collapse of archean orogens and the generation of late to postkinematic granitoids intrusion of late stage granitoids into archean granite greenstone terranes may be spatially and temporally related to late orogenic upper crustal extensional collapse and formation of metamorphic core complexes the late to postkinematic granitoid magmas are derived through partial melting of upper mantle and lower crustal sources and are locally associated with late stage extensional structures thick roots of depleted lithospheric mantle of archean age tectosphere beneath archean cratons makes melting of the upper mantle difficult it is proposed that the archean tectosphere is made of imbricated slabs of buoyant archean oceanic crust and lithosphere together with trapped wedges of fertile mantle this would explain several seemingly contradictory phenomena decompression from upper crustal extension generates significant amounts of basaltic melts from the trapped wedges of fertile mantle these basaltic magmas intrude and partially melt the lower crust becoming more silicic in composition they migrate upward to solidify in the middle and upper crust forming the late to postkinematic granitoid suite the tectosphere in this model is less dense and colder than surrounding asthenosphere and is consistent with eclogitic xenoliths from subducted oceanic lithosphere found entrained in kimberlites the direction and magnitude of shear wave anisotropy in the upper mantle is also explained in this model through the orientation of olivine crystals in the underplated slabs of oceanic lithosphere

proterozoic contraction extension tectonics of the southern sao francisco region minas gerais brazil the southern sao francisco region comprises the southern portion of the sao francisco craton which is underlain by archean basement and its fringing proterozoic orogenic belts included in this region are three geologic provinces the quadrilatero ferrifero qf the sao francisco basin and the cordilheira do espinhaco we synthesize preliminary results of ongoing structural analysis reconnaissance field studies and published reports to provide a concise review of the geology of the region and to propose a working tectonic model for its proterozoic evolution interpretation of superimposed folds faults and foliations leads to the conclusion that the southern sao francisco region records the effects of four principal tectonic events that occurred subsequent to deposition of the lower proterozoic minas supergroup the first event resulted in formation of a northwest verging fold thrust belt the second event led to development of high angle reverse faults open folds and basement uplift the third event is manifested in the qf by the formation of normal faults and by the intrusion of mafic dikes these extensional structures may be associated with formation of the sedimentary basins east and south of the sao francisco craton the final event resulted in formation of a west verging fold thrust belt that affected the qf the cordilheira espinhaco and the sao francisco basin the events that we describe can be tentatively associated with regionally recognized orogenic periods trans amazonian uruacuano brasiliano

depositional age and sediment source of the auriferous moeda formation quadrilatero ferrifero of minas gerais brazil new constraints from u pb hf isotopes in zircon and xenotime abstract the moeda formation is a metasedimentary clastic sequence of witwatersrand like metaconglomerate and quartzite at the base of the minas supergroup quadrilatero ferrifero of minas gerais brazil which is also known for its world class iron ore deposits the moeda formation can be subdivided into three units units i to iii from bottom to top with each unit consisting of metaconglomerates at their base mostly auriferous grading into quartzite and locally into phyllite this study presents the first combined u pb and lu hf isotope data from detrital zircons and xenotimes of auriferous metaconglomerates and quartzites from units ii and iii these data indicate that deposition of sediments occurred 2 68 ga for unit ii and 2 62 ga for unit iii as supported by the youngest magmatic zircon grains the data further provide evidence that the source area of sediments was affected by crust re working during two major magmatic metamorphic events at 2 89 2 87 and 2 72 2 69 ga and minor events between 3 3 and 3 0 and at 2 80 and 2 65 ga this interpretation is supported by mainly subchondritic hf t between 10 and 1 4 and large hf t variations including all magmatic events a few superchondritic hf t obtained for the magmatic events at 3 3 2 9 and 2 7 ga point to limited juvenile crust addition the new dataset indicates that the clastic sediments of the auriferous moeda formation were mainly derived from an evolved continental crust in contrast to primitive magmatic arc sources which are suggested for the mesoarchaean auriferous sediments of the witwatersrand basin of south africa and those of the eastern goldfield province of australia

statistical analysis of the induced basel 2006 earthquake sequence introducing a probability based monitoring approach for enhanced geothermal systems summary geothermal energy is becoming an important clean energy source however the stimulation of a reservoir for an enhanced geothermal system egs is associated with seismic risk due to induced seismicity seismicity occurring due to the water injection at depth have to be well recorded and monitored to mitigate the seismic risk of a damaging event an appropriate alarm system needs to be in place for each individual experiment in recent experiments the so called traffic light alarm system based on public response local magnitude and peak ground velocity was used we aim to improve the pre defined alarm system by introducing a probability based approach we retrospectively model the ongoing seismicity in real time with multiple statistical forecast models and then translate the forecast to seismic hazard in terms of probabilities of exceeding a ground motion intensity level one class of models accounts for the water injection rate the main parameter that can be controlled by the operators during an experiment by translating the models into time varying probabilities of exceeding various intensity levels we provide tools which are well understood by the decision makers and can be used to determine thresholds non exceedance during a reservoir stimulation this however remains an entrepreneurial or political decision of the responsible project coordinators we introduce forecast models based on the data set of an egs experiment in the city of basel between 2006 december 2 and 8 approximately 11500 m 3 of water was injected into a 5km deep well at high pressures a six sensor borehole array was installed by the company geothermal explorers limited gel at depths between 300 and 2700 m around the well to monitor the induced seismicity the network recorded approximately 11200 events during the injection phase more than 3500 of which were located with the traffic light system actions where implemented after an ml 2 7 event the water injection was reduced and then stopped after another ml 2 5 event a few hours later an earthquake with ml 3 4 felt within the city occurred which led to bleed off of the well a risk study was later issued with the outcome that the experiment could not be resumed we analyse the statistical features of the sequence and show that the sequence is well modelled with the omori utsu law following the termination of water injection based on this model the sequence will last 31 29 14 years to reach the background level we introduce statistical models based on reasenberg and jones and epidemic type aftershock sequence etas models commonly used to model aftershock sequences we compare and test different model setups to simulate the sequences varying the number of fixed and free parameters for one class of the etas models we account for the flow rate at the injection borehole we test the models against the observed data with standard likelihood tests and find the etas model accounting for the on flow rate to perform best such a model may in future serve as a valuable tool for designing probabilistic alarm systems for egs experiments

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

kinematics of orocline and arc formation in thin skinned orogens the regional traces of folds faults and foliations found in many fold thrust belts are bent in plan view it proves valuable to distinguish between two types of bent orogens based on their kinematic evolution oroclines or rotational arcs are bent orogens in which segments of the orogen change strike during the evolution of the bend and nonrotational arcs are bent orogens in which segments of the orogen do not change strike during development of the bend the kinematic evolution of a bend that forms in a thin skinned orogen can be described in terms of three parameters the displacement path trajectories followed by points along the strike of the orogen the magnitude and distribution of tangential extension along the strike of the orogen and the change in position of the endpoints of the orogen with respect to a reference line only slight differences in the displacement path trajectory pattern determine whether an orogen evolves as a nonrotational arc or as an orocline thus the distinction between these two types of bends is not always of major tectonic significance compressional deformation along irregular continental margins or the impact of indentors on continental margins during collisional orogenies more likely leads to formation of nonrotational arcs as can be simulated with a sand wedge model interaction between a fold thrust wedge with obstacles in the foreland or with a wrench fault more likely leads to orocline formation orocline formation is also associated with noncoaxial reactivation of thrust faults tangential extension is a necessary consequence of certain displacement path trajectory patterns and can accompany development of either nonrotational arcs or oroclines two examples one from the umbrian apennines and one from the makran range demonstrate how movement on cross strike fault arrays can accommodate tangential extension in thin skinned bends

tectonic evolution of south america during the late proterozoic abstract this paper discusses the evolution and the late proterozoic assembly of the south american continent i e the west gondwana under the light of global tectonics the northern portion of the continent the amazonian craton is a pre brasiliano domain it may be a fragment derived from laurentia this domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent brasiliano domain the brasiliano domain had a more complex history and composition during the brasiliano cycle middle proterozoic to early paleozoic in the brasiliano domain several late proterozoic continental plates have been identified sao luiz west africa congo kasai sao francisco rio de la plata arequipa antofalla which acted as forelands to the brasiliano orogenic belts that surround and amalgamate these plates smaller fragments of pre brasiliano continental lithosphere are common in the interior of the brasiliano orogenic belts and played diversified roles microplates microcontinents structural highs some of them have acted as true backlands hinterlands flanked by arc magmatism two principal types of brasiliano orogenic belts are recognized between the continental plates a marginal orogenies to the late proterozoic plates with pelitic carbonatic rocks and discrete volcanism which were thrust by collision onto the margin of the plates and which seem to represent final products of previous passive margin sequences b distal orogenics internal belts within the continental masses sometimes a branching system of orogenics that contain varied clastic and minor carbonate sedimentary rocks accompanied by important bimodal volcanism and calc alkaline plutonism for these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized the amalgamation of the continent during the late proterozoic i e the formation of the brasiliano structures indicates the existence of practically synchronous multilateral stresses within lithospheric plates this can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere analogous to modern continental collisions two large and important tectonic zones may be candidates to such a scenario the hijaz mozambique zone site of connection of east and west gondwana and the transbrasiliano lineament which cuts across south america and extends to north africa following the zone of connection between pre brasiliano and brasiliano domains other small ocean openings and closings inside the various brasiliano areas dom feliciano gariep for instance were minor events and complementary to the other two major tectonic zones mentioned above

pb sr and nd isotope constraints on the archaean evolution of gneissic granitoid complexes in the southern sao francisco craton brazil abstract three archaean gneissic complexes bonfim belo horizonte and campo belo in the southern part of the sao francisco craton were studied by means of u pb zircon and titanite monazite and sm nd rb sr and pb pb whole rock methods in the bonfim metamorphic complex the period 2780 2700 ma is marked by in situ anatexis of gneisses intrusion of tonalites mafic dikes and late granites and deposition of the rio das velhas greenstone belt sm nd tdm model ages on its gneisses amphibolites and granites range from 2800 to 3000 ma indicating involvement of older sialic crust in the 2780 2700 ma event in the belo horizonte metamorphic complex u pb zircon geochronology has yielded ages of 2860 2776 and 2712 ma and rb sr whole rock isochrons on its gneisses and migmatites yielded ages in the range 2800 2750 ma with large uncertainties and 87sr 86sr initial ratios of 0 700 0 710 the highest initial ratio 0 710 obtained for the migmatites suggests that ensialic episodes participated in the tectonomagmatic evolution existence of older crust is suggested by u pb zircon ages of 3030 2920 and 2880 ma from inherited grains in metavolcanics and orthogneisses rb sr ages of 2250 2130 ma on granitoids and u pb titanite monazite ages from 2320 to 2030 ma on granitoids felsic veins and amphibolite enclaves indicate reworking of this complex in the palaeoproterozoic in the campo belo metamorphic complex granite and greenstone lithologies have yielded ages of 3380 3000 2900 and 2650 ma by u pb zircon and rb sr and pb pb whole rock methods tdm ages on its granites gneisses and granulites have yielded ages from 3070 to 2780 ma indicating involvement of older sialic material during the 2650 ma event these complexes indicate major events during the period 2860 2700 ma including reworking of established sialic crust with components up to 3380 ma old late granites were injected between 2700 2600 ma preceding final assembly and stabilization of the archaean crust

building robust models to forecast the induced seismicity related to geothermal reservoir enhancement abstract near real time seismicity forecasting is critically important for decision making during the reservoir creation phase of enhanced geothermal systems egs to compute within a probabilistic framework the seismic hazard and risk posed by induced seismicity it is necessary to calibrate robust forecast models quantitatively assess their reliability and capture the uncertainty of the process here we test the performance of three model classes each forecasting the induced seismicity observed during the failed 2006 2007 egs reservoir creation underneath basel switzerland each model class represents a logic tree branch that captures the epistemic uncertainty of the process the aleatory variability is accounted for by bootstrapping of the observed seismicity we calibrate each model using a range of updating strategies and quantitatively test the performances of the models using likelihood tests we then define a combined model based on akaike weights and show that the combined model outperforms in our pseudoprospective testing environment all of the individual models we also show that seismicity recorded during the initial one to three days of seismicity several days before the onset of felt seismicity is sufficient for deciding with confidence that the prescribed injection regime will eventually lead to unacceptably high levels of seismicity

tectonic evolution of the bindura shamva greenstone belt northern zimbabwe progressive deformation around diapiric batholiths abstract the bindura shamva greenstone belt northern zimbabwe and adjacent batholiths are regarded as one of the type areas for granite greenstone tectonics structural observations and three dimensional strain analyses have been used to propose a model for the tectonic evolution of this late archaean greenstone belt the data indicate that pluton emplacement triggered the deformation of the greenstone sequences and that the deformation history is single phase progressive rather than polyphase they do not however lead to an unambiguous interpretation of the emplacement mechanism structural features are best explained by a combination of pluton diapirism and ballooning plutonism criteria used as evidence for pluton related deformation include the pattern of foliation and stretching lineation trajectories the variation in strain type and strain intensity fold types sense of shear at the batholith greenstone interface metamorphic zoning and timing of porphyroblast growth specific structural and strain patterns within cleavage triple points interdomal synclinoria saddle culminations and batholith margins demonstrate interference between strain fields developed around each batholith

detrital rutile ages can deduce the tectonic setting of sedimentary basins abstract sedimentary sequences contain a detailed record of the evolution of the earth s surface detrital mineral grains provide for example evidence of source redox conditions insights into the age geochemistry and petrology of basement sources and can also be used to elucidate tectonic environments detrital zircon has been used as a means of analysing the tectonic setting of host sedimentary successions but with potentially ambiguous results it is important to find additional ways to discriminate depositional settings particularly in precambrian sequences where other proxies are either not available or have been lost in this contribution we provide a new way to discriminate between different sedimentary tectonic environments using the mineral rutile we present a large compilation of detrital rutile data to show that the u pb age distribution is sensitive to the tectonic setting of the basin in which the host sediments were deposited we then apply this new approach to two case studies where the depositional setting and age are well constrained siliciclastic units of nw scotland which were deposited in the neoproterozoic foreland of the grenville orogen and on the cambrian passive margin of the iapetus ocean and the brazilian sabara basin located within the palaeoproterozoic foreland of the minas orogen we compare the detrital rutile and zircon age distributions of these successions showing that in some cases they are different and that rutile is most sensitive to the youngest metamorphic events affecting the sources reinforcing the applicability of this tool by testing this new method on polyphasally deformed successions sabara basin we show that detrital rutile can still inform the tectonic setting even at medium grades of metamorphism

a three stage structural evolution of the quadrilatero ferriifero consequences for the neoproterozoic age and the formation of gold concentrations of the ouro preto area minas gerais brazil abstract the quadrilatero ferrifero occurs in the southern part of the sao francisco craton brazil where three major proterozoic supracrustal units the rio das velhas minas and itacolomi supergroups are exposed together with archaean to palaeoproterozoic granito gneissic basement microtectonic and kinematic analyses have been undertaken in the area close to ouro preto in order to precise the tectonic evolution and to distinguish the tectonic features related to the transamazonian and brasiliano orogenies respectively 2000 and 600 ma three main tectonic events have been recognized the earlier event de is clearly expressed within basement rocks of the bacao complex and surrounding supracrustal units a stretching lineation le lineation roughly oriented n s can be observed numerous shear criteria such as drag folds and asymmetric pressure shadows indicate a top to the south sense of movement along the southern border of the bacao complex this event was developed within amphibolite facies conditions fact confirmed by quartz c axis fabrics this earlier deformation is related to plutonic activity and reactivation of the archaean basement expressed by the occurrence of scarce leucocratic granitoid rocks intruded within the bacao gneisses it is here attributed to the transamazonian orogeny in agreement with recent u pb dating 2000 ma the large scale folds i e mariana anticline dom bosco syncline which define the dominant structural geometry of the quadrilatero ferrifero were formed during this event the second tectonic event dm has been defined as a major thrusting towards the wnw related to the brasiliano cycle a well expressed stretching and mineral lineation oriented e w to nw se lm lineation and associated nw verging shear criteria characterize this event occurrence of muscovite biotite and more scarcely sillimanite indicate that deformation developed in upper greenschist lower amphibolite facies quartz c axis fabrics confirm the sense of shearing but the large scattering of several fabrics traduces a weak intensity of strain all supracrustal and gneiss formations were affected by the thrusting which is developed on a folded surface previously formed during the de event the third event dm is characterized by hangingwall down movement of the nappe pile in response to relaxation of the brasiliano compressional dm forces this event developed in greenschist facies is marked by the occurrence of metre scale drag folds sometimes associated to the development of a new lineation lm gold mineralization which occurs along the main mineralized contact located between the minas and rio das velhas supergroups is found within late veins which comprise large recrystallized quartz carbonate sulphide chlorite and tourmaline microscopic analysis and veins geometry allow us to conclude that the veins were emplaced mostly after the ductile dm deformation it is proposed that the veins were formed by filling of open cavities created during dm tectonics this study highlights that the northwest verging thrusting which is the main tectonic event of the quadrilatero ferrifero is only relevant of the brasiliano orogeny a model of structural evolution which involves plutonic activity at 2000 ma within the archaean bacao complex followed by neoproterozoic thrust tectonics brasiliano is proposed and discussed the age of gold concentration assumed to be achieved during the relaxation that occurred in response to the thrusting event is re interpreted and related to the late stage of the brasiliano tectonics

package fluids 1 computer programs for analysis of fluid inclusion data and for modelling bulk fluid properties the computer package fluids contains five sets of computer programs written in c for the calculation of fluid properties both model fluids and data from fluid inclusions can be analysed in the programs bulk density molar volume and composition of fluid inclusions are calculated with the program bulk which uses microthermometric data micro raman spectroscopic data cation ratios and volume fraction estimations as input the program offers a broad variety of equations of state which are selected according to the fluid system or the temperature and pressure conditions of interest the isochores that correspond to the output or test v x properties of fluid inclusions are calculated by the program isoc in calculating the isochores this program takes account of the compressibility and expansion of selected host minerals the program test allows different equations of state to be compared directly with experimental data thereby facilitating selection of the most adequate model in the programs bulk and isoc loner is a group of programs that handle individual equations of state eos for all fluid systems presented in the programs bulk and isoc aqso is a group of programs that allows purely empirical modelling of electrolyte bearing aqueous solutions henry s law for dilute gas bearing aqueous solutions and osmotic coefficients for dilute electrolyte bearing aqueous solutions the programs are available on the university of leoben web site at http www unileoben ac at buero62 minpet ronald

insights into orogenic processes from drab schists and minor intrusions southern sao francisco craton brazil abstract minor altered intrusions and drab retrogressed schists can easily be overlooked in geological studies but this contribution explores these rocks within the archaean and palaeoproterozoic southern sao francisco craton ssfc brazil using geological relationships and accessory mineral in situ analyses in the context of cratonic assembly three magmatic pulses are documented i archaean and ii palaeoproterozoic felsic intrusions both hosted by archaean protoliths and iii palaeoproterozoic felsic intrusions in palaeoproterozoic supracrustal sequences archaean felsic intrusions confirm the palaeoarchaean age of the mafic ultramafic sequence of the rio das velhas greenstone belt and rhyacian intrusions mark the collisional stage of the mineiro belt with the sfc at c 2130 ma greenstone belt schists show a wide distribution of rounded soccer ball archaean detrital and metamorphic zircon grains ranging in age from 3200 to 2750 ma with an interpreted overprinting high grade metamorphic event at c 2700 to 2680 ma most high grade metamorphic rims have th u 0 1 negative ehf t values and ree pattern consistent with eclogite granulite metamorphic facies reinforcing the hypothesis of a dehydrated refractory crust formed during the stabilization of the ssfc even though no such protoliths are preserved this event links crustal thickening and partial melting of archaean lower crust archaean rutile crystals from the greenstone belt schist were reset during the palaeoproterozoic event but still preserve the early archaean high grade metamorphic signature the presence of unstable ilmenite replaced by rutile in the schist associated to felsic intrusions with the same age at c 2130 ma suggest high pressure low temperature prograde metamorphism during the collisional stage of the palaeoproterozoic orogen elongate and prismatic zircon grains from the rhyacian intrusions have low ehf t signature and crystallised from partial melting of sedimentary protoliths accretionary events produced thicker and more differentiated crust by the end of rhyacian time easily overlooked rocks in this study when studied have revealed a rich multi event history of cratonic evolution

earthquakes as multiscale dynamic ruptures with heterogeneous fracture surface energy 1 we propose a model of the wide scale growth of dynamic rupture during an earthquake based on our multiscale simulation of a planar crack in a three dimensional homogeneous elastic space a simple slip weakening law governs the fracture friction processes and its characteristic parameters slip weakening distance and fracture surface energy have multiscale heterogeneous distributions we consider a set of randomly distributed circular patches whose diameter is proportional to the fracture surface energy each patch represents an asperity between irregular fault surfaces and the size number relation of the patches obeys power law statistics we assess rupture propagation from a small instability using a boundary integral equation method with a renormalization technique although most events stop shortly after their initiation some grow triggering neighboring patches of similar size small and large events show statistically self similar properties of rupture growth and stop spontaneously without requiring a special stopping mechanism the rupture velocity locally exceeds the shear wave speed but globally remains subshear speed due to the increase of the average fracture energy as the rupture grows the relation between size and frequency of events is a power law which is explained by the triggering probability between patches as a consequence of statistically self similar random triggering growth we observe a distinct main phase in seismic waves similar to those of natural earthquakes but we cannot estimate the final size of the event from the initial part of the seismic waves if this is true for the real earthquakes predicting the size of a future earthquake would be quite difficult

age of felsic volcanism and the role of ancient continental crust in the evolution of the neoarchean rio das velhas greenstone belt quadrilatero ferrifero brazil u pb zircon dating of volcaniclastic graywackes abstract a felsic eruptive event within the rio das velhas greenstone belt is mainly represented by volcaniclastic and resedimented volcaniclastic rocks besides rare lava flows of dacitic composition the volcaniclastic rocks comprise four lithofacies 1 monomictic and polymictic breccias 2 conglomerate graywacke 3 graywacke sandstone and 4 graywacke argillite shrimp and id tims u pb zircon dating of three graywacke samples dated the eruptive event at 2792 11 2773 7 and 2751 9 ma indicating a range of about 40 ma for the felsic magmatism this time range may also encompass the whole magmatic activity within the belt because of the association of the felsic rocks with the more extensive basaltic flows volcaniclastic graywackes are characterized by an abundance of detritic xenocrystic zircon grains displaying a polymodal age spectrum ranging from 2 8 to 3 4 ga older zircon grains must be derived from magma contamination and or post eruptive sedimentary reworking and mixing with felsic detritus from other sources it may be concluded that felsic magmas were in contact with older granitoid crust and that this basement was also exposed as source for detritic material in the quadrilatero ferrifero the ancient complexes of ttg gneisses and migmatites make up most of the exposed archean terrains and its present juxtaposition with the supracrustal assemblage suggests an authochthonous to parauthocthonous development for the rio das velhas greenstone belt in a continental or near continental tectonic setting

control of hazard due to seismicity induced by a hot fractured rock geothermal project in 2003 hydraulic stimulations were carried out in a geothermal field in eastern el salvador central america as part of a project to explore the feasibility of commercial hot fractured rock energy generation a key requisite for this environmentally friendly energy source is that the fracturing of the hot rocks at depths of 1 2 km must not produce levels of ground shaking at the surface that would present a serious disturbance or threat to the local population thresholds of tolerable ground motion were inferred from guidelines and regulations on tolerable levels of vibration and from correlations between instrumental strong motion parameters and intensity considering the vulnerability of the exposed housing stock the thresholds were defined in terms of peak ground velocity pgv and incorporated into a traffic light system that also took account of the frequency of occurrence of the induced earthquakes the system was implemented through a dedicated seismograph array and locally derived predictive equations for pgv the traffic light was used as a decision making tool regarding the duration and intensity of pumping levels during the hydraulic stimulations the system was supplemented by a small number of accelerographs and re calibrated using records obtained during the rock fracturing

where are the eburnian transamazonian collisional belts the reconstruction of early proterozoic crustal evolution and geodynamic environments in africa and south america is incomplete if cratonic areas alone are studied if the presence of high grade

the itiuba alkaline syenite massif bahia state brazil mineralogical geochemical and petrological constraints relation to the genesis of rapakivi magmatism abstract numerous alkaline massifs occur throughout the state of bahia brazil isotopic dates fall into two age groups brasiliano 0 45 0 70 ga and trans amazon 1 8 2 1 ga ones brasiliano alkaline provinces comprise a silica undersaturated association with related volcanic rocks their emplacement is always controlled by fault zones trans amazon alkaline massifs are characterized by large plutons more than 100 km2 of k rich syenite and granite associated with mafic cumulates and abundant dyke swarms no associated volcanic rocks have been so far recorded the shape of the plutons varies as a function of their location within the sao francisco craton in its northeastern and southern parts syenite massifs are elongated trending n s while in its western part they are emplaced as circular bodies displaying contact metamorphic aureoles the presence of older archaean alkaline rocks cannot be ruled out as some granulitic facies display syenite compositions the itiuba massif provides a good example of proterozoic alkaline syenite located in the northern part of the sao francisco craton this 150 km long pluton covers 1800 km2 in area whole rock rb sr isotopic data yield a lower proterozoic age a n s trending foliation at the margins is gradually replaced by isotropic textures toward the core two sets of faults both of trans amazonian age have been defined n s trending reverse faults accompanied by intense mylonitization and younger nw trending arcuate transcurrent faults related to a ne sw compressive regime alkaline syenites constitute 98 of the exposures and are medium to coarse grained clinopyroxene amphibole hypersolvus leucratic rocks cumulates are represented by mafic layers and clinopyroxene apatite enclaves dykes are composed of alkaline syenite hypersolvus and transsolvus quartz syenites and alkaline granites syenites are metaluminous and rich in k mg p ti and ba and their geochemical trends are controlled by alkali feldspar clinopyroxene apatite and fe ti oxide fractionation oxidizing conditions are reflected by the weak variation in the mg ratio the reverse fe mg mineral zonation the synchronous precipitation of oxides and clinopyroxene in mafic layers and the late development of uralitic amphibole and low ti phlogopite temperatures for oxide equilibration of 930 880 c are compatible with the hypersolvus feldspar mineralogy the evolution of feldspar mineralogy is a good indicator of varying thermodynamical conditions during late stage crystallization of the pluton minimum values for initial crystallization temperature have been evaluated at 950 c structural and petrological results indicate that the presently exposed syenite massif represents the roof of a proterozoic magma chamber emplaced in granulite gneiss formations rapakivi magmatism may represent disrupted and floating portions of the roof of earlier magma chambers refilled by new syenite granite melts

stratigraphy of eastern bay of exploits newfoundland fossils from the fanners island formation new name show that southwest of new world island newfoundland a thick over 3 km sequence of late llandeilo turbidites and basalts overlies the dunna

transamazonian orogeny in the southern sao francisco craton region minas gerais brazil evidence for paleoproterozoic collision and collapse in the quadrilatero ferrifero abstract the paleoproterozoic transamazonian orogeny yielded a series of discrete orogens in brazil the present field study in the quadrilatero ferrifero qf indicates that the southern sao francisco craton region of the brazilian highlands preserves a portion of one of these orogens two sets of transamazonian structures occur in this region the first consists of northwest verging folds and thrusts affecting supracrustal sequences it is suggested that this set formed in a fold thrust belt setting shortly after 2 125 ga during the closure of a passive margin basin that had initiated along the margins of a preexisting continental mass at ca 2 5 ga the second set consists of structures defining the prominent dome and keel architecture of the qf this set a consequence of the emplacement of basement domes against supracrustal rocks at 2 095 ga may reflect the consequences of orogenic collapse narrow conglomerate filled intermontane basins may have formed coevally with dome emplacement formation of an ocean basin east of the present day sao francisco craton eventually occurred in late mesoproterozoic in effect the transamazonian orogen of the qf represents the collision and collapse stages of a paleoproterozoic wilson cycle the contractional phase of the transamazonian orogeny probably represents accretion of an offshore arc to the eastern and southeastern margin of the present day sao francisco craton region the arc and an associated suture may be traced in the brasiliano pan african orogen east of the sao francisco craton northwards into the northeastern lobe of the sao francisco craton clearly initial assembly of crustal blocks to form a larger continent involving south america occurred during the paleoproterozoic 2 1 ga post transamazonian rifting of this continent created the basins which were later inverted during the brasiliano assembly of gondwana

brazil s premier gold province part i the tectonic magmatic and structural setting of the archean rio das velhas greenstone belt quadrilatero ferrifero rocks of the rio das velhas supergroup comprise one of the most significant archean greenstone belt successions in brazil in both their appreciable mineral productivity and extensive mineral potential a large part of this greenstone belt is contained within the quadrilatero ferrifero iron quadrangle region minas gerais state southeastern brazil which occupies the southernmost portion of the sao francisco craton the nova lima group rocks at the base of the rio das velhas greenstone belt host important orogenic gold deposits the group contains lithological associations from bottom to top as follows 1 mafic ultramafic volcanic 2 volcanic chemical 3 clastic chemical 4 volcaniclastic and 5 resedimented rocks rocks of the resedimented volcanic chemical and mafic ultramafic volcanic associations mainly host the most important gold deposits an early compressional deformation occurs in the rocks of the rio das velhas greenstone belt and basement gneisses with tangential thrusting from the north to the south or southwest structures generated during a second compressional deformation encompass nw striking thrust faults and sw vergent tight to isoclinal folds inferring a general southwest transport direction in the central portion of the quadrilatero ferrifero the paciencia lineament which strikes northwest and dips to the northeast in the south or strikes northeast and dips to the southeast in the north is a thrust related oblique ramp fault that hosts important gold deposits the convergence of these two trends in the nova lima region is accommodated by roughly e w striking transcurrent faults which are the most favored sites for large gold concentrations intracratonic extension in late archean to early paleoproterozoic times and nw vergent trans amazonian compressional deformation post date gold deposition late extension during the paleoproterozoic led to basin formation and the prominent dome and keel architecture of the quadrilatero ferrifero region deformation related to the west vergent thrust and fold belts of the brasiliano orogeny is recognized at the eastern margin of the quadrilatero ferrifero region defining structures as archean trans amazonian and brasiliano in age is still difficult although it is accepted that the gold related archean structures are best preserved in the central and western parts of the quadrilatero ferrifero the principal source for the rio das velhas sedimentary rocks was probably the trondhjemite tonalite granodiorite crust that formed in multiple episodes after 3 500 ma and was widely metamorphosed and intruded at 2 880 2 850 ma felsic volcanism at 2 772 6 ma formed the rio das velhas greenstone belt the volcanic succession was a source for some overlying sedimentary rock units as indicated by the presence of detrital zircons dated at 2 777 2 771 ma strongly foliated granitic plutons range between 2 712 5 4 and 2 698 18 ma the age of gold mineralization is inferred between 2 698 and 2 670 ma a metamorphic overprint during the trans amazonian orogeny is estimated at 2 050 ma there is evidence of local isotopic disturbances because of post trans amazonian tectonic events at 1 8 1 7 and 0 6 ga

rhyacian evolution of subvolcanic and metasedimentary rocks of the southern segment of the mineiro belt sao francisco craton brazil abstract the tiradentes suite in conjunction with the coeval estacao tiradentes metasedimentary sequence makes up a distinct tectonic element within the paleoproterozoic framework of the mineiro accretionary belt in the southern sao francisco craton this suite is the second occurrence of volcanic and subvolcanic rocks in the mineiro belt and shows petrographic and geochemical similarities with the nearby serrinha suite 2 23 2 21 ga the first reported rhyacian volcanism in the mineiro accretionary belt the tiradentes suite is represented by mafic andesites dacites granophyres and tonalites and shows petrographic features of volcanic rocks such as spherulites amygdales granophyric intergrowths and porphyritic textures with plagioclase phenocrysts the rocks of this suite are metaluminous to peraluminous enriched in alkaline elements compatible with low al2o3 trondhjemites and show primary fractionation of plagioclase apatite hornblende biotite and ilmenite high hree content coupled with incipient negative eu anomaly and high y values are consistent with a garnet amphibolite protolith with plagioclase and hornblende as present in the residual phases the samples of the tiradentes suite show geochemical signatures of magmatic arcs which are also observed in rocks from the serrinha suite rocks of the tiradentes suite yield the following u pb zircon crystallization ages i mafic andesite 2217 23 ma ii dacitic dike 2204 11 ma iii subvolcanic tonalite 2213 9 ma the deposition of the estacao tiradentes metasedimentary sequence probably took place after ca 2088 12 ga although it may be as young as 1990 39 ma according to the u pb detrital zircon geochronology of selected samples diamictite ferruginous quartzite and carbonaceous phyllite the tiradentes rocks show three tdm model ages ranging between 2 4 and 2 3 ga indicating short crustal residence for the magmatic protoliths this inference is also supported by the nd 2 2 ga values 1 1 to 2 3 whereas the respective 87sr 86sr t ratios 0 7018 0 7032 are consistent with the involvement of low rb sr material in the magma genesis such as a paleoproterozoic oceanic lithosphere from a tectonic perspective the above data are consistent with the idea that a paleoproterozoic seafloor developed outboard of the archean siderian passive margin setting minas basin of the sao francisco proto craton the 2 23 2 21 ga volcanic subvolcanic rocks of the tiradentes and serrinha suites were associated with a rhyacian oceanic arc the estacao tiradentes metasedimentary sequence turbidites and distal phyllites probably formed in back arc setting concomitant with the tiradentes suite as indicated by detrital zircon geochronology

texture development during progressive deformation of hematite aggregates constraints from vpsc models and naturally deformed iron oxides from minas gerais brazil abstract we show that naturally deformed hematite from the quadrilatero ferrifero province minas gerais brazil develops cpos by dislocation creep strongly influenced by basal plane parallel glide even when this is not the favored slip system characterization of microstructure and texture particularly intragranular misorientations of naturally deformed hematite aggregates by ebsd allowed us to determine the importance of different slip systems and confirm dislocation creep as the dominant deformation mechanism viscoplastic self consistent vpsc models were constructed to constrain the slip systems required to operate for the observed cpo to develop and its rheological implications changes in the crss ratio of hematite prism and basal slip systems and deformation regime lead to the development of distinct patterns of hematite crystallographic orientations the basal slip dominated simple shear model is the only one that can develop quasi single crystal cpo of the kind observed in highly deformed rocks from quadrilatero ferrifero comparison between naturally deformed hematite aggregates and vpsc models shows that cpo development of hematite is strongly influenced by a highly viscoplastic anisotropy through dislocation creep on hematite basal plane nonetheless our results demonstrate that even the unfavorable slip systems should be regarded when the bulk rheology of mineral aggregates is evaluated

geochronology and thermochronology of gold mineralization in the turmalina deposit ne of the quadrilatero ferrifero region brazil abstract the turmalina gold deposit comprises three epigenetic domains whose development is related to the propagation reactivation events of the nw se pitangui shear zone psz the lodes are hosted in a late archaean sedimentary sequence on top of a strongly deformed mafic dominated metavolcanic pile metamorphosed under 3 5 4 kbar and 540 610 c the association forms the upper part of a lithostratigraphic succession pitangui group that overlies an older ttg gneissic basement according to field evidence and petrographic observations the ore forming process is polyphasic starting at the time when the psz crossed the metamorphic quartz staurolite biotite almadine hornblende isograde the main evolving stages however mostly took place throughout the metamorphic retrogression path fluid inclusion microthermometry also shows that metamorphogenic aqueous carbonic solutions initially with 16 20 eq wt nacl and circulating at approximately 4 kbar and 550 c were subjected to repeated boiling and mixing with cooler aqueous solutions at approximately 1 2 kb and 300 350 c these boiling events which were triggered by depressurization were contemporaneous with gold and later sulfide deposition preceding a late stage of hydrothermal activity under lower pt conditions

major brazilian gold deposits 1982 to 1999 brazil has been a major but intermittent producer of gold since its discovery in 1500 brazil led the world in gold production during the 18th and early 19th centuries from the late 19th century to the late 20th century total mining company and garimpeiro production was small and relatively constant at about 5 to 8 t year the discovery of alluvial deposits in the amazon by garimpeiros in the 1970s and the opening of eight mines by mining companies from 1983 to 1990 fueled a major boom in brazil s gold production exceeding 100 t year in 1988 and 1989 however garimpeiro alluvial production decreased rapidly in the 1990s to about 10 t year by 1999 company production increased about tenfold from about 4 t year in 1982 to 40 t in 1992 production from 1992 to the present remained relatively stable even though several mines were closed or were in the process of closing and no new major mines were put into production during that period

3230 3200 ma post orogenic extension and mid crustal magmatism along the southeastern margin of the barberton greenstone belt south africa abstract the barberton granitoid greenstone terrain south africa preserves a complex and protracted evolution involving several events of magmatism and terrain accretion along convergent tectonic boundaries recent studies propose that the main period of tectonic accretion and arc related magmatism is linked to a system of divergent subduction zones above which voluminous ttg magmas were emplaced between ca 3236 and 3227 ma our structural and la icp ms u pb geochronology study along the southeastern margin of the barberton greenstone belt bgb ties the waning stages of this ttg magmatism to a short ca

trace elements and isotope geochemistry c o fe cr of the caue iron formation quadrilatero ferrifero brazil evidence for widespread microbial dissimilatory iron reduction at the archean paleoproterozoic transition abstract the banded iron formation bifs of the caue formation quadrilatero ferrifero brazil are part of the great volume of bifs deposited worldwide around 2 45 ga samples of carbonate ankerite dolomite bif from this unit were collected from a drill core ca 600 m deep in the alegria region geochemical data suggest low clastic contamination y ho 37 7 and seawater like signatures with positive la and y anomalies no true ce anomalies and mean eu eu 1 62 u th shows a sharp increasing downwards pattern up to ca 15 below the 535 m mark values of 13c and 18o are all negative reaching a minimum of 10 0 and 24 6 respectively in the 540 560 m interval values of 56fe are mostly positive between 0 5 and 1 3 the higher values are attained in the same interval values of authigenic 53cr range between 0 01 and 0 26 with an increasing downwards pattern which mirrors the u th variation a possible explanation for both the low 13c and the high 56fe is the activity of microorganisms in the dissimilatory iron reduction dir of the precursor ferric oxides thus we put forward a model for deposition of the caue bifs that starts with the generation of fe ii aq through hydrothermal input to the deeper basin which upon reaching a chemocline in the platform border oxidizes to fe iii and settles as iron oxy hydroxides to the basin floor when reaching the sediment layer fe iii is then reduced by microbial activity to generate magnetite and iron rich carbonates with negative 13c and positive 56fe an important issue is how u vi and cr vi were mobilized to the basin to form the positive u th and 53cr excursions these could reflect oxidative continental weathering but an alternative possibility is that those elements became available through local oxygenation in continental microenvironments which would better explain the lack of ce redox cycling in either case our results together with a growing body of data for bif sequences of similar age around the world e g kuruman south africa and hamersley australia suggest that microbial mediated dir was a common and widespread mechanism for the genesis of bifs at the archean paleoproterozoic transition

induced seismicity during the stimulation of a geothermal hfr reservoir in the cooper basin australia a long term fluid injection experiment was performed in the cooper basin australia in 2003 to stimulate a geothermal reservoir more than 20 000 m 3 of water were injected into the granitic crust at 4250 m depth during reservoir stimulation about 27 000 induced seismic events were detected by a local eight station seismic monitoring system deployed in nearby boreholes hypocenter locations for 11 068 events were determined by using an averaged velocity model that was calibrated by associating early events with the injection point the spatial hypocenter distribution forms a nearly subhorizontal structure with a lateral extension of 2 km 1 5 km and an apparent thickness of approximately 150 200 m which is in the order of the hypocenter location confidence limits the hypocenter distribution exhibits a high degree of spatiotemporal ordering with the seismic activity systematically migrating away from the injection well with increasing time previously activated regions become seismically quiet indicating relaxation processes high resolution relative hypocenter locations determined for clusters of similar events locally reduce the apparent thickness of the structure to the level of a few tens of meters indicating that the reservoir is dominated by a single fracture zone only consistent with these findings a subsequently drilled well intersects a dominating high permeable fracture within 15 m of the predicted intersection depth based on drilling and logging information the fracture zone is interpreted as a preexisting possibly tectonically formed feature that partly sheared during stimulation triggering of the induced seismicity is found to be predominantly controlled by the increase of fluid pressure implicating a local reduction of the effective normal stress resolved on the fracture plane additionally perturbations of the stress field caused by the largest magnitude events may trigger seismicity aftershocks on a local short ranging scale

using oxygen isotope chemistry to track hydrothermal processes and fluid sources in itabirite hosted iron ore deposits in the quadrilatero ferrifero minas gerais brazil the quadrilatero ferrifero brazil is presently the largest accumulation of single itabirite hosted iron ore bodies worldwide detailed petrography of selected hypogene high grade iron ore bodies at e g the aguas claras conceicao pau branco and pico deposits revealed different iron oxide generations from oldest to youngest magnetite martite hematite pseudomorph after magnetite granoblastic recrystallised microplaty fine grained 100 m hematite laser fluorination oxygen isotope analyses of selected iron ore species showed that the 18o composition of ore hosted martite ranges between 4 4 and 0 9 and is up to 11 depleted in 18o relative to hematite of the host itabirite during the modification of iron ore and the formation of new iron oxide generations e g microplaty and specular hematite an increase of up to 8 in 18o values is recorded calculated 18o values of hydrothermal fluids in equilibrium with the iron oxide species indicate 1 the involvement of isotopically light fluids e g meteoric water or brines during the upgrade from itabirite hosted hematite to high grade iron ore hosted martite and 2 a minor positive shift in 18ofluid values from martite to specular hematite as result of modified meteoric water or brines with slightly elevated 18o values and or the infiltration of small volumes of isotopically heavy metamorphic and or magmatic fluids into the iron ore system the circulation of large fluid volumes that cause the systematic decrease of 18o 16o ratios from itabirite to high grade iron ore requires the presence of e g extensive faults and or large scale folds

the shear zone related gold mineralization at the turmalina deposit quadrilatero ferrifero brazil structural evolution and the two stages of mineralization turmalina is an important orogenic gold deposit located in the nw region of the quadrilatero ferrifero the deposit is hosted in an archean greenstone belt composed of ortho amphibolites and pelites with interleaved tuffs metamorphosed under amphibolite facies conditions and intruded by a granite stock the orebodies are controlled by wnw ese trending shear zones associated with hydrothermal alteration three deformation events are recognized in the turmalina gold deposit d1 and d2 are the result of a progressive archean deformation under ductile conditions between 2749 7 and 2664 35 ma d3 is characterized by a transpressional event under ductile brittle conditions with the age still unclear the three generations of garnet observed show that grt1 blastesis is pre to syn d1 and grt2 growth during the late to post deformation stages of the d2 event the initial temperature grt1 core is around 548 600 c whereas during late d2 the temperatures reached 633 c metamorphic peak grt2 rim likely as a result of granite intrusion the grt3 resulted from re equilibration under retrograde conditions two gold bearing sulfide stages were identified pyrrhotite arsenopyrite lollingite chalcopyrite gold stage i precipitated below a metamorphic peak temperature of 598 19 c associated with s1 foliation d1 and pyrrhotite pyrite arsenopyrite chalcopyrite gold stage ii is located commonly along v3 quartz carbonate veinlets with a temperature range between 442 9 and 510 30 c we suggest that the granite intrusion imposed an additional thermal effect that promoted further dehydration of country rocks the au derived mainly from a metamorphic fluid source but potentially mixed with magmatic fluids from the granite

paleoecology of selected conodontophorid species from the cobbs arm formation middle ordovician new world island north central newfoundland the upper chazyan llandeilian cobbs arm formation of new world island north central newfoundland consists of pure limestones and limestones with varying amounts of volcaniclastic admixture inte

braidplain sedimentation in a proterozoic rift basin the sao joao da chapada formation southeastern brazil abstract the sao joao da chapada formation 100 200 m thick is the basal unit of the middle proterozoic espinhaco supergroup this up to 3000 m thick and slightly metamorphosed sedimentary succession is exposed in the central part of the southern serra do espinhaco southeastern brazil detailed lithofacies studies were carried out to characterize the sedimentation and paleogeography of the sao joao da chapada formation six lithofacies are present horizontal to low angle cross stratified sandstones lithofacies sh sl trough cross stratified sandstones lithofacies st planar cross stratified sandstones lithofacies sp ripple cross laminated sandstones lithofacies sr massive to horizontally stratified conglomerates lithofacies gm and fine grained rocks lithofacies f these are usually arranged in fining and thinning upward successions characterizing waning flood cycles which are always bounded by erosional surfaces no complete cycles have been found however and the cycles are normally composed only of lithofacies sh sl and st the other lithofacies are rare the observed sedimentary structures and their arrangement the geometry of the bodies mainly sheet like to slightly lenticular the low relief morphology of the erosion surfaces bounding the waning flood cycles the coarse and poorly sorted texture of the sediments and the measured paleocurrent pattern unidirectional with little dispersion throughout the study area point to deposition on a wide ephemeral braidplain characterized throughout by relative uniformity of facies and paleocurrent directions water depths on this subsiding braidplain were maintained by high discharge streams debouching onto it from the upland catchment areas as these rivers spread onto the depositional plain the changes in gradient and substratum transformed them into channels of remarkable width and very large width depth ratios these rivers ran in braided tracts and floods were temporarily confined within banks in these tracts channel switching events associated with tilting and subsidence of the basin floor resulted in the accumulation of extensive sandstone sheets interfingering laterally with one another and succeeding one another vertically which built up a braidplain succession extending up to 35 km transverse to the paleoslope the sao joao da chapada braidplain flanked the west margin of a n s elongated rift basin

the platiniferous au pd belt of minas gerais brazil and genesis of its botryoidal pt pd aggregates we delineate an approximately 240 km long north trending au pd pt belt in minas gerais brazil which hosts hydrothermal pd pt bearing gold mineralization in its southern part quadrilatero ferrifero and itabira district and alluvial pd pt au mineralization in its northern part southern serra do espinhaco the hydrothermal pd pt bearing gold mineralization locally known as jacutinga occurs as late orogenic quartz hematite talc kaolinite veins which truncate the brasiliano regional tectonic foliation of the itabiritic host rocks of the paleoproterozoic itabira iron formation the alluvial pd pt au mineralization occurs as botryoidal pd pt nuggets and delicate palladiferous gold aggregates in clastic and organic rich valley infill on quartzite and hematitic rocks of the paleo to mesoproterozoic espinhaco supergroup the au pd pt belt follows the regional trend of major thrust faults related to the 0 6 ga brasiliano orogeny and the hydrothermal jacutinga mineralization could be regarded as an oxidized variant of the orogenic gold deposit spectrum secondary botryoidal and arborescent pt pd aggegates and primary detrital au pd grains are peculiar to the alluvial platiniferous deposits of the northern part of the belt in the serra do espinhaco we suggest that their source was jacutinga style vein mineralization as deduced from abundant detrital hematite and rutile and the discovery of detrital hongshiite a pt cu alloy that forms bonanza pockets within jacutinga at itabira reconnaissance laser ablation inductively coupled plasma mass spectrometry analyses for trace metals in the botryoidal pt pd grains show that they have elevated tl and se contents of 0 9 to 354 and 54 to 2 700 ppm respectively these elements correlate positively with a tl se ratio of about 0 08 a value that contrasts with tl se 1 for hongshiite from the hydrothermal jacutinga system at itabira the low and roughly constant tl se ratios of the alluvial samples are close to the average value of 0 12 for fluvial waters we suggest that the botryoidal pt pd aggregates grew in situ by electrochemical metal accretion from dilute solutions carrying pt and pd from the dissolution of detrital pt pd minerals from jacutinga the connection between jacutinga and platiniferous alluvia indicates that there is a prospective trend of about 240 km for pd pt bearing gold mineralization reaching from ouro preto to diamantina the very high oxidation state required for efficient low temperature aqueous mobilization of pt pd and au was apparently controlled by the quartzitic and hematitic rocks in the serra do espinhaco but not in the southern belt where greenstone rocks buffered fluids to moderate ph and lower oxidation states

brazil s premier gold province part ii geology and genesis of gold deposits in the archean rio das velhas greenstone belt quadrilatero ferrifero orogenic gold deposits are hosted by rocks of the archean rio das velhas greenstone belt in the quadrilatero ferrifero region minas gerais state brazil one of the major gold provinces in the world the gold deposits occur at the base of the mafic ultramafic succession with the most important orebodies controlled by e w striking strike slip faults the main mineralization styles are 1 structurally controlled sulfide replacement zones in banded iron formation bif 2 disseminated sulfide minerals and gold in hydrothermally altered rocks along shear zones and 3 auriferous quartz carbonate sulfide veins and veinlets in mafic ultramafic and felsic volcanic rocks and also in clastic sedimentary rocks the most common host rocks for ore are metamorphosed oxide and carbonate facies banded iron iron rich metachert formations e g the cuiaba sao bento and raposos deposits and the lapa seca unit which is a local term for intensely carbonatized rock e g the giant morro velho mine with 450 t of contained gold metabasalts host most of the remaining gold deposits mineralogical characteristics and fluid inclusion studies suggest variations in the h2o co2 ratio of a low salinity near neutral reducing sulfur bearing ore fluid the presence of abundant ch4 rich inclusions is related to reduction of the original h2o co2 fluid via interaction with carbonaceous matter in the wallrocks oxygen fugacity was close to that of graphite saturation with variations likely to have been influenced by reaction with the carbonaceous matter carbon rich phyllites and schists which commonly bound ore bearing horizons seem to have played both a physical and chemical role in localizing hydrothermal mineral deposition microtextural studies indicate that gold deposition was mainly related to desulfidation reactions and was paragenetically coeval with precipitation of arsenic rich iron sulfide minerals carbon isotope data are compatible with dissolution of co2 from pre existing mantle derived carbonation zones and indicate fluids of metamorphic origin a major episode of hydrothermal fluid introduction into different rock types caused epigenetic gold formation and wallrock alteration at about 300 to 400 c during the late stages of regional deformation and metamorphism of the greenstone belt the age of gold mineralization is constrained to be younger than 2 698 and perhaps closer to 2 670 ma

using the migration of the induced seismicity as a constraint for fractured hot dry rock reservoir modelling the challenge for hot dry rock technology is to develop a heat exchanger in deep hot rock masses and to circulate a fluid in order to extract its energy to be used at the ground surface the present day strategy is to take advantage of natural fractures that pre exist at these depths and to improve their hydraulic properties the extension of the area with enhanced properties must then be evaluated so that the best locations for further boreholes can be proposed to date this development procedure is based on fluid injection at high rates forcing hydro mechanical interactions to take place along pressurised fractures these pore pressure driven mechanisms are accompanied by seismic activity assuming the validity of poro elastic theory in the fractured host rock some authors have derived the virgin hydraulic diffusivity of the fractured reservoir from the analysis of the spatio temporal growth of the induced seismicity the present work is aimed at verifying this approach using a numerical code to solve directly for hydro mechanical interactions in random fracture networks our approach assumes that the seismic activity is controlled by a coulomb shear criterion and we show how the interpretation of spreading rate of the modelled shear activity in a given network coincides with the upscaled virgin hydraulic diffusivity of the same fracture network calculated from an independent numerical procedure at the reservoir scale therefore it is shown that the direct analysis of the seismicity migration is appropriate to give reliable estimates of virgin hydraulic and mechanic parameters these parameters can then be used for performing any further quantitative analysis of a reservoir open to the far field this is of importance as fluid mass balance in multi well exploitation systems will include the contribution from areas surrounding the stimulated zone

the tectonic evolution of the quadrilatero ferrifero minas gerais brazil abstract the structural framework of the quadrilatero ferrifero is the result of two main deformational events the first is an extensional event of palaeoproterozoic age between 2 100 and 1700 ma the transamazonian event which resulted in the formation of archaean granite gneiss domes and the nucleation of regional synclines in the overlying rio das velhas and minas supergroups strata such regional synclines are continuous with each other and are bordered by structural granite gneiss highs at the contact between these units ductile brittle to ductile extensional shear zones are developed representing the dislocation surfaces of archaean blocks this tectonism is probably related to the evolution of a metamorphic core complex formed in the hinterland to the west of a transamazonian collision zone which structured the paramirim craton the second event is compressive and associated with the closure of the pan african brasiliano proto ocean 650 500 ma situated to the east of quadrilatero ferrffero qf during this event a west verging fold and thrust belt ftb developed causing inversion amplification translation and rotation of the basinal synclines the ftb affected mainly the eastern portion of qf and obliterated many of the tectonic features of the extensional event

deciphering the geology of some darriwilian sandbian ordovician ghost formations in the uk and north america using olistoliths in marine debris flows olistostromes with calcareous olistoliths are rare components in the ordovician successions in nw europe and north america having been described from only a small number of localities one of the best exposed but least known is in the garn formation in coastal outcrops in anglesey in northwestern wales here in the graptolite bearing shales of the garn formation there are numerous limestone olistoliths that are derived from an otherwise unknown ghost formation whose original depositional site remains an enigma these olistoliths contain a baltoscandian type of conodont fauna that is otherwise unknown in wales and england it represents the baltoniodus variabilis subzone of the amorphognathus tvaerensis zone similar but slightly older conodont faunas are recorded from olistoliths in the tweeddale member of the shinnel formation in southern scotland and in probable olistoliths of the cobbs arm limestone in northeasternmost newfoundland approximately coeval conodont faunas are present in calcareous olistoliths in the woods hollow shale of west texas and the womble shale in the ouachita mountains of arkansas usa lithological and conodont evidence indicates that the calcareous olistoliths were derived from carbonate sediments deposited in relatively shallow water it is concluded that the study of ghost formation olistoliths may provide otherwise unavailable but important data bearing on the marine depositional history of a particular region

proterozoic crustal extension and the generation of dome and keel structure in an archaean granite greenstone terrane archaean granite greenstone terranes1 3 in which narrow belts of greenstone ultramafic and mafic volcanics and overlying sedimentary rocks occur in association with broad provinces of granite granitic igneous rocks gneiss and migmatite have long puzzled geologists because of the lack of any clear modern analogues not only is there uncertainty about how the greenstone formed in the first place4 6 but there is continuing debate about how and when the terranes developed their distinctive structural and metamorphic architecture many granite greenstone terranes display a dome and keel geometry in which belts of supracrustal volcanic and sedimentary rocks occur as structural troughs wedged between dome shaped bodies of the granite gneiss migmatite complex a metamorphic aureole typically occurs in the supracrustals adjacent to their contact with the domes and the contact itself is a shear zone here we report the results of field studies of a granite greenstone terrane in brazil showing that this architecture originated during the proterozoic more than 500 million years after extrusion of the greenstone we suggest that the thermal regime and deformation kinematics necessary to create this architecture could be generated during an episode of crustal extension when hot basement rocks were transported upwards along a transcrustal normal fault system to the base of the supracrustals

a review of the geochronology of the amazonian craton tectonic implications abstract the available geochronological data for the amazonian craton permit delineation of its main age provinces and their respective tectonic development demarcation of the boundaries of each age province is primarily based on rb sr whole rock isochrons supported by k ar determinations on minerals some u pb results on zircons pb pb whole rock isochrons and sm nd model ages complement these data in key areas at the center of the amazonian craton lies the archean age central amazonian province bounded in its northern part by the maroni itacaiunas mobile belt which developed during early proterozoic times the western and southern margins of the archean core are bordered by three tectonic provinces 1 the rio negro juruena magmatic arc 2 the rondonian mobile belt and 3 the sunsas mobile belt each belt borders the western margin of the preceding belt and each records a successively younger middle proterozoic orogeny within the central amazonian province 2 7 ga age determinations have been obtained from the serra dos carajas area although the maroni itacaiunas belt includes several reworked archean nuclei rock units formed during the 2 2 1 9 ga trans amazonian orogeny predominate the isotopic evidence suggests that a mantle derived component constituted a significant portion of the crust accreted at that time the rio negro juruena province is of mid proterozoic age 1 75 1 50 ga and includes almost entirely mantle derived material juxtaposed with the pre existent continental mass along a series of successive magmatic arcs the rondonian and sunsas provinces both exhibit older sialic basements 1 5 ga the former is characterized by important tectonomagmatic events in the 1 45 1 25 ga interval whereas the latter records tectonic reworking at 1 1 0 9 ga finally the amazonian craton is bordered to the east by the late proterozoic paraguai araguaia or araguaia tocantins fold belt

selecting ground motion models developed for induced seismicity in geothermal areas we present a case study of the ranking and weighting of ground motion prediction equations gmpes for seismic hazard assessment of enhanced geothermal systems egss the study region is cooper basin australia where a hot fractured rock project was established in 2002 we test the applicability of 36 gmpes based on stochastic simulations previously proposed for use at egss each gmpe has a set of corresponding model parameters describing stress drop regional and local near surface attenuation to select suitable gmpes for cooper basin from the full set we applied two methods in the first seismograms recorded on the local monitoring network were spectrally analysed to determine characteristic stress and attenuation parameters in a second approach residual analysis using the log likelihood llh method was used to directly compare recorded and predicted short period response spectral accelerations the resulting ranking was consistent with the models selected based on spectral analysis with the advantage that a transparent weighting approach was available using the llh method region specific estimates of variability were computed with significantly lower values observed compared to previous studies of small earthquakes this was consistent with the limited range of stress drops and attenuation observed from the spectral analysis

sm nd and sr isotope fingerprinting of iron mining tailing deposits spilled from the failed samarco fundao dam 2015 accident at mariana se brazil abstract this work presents the first sm nd and sr isotope tims data on the fine grained fraction 3 of a quartz and iron oxide rich slurry composed of water fine sand and silt that flowed into the gualaxo do norte and carmo rivers and eventually along the whole doce river samples were collected between 5 and 80 km downstream from the fundao dam along the santarem creek and the gualaxo do norte and carmo rivers xrd analyses show that samples are 90 composed of quartz and iron oxides hematite goethite and magnetite with minor phyllosilicates kaolinite and mica owing to sediment transport processes heavy hematite goethite magnetite to light quartz phyllosilicates mineral fractionation is observed as with xrd data major and trace element compositions also closely match those of the original paleoproterozoic caue formation bif iron ore the studied samples yielded nd between 17 0 and 19 7 pptt part per ten thousand with the exception of one sample with nd of 23 2 pptt strontium 87 sr 86 sr ratios show variability between 0 7351 and 0 7729 the t dm ages scatter between 1 94 ga and 2 75 ga with a negative correlation with phyllosilicate contents indicating that the latter has preferential provenance from younger more juvenile sources than the caue fm itabirites this interpretation is corroborated by two component mixing models of nd and sr isotope ratios where the nd sr isotopic signatures of the studied samples are compatible with 98 of the caue fm itabirite component mixed with a neoproterozoic juvenile mafic rocks component such as those from the ophiolitic assemblages of the distal macaubas group or tholeiitic mafic rocks of the pedro lessa suite the reported sm nd and sr isotope compositions closely match those of the original iron ore and contrast with either those of the upriver or downriver precambrian to ordovician bedrock units the firstly defined isotope signature can thus be potentially used as a proxy for ongoing suspended sediment dispersion studies farther downstream along the doce river and in adjacent marine environment

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

using estimated risk to develop stimulation strategies for enhanced geothermal systems enhanced geothermal systems egs are an attractive source of low carbon electricity and heating consequently a number of tests of this technology have been made during the past couple of decades and various projects are being planned or under development egs work by the injection of fluid into deep boreholes to increase permeability and hence allow the circulation and heating of fluid through a geothermal reservoir permeability is irreversibly increased by the generation of microseismicity through the shearing of pre existing fractures or fault segments one aspect of this technology that can cause public concern and consequently could limit the widespread adoption of egs within populated areas is the risk of generating earthquakes that are sufficiently large to be felt or even to cause building damage therefore there is a need to balance stimulation and exploitation of the geothermal reservoir through fluid injection against the pressing requirement to keep the earthquake risk below an acceptable level current strategies to balance these potentially conflicting requirements rely on a traffic light system based on the observed magnitudes of the triggered earthquakes and the measured peak ground velocities from these events in this article we propose an alternative system that uses the actual risk of generating felt or damaging earthquake ground motions at a site of interest e g a nearby town to control the injection rate this risk is computed by combining characteristics of the observed seismicity of the previous 6 h with a potentially site specific ground motion prediction equation to obtain a real time seismic hazard curve this is then convolved with the derivative of a potentially site specific fragility curve based on the relation between computed risk and pre defined acceptable risk thresholds the injection is increased if the risk is below the amber level decreased if the risk is between the amber and red levels or stopped completely if the risk is above the red level based on simulations using a recently developed model of induced seismicity in geothermal systems which is checked here using observations from the basel egs in this article it is shown that the proposed procedure could lead to both acceptable levels of risk and increased permeability

u pb evidence of late archean tectono thermal activity in the southern sao francisco shield brazil the sao francisco craton in eastern brazil is one of the major shield areas in south america in the quadrilatero ferrifero area southern sector of the craton the archean rio das velhas greenstone belt is surrounded by granite gneiss terrane and is overlain by proterozoic sedimentary successions the bonfim metamorphic complex is the only area of the granite gneiss terrane adjacent to the greenstone belt that has been mapped it comprises two gneissic units tonalites and late granitic dykes which crosscut the regional north south shear fabric samples of gneiss two tonalites and a granitic dyke were dated by u pb a zircon core from the alberto flores gneiss yielded a minimum age of 2920 ma whereas the overgrowth is concordant at 2772 6 ma two tonalites from the vicinity of serra da moeda are old and a late dyke yielded an age of these data together with previously published u pb ages show that i greenstone belt volcanism was coeval with granitoid intrusion and with metamorphism of older

u pb shrimp monazite ages of the giant morro velho and cuiaba gold deposits rio das velhas greenstone belt quadrilatero ferrifero minas gerais brazil abstract u pb shrimp results of 2672 14 ma obtained on hydrothermal monazite crystals from ore samples of the giant morro velho and cuiaba archean orogenic deposits represent the first reliable and precise age of gold mineralization associated with the rio das velhas greenstone belt evolution in the quadrilatero ferrifero brazil in the basal nova lima group of the rio das velhas greenstone belt felsic volcanic and volcaniclastic rocks have been dated between 2792 11 and 2751 9 ma coeval with the intrusion of syn tectonic tonalite and granodiorite plutons and also with the metamorphic overprint of older tonalite trondhjemite granodiorite crust since cratonization and stable shelf sedimentation followed intrusion of neoarchean granites at 2612 3 2 ma it is clear that like other granite greenstone terranes in the world gold mineralization is constrained to the latest stages of greenstone evolution

large earthquakes during hydraulic stimulations at the geothermal site of soultz sous forets abstract several deep wells were drilled in the rhine graben soultz sous forets france to evaluate the geothermal hot dry rock potential of a deep fractured granite reservoir three main boreholes which reached about 5 km depth intersected a crystalline basement overlain by 1 4 km of cenozoic and mesozoic sediments stimulations of these three wells were carried out in 2000 for gpk2 2003 for gpk3 and 2004 and 2005 for gpk4 during these stimulations and other hydraulic activities a seismological surface network was installed in order to monitor the seismicity induced by the massive fluid injection here we analyse the seismicity of magnitude larger than or equal to 1 4 which is the lowest magnitude felt by the population based on a spectral analysis of the displacement recorded by a guralp velocimeter at a depth of 200 m we know that the source dimensions range from tens to hundreds of metres we analyse several parts of the reservoir where obvious correlation between the fluid path and tectonic features has been pointed out based on seismological arguments such as location and focal mechanisms we show that this activity is linked with tectonic features or at least with large fractures that control the behaviour of the geothermal reservoir to constrain the hypothesis we study 391 events and focal mechanisms to argue in favour of the existence and stability of tectonic features that can be compared to geological data we show that the largest events recorded on the site occurred after the shut in their spatial distribution appears not to be random within the reservoir and the focal mechanisms of these events also confirm the non randomness of their distribution given their source dimensions the largest events can only occur on large structures such as tectonic ones therefore the behaviour of the reservoir is controlled by these main fractured zones which either lead the fluid or hinder its path

developing an application specific ground motion model for induced seismicity abstract a key element of quantifying both the hazard and risk due to induced earthquakes is a suite of appropriate ground motion prediction equations gmpes that encompass the possible shaking levels due to such events induced earthquakes are likely to be of smaller magnitude and shallower focal depth than the tectonic earthquakes for which most gmpes are derived furthermore whereas gmpes for moderate to large magnitude earthquakes are usually derived to be transportable to different locations and applications taking advantage of the limited regional dependence observed for such events the characteristics of induced earthquakes warrant the development of application specific models a preliminary ground motion model for induced seismicity in the groningen gas field in the netherlands is presented as an illustration of a possible approach to the development of these equations the gmpe is calibrated to local recordings of small magnitude events and captures the epistemic uncertainty in the extrapolation to larger magnitude considered in the assessment of the resulting hazard and risk

multistage mineralization at the hypozonal sao sebastiao gold deposit pitangui greenstone belt minas gerais brazil abstract sao sebastiao is a recently discovered archean gold deposit at the northwesternmost part of the quadrilatero ferrifero qf southern sao francisco craton brazil the gold ore is strata confined in two levels of banded iron formation bif of the basal stratigraphic unit in the pitangui greenstone belt 2 8 ga pitangui is correlated to the widely known rio das velhas greenstone belt which hosts several orogenic gold deposits such as the world class cuiaba the gold mineralization at sao sebastiao is associated to replacement style sulfidation over magnetite where the main developed phases are pyrrhotite chalcopyrite arsenopyrite pyrite and arsenian pyrite native au occurs as inclusions in pyrrhotite formed at dilatation domains e g breccias fold hinges during compressional events rio das velhas orogeny at 2 8 2 75 ga but mainly as inclusions in late to post kinematic pyrite and arsenopyrite as fracture infills in arsenopyrite and in contact with gangue minerals electron microprobe analyses in arsenopyrite reveal a 465 560 c precipitation temperature interpreted from as atomic percentage and indicate that high temperature fluids were active after the deformational event the temperature increase caused the incorporation of la icp ms detected lattice bound ni and co in arsenian pyrite and arsenopyrite related to the mafic ultramafic signature of boundary rocks high temperature conditions are supported by pyrite and iss intermediate solid solution in the cu fe s system formation at 600 c and melting derived textures resulting from the precipitation of bi and as melts typical of amphibolite facies environments with high fs2 whole rock geochemical assays of drill core samples display two clear trends on ag bi cu vs au binary plots and on principal component analysis results the trends are a product of the heterogeneous spatial distribution of melts and high temperature fluids melting features and superposition of mineralization styles are common in hypozonal gold deposits experiencing higher heat influx at sao sebastiao the thermal effect is likely related to the 2 7 ga granitic intrusions that surround the pitangui belt sao sebastiao shows distinct characteristics than those described for the mesozonal gold deposits in the rio das velhas greenstone belt and may depict the deeper roots of the qf gold system

dome and keel provinces formed during paleoproterozoic orogenic collapse core complexes diapirs or neither examples from the quadrilatero ferrifero and the penokean orogen paleoproterozoic dome and keel provinces in which troughs of deformed and metamorphosed paleoproterozoic supracrustal rocks surround domes of archean basement continue to puzzle geologists in current literature some authors refer to the domes as diapirs implying basement flowed vertically upward and others consider them to be cordilleran type metamorphic core complexes implying that the contact between basement and cover is an upwarped detachment geochronological studies suggest that dome emplacement occurred during extensional collapse of contractional orogens seemingly supporting the core complex analogy however structural analyses in the quadrilatero ferrifero brazil and the penokean orogen michigan demonstrate that the domes resemble diapirs in shape and in terms of surface kinematics the domes differ from diapirs however in that they did not flow penetratively within but were emplaced by movement along steeply dipping shear zones we suggest that contrasts between dome and keel and core complex provinces reflect contrasts between paleoproterozoic and phanerozoic crustal structure specifically during the paleoproterozoic when crust was warmer and supracrustal assemblages denser core complex like detachment faulting and associated plutonism resulted in juxtaposition of hot basement beneath a denser tectonically thickened supracrustal layer the resulting viscosity contrast and density inversion triggered vertical rise of diapir shaped basement domes by slip on steep shear zones

structures indicative of helicoidal flow in a migmatitic diapir bacao complex southeastern brazil abstract the migmatitic bacao complex southeastern brazil shows foliation and lineation patterns that depict an internal flow structure with helicoidal geometry microstructures show that a rheological transition exists from a predominantly magmatic flow represented by isotropic undeformed granitic textures to solid state flow mylonites towards the diapir margins nebulites and contorted migmatites occur in the diapir core whereas the peripheral domains show different kinds of banded migmatites gneisses and high temperature mylonites along the margins low temperature mica rich mylonites and phyllonites occur two major flow domains were recognized the central cell and the peripheral cell each cell is characterized by one or more systems of concentric foliations subcells the mineral lineation produced by the morphological alignment of biotite flakes on the foliation planes defines helicoidal geometries in all subcells this lineation was interpreted as being parallel to the local flow vector the foliation and lineation patterns suggest that the principal ascent trunk occurred along a medium plunge spiral in the central cell a dip flow regime the constant injection of molten material in the central cell pushed the material previously emplaced into the peripheral cell where flow upwards continued through a shallower plunge spiral a strike flow regime

complexity in earthquake sequences controlled by multiscale heterogeneity in fault fracture energy 1 a series of dynamic rupture events under constant tectonic loading is simulated on a fault with multiscale heterogeneity and a stochastic rupture initiation process the fracture energy of the fault plane is assumed to have multiscale heterogeneous distribution using fractal circular patches the stochastic rupture initiation process as a function of the accumulated stress is introduced in order to take account of unknown smaller scale heterogeneity and variability five realizations of a statistical spatial distribution of fracture energy fault heterogeneity maps are tested for the simulations of earthquake sequences during a few seismic cycles the diversity of earthquake sequences is principally controlled by the spatial distribution of the patches the effect of dynamic rupture appears in the residual stress after the characteristic events because of their directivity and this localizes the subsequent sequences although the characteristic earthquakes occur rather regularly in time and similarly in different seismic cycles some irregular behavior is found based on the heterogeneity maps and the randomness of the preceding earthquake sequence leading to a visible anomaly in the seismicity such anomalies are not predicable but understandable through the analysis of the considered earthquakes during the cycle the similarity and the diversity simulated in this study governed by the structure of an inherent distribution of multiscale heterogeneity suggest the importance of the pre existing heterogeneity field along the fault for the appearance of earthquake sequences including those that are characteristic

seleniferous minerals of palladium and platinum from ouro preto bearing mineralisation in brazil abstract selenium is a trace but characteristic element in the bonanza style mineralisation of palladiferous gold that triggered two major gold rushes in brazil minas gerais and serra pelada the auriferous mineralisations of gongo soco and itabira minas gerais and serra pelada para have pd o bearing dark coloured ferruginous coatings on gold ouro preto depending upon the amount of palladium alloyed with gold a number of seleniferous minerals of palladium and platinum can be found chrisstanleyite ag 2 pd 3 se 4 a phase analogous to tischendorfite pd 8 hg 3 se 9 an argentiferous palladseite like phase empirical pd 5 hg sb ag 2 se 6 and pd sb ag hg 5 se 4 at gongo soco palladseite ideally pd 17 se 15 and sudovikovite ptse 2 at itabira and exclusively to serra pelada empirical pd 9 ptse 2 pd 9 se 2 pd 3 se bi and pd hg pb 3 se this mineralogical signature of selenium is typical of hydrothermal vein type deposits formed at relatively low temperatures although the synthetic pd 9 se 2 equivalent is stable at temperatures over 390 c fluid inclusion data suggest lower temperatures of formation

u pb geochronology of archean magmatism and proterozoic metamorphism in the quadrilatero ferrifero southern sao francisco craton brazil the quadrilatero ferrifero is a metallogenic district au fe mn located in the southernmost sao francisco craton in eastern brazil it is composed of archean granitoids gneisses and a greenstone belt sequence rio das velhas supergroup overlain by a proterozoic clastic chemical sedimentary sequence hosting a thick lake superior type banded iron formation minas supergroup a u pb study of the main units leads to the following conclusions granitoid emplacement was dated at 2776 7 6 ma and 2721 3 ma two felsic volcanic rocks from the rio das velhas supergroup yielded ages of 2776 23 10 ma and 2772 6 ma these data indicate that greenstone belt volcanism was coeval with granitoid emplacement a common feature of greenstone belt terrains but so far undocumented in the southern sao francisco shield the younger granitoid intrusion marks the stabilization of the shield in late archean time zircon cores older than 2776 ma together with other data indicate the presence of continental crust 2 88 3 1 ga old in the quadrilatero ferrifero a detrital zircon from the uppermost formation in the minas supergroup sabara formation is concordant at 2125 4 ma and provides the first age constraint on the deposition of the supergroup monazite from two pegmatites and titanite from an amphibolite enclave in gneisses from the bacao complex in central quadrilatero ferrifero yielded ages between 2060 ma and 2030 ma these data may constrain the minimum age for the deposition of the minas supergroup in addition they indicate that the early proterozoic transamazomian orogeny is represented by amphibolite facies metamorphism and partial melting of archean crust

from induced seismicity to direct time dependent seismic hazard the growing installation of industrial facilities for subsurface exploration worldwide requires continuous re nements in understanding both the mechanisms by which seismicity is induced by eld operations and the related seismic hazard particularly in proximity of densely populated areas induced low to moderate magnitude seismicity characterized by high frequency content can be clearly felt by the surrounding inhabitants and in some cases may produce damage in this respect we propose a technique for time dependent probabilistic seismic hazard analysis to be used in geothermal elds as a monitoring tool for the e ects of on going eld operations the technique integrates the observed features of the seismic ity induced by uid injection and extraction with a local ground motion prediction equation the result of the analysis is the time evolving probability of exceedance of peak ground acceleration which can be compared with selected critical values to manage eld operations to evaluate the reliability of the proposed technique we applied it to data collected in the geysers geothermal eld in northern california between 1 september 2007 and 15 november 2010 we show that in the period considered the seismic hazard at the geysers was variable in time and space which is a consequence of the eld operations and the variation of both seismicity rate and b value we conclude that for the exposure period taken into account i e 2 months as a conservative limit peak ground acceleration values corresponding to the lowest probability of exceedance e g 30 must not be exceeded to ensure safe eld operations we suggest testing the proposed technique at other geothermal areas or in regions where seismicity is induced for example by hydrocarbon exploitation or carbon dioxide storage

tectonic implications of precambrian sm nd dates from the southern sao francisco craton and adjacent aracuai and ribeira belts brazil abstract the archean and paleoproterozoic sao francisco craton of eastern brazil is surrounded on all sides by brasiliano pan african orogens the n ne trending orogen that separates the eastern edge of the southern sao francisco craton from the atlantic coast can be divided into the largely greenschist and amphibolite facies aracuai belt on the west and the largely granulite facies ribeira belt on the east a pronounced linear gravity and magnetic anomaly the abre campo discontinuity defines the boundary between these two belts we obtained sm nd mineral ages and whole rock sm nd model ages for garnet bearing metamorphic rocks along an e w transect across the southern sao francisco craton the southern aracuai belt and the ribeira belt at about latitude 20 s a recrystallization age of 2 1 ga from metasediments recrystallized during the development of the classic dome and keel province of the southern sao francisco craton the quadrilatero ferrifero indicates that dome emplacement occurred during the waning stages extensional collapse of the transamazonian collisional orogeny seven mineral ages from the southern aracuai and ribeira belts date the thermal peak of metamorphism at between 538 and 589 ma confirming that these belts were pervasively remobilized during the brasiliano event samples from the aracuai belt yield either archean 2 6 ga or transamazonian 2 1 2 3 ga tdm model ages indicating that the protoliths were either fragments of the sao francisco craton crust or were sediments derived from that craton which presumably had been stretched to form a thinned continental margin during meso and neo proterozoic rifting events notably a mixed meta pelite and metabasite sequence in the southern aracuai belt the dom silverio group has transamazonian ancestry and thus may represent oceanic sediments deposited on or east of this stretched margin and then thrust back onto the continent to mark a collisional suture between the sao francisco block and an archean crustal sliver to the east the rocks of the ribeira belt i e the region east of the abre campo discontinuity have younger model ages tdm 1 6 2 0 ga indicating that this belt was not originally part of the sao francisco craton it may represent an accreted transamazonian terrane therefore the abre campo discontinuity marks an important crustal boundary possibly a suture brasiliano shear zones appear to steepen and root into the abre campo discontinuity suggesting that it formed in brasiliano time

lithofacies associations and structural evolution of the archean rio das velhas greenstone belt quadrilatero ferrifero brazil a review of the setting of gold deposits abstract the rio das velhas greenstone belt is located in the quadrilatero ferrifero region in the southern extremity of the sao francisco craton central southern part of the state of minas gerais se brazil the metavolcano sedimentary rocks of the rio das velhas supergroup in this region are subdivided into the nova lima and maquine groups the former occurs at the base of the sequence and contains the major au deposits of the region new geochronological data along with a review of geochemical data for volcanic and sedimentary rocks suggest at least two generations of greenstone belts dated at 2900 and 2780 ma seven lithofacies associations are identified from bottom to top encompassing 1 mafic ultramafic volcanic 2 volcano chemical sedimentary 3 clastic chemical sedimentary 4 volcaniclastic association with four lithofacies monomictic and polymictic breccias conglomerate graywacke graywacke sandstone graywacke argillite 5 resedimented association including three sequences of graywacke argillite in the north and eastern at greenschist facies and in the south at amphibolite metamorphic facies 6 coastal association with four lithofacies sandstone with medium to large scale cross bedding sandstone with ripple marks sandstone with herringbone cross bedding sandstone siltstone 7 non marine association with the lithofacies conglomerate sandstone coarse grained sandstone fine to medium grained sandstone four generations of structures are recognized the first and second are archean and compressional driven from nne to ssw the third is extensional and attributed to the paleoproterozoic transamazonian orogenic cycle and the fourth is compressional driven from e to w is related to the neoproterozoic brasiliano orogenic cycle gold deposits in the rio das velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along archean thrust shear zones of the second generation of structures sedimentation occurred during four episodes cycle 1 is interpreted to have occurred between 2800 and 2780 ma based on the ages of the mafic and felsic volcanism and comprises predominantly chemical sedimentary rocks intercalated with mafic ultramafic volcanic flows it includes the volcano chemical sedimentary lithofacies association and part of the mafic ultramafic volcanic association the cycle is related to the initial extensional stage of the greenstone belt formation with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains cycle 2 encompasses the clastic chemical sedimentary association and distal turbidites of the resedimented association in the eastern sector of the quadrilatero ferrifero it was deposited in the initial stages of the felsic volcanism cycle 2 includes the coastal and resedimented associations in the southern sector in advanced stages of subduction in this southern sedimentary cycle it is also possible to recognize a stable shelf environment following the felsic volcanism cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations largely in the northern sector of the area the characteristics of both associations indicate a submarine fan environment transitional to non marine successions related to felsic volcanic edifices and related to the formation of island arcs cycle 4 is made up of clastic sedimentary rocks belonging to the non marine lithofacies association they are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles

geochronological framework of the quadrilatero ferrifero with emphasis on the age of gold mineralization hosted in archean greenstone belts abstract archean terrains of the quadrilatero ferrifero comprise a greenstone belt association surrounded by granitoid gneiss complexes mainly composed of banded ttg gneisses whose igneous protoliths are older than 2900 ma this early continental crust was affected by three granitic magmatic episodes during the neoarchean ca 2780 to 2760 ma 2720 to 2700 ma and 2600 ma dating of felsic volcanic and volcaniclastic rocks defines a felsic magmatic event within the greenstone belt association around 2772 ma contemporaneous with emplacement of several of the granitic plutons and constrains a major magmatic and tectonic event in the quadrilatero ferrifero lead isotopic studies of lode gold deposits indicate that the main mineralization episode occurred at about 2800 to 2700 ma proterozoic evolution of the quadrilatero ferrifero comprises deposition of a continental margin succession hosting thick lake superior type banded iron formations at ca 2500 to 2400 ma followed by deposition of syn orogenic successions after 2120 ma the latter is related to the transamazonian orogeny the western part of the quadrilatero ferrifero was also affected by the brasiliano orogeny 600 to 560 ma

geology and paleobiology of islands in the ordovician iapetus ocean review and implications a history of islands in the iapetus ocean and their destruction is recorded by widespread occurrences of lower to upper ordovician volcanic and epiclastic rocks and some limestone through the axial parts of the appalachian caledonide orogen in north america and northwestern europe assemblages of arenigian and llanvirnian benthic shelly fossils chiefly brachiopods from these rocks constitute the celtic biogeographic province that is distinguished by its endemic genera and by some that are older than in other biogeographic provinces some that are younger than elsewhere and others that have not hitherto been found together evidence from these fossils and from the geology of the rocks pertaining to them suggests that north american terranes that included islands of arenigian llanvirnian age lay at middle to high latitudes both in mid ocean and within the armorican continental margin whereas coeval rocks in west central ireland and norway that have toquima table head shelly fossils were originally part of the laurentian continental margin eradication of the celtic assemblage and the reduction of ordovician faunal provinciality in the appalachian caledonide region coincide with llanvirnian obduction subduction of hundreds of kilometres of iapetus ocean crust late llanvirnian and younger ordovician rocks in the remnant basin including some of peri insular origin have brachiopods of scoto appalachian affinities no discrete islands of ashgillian age have been recognized although fossiliferous volcaniclastic and polymictic conglomerates indicate intermontane basins in some places shelly fossils in these rocks and those in the siltstones and limestones from the gaspe peninsula are like those of ashgillian age elsewhere

the 2 ga peraluminous magmatism of the jacobina contendas mirante belts bahia brazil geologic and isotopic constraints on the sources abstract a 500 km north south alignment of granitic intrusions cross cuts the central part of the sao francisco craton these late to post tectonic granitic bodies are emplaced between two archean blocks jequie and gaviao blocks and cross cut the contendas mirante volcano sedimentary sequence and the metasedimentary rocks of the serra de jacobina they are two mica or muscovite garnet bearing peraluminous granites rb sr systematics show that these granites were emplaced during the transamazonian orogeny 1 9 ga the high initial 87 sr 86 sr ratios 0 706 0 748 and the very low nd t 13 to 5 indicate a crustal origin possible sources are 1 the jequie block 2 the contendas mirante sequence and 3 the gaviao block including some early archean domes trondhjemitic tonalitic granodioritic ttg boa vista type which were tectonically emplaced within the volcano sedimentary sequence the currently available rb sr and sm nd data are not consistent with reworking of the 3 5 ga ttg the combination of field and isotopic constraints preclude the jequie block and the contendas mirante sequence and favour instead the gaviao block medium grade terrains as the source for the peraluminous granitic line the available data suggest that a continent continent collision occurred during the transamazonian orogeny which followed subduction obduction marked by volcanism arc tholeiitic calc alkaline and shoshonitic and related plutonism

iron quadrangle stream sediments brazil geochemical maps and reference values geochemical mapping and the determination of reference values receive increasing attention the need to know regional geochemical background data as well as the spatial allocation of areas with anomalous geochemical concentrations and chances to identify their major sources motivate the interest here results from a high density geochemical mapping exercise and derived related reference values are presented 541 stream sediment samples have been collected across the iron quadrangle iq brazil with a density of one sample per 13 km2 geochemical maps were compiled using the inverse distance weighted interpolation method normal concentrations were distinguished from anomalies with the separation by concentration range technique where positive anomalies are defined by the boxplot upper inner fence uif rule more than 70 of the iq show exclusively geogenic concentration ranges for the elements studied about 20 represent positive anomalies likely related to both near surface rock types and to human interference distinct anthropogenic anomalies notably from mining activities explain 5 10 of all samples this first high density sampling in the iq allows delineating the role of lithology on the elemental composition of stream sediments and to establish reference values for the main river basins thus pinpointing localities cities and fluvial basins that are exposed to environmental risks and need to be protected

multi process and multi scale spatial predictive analysis of an orogenic archean gold system rio das velhas greenstone belt brazil abstract there has always been a need for new methodologies and research to improve the decision making process at the early stages of mineral exploration this article presents a novel approach to integrating geodata in support of a mineral systems based spatial analysis of orogenic gold deposits in the rio das velhas greenstone belt rvgb quadrilatero ferrifero province brazil the gold mineralization in the rvgb is spatially associated with thrust faults and shear zones and mainly hosted by iron rich rocks such as mafic ultramafic sequences and banded iron formations to best represent the targeting elements of this mineral system spatially a knowledge based fuzzy logic method was employed to map the expressions of the gold depositional processes at the province 1 500 000 district 1 100 000 and camp 1 50 000 scales at each scale multivariate statistical techniques served to enhance multiple geological geophysical and geochemical datasets and extract from these data spatial proxies of the gold depositional processes the results of this multi scale predictive analysis were as follows the first province scale model m1 identified the entire gold prospective tract and the areas within it that may be of greatest relevance to future exploration the second district scale model m2 identified the different gold camps within the prospective tract and mapped the areas of gold favorability in a more detailed manner the third camp scale model m3 identified areas that based on the current knowledge and distribution of high resolution geodata are the most favorable whilst also being small enough as to permit target testing using conventional mineral exploration tools such as geophysics geochemistry and or drilling the results obtained from our predictive models were validated by comparing them against the known gold occurrences using roc receiver operating characteristics curves and auc area under the curve graphs according to these validations model m1 scored an accuracy of 93 38 whereas models m2 and m3 scored accuracies of 88 31 and 93 38 respectively a key observation made in the course of this study is that the gold prospective area as predicted by models m1 m2 and m3 varies according to the scale of the analysis a novel factor in our approach is that we aimed assess the targeting criteria and spatial datasets that underpin them according to their spatial resolution and presented the results in form of integrated maps in addition the tools developed in this study have the capacity to reduce the cost of direct detection technologies regarding the transition from broad regional to camp scale at the early stages of mineral exploration where the most initial decisions in search and area reduction are critical

probability of inducing given magnitude earthquakes by perturbing finite volumes of rocks 1 fluid induced seismicity results from an activation of finite rock volumes the finiteness of perturbed volumes influences frequency magnitude statistics previously we observed that induced large magnitude events at geothermal and hydrocarbon reservoirs are frequently underrepresented in comparison with the gutenberg richter law this is an indication that the events are more probable on rupture surfaces contained within the stimulated volume here we theoretically and numerically analyze this effect we consider different possible scenarios of event triggering rupture surfaces located completely within or intersecting only the stimulated volume we approximate the stimulated volume by an ellipsoid or cuboid and derive the statistics of induced events from the statistics of random thin flat discs modeling rupture surfaces we derive lower and upper bounds of the probability to induce a given magnitude event the bounds depend strongly on the minimum principal axis of the stimulated volume we compare the bounds with data on seismicity induced by fluid injections in boreholes fitting the bounds to the frequency magnitude distribution provides estimates of a largest expected induced magnitude and a characteristic stress drop in addition to improved estimates of the gutenberg richter a and b parameters the observed frequency magnitude curves seem to follow mainly the lower bound however in some case studies there are individual large magnitude events clearly deviating from this statistic we propose that such events can be interpreted as triggered ones in contrast to the absolute majority of the induced events following the lower bound