism caused by granite plutons this spatial sulfide amphibole relationship indicates that gold mineralization likely occurred during granite intrusion consistent with prior studies on randalls the trace element characteristics of the sulfides show that they were not inherited from replaced magnetite further these characteristics suggest that the fluid from which sulfides crystallized was not indigenous to the bifs and that this s rich fluid either contained s in abundance originally or interacted with a lithology lithologies that were rich in s before reaching the bifs black shale containing diagenetic nodular pyrite occurs in the upper black flag group which unconformably underlies the belches supersequence at depth 3 km the trace element composition of nodular pyrite from this unit fits well with the trace element characteristics of the bif pyrrhotite and arsenopyrite and the nodular pyrite contains an average of 0 5 ppm au however the nodular pyrites have an average 34s value of 5 6 n 13 which contrasts markedly with the 34s average of pyrrhotite pyrite and arsenopyrite in mineralized bifs 0 8 n 17 thus a definitive source of gold arsenic silver and tellurium in the randalls system remains elusive although we would contend that the upper black flag group should be considered as a possible metal and s source in areas of the yilgarn where it forms a thick substrate to ore camps

holocene activity and seismogenic capability of intraplate thrusts insights from the pampean ranges argentina abstract trench excavations across the el molino fault in the southeastern pampean ranges of central western argentina have revealed a deformation zone composed of opposite verging thrusts that deform a succession of holocene sediments the west verging thrusts place precambrian basement over holocene proximal scarp derived deposits whereas the east verging thrusts form an east directed fault propagation fold that deforms colluvium fluvial and aeolian deposits ages for exposed fault related deposits range from 7 1 0 4 to 0 3 ka evidence of surface deformation suggests multiple rupture events with related scarp derived deposits and a minimum of three surface ruptures younger than 7 1 0 4 ka the last rupture event being younger than 1 ka shortening rates of 0 7 0 2 mm a are near one order of magnitude higher than those estimated for the faults bounding neighboring crustal blocks and are considered high for this intraplate setting these ground rupturing crustal earthquakes are estimated to be of magnitude mw 7 0 a significant discrepancy with the magnitudes mw

magnetotelluric evidence of the tectonic boundary between the rio de la plata craton and the pampean terrane chaco pampean plain argentina the extension of the transbrasiliano lineament abstract the transbrasiliano lineament represents a continental shear belt that transversely intersects the south american platform from nne to ssw much evidence of this lineament exists to the north but it remains uncharacterized in the distal andean foreland to the south where it is associated with the tectonic boundary between the rio de la plata craton and the pampean terrane this tectonic boundary is mostly unexposed in the chaco pampean plain argentina debate continues about the existence of an east or west dipping subduction zone and a later collisional event between the terranes here we report the results of a magnetotelluric survey along a w e profile at 27 s between 63 45 and 60 30 w to characterize the geoelectric structure the magnetotelluric data were processed the dimensionality and distortion of the geoelectrical medium were analyzed and a 2 d inversion model was developed the distribution of the resistivities at the lithospheric scale indicates a highly resistive crust 20 000 m on the east side of the profile that is correlated with the rio de la plata craton on the west side of the profile a less resistive but still highly resistive crust 12 000 m is correlated with the pampean terrane the highly resistive blocks are separated by an east dipping conductive anomaly 150 250 m that is correlated with the transbrasiliano lineament this conductive feature is consistent with the east dipping subduction model and can be explained by the presence of graphite in paleosutures in long stable geological terranes oblique convergence between the terranes may have developed the transpressional shear belt the results provide new geophysical evidence of the tectonic boundary between the rio de la plata craton and the pampean terrane beneath the chaco pampean plain that extends southward into the transbrasiliano lineament the study also improves the knowledge of the amalgamation of western gondwana

structural mineralogical and geochemical constraints on the atypical komatiite hosted turret deposit in the agnew mt white district western australia in the norseman wiluna belt yilgarn craton the agnew mt white district is the host of many gold deposits located in the hinge of the regional lawlers anticline the turret gold deposit is structurally controlled by the table hill shear zone that transects the agnew ultramafic unit geochemistry coupled with petrographic data allowed the delineation of the paragenetic sequence associated with gold mineralisation and include 1 a pervasive talc carbonate alteration assemblage 2 a pre mineralisation stage associated with pervasive arsenopyrite chalcopyrite pyrrhotite pyrite alteration followed by 3 a late deformation event along a dilatational segment of the main table hill shear zone leading to the formation of a breccia hosting a cu bi mo au ag zn te w metal assemblage the presence of au ag cu alloys native bismuth chalcopyrite and other bi te s phases in the mineralisation stage suggest that gold may have been scavenged from the hydrothermal fluids by composite bi te cu au ag s liquids or melts using this mineral paragenetic sequence together with mineralogical re equilibration textures observed we show that the gold deposition at turret occurred over a temperature range approximately between c 350 and 270 c this temperature range together with the structural control and typical mesothermal alteration pattern including carbonate chlorite alteration shows that the turret deposit shares common characteristics with the orogenic gold deposit class however the metal association of cu au bi and mo the quartz poor and high copper sulphide content up to 15 are characteristics that depart from the typical orogenic gold deposit mineralogy through comparison with similar deposits in the yilgarn craton and worldwide we propose that the turret deposit represents an example of a porphyry derived au cu bi mo deposit

structural setting textures and timing of hydrothermal vein systems in the val d or camp abitibi canada implications for the evolution of transcrustal second and third order fault zones and gold mineralization in the val d or camp archean abitibi greenstone belt canada numerous gold mineralized second and third order fault zones are spatially associated with the transcrustal cadillac tectonic zone ctz this situation is used to test whether fluid systems in the ctz have a similar structural timing to those in the gold hosting structures and hence the ctz could represent the main fluid conduit in the camp the transcrustal ctz at orenada no 2 contains structurally complex vein systems with mineralized quartz tourmaline veins related to both d2 oblique reverse faulting and f3 dextral asymmetric folding both of which have been overprinted by unmineralized subhorizontal and subvertical quartz veins quartz tourmaline veins within second and third order shear zones at paramaque and riviere heva also formed during d2 deformation and have been at least at riviere heva deformed by f3 asymmetric folding in contrast mineralized quartz vein systems at cartier malartic are controlled by f3 folding and over

late holocene faulting in the southeast sierras pampeanas of argentina the seismic history of extra andean south america is poorly documented between lat 26 s and 33 s about 375 km east of the fold and thrust belt that marks the present orogenic front the block faulted crystalline basement uplifts of the sierras pampeanas in argentina reveal extensive quaternary reverse faulting 14c dates of 1080 70 and 1310 40 yr b p have been determined on organic material from colluvium on the footwall of the east dipping los molinos branch of the comechingones fault they indicate at least 2 1 m of slip probably coseismic during the past 1300 yr in an area for which no significant deformation has been reported in association with historical earthquakes

crustal motion in the southern andes 26 36 s do the andes behave like a microplate 1 a new global positioning system gps derived velocity field for the andes mountains 26 36 s allows analysis of instantaneous partitioning between elastic and anelastic deformation at the orogen s opposing sides adding an andes microplate to the traditional description of nazca south america plate convergence provides the kinematic framework for nearly complete explanation of the observed velocity field the results suggest the oceanic nazca boundary is fully locked while the continental backarc boundary creeps continuously at 4 5 mm yr the excellent fit of model to data 1 7 mm yr rms velocity misfit and the relative aseismicity of the upper crust in the interior andean region in comparison with its boundaries supports the notion that the mountains are not currently accruing significant permanent strains additionally the model implies permanent deformation is not accumulating throughout the backarc contractional wedge but rather that the deformation is accommodated only within a narrow deformational zone in the backarc

the origin of fluids and nature of fluid rock interaction in mid crustal auriferous mylonites of the renco mine southern zimbabwe geothermometric constraints on auriferous shear zones of the renco mine in the northern marginal zone of the late archaean granulite facies limpopo belt in southern zimbabwe indicate that deformation and associated mineralization occurred at temperatures of at least 600 c up to more likely 700 c mid to upper amphibolite facies conditions during mineralization correspond to the regional scale retrogression of granulite facies wall rocks during the late archaean thrusting of high grade metamorphic rocks of the northern marginal zone onto low to medium grade granite greenstone terrains of the zimbabwe craton mineral assemblages indicate that the ore fluid was moderately oxidized with log fo2 values between 10 17 and 10 18 bars with high h2s activities of 0 25 0 75 elements enriched in the shear zones include au s fe cu mo bi te ni co and h2o au and cu being the most enriched geochemically au correlates with cu but not with s which together with the fact that gold is only rarely intergrown or in direct contact with sulfides possibly indicates a transport of gold as a chloride complex the siting of gold along fractures or within implosion breccias suggests that gold was precipitated due to fluid immiscibility induced by catastrophic fluid pressure drops during seismic slip events fluid inclusions are predominantly co2 ch4 n2 rich but petrographic work indicates that fluid inclusions have undergone extensive post entrapment modifications due to the pervasive recrystallization of mineral textures in the high temperature shear zones the mineralized shear zones are enriched in 18o compared to wall rock enderbites which is interpreted to represent an influx of externally derived fluids of probably metamorphic origin based on temporal and spatial relationships between mineralization late archaean overthrusting of the northern marginal zone onto the zimbabwe craton and coeval amphibolite facies hydration of granulites we suggest that the renco mineralization formed in a mid crustal environment from metamorphic fluids that were generated from dehydration of subcreted greenstone terrains of the zimbabwe craton

hydrothermal fluid evolution within the cadillac tectonic zone abitibi greenstone belt canada relationship to auriferous fluids in adjacent second and third order shear zones the cadillac tectonic zone in the abitibi greenstone belt canada is spatially associated with world class gold deposits that are hosted in second and third order faults five types of hydrothermal quartz veins with distinct structural timing are distinguished in the cadillac tectonic zone in the val d or camp and are related to d 2 steep reverse faulting and d 3 oblique transcurrent movement fluid inclusions in gold bearing and barren hydrothermal quartz veins in the transcrustal cadillac tectonic zone record the progressive evolution of the hydrothermal fluids within the cadillac tectonic zone oldest h 2 o co 2 ch 4 nacl fluid inclusions which predate gold mineralization are superseded by gold associated dominantly carbonic co 2 ch 4 h 2 o nacl fluid inclusions on the basis of variable phase ratios and similar total homogenization temperatures in contemporaneous fluid inclusions trails the oldest h 2 o co 2 ch 4 nacl fluids are interpreted to have undergone phase separation the ch 4 content increases from the older x ch 4 0 03 0 24 to the younger x ch 4 0 15 0 71 fluid inclusions and the salinity is low to moderate in the older fluid inclusions mean 1 7 2 6 2 wt nacl equiv and increases in the younger fluid inclusions mean 1 16 2 6 0 wt nacl equiv trails of late stage co 2 ch 4 n 2 h 2 o and ch 4 c 2 h 6 c 3 h 8 fluid inclusions crosscut earlier carbonic aqueous fluid inclusion trails in all veins the texturally latest fluid inclusions occur in trails that crosscut earlier fluid inclusion trails and quartz deformation bands and are composed of h 2 o nacl cacl 2 postentrapment modifications are considered unlikely for most fluid inclusions in the cadillac tectonic zone the hydrothermal fluids in the cadillac tectonic zone were compared with those in second and third order fault zones in the world class sigma deposit to evaluate whether the two fluid systems are of similar derivation carbonic aqueous fluids that deposited quartz in veins and some gold in the cadillac tectonic zone are distinct from those that are hosted in structures of similar timing at sigma cadillac fluids have higher co 2 content and marginally higher salinity 0 6 15 3 wt nacl equiv and 1 10 wt nacl equiv in the cadillac tectonic zone and at sigma respectively but contain similar amounts of ch 4 the high salinity aqueous inclusions at sigma which are interpreted to represent the unmixed h 2 o rich end member of a combined co 2 ch 4 h 2 o nacl fluid are absent in the cadillac tectonic zone veins of d 3 timing which contain a high ch 4 component in the cadillac tectonic zone to 70 mole have not been documented at sigma the magnitude of the hydrothermal quartz vein field and the homogeneity of quartz vein mineralogy and textures in the val d or camp strongly suggest that the fluids that precipitated auriferous quartz veins in second and third order shear zones were not locally derived it is suggested that ore bearing fluids ascended along the cadillac tectonic zone and infiltrated the second and third order fault network an unknown amount of metamorphic h 2 o from the greenstone belt probably mixed with these fluids differences in the fluid composition in the cadillac tectonic zone and those in second and third order shear zones are explained by phase separation with preferential vapor loss into the upper portions of the cadillac tectonic zone and by in situ phase separation without vapor loss during ore deposition at sigma methane enrichment in the late fluids in the cadillac tectonic zone was most likely controlled by an increased fluid interaction with reduced carbon in sedimentary rocks but increasing mantle outgassing and cooling of an f o 2 buffered ascending fluid may also have contributed to the ch 4 budget the youngest aqueous brines in the cadillac tectonic zone are similar to fluids in late fracture fillings and to canadian shield basement brines

rare earth element variations in mid archean banded iron formations implications for the chemistry of ocean and continent and plate tectonics abstract abundances of major and rare earth elements rees are reported for mid archean 3 3 3 2 gyr sedimentary rocks including banded iron formations bifs and ferruginous siliceous mudstone from the cleaverville area in the pilbara craton western australia geological lithological and geochemical lines of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments ranging from that typical of mid oceanic spreading centers to convergent plate boundary settings a range of environments most likely caused by plate movements except for the mudstone the ree content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons europium anomalies decrease up section eu eu values normalized to nasc change from 3 5 to 1 1 as the ree contents and lree hree ratios increase the striking similarity in these ree signatures of bifs and modern hydrothermal sediments leads us to propose that the bifs were in situ hydrothermal precipitates near a mid ocean ridge significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the uppermost horizon the observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothermal to terrigenous suggests that plate tectonics dominated by horizontal movement was already operating in the mid archean distal hydrothermal sediments without a eu anomaly when normalized to nasc suggest that mid archean seawater had already been strongly influenced by a riverine flux from an upper continental crust and that this component bore no eu anomaly i e it had a negative eu anomaly when normalized to chondrite in addition to an absence of a eu anomaly mid archean seawater did not have a ce anomaly suggesting less oxic conditions in the mid archean than in the modern ocean

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

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

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

crustal structure of the eastern sierras pampeanas of argentina using high frequency local receiver functions abstract the eastern sierras pampeanas are basement cored outcrops uplifted in the andean foreland where the easternmost segment of the pampean flat slab segment starts dipping more steeply into the deeper mantle these ranges of central argentina known as the sierras de cordoba have an enriched quartz composition and are bounded by a series of reverse faults different models have been suggested to represent the style of the thick skinned deformation in this area however the overall structure linking the exposed faults and terrane boundaries with their probable continuation at depth is unknown in this paper we present images of the crustal structure beneath the sierras de cordoba using the common conversion point stacking method of high frequency local receiver functions recorded by the esp broadband seismic array the work consists of two transects located around 31 s and 32 s across the sierras de cordoba the results show a consistent sharp moho signal associated with a high contrast in seismic velocities in good agreement with the granitic character of the crust lying above the mafic upper mantle the moho morphology varies exhibiting thicknesses of 38 km in the west to 35 km in the east with a vertical shifting under the sierra chica we relate this variable character to the presence of the boundary between the pampia terrane and the rio de la plata craton our results for the intra crustal structure indicate the presence of three discontinuities in the northern transect and at least two discontinuities in the southern transect these discontinuities appear vertically displaced beneath the surface traces of the major range bounding faults providing evidence for the continuation to mid crustal depths of the exposed reverse faults thus the reverse faults seem to have displaced several horizontal intra crustal interfaces finally in the lower crust we found a region which seems to be aseismic

neogene and quaternary tectonics of the eastern sierras pampeanas argentina active intraplate deformation inboard of flat slab subduction 1 the ranges of the eastern sierras pampeanas are located 600 km east of the andean cordillera in central argentina and have been interpreted to be a response to shortening related to flat slab subduction of the nazca plate uplift of the ranges has been broadly documented to occur during neogene time but many questions remain regarding the timing and style of deformation and the subsurface structural configuration in this study we address these unknowns with observations at multiple scales integrate our results into a tectonic model for the area and discuss how our structural interpretation fits with more regional tectonic models our major findings are 1 the range bounding faults thrust late proterozoic to cambrian schist and gneiss over poorly dated pliocene to pleistocene alluvial strata the timing of fault displacement and age of footwall strata suggest that deformation may have been active at least by pliocene time 2 apatite and zircon u th he thermochronometry exhibits cooling ages that range from permian to early jurassic time and suggests that rock exhumation in the area is less than 2 3 km since that time 3 deploying a local seismic array allowed for locating seismicity and calculating receiver functions these observations indicate that the moho lies at a depth of 37 km and that a midcrustal discontinuity appears to correspond to a detachment zone between 15 and 20 km depth and aligns with a plane of seismicity in our tectonic model the craton appears to act as a rigid backstop to the eastward propagation of stresses from the shallowly subducting slab deformation then propagates back to the west via westward verging faults along a midcrustal detachment

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

the mar chiquita lake an indicator of intraplate deformation in the central plain of argentina abstract the mar chiquita saline lake is located in the lowest part of one of the largest endorreic saline basins of south america with a surface area of 6000 6500 km2 the lake is located in a tectonic depression with an asymmetric cross section the sierras pampeanas foothills with altitudes near 1500 m are to the west and a 100 m topographic high san guillermo high is to the east whose western border is bounded by a buried middle pleistocene fault the tostado selva fault the main tributary of the lake is the dulce river which flows from north to south the southward flow of the river was impeded by an obstacle that closed the dulce valley generating the mar chiquita lake the megafans of the primero segundo and tercero rivers deposited a large amount of sediment against the faulted border of the san guillermo high generating an obstacle that impeded the normal flow of the rivers and diverted the dulce and the salado rivers to their present positions precise data concerning the age of the impounding of mar chiquita does not exist but lacustrine conditions are undoubtedly younger than the uplift of the san guillermo high which occurred in the middle pleistocene the well preserved dry valley of the dulce river located southward of mar chiquita is still visible in satellite images and confirms the youth of the impounding the observations introduced in this paper allow us to understand the origin of a significant feature of the central plains of south america the generation of mar chiquita lake and upstream wetlands produced a pronounced environmental change in the arid chaco pampeana plain which favored human life by introducing changes in vegetation and fauna

structural style deformation mechanisms and paleodifferential stress along an exposed crustal section constraints on the rheology of quartzofeldspathic rocks at supra and infrastructural levels bohemian massif abstract investigations of variscan shortening of quartzofeldspathic rocks have been carried out along an exposed crustal section located in the western part of the tepla barrandian unit bohemian massif the study focuses on determining differential stress deformation mechanisms and geometry of cadomian metagreywackes and cambrian granitoids which show different grade of variscan barrovian type metamorphism ranging from the greenschist subgreenschist facies boundary semibrittle deformation regime to the high grade amphibolite facies the semibrittle deformation regime t ca 300 c is characterized by large scale thrusting within a folding environment and by similar intensities of cataclasis diffusive mass transfer and crystal plasticity of quartz the latter including low temperature migration recrystallization within cadomian quartz veins of the greenschist facies part dislocation creep of quartz operated at differential stresses ranging from ca 70 mpa in the lowermost greenschist facies to ca 20 mpa in the uppermost greenschist facies these values have been derived by paleopiezometry using the grain size of recrystallized quartz they are interpreted as upper bounds for the bulk stress because of intense diffusive mass transfer within the adjacent metagreywackes a marking increase in recrystallized grain size of quartz occurs within the lower greenschist facies where low temperature grain boundary migration is replaced by subgrain rotation thus causion is needed when applying the recrystallized grain size paleopiezometer to this crustal level pressure solution of quartz and development of discrete crenulation cleavage was common in fine grained metagreywackes of the lower greenschist facies whereas the upper greenschist facies t ca 500 570 c is characterized by increasing intensity of metamorphic reactions grain coarsening and development of a schistosity the entire greenschist facies level forms a suprastructure where viscosity contrasts between mica and quartz rich layers were sufficiently high for buckle folding strain protected intercalated metagranitoids on the other hand do not show pervasive deformation fabrics the amphibolite facies part forms an infrastructure characterized by pervasive mylonitic shearing of both cambrian granitoids and metagreywackes it is suggested that the combination of the following processes has contributed to weaken the amphibolite facies level making nonlocalized flow possible 1 increased recrystallization of feldspar 2 crystal plastic softening of quartz due to high temperature grain boundary migration and activity of prism slip and 3 enhanced diffusive mass transfer in the form of metamorphic reactions the structural breaks at the semibrittle crustal level and at the greenschist amphibolite facies boundary supports the view that rheological boundaries are prone to form major detachment horizons in the continental crust

uplift analysis on a pericratonic region an example in the sierras de cordoba 29 34 s argentina abstract the sierras de cordoba argentina is a mountain belt located in the most distal pericratonic part of the south central andean foreland in the easternmost end of the sierras pampeanas pampean ranges broken foreland although this region is considered an excellent example for settings where basement thrusting generates relief and intermontane basin systems hundreds of kilometers away from the cordilleran front the driving mechanisms and main controls on topography are still poorly understood given that the landscape and particularly the rivers react to changes in climate and tectonics on timescales between 103 and 106 yrs we used dem based morphometry elevation patterns local relief and analysis of 242 longitudinal stream profiles to evaluate the main controls on modern topography we focus on the morphologies of the main streams and their tributaries using metrics of the channel concavity and normalized channel steepness index ksn the distribution of local relief concavity and ksn values was compared with the present day topography tectonic structures rock type and modern rainfall patterns the stream longitudinal profiles show homogeneous concavities and ksn background values with some local anomalies local structures east and west vergent thrusts in the sierras de cordoba seem to be the most fundamental controls on the formation of these anomalies slope break knickpoints as well as deep incision high local relief this suggests localized and accelerated rock uplift rates which produce localized increases in the gradient along the longitudinal stream profile it is important to notice that the highest ksn values were reported on the eastern range flanks coincident with the highest values of mean annual rainfall however high ksn values appear to be maintained even with greater rainfall our study provides new insights on the neogene sierra de cordoba uplift evolution which will assist future sampling and modelling cosmogenic nuclides analysis thermochronology and gps analysis

compressional and transpressional stress pattern for pliocene and quaternary brittle deformation in fore arc and intra arc zones andes of central and southern chile kinematic analysis of fault slip data for stress determination was carried out on late miocene to quaternary rocks from the fore arc and intra arc regions of the chilean andes between 33 and 46 south latitudes studies of neogene and quaternary infilling the central depression as well as plutonic rocks of the north patagonian batholith along the liquine ofqui fault zone have revealed various compressional and or transpressional states of stress in the pliocene the maximum compressional stress 1 was generally oriented east west during the quaternary the deformation was partitioned into two coeval distinctive states of stress in the fore arc zone the state of stress was compressional with 1 oriented in a n s to nne ssw direction in the intra arc zone the state of stress was transpressional with 1 striking ne sw along the coast in one site 37 30 s the quaternary strain deformation is extensional with an e w direction which can be explained by a co seismic crustal bending readjustment

the dharwar craton southern india interpreted as the result of late archaean oblique convergence abstract the dharwar craton comprises two distinct parts separated by a steep n s sinistral shear zone in the western part a pre 2900 ma complex of orthogneisses granodiorites and narrow tracts of supracrustal rocks sargur group forms the basement to volcanic and sedimentary basins of the dharwar supergroup ca 2800 2550 ma late archaean deformation is characterised by ne sw crustal shortening and steep n s sinistral shear zones the eastern part is underlain by parallel steep n s or nw se linear belts of calc alkaline anatectic and juvenile granites and granodiorites dharwar batholith ca 2750 2510 ma with intervening volcanic and sedimentary schist belts ca 2800 2550 ma the plutonic belts are 15 25 km wide up to 150 km long and bounded by steep nw se high strain zones up to 2 km wide with sinistral shear sense except one which is dextral magmatic state fabrics and structures in the plutonic rocks are parallel to solid state sinistral shear fabrics in the high strain zones but diffuse magmatic banding is commonly oblique to these zones and coincides with the plane of instantaneous shortening in sinistral shear magmatic state structures swarms of vertical nw se dykes of granite and the vertical wedge shape of the linear belts are consistent with emplacement of the batholith during sinistral shear when magma pressure exceeded regional horizontal compressive stress upright folds and schistosity steep reverse faults and effects of regional ht lp metamorphism show that deformation was partitioned into ne sw shortening in the schist belts during emplacement of the plutonic belts in the sinistral shear regime the western part of the craton is interpreted as the foreland to an accretionary arc represented by the batholith and schist belts intra arc basins in the east ne sw shortening and sinistral transcurrent displacements in the foreland and arc are consistent with arc normal and arc parallel displacements during oblique convergence analogous to mesozoic cenozoic convergent settings

diversity within a unified model for archaean gold mineralization in the yilgarn craton of western australia an overview of the late orogenic structurally controlled gold deposits abstract the archaean yilgarn craton has produced 3000 tonnes of gold mainly from structurally controlled deposits that formed during the latest stages of an orogenic event that affected the entire craton and culminated in the period 2 66 2 63 ga as a group these late orogenic deposits encompass a wide range of host rocks structural settings and structural styles and alteration types however several consistent features justify grouping the deposits into a single class of mineralization these features include timing relative to orogenesis high gold to base metal ratios an association with potassium metasomatism and carbonation of host rocks low sulfide contents and a low salinity h 2 o co 2 ch 4 ore fluid most deposits formed in domains of low mean stress during east west regional compression and preserve evidence in the form of quartz vein arrays and hydraulic breccias for high fluid pressures and rock dilation the range of ore depositional conditions implied by alteration assemblages at various deposits makes the term mesothermal an inappropriate description for this class of mineralization the term late orogenic structurally controlled gold deposits is proposed as one which more completely encompasses the class of mineralization described in this overview late orogenic structurally controlled gold deposits formed in thermal equilibrium with high grade metamorphic rocks at the margins of the greenstone belts but were retrograde with respect to lower grade host rocks in the cores of greenstone belts these relationships probably reflect diachronous metamorphism and a single gold mineralization event which was contemporaneous with the later high grade metamorphism geochronological data indicate gold deposition at ca 2 63 ga in the western part of the yilgarn craton isotopic data suggest similar depositional ages in the eastern part of the craton eastern goldfields superterrane but this timing is difficult to reconcile with field relationships that suggest broad contemporaneity between regional metamorphism and gold mineralization at ca 2 66 ga the main competing genetic models for late orogenic structurally controlled gold mineralization in the yilgarn craton propose an orthomagmatic deeply sourced or metamorphogenic ore fluid none of these models can account satisfactorily for all the features of the late orogenic structurally controlled gold deposits

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

late paleozoic deformation and exhumation in the sierras pampeanas argentina 40 ar 39 ar feldspar dating constraints systematic 40ar 39ar feldspar data obtained from the sierras pampeanas are presented filling the gap between available high 300 c and low temperature 150 c thermochronological data results show silurian devonian exhumation related to the late stages of the famatinian ocloyic orogeny for the sierra de pocho and the sierra de pie de palo regions whereas the sierras de san luis and the sierra de comechingones regions record exhumation during the carboniferous comparison between new and available data points to a carboniferous tectonic event in the sierras pampeanas which represents a key period to constrain the early evolution of the proto andean margin of gondwana this event was probably transtensional and played a major role during the evolution of the paganzo basin as well as during the emplacement of alkaline magmatism in the retroarc

pie de palo argentina a cataclastic diapir the term cataclastic diapirism is proposed for the low temperature extrusion of highly fractured rocks through more competent strata to produce domed topographies at the surface the process is illustrated by reference to the geomorphology neotectonics and microseismicity of the pie de palo an elongated ridge in the western sierras pampeanas of argentina composed of shattered and sheared lower palaeozoic rocks and subject to coseismic uplift the pie de palo is conventionally interpreted as a fault driven basement fold linked to low angle eastward subduction of the nazca plate beneath south america the diapiric model implies instead that deformation is powered by regional compression from west verging near surface crustal shortening which results ultimately from atlantic spreading

geochemistry of precambrian and paleozoic siliciclastic rocks from the iberian range ne spain implications for source area weathering sorting provenance and tectonic setting abstract the major and trace element chemical composition of precambrian and paleozoic shales and sandstones from the iberian range spain has been investigated to determine the provenance and tectonic setting of these rocks as well as to appraise the influence of the weathering hydraulic sorting and recycling processes upon source rock signature the samples studied belong to a prograde sequence from diagenesis to anchizonal grade and are mainly composed of quartz illite mica phases and or kaolinite with chlorite feldspars and carbonates as minor components the major element distribution chemical index of alteration cia indices reflects that recycling processes have been important in homogenizing the composition of shales and sandstones the higher cia indices observed in the silurian and devonian shales 80 and 85 respectively compared with those of precambrian to ordovician and the carboniferous age about 70 indicate that their source area underwent more intense chemical weathering processes possibly due to climatic and or tectonic variations alternatively the source area of the silurian and devonian shales may have been composed of recycled sedimentary materials mineral fractionation is mainly observed in coarser rocks through zircon apatite and xenotime accumulation although the shales are not free of these phases the zircon content of the fine grained rocks is not high enough to affect rare earth element ree contents but phosphate minerals apatite and xenotime at least partially control the ree distribution the influence of apatite and xenotime are slightly higher in the precambrian shales as the phosphate concentration produces a decrease in the la sm ratios chondrite normalized ree patterns and negative eu anomaly size of the studied rocks are similar to that of post archean australian shales paas indicating that they originally come from a differentiated silicic source the higher ree contents in the studied shales in relation to paas indicate that recycling processes in the iberian range sources were probably more intense than that of paas the slight differences among the ree patterns of the different groups of shales probably do not reflect changes in source area composition but instead suggest variations in mineral sorting chemical weathering and or sediment recycling the th sc co th cr th cr v v ni ratios support a primitive silicic source for these rocks with the higher th sc and lower co th in cambrian and ordovician shales indicating a higher proportion of felsic material in their primitive source area the compositional maturity of analyzed sandstones is typical of cratonic environments and their la sc th and zr contents reflect their passive continental margin setting

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

trace rare earth element and oxygen isotopic systematics for the genesis of banded iron formations evidence from kushtagi schist belt archaean dharwar craton india abstract the ree distribution pattern shapes free 5 92 and 27 87 ppm in chert and hematite respectively positive eu anomalies eu eu 3 25 and 2 1 and la n yb n ratios 0 75 and 0 87 of the chert and hematite layers of the oxide facies bifs of kushtagi schist belt indicate that these constituents were derived from hydrothermal solutions generated and emplaced at an archaean ocean ridge however their oxygen isotopic composition have been modified by mixing with the ambient ocean water and diagenetic reactions 18o values 13 9 17 4 c and 8 6 to 3 3 in chert and hematite respectively yield 150 250 c temperature in the diagenetic and lowest greenschist facies field the mixing model of hydrothermal solutions with ocean water of 1 1000 ratio will not significantly change the oxygen isotope ratio of the ambient ocean water therefore while the trace element data have retained hydrothermal signatures oxygen isotopic signatures are imprinted by diagenetic processes

mio pliocene volcaniclastic deposits in the famatina ranges southern central andes a case of volcanic controls on sedimentation in broken foreland basins abstract a well constrained record of miocene pliocene explosive volcanism is preserved within the broken foreland of western argentina along the famatina ranges this paper focuses on the volcaniclastic record known as the rio blanco member of the el durazno formation three facies can be recognized in the study area 1 massive tuffs 2 volcaniclastic conglomerates and 3 pumiceous sandstones these facies are interpreted as primary pyroclastic flow deposits ignimbrites and reworked volcanogenic deposits within interacting volcanic fluvial depositional systems alternation between ignimbrites and volcanogenic sandstones and conglomerates suggest a recurrent pattern of sedimentation related to recurrent volcanic activity considering the facies mosaic and relative thicknesses of facies short periods of syn eruption sedimentation volcaniclastic deposits seem to have been separated by longer inter eruption periods where normal stream flow processes were dominant the volcaniclastic component decreases up section suggesting a gradual reduction in volcanic activity the mean sedimentation rate of the rio blanco member is higher 0 44 mm year than those obtained for the underlying and overlying units this increase cannot be fully explained by foreland basement deformation and tectonic loading hence we propose subsidence associated with volcanic activity as the causal mechanism volcanism would have triggered additional accommodation space through coeval pyroclastic deposition modification of the stream equilibrium profile flexural loading of volcanoes and thermal processes these mechanisms may have favored the preservation of volcaniclastic beds in the high gradient foreland system of famatina during the mio pliocene thus the rio blanco member records the response of fluvial systems to large volcanism induced sediment loads

crustal wedging triggering recent deformation in the andean thrust front between 31 s and 33 s sierras pampeanas precordillera interaction 1 we document a new model of crustal structure of the andean front in argentina where numerous historic earthquakes destroyed the cities of mendoza in 1861 ms 7 and san juan in 1944 mw 7 0 the cerro salinas anticline is formed above the west directed cerro salinas thrust an east facing monocline with an amplitude of about 2 km folds the cerro salinas thrust and overlying neogene succession this monocline is formed above a blind crustal thrust in the basement its dip of 14 west is inferred from fold geometry this thick skinned east directed blind thrust and the thin skinned west directed cerro salinas thrust define a tectonic wedge the wedge tip occurs at a depth of 5 4 km growth of the monocline after 8 5 ma is revealed on multichannel seismic msc profile 31017 repsol ypf rates of cerro salinas thrust displacement are of the order of 1 mm yr whereas vertical uplift of 0 45 mm yr results from the combined displacement on the cerro salinas thrust and growth of east facing monocline the lateral extent of the east directed crustal blind ramp corresponds with the along strike extent of the eastern precordillera when combined with the low displacement rate a long earthquake recurrence interval is implied smaller magnitude earthquakes however indicate that segments of the blind thrust ramps ruptured in historic events if all the segments of the blind thrust ruptured together the fault area is 7000 km 2 and could produce a mw 7 7 earthquake the crustal wedge model provides new constraints on the origin and potential size of earthquakes that threaten the densely populated region

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material

phase equilibria in the system k 2 o feo mgo al 2 o 3 sio 2 h 2 o co 2 and the stability limit of stilpnomelane in metamorphosed precambrian iron formations the phase relations of al and fe bearing silicates in the system k2o feo mgo al2o3 sio2 h2o co2 in the presence of quartz and magnetite are discussed on the basis of mineralogic and petrologic data from precambrian iron formations and blueschist facies meta ironstone from the franciscan formation california these relations allow an estimation of the physiochemical conditions during low grade metamorphism of iron formations petrologic data together with available experimental and predicted thermodynamic data on the associated minerals place the upper stability limit of stilpnomelane in iron formations at about 430 470 c and 5 6 kilobars fe end member stilpnomelane can persist to a maximum temperature of 500 c and pressures up to 6 7 kilobars although it is unlikely to occur in metamorphosed iron formations in iron formation occurrences the stilpnomelane stability field is bordered by four equilibrium reactions with the assemblages stilpnomelane zussmanite chlorite minnesotaite stilpnomelane zussmanite chlorite grunerite stilpnomelane biotite chlorite grunerite and stilpnomelane biotite almandine grunerite the stability field is reduced by increasing x co2 and xmgstil and is also a function of a k a h in the metamorphic fluid if the value of a k a h is smaller than that defined by the above assemblages stilpnomelane decomposes to chlorite but if larger it is replaced by biotite at pressures less than 4 kilobars the zussmanite field is restricted to a very high value of a k a h 5 0 in log units at 1 0 kilobar where iron formation assemblages are not stable

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