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

the origin of hematite in high grade iron ores based on infrared microscopy and fluid inclusion studies the example of the conceicao mine quadrilatero ferrifero brazil petrographic and textural analysis combined with fluid inclusion studies by infrared microscopy of high grade 65 fe hematite ore samples from the conceicao deposit in the northeastern part of the quadrilatero ferrifero brazil indicate a complex process of oxidation and mineralization during two orogenic events each developed under different conditions and involving distinct fluids the earliest mineralization formed massive magnetite rich orebodies under relatively reducing conditions in the early stages of the transamazonian orogeny magnetite was oxidized martitized with the development of porous hematite crystals hematite i possibly during this stage new hematite crystals were also formed from low temperature low to medium salinity fluids as indicated by two phase fluid inclusions the origin of these fluids is still uncertain but tentatively interpreted as being modified surface water the fluids were transported along normal faults and fractures during post tectonic collapse following the transamazonian orogeny 2 1 2 0 ga and creation of the dome and keel structural pattern of the quadrilatero ferrifero these solutions were also likely responsible for the initial oxidation of the iron formations and the development of hematite i subsequent uplifted hot basement rocks or post tectonic plutons were probable heat sources for the regional metamorphism and development of a granoblastic fabric of hematite ii grains in the iron formations and high grade orebodies however the ore was only partially recrystallized as several relics of the early magnetite martite and hematite are still preserved in the granular hematite ii crystals during the brasiliano pan african orogeny 0 8 0 6 ga high salinity fluids with temperatures varying from 120 to a maximum of approximately 350 c penetrated the iron formations along shear zones crystallizing initially tabular and thereafter platy hematite crystals hematite iii and specularite forming schistose orebodies quartz veins that cut across the ore and envelop specularite plates and ore fragments formed from late stage high temperature and low salinity fluids containing co2 these later fluids did not alter the ore each of these stages of mineralization produced orebodies with distinct features recurrent hydrothermal mineralization is thought to have been responsible for the development of giant high grade iron ore deposits in structurally favorable sites fold hinges with enhanced permeability and deep faults able to conduct the fluids to the surface repeatedly over time should be important targets for exploration of new resources

a pressure solution creep law for quartz from indentation experiments 1 indenter experiments have been performed on quartz crystals in order to establish a pressure solution creep law relevant at upper to middle crustal conditions this deformation mechanism contributes to earth s crust geodynamics controlling processes as different as fault permeability strength and stress evolution during interseismic periods or mechanochemical differentiation during diagenesis and metamorphism indenter experiments have been performed at 350 c and 20 120 mpa during months with differential stress varying from 25 to 350 mpa several experimental parameters were varied nature of quartz synthetic or natural nature of fluid manner in which the solid solution solid interface was filled and orientation of the indented surfaces versus quartz crystallographic c axis significant strain rates could only be obtained when using high solubility solutions naoh 1 mol l 1 displacement rates of the indenter were found activated by differential stress with exponential dependence as theoretically predicted the mean thickness of the trapped fluid phase below the indenter was estimated in the range 2 10 nm moreover the development of this trapped fluid phase was relatively fast and allowed fluid penetration into previously dry contact regions by marginal dissolution the indenter displacement rate was driven by differential stress and its kinetics was controlled by diffusion along the trapped fluid and the development of a morphological roughness along the interface conversely marginal strain energy driven dissolution was observed around the indenter and its kinetics was controlled by free surface reaction these experimental results are applied to model the interactions between pressure solution and brittle processes in fault zones providing characteristic time scales for postseismic transitory creep and sealing processes in quartz rich rocks

experimental pressure solution in halite the effect of grain interphase boundary structure to investigate the mechanisms and kinetics of pressure solution we measured deformation at contacts between polished convex lenses of halite and polished flat lenses of either halite or fused silica in saturated brine at 50 2 0 2 c and fluid pressures of 0 1 mpa loads applied using small springs ranged from 0 1 4 2 n mean effective normal stresses within the contact zone ranged from 1 14 mpa the geometry and diametric growth rate of the contact spot neck growth and the rate at which the lenses approached one another convergence were monitored during deformation using transmitted and reflected light photomicrography time dependent convergence did not occur at measurable rates when two halite lenses were pressed together in saturated brine but did occur when halite and fused silica lenses were pressed together in brine convergence rates in the halite silica experiments were 0 01 0 05 m day although the initial deformation during loading involved elastic and plastic processes control experiments without brine showed no time dependent convergence indicating that dislocation creep did not contribute to the observed rates no undercutting or cataclasis was observed in any experiments residual fluid inclusions were formed along grain boundaries between two halite lenses loaded in brine indicating non zero wetting angles

the lomagundi jatuli carbon isotopic event recorded in the marble of the tandilia system basement rio de la plata craton argentina abstract the lomagundi jatuli event corresponds to the most important 13c positive anomaly 5 globally reported in palaeoproterozoic marine carbonates between 2 30 and 2 06 ga in the tandilia system argentina rio de la plata craton this event was recorded in the basement marble of the san miguel area the calcite diopside marble hosted by biotite gneiss and intruded by 2 12 ga garnet leucogranite was metamorphosed in amphibolite facies during the transamazonian cycle paas normalised rare earth elements ree and y for the carbonate rocks are hree enriched and display positive eu and y anomalies typical of primary precipitates from a mixed hydrothermal marine environment carbonate additionally a truly negative ce anomaly for all the samples indicates that the depositional environment was oxidising positive 13c values ranging from 5 90 to 4 30 v pdb and 18o from 17 45 to 13 84 v smow were determined in this marble both gradually decreasing towards the contact with the leucogranites these values indicate that devolatilization reactions took place during the crystallisation of a wollastonite vesuvianite grossular diopside skarn generated by the leucogranite intrusions into the marble 18o values obtained from diopside and calcite crystals in the marble sectors furthest from the contacts with leucogranite allowed a 663 623 c formation temperature to be calculated considering oxygen in a calcite diopside geothermometric pair these temperatures are consistent with the metamorphic degree amphibolite facies reached in this portion of the basement although the san miguel marble shows petrographic and mineralogical evidence of regional and contact metamorphism important geochemical and isotopic characteristics together with its estimated palaeoproterozoic age indicate that the marble protolith was a marine carbonate deposited during the lomagundi jatuli event

mixed brittle and viscous strain localization in pelagic sediments seaward of the hikurangi margin new zealand calcareous pelagic input sediments are present at several subduction zones and deform differently to their siliciclastic counterparts we investigate deformation in calcareous pelagic sediments drilled 20 km seaward of the hikurangi megathrust toe at site u1520 during iodp expeditions 372 and 375 clusters of normal faults and subhorizontal stylolites in the sediments indicate both brittle faulting and viscous pressure solution operated at 150 c where frictional possibly seismic slip likely predominates

celebrating the centenary of the geology of central minas gerais brazil an insight from the sitio largo amphibolite abstractone century ago in 1915 harder and chamberlin established the stratigraphy on which the geology of the quadrilatero ferrifero of minas gerais is based one of their key observations was the occurrence of metavolcanic rocks in spatial association with the itabira iron formation an observation that has been ignored by most authors crucial for the understanding of the geology of the quadrilatero ferrifero but equally ignored is a 400 m thick unit of amphibolite that rests immediately on the itabira iron formation also known as the caue itabirite here we revitalize the sitio largo amphibolite by indicating its basaltic affiliation we present new results of u pb dating of zircon and rutile that show that the amphibolite protolith which covered an erosional surface on the caue itabirite is ca 2 18 ga old and underwent metamorphic overprint at ca 500 ma

two billion years of evolution enclosed in hydrothermal rutile recycling of the sao francisco craton crust and constraints on gold remobilisation processes abstract the quadrilatero ferrifero in the sao francisco craton brazil is known for its world class gold deposits hosted by the rio das velhas greenstone belt rdv as in other greenstone belts gold is also concentrated in late archean early paleoproterozoic supracrustal units in this case within the moeda formation this unit has been compared to the witwatersrand basin where there has been a long term debate over the nature of the gold and whether it is detrital placer hydrothermal or hybrid modified placer presently links between gold in the moeda formation and rdv are tenuous to enlighten this area of ambiguity a new approach using chemical isotopic and mineral inclusion data from detrital rutile found in the gold bearing horizons of the moeda formation is presented grain textures and mineral inclusions have contributed to establishing a primary hydrothermal origin for the rutile related to as fe cu sb pb w rich mineralising fluids fe nb and v indicate that the likely source of the gold related rutiles is the archean banded iron formations within the rdv where most of the lode gold occurs average nb ta values of these rutiles are lower than average continental crust ratios which suggests fractionation driven by fluid precipitation and or competing mineral phases u pb data yield primary crystallization ages older than deposition ages 2 58 ga supporting the detrital nature of these rutiles rutiles record either resetting or new growth at 2 1 ga and at ca 500 ma during which gold was remobilised hence this study supports a modified placer model for the gold bearing horizons of the moeda formation these fluid assisted gold remobilisation events can be found in other correlated paleoproterozoic gold bearing horizon units that represent intracratonic basins related to the rifting of the kenorland supercontinent this study reinforces the suitability of rutile and mineral inclusions as a powerful tool to elucidate processes and provenance

experimental compaction of quartz sand at low effective stress and temperature conditions experiments have been carried out on dry and fluid saturated quartz sands to investigate the behaviour during compaction under diagenetic conditions and to evaluate the importance of stress induced solution transfer 9pressure solution9 as a compaction mechanism the experiments were performed at temperatures t in the range 150 350 c applied effective stresses e up to 20 7 mpa and pore fluid pressures p 1 of 12 5 and 15 5 mpa using material with a grain size d in the range 20 100 m and an initial porosity of 45 52 dry quartz sands underwent significant compac tion during the loading stage but showed very little compaction creep once full load was achieved i e essentially time independent compaction in contrast fluid saturated material at constant applied effective stress showed substantial time dependent compaction i e creep with increasing temperature there is a decreasing number of grains in the wet compacted sand which show intragranular cracks and an increasing number of grains which show dissolution features at contacts with adjacent grains in addition there is a decreasing dependence of the volumetric strain rate on e with both increasing volumetric strain e v and increasing e and a decreasing dependence of on e v with increasing temperature these observations suggest that for wet quartz sand a gradual change might occur from compaction creep controlled by time dependent microcracking at t 250 300 c to compaction creep controlled by stress induced intergranular solution transfer at t 300 350 c

global nature of the paleoproterozoic lomagundi carbon isotope excursion a review of occurrences in brazil india and uruguay abstract positive carbon isotope excursion is reported from paleoproterozoic carbonates of the aravalli supergroup northwestern india the minas supergroup brazil and new sections of the paso severino formation uruguay the 2 42 ga gandarela formation minas gerais brazil contains red carbonate facies bif grading into dolostones and limestones and yielding 13 c values ranging from 1 6 to 0 4 v pdb the positive c isotope excursion up to 11 v pdb in marine shallow water carbonates in india and brazil jhamarkotra formation in northwestern india and cercadinho and fecho do funil formations in minas gerais state brazil is comparable to that observed in 2 22 2 1 ga carbonate successions worldwide that were deposited during the lomagundi excursion in uruguay 13 c values up to 11 6 v pdb in the deep water paso severino formation of the piedra alta terrane are compatible with deposition at ca 2 15 ga as indicated by the 2146 7 ma u pb age of dacites occurring at the top of the unit negative 13 c values are also present in carbonates of the paso severino formation but an origin related to organic matter remineralization cannot be ruled out thin carbonate beds in the rio itapicuru greenstone belt bahia state brazil are associated as in the paso severino formation with deep water black shales and have carbon isotope values up to 9 v pdb high metamorphic grade carbonates of the jacurici terrane in the medrado ipueira area bahia brazil have carbon isotope values up to 6 9 v pdb consistent with their minimum age of 2085 5 ma inferred from the intrusive contact with and the age of the medrado norite no evidence was found in india brazil or uruguay for paleoproterozoic glacial events recognized in the 2 45 2 22 ga sedimentary successions worldwide unconformities between the gandarela and cercadinho formations in brazil and the banded gneissic complex and the lower aravalli supergroup in india might explain the absence of glacial record compositional and isotopic data presented here for studied paleoproterozoic carbonate successions allow their integration into the global record of the paleoproterozoic evolution as well as correlation with other successions of similar age the study highlights the global nature of the lomagundi excursion furthermore it indicates that the lomagundi excursion is recorded in both shallow water aravalli and minas supergroups and deep water carbonates paso severino formation and rio itapicuru greenstone belt negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow water open marine and isolated basins

shear zone related gold mineralization in quartz carbonate veins from metamafic rocks of the bif hosted world class cuiaba deposit rio das velhas greenstone belt quadrilatero ferrifero brazil vein classification and structural control abstract the cuiaba deposit located in the quadrilatero ferriferro district qf in the southeastern part of the minas gerais state is the largest underground mine in brazil 5 78 moz resources of 10 26 g t au 1 18 moz of ore reserves and 6 moz of overall production this world class orogenic gold deposit is part of the rio das velhas greenstone belt a typical association of mafic volcanic rocks banded iron formation bif carbonaceous and micaceous phyllites metamorphosed to the greenschist facies conditions historically economic gold production at cuiaba derives from bif hosted replacement style sulfide pyrite and pyrrhotite ores more recently economically significant shear zone related gold mineralization in quartz carbonate veins systems hosted in metamafic rocks were identified they are associated with distal chlorite intermediate carbonate and proximal sericite dominant hydrothermal alteration zones structures related to the gold bearing quartz carbonate veins were generated during a single progressive ductile to brittle deformational event d1 with the compositional banding sb and the flexural foliation sf created in the early stages of deformation as deformation progressed shear zones isoclinal folds the mylonitic foliation sm the mineral lineation lmin and the intersection lineation lint were formed simultaneously to the main stage of hydrothermal fluid input flanking folds and flanking shear bands represent the late stage of deformation and the final hydrothermal fluid pulse four quartz carbonate vein systems are identified v1 v2 v3 and v4 the v1 mineralized system is an sm controlled fault fill type associated with the principal gold bearing fluid input stage the v2 veins are also mineralized controlled by the sf foliation and subdivided in three sub types according to internal crystal orientation v2a extensional v2b oblique extensional and v2c a fault fill vein the extensional array v3 veins are also controlled by the sm foliation whereas the extensional breccia style v4 system has veins associated with flanking structures the structural analysis of veins indicates a shortening vector rotation during d1 accompanied by hydrothermal fluid input since all vein systems are related to the d1 event each one defining an incremental deformational stage they provide information about the hydrothermal fluid evolution at cuiaba a proposed evolutionary model for the vein systems suggest that the ore forming fluid responsible for gold mineralization was channeled along shear zones and dispersed out into the sb and sf structures the internal vein features and the trapping temperatures of quartz from each vein system indicate a progressively decrease in tectonic activity

the origin of coexisting carbonates in banded iron formations a micro mineralogical study of the 2 4 ga itabira group brazil this is the first petrographic and chemical study on the origin of co existing calcite dolomite ankerite and sideroplesite in the 2 4 ga itabira group brazil the itabira group hosts silicic and dolomitic bifs known as quartz itabirites and carbonate itabirites which are composed of micritic quartz and dolomite with inclusions of nano and micro hematite nanometric si bearing ferrihydrite clusters are observed in the micritic dolomite mineralogical and textural features indicate a primary oxidizing depositional environment for the itabira group at 2 4 ga limestones of the itabira group include early diagenetic hematite nodules intergrown with dolomite silica in the ferrihydrite structure increases its stability up to 500 c and the nano and micro hematite included in dolomite and quartz were partially protected from later transformation by being enclosed within the dolomite and quartz crystals positive eu and y anomalies and heavy rare earth element enrichments of all lithofacies along with their low terrestrial element abundance excluding the shales indicate their primary precipitation from a mixed hydrothermal marine environment in the case of the carbonate itabirites fluids were co2 rich dolomite precipitated abiotically due to higher temperatures 100 c and high mg ca ratios the facies changes from carbonate rich to quartz rich are attributed to transgression and regression episodes and or a change in the upwelling hydrothermal fluid composition post depositional reductive dissolution affected the quartz itabirites facilitating the precipitation of ankerite sideroplesite and magnetite sulphides from a reducing diagenetic solution rich in mn mg and fe either as lenses or disseminated in the quartz itabirites and shales the sideroplesite richest samples have the most negative 13c values quartz itabirite 9 49 shale 5 76 indicating that at least part of the c is of organic origin either through the oxidation of organic matter or introduced via the diagenetic fluids specularite is replacive and likely related to a post depositional hydrothermal event this study shows that careful mineralogical investigations combined with ree y and trace element geochemistry is necessary in order to decipher the depositional environments which are oxidizing mixed marine hydrothermal post depositional reductive diagenesis masks in part these primary conditions

strain localization on an oceanic detachment fault system atlantis massif 30 n mid atlantic ridge 1 microstructural observations mineral chemistry and the spatial distribution of deformation fabrics recorded in outcrop samples collected from atlantis massif the active inside corner high at 30 n mid atlantic ridge suggest that strain is localized near the subhorizontal domal surface hypothesized to be an exposed detachment fault deformation textures in peridotite and gabbro indicate that high temperature 500 c strain occurred via crystal plastic flow and diffusive mass transfer low temperature 400 c shear zones containing brittle and semibrittle microboudinage textures in which tremolite chlorite and or talc replace fractured serpentine or hornblende cut earlier formed high temperature deformation fabrics in peridotite textures indicate strain was localized by cataclasis and reaction softening into zones of intense greenschist and subgreenschist grade metamorphism gabbro is only weakly deformed below amphibolite facies 500 c indicating that strain was partitioned into altered peridotite at low temperature there is a clear relationship between deformation intensity and structural depth beneath the subhorizontal surface of the massif discontinuous high intensity crystal plastic deformation fabrics are found at all structural depths 0 520 m beneath the surface indicating that high temperature granulite and amphibolite grade deformation was not localized in a single shear zone in contrast semibrittle and brittle low temperature shear zones are concentrated less than 90 m structurally beneath the surface and the most intensely brittlely deformed samples concentrated in the upper 10 m localization of brittle deformation fabrics near the upper surface of the massif supports the hypothesis that it is the exposed footwall of a detachment fault the structural evolution of atlantis massif is therefore analogous to a continental metamorphic core complex strain was localized onto the fault by reaction softening and fluid assisted fracturing during greenschist and subgreenschist grade hydrothermal alteration of olivine clinopyroxene serpentine and hornblende to tremolite chlorite and or talc

kinetics of pressure solution at halite silica interfaces and intergranular clay films pressure solution is widely regarded as a potentially important deformation mechanism along crustal faults and during aliagenesis yet the mechanisms and kinetics of this process remain highly controversial to better understand the fundamental factors controwing the rates of pressure solution at the grain to grain scale we conducted experiments in which convex halite lenses were pressed against fiats of fused silica in brine fluid pressures were maintained at 0 1 mpa temperatures and mean contact normal stresses ranged from 8 3 o to 90 2oc and 0 5 to 13 5 mpa respectively the geometry and growth rate of the contact spot between the two lenses and the rate at which the lenses approached one another convergence were monitored using reflected light interferometry and transmitted light photomicrography convergence occurred when halite and silica lenses were pressed together in brine halite silica experiments no undercutting was observed and dry control experiments indicated negligible dislocation creep convergence rates in experiments at 50 2oc ranged from 0 01 to 0 05 m d depending on mean normal stress and contact spot radius the data are consistent with intergranular pressure solution ips rate limited by diffusion through an intergranular film with a very high diffusion coefficient 10 5 10 7 cm2 s the data further suggest that the diffusion coefficient and or thickness of this film increases with decreasing normal stresses at least for normal stresses less than about 4 mpa as no island channel boundary structures were observed we propose that this film consists of a continuous layer of strongly adsorbed i e structured water that is maintained between the halite and silica lenses during deformation convergence rates in similar experiments conducted at a constant load of 0 11 n but at 8 3 o 50 2 o and 90 2oc were approximately constant at a given normal stress and contact spot size the cause of this temperature insensitivity is unknown but might result from changes in interphase boundary structure or thickness with increasing temperature that are sufficient to offset the expected thermal activation an experiment was also conducted in which a halite lens was pressed against a fused silica fiat coated with an 0 8 m thick film of na montmorillonite in brine this clay film produced an approximately fivefold increase in convergence rates over those observed in a halite silica experiment conducted without clay at the same load and temperature the strong sensitivity of ips rates both to contact spot radius and to the presence of second phases along grain boundaries suggests that fine grained clay rich fault gouges and multiphase granular aggregates should be particularly susceptible to pressure solution creep in the middle to upper crust

first direct radiometric dating of archaean stromatolitic limestone here we report the first direct dating of the depositional age of a sedimentary carbonate rock using long lived radioactive decay schemes the mushandike stromatolitic limestone from the masvingo formerly fort victoria greenstone belt of southern zimbabwe1 2 yields a pb pb isochron age of 2 839 33 myr published claims3 for an age of 3 5 gyr for the mushandike limestone were based on its supposed intrusion by the mushandike granite for which a rb sr whole rock age of 3 445 260 myr had been reported4 new rb sr and pb pb isotope data for the mushandike granite indicate a probable emplacement age of 2 900 myr and new field evidence j l o p t j f w in preparation suggests that the granite forms part of the basement on which the limestone was deposited thus the new dates do not support an age of 3 5 gyr for the algal stromatolites at mushandike the pb pb isochron method may have wide applicability to dating the time of deposition of precambrian fossiliferous and unfossiliferous limestones and thus to the problem of the antiquity of life

an outline of early proterozoic crustal evolution in the sao francisco craton brazil a review abstract the early proterozoic evolution of the sao francisco craton sfc occurred between 1 8 2 5 ga and includes the transamazonian cycle 1 9 2 2 ga one of the main periods of crustal growth affecting the south american continent this synthesis of the available data is largely concerned with the early proterozoic terranes of the sfc and presents a modeled crustal evolution integrated with that of the west congo craton interpretation of radiometric ages geochemical and structural data metamorphism p t conditions and lithostratigraphical correlations leads to the characterization of three early proterozoic belts in the sfc named the correntina guanambi itabuna and mineiro belts these belts are here described following a geographic scenario as given by their representatives medium to high grade sequences low grade supracrustal sequences and intrusive igneous rocks the anatomy and regional structures of these belts indicate that they developed along the margin of a large archean protocontinent although characteristic older mantled gneiss domes are also found within the itabuna and mineiro belts available isotopic evidence suggests an episodic growth of the continental crust during the late archean 2 6 3 2 ga with the formation of microcontinents that gradually coalesced around 2 6 ga ago after the archean proterozoic transition the tectonic evolution of the proterozoic mobile belts evolved by mantle accretion differentiation and crustal reworking the main early proterozoic magmatic arc of the sfc is represented by the itabuna mobile belt whereas the minciro belt is chiefly ensialic the nature of the salvador correntina belt is still poorly constrained by geochronological and geochemical data by the end of the transamazonian cycle around 1 8 1 9 ga ago the sfc was fully cratonized

development and scaling relationships of a stylolite population abstract the frequencies amplitudes and insoluble residue thicknesses of 4639 stylolites have been measured from 674 3 m of core from four vertical wells in the khuff formation a permo triassic carbonate reservoir offshore abu dhabi although there are similar numbers of stylolites per metre of core in dolomites and limestones the stylolites in the limestones have approximately double the cumulative amplitudes and insoluble residue thicknesses than the stylolites in dolomites this indicates that stylolites in the limestones have grown at approximately twice the speed or for twice as long as the stylolites in the dolomites stylolite amplitudes in dolomites and limestones together appear to obey a power law scaling relationship over about one order of magnitude 20 150 mm stylolite amplitudes in dolomites however have a higher power law exponent than those in the limestones and appear to obey a power law down to 10 mm this indicates that stylolites in the limestones have merged more than the stylolites in the dolomites the amount of pressure solution and possibly the scaling of a population of stylolites may also be controlled by location within the fold with less pressure solution in the hinge region into which hydrocarbons migrated earlier than in the limbs

smaller gold deposits in the archean rio das velhas greenstone belt quadrilatero ferrifero brazil abstract the quadrilatero ferrifero is a classic gold province on a global scale with hundreds of individual gold occurrences in the archean greenstone belt comprising the rio das velhas supergroup there are numerous small gold deposits including faria esperanca iii bicalho bela fama juca vieira brumal boa vista fernandes moita roca grande bico de pedra and pari as well as the world class deposits of morro velho and cuiaba all these deposits whether large or small are structurally controlled and related to either shear zones or folds extensive down plunge continuity is a consistent feature of the deposits in the rio das velhas greenstone belt six main styles of gold mineralization are recognized these are deposits hosted within 1 lapa seca e g morro velho bicalho bela fama 2 banded iron formations e g cuiaba sao bento raposos faria brumal roca grande 3 quartz veins e g juca vieira fernandes 4 disseminated sulfides with quartz veinlets e g moita 5 amphibolites e g pari and 6 disseminated to massive base metal sulfides e g bico de pedra the first four types of deposits are epigentic orogenic gold deposits similar to those found in greenstone belts worldwide the last two are unusual types of gold deposit peculiar to the quadrilatero ferrifero bico de pedra is a polymetallic au ag zn pb cu deposit related to an aplite intrusion whereas pari is a stratiform au bearing banded iron formation metamorphosed to epidote amphibolite metamorphic facies

iron oxide mineralization at the contact zone between phyllite and itabirite of the pau branco deposit quadrilatero ferrifero brazil implications for fluid rock interaction during iron ore formation this study examines the whole rock geochemistry and laser ablation inductively coupled plasma mass spectrometry la icp ms iron oxide chemistry of itabirite and related iron ore as well as underlying phyllitic wall rock of the pau branco iron ore deposit quadrilatero ferrifero brazil the phyllite and iron ore of pau branco extend from hypogene deep seated into supergene weathered zones the phyllitic sequence that underlies the itabirite hosted iron ore along a sheared contact includes from the bottom to top carbonaceous and carbonate chlorite and fe rich zones which likely reflect distal to proximal alteration halos the fe rich phyllite and iron ore comprise a complex iron oxide mineralogy 1 magnetite martite is hosted in itabirite and related medium grade iron ore and present as disseminated minerals in phyllite 2 microplaty hematite replaces chlorite and carbonate in the phyllite 3 granoblastic hematite is a recrystallization precipitation product and the dominant iron oxide of the hard iron ore 4 schistose specular hematite overgrows earlier iron oxides and 5 goethite replaces earlier hematite and amphibole within the weathering horizon whole rock geochemistry and related mass balance calculations reveal that unlike the itabirite hosted iron ore interpreted to be the result of a solely residual iron enrichment via depletion of most major oxides and trace elements the iron enrichment of the phyllite is caused by addition of fe 2 o 3 the fe 2 o 3 is likely sourced from the overlying itabirite iron ore and is mineralogically reflected in the replacement of carbonate and chlorite by hematite the rare earth elements ree abundances of the different ore and phyllite zones reveal seawater like ree signatures and y ho ratios of the itabirite hypogene and supergene iron ore shale atypical ree patterns in the unaltered phyllite and eu fractionation trends this suggests a distinct input of hydrothermal fluids during the fe enrichment of the phyllite locally restricted ce fractionation is attributed to distinct e g mn rich lithostratigraphic intercalations within the itabirite laser ablation icp ms mineral chemistry of phyllite hosted martite and microplaty hematite suggests a strong chemical inheritance by the host rock and precursor mineral positive ce anomalies are restricted to phyllite and itabirite hosted martite only and therefore may reflect the highly oxidative conditions during martitization at pau branco the fluid rock interaction affected not only the itabirite but also the underlying phyllite resulting in distinct alteration halos that extend beyond the itabirite toward the footwall the significant chemical influence and importance of country rock lithologies as a possible elemental source for example must be taken into account when interpreting whole rock geochemical data and investigating an itabirite banded iron formation hosted iron ore system

gold deposit formation during collapse tectonics structural mineralogical geochronological and fluid inclusion constraints in the ouro preto gold mines quadril atero ferrifero brazil the gold deposits of the ouro preto area minas gerais brazil occur along the main tectonic contact responsible for the thrusting of the paleoproterozoic units minas supergroup onto the gneissic and metasedimentary archean units rio das velhas supergroup during the brasiliano ca 600 ma tectonic event these deposits are claimed to be the most complex of the quadrilatero ferrifero with orebodies lying parallel to the thrust related stretching lineation based on structural mineralogical geochronological and fluid inclusion constraints our study demonstrates that the gold bearing quartz veins were formed in response to late orogenic collapse tectonics that led in this area to gravity backsliding of the nappes during these movements sigmoidal voids were created and filled by elongate quartz grains associated with sulfides tourmaline and carbonates inferred hydrothermal effects were responsible for the formation of illite sulfides carbonate garnet and green biotite within the matrix large arsenopyrites within the veins were affected by three stages of fracturing during which the metallic phases and associated gold were deposited gold wherever it is in contact with bi related minerals occurs in the form of electrum maldonite or aurostibite fluid inclusion studies show that this event was associated with the entrapment of both co2 rich and h2o salt rich fluids the process was initiated after the brasiliano thrusting event ca 500 c 8 kbars and in p t conditions close to 300 to 400 c and 2 kbars an age of ca 485 to 490 ma has been obtained by 40ar 39ar dating of hydrothermal biotite and muscovite this study demonstrates that the ouro preto gold deposits were formed in a context of late orogenic collapse tectonics drastically different from the conventional auriferous shear zone model this could account for their relative paucity compared to other mesothermal gold deposits

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

pb pb dating and pb isotope geochemistry of neoproterozoic carbonate rocks from the sao francisco basin brazil implications for the mobility of pb isotopes during tectonism and metamorphism abstract a u pb study was carried out on carbonate rocks from the neoproterozoic bambui group in the southern part of the sao francisco basin brazil pb isotopic compositions and u and pb concentrations were determined on more than 90 samples from different parts of the basin these samples were found to contain four distinct types of pb here called types i ii iii and iv type i pb was found in samples with low pb concentrations and relatively high u concentrations it represents in situ growth of radiogenic pb and has a large enough variation to define pb pb isochron ages type ii pb is present in samples with relatively high pb concentrations and low u concentrations it is nonradiogenic crustal pb that could either represent average crustal pb at the time of deposition or as interpreted here at the time of deformation of these rocks type iii pb is also found in samples with high pb concentrations and low u concentrations but it apparently is radiogenic crustal pb from the archean or paleoproterozoic basement that was incorporated into the carbonates during the 600 ma brasiliano orogeny type iv pb is intermediate in composition between type iii and type i pb it represents a mixture of these two types pb pb isochron ages obtained from mesoscopically undeformed carbonates containing type i pb range from 686 69 to 520 53 ma the older age is the minimum depositional age for carbonate rocks from the sete lagoas formation during the interval from 690 to 500 ma the pb isotope system of carbonates from the sao francisco basin was disturbed and in some areas it was totally reset type iii pb compositions define a straight line type ii pb falls on the lower end of this line which intercepts the stacey and kramers s k pb growth curve at about 520 and 2100 ma this line is interpreted as representing a stage of crustal pb evolution beginning about 2100 ma and ending about 550 to 500 ma when variably radiogenic pb from the basement was incorporated into these carbonates the ages determined in this study are in agreement with most published ages for brasiliano fold belts marginal to underlying sao francisco craton showing that isotopic systems of sao francisco basin rocks were largely affected by brasiliano tectonism

first results on the lpo derived seismic properties of iron ores from the quadrilatero ferrifero region southeastern brazil abstract determinations of the lpo derived seismic properties of iron ores were carried out in five samples with contrasting mineralogy hematite magnetite and quartz and deformed in different conditions all the samples are seismically quasi isotropic or weakly anisotropic which reflect i an absent or weak crystallographic preferred orientation cpo in some samples ii the high modal content of magnetite iii the relatively weak anisotropy of elastic stiffness of hematite single crystal such variables induce a low anisotropic seismic behavior even in high strained iron ores with strong preferred orientation of hematite a plane of seismic transversal isotropy parallel to the foliation of the aggregates is developed in hematite magnetite aggregates and in itabirites with strong cpo of hematites in these high strained aggregates some relationships between the crystallographic axes of hematite and the propagation velocities can be observed the magnitudes of p and s wave velocities derived of hematite cpo are lower than the values experimentally determined in iron ores such a difference probably reflects other microstructural variable which were not taken into account in the present contribution e g shape preferred orientation

a model for intergranular pressure solution in open systems abstract intergranular pressure solution ips represents a well known mechanism of bulk deformation and pore volume change especially in sedimentary rocks which has been documented in the field and reproduced in laboratory experiments theoretical descriptions of the process in terms of simple pressure solution creep laws can explain features such as grain size dependence of creep rates but have typically remained restricted to closed system conditions with negligible long range transport in the fluid phase and somewhat biased toward grain boundary diffusion controlled creep the recasting of the classical weyl rutter model of ips considered in this paper emphasizes open system conditions and the competition between diffusion and dissolution reaction rate processes the thermodynamic force which drives ips is expressed in terms of an effective grain boundary normal traction and a solute supersaturation this suggests the occurrence of a transition from ips to grain boundary healing at a certain level of supersaturation of the pore fluid although the process of healing is not studied here the explicit appearance of the solute concentration in the driving force highlights the coupling of the ips deformation process to the long range transport of dissolved material and the need to model the diagenesis of compacting sedimentary sequences accordingly the conceptual grain boundary model used in this paper is that of a fluid invaded grain to grain contact zone which is visualized to remain in a roughened state during continued ips the use of simple linear phenomenological rate laws for dissolution and grain boundary diffusion allows the ranges of rate control by dissolution or diffusion to be succinctly characterized in terms of the ratio of corresponding time constants the associated compaction behaviour is explored quantitatively in this paper by presenting solutions for the uniaxial and isotropic compaction creep strain vs time for a simple cubic packing of spheres

geology of the pitangui greenstone belt minas gerais brazil stratigraphy geochronology and bif geochemistry abstract based on drillcore description of rocks at the sao sebastiao gold deposit and surroundings a new geological map and a detailed stratigraphic column are presented for the pitangui greenstone belt which is located northwest of the quadrilatero ferrifero southern sao francisco craton brazil this belt comprises three units a lower unit predominantly of metavolcanic ultramafic and mafic rocks interbedded with thinner metasedimentary intervals of banded iron formations bifs turbiditic metasandstones and metapelites a middle unit of turbiditic metasediments interbedded with metavolcanic intermediary rocks minor bifs and quartzites and an upper unit composed of quartzites and polymictic metaconglomerates rocks of all three units are metamorphosed under upper greenschist to lower amphibolite facies conditions and are bounded by domes ttgs and late to post tectonic alkaline granitoids the pitangui greenstone belt records the geologic evolution of an ancient oceanic basin that evolved from a thick succession of metavolcanic ultramafic and mafic rocks interbedded with metasedimentary intervals of bifs and turbidites the latter representing moments of quiescence in volcanic activity this basin experienced a transient period dominated by andesitic volcanism turbidite deposition and minor bif and a final stage of clastic only sedimentation at the same time when volcanic activity ceased combined u pb and lu hf isotope data of detrital zircon from lower unit metasandstone revealed a maximum depositional age of 2859 11 ma this age is similar to those found in the basal unit of the nova lima group 2 90 2 78 ga in the rio das velhas greenstone belt located approximately 100 km southeast both belts can be correlated on the basis of similarities in stratigraphy and isotopic ages indicating that they possibly represent parts of an ancient larger domain geochemical data of bif coupled with stratigraphic analysis place constraints on paleoenvironmental conditions during the development of the mesoarchean pitangui greenstone belt shale normalized rare earth elements ree y in some bif samples display negative cerium ce anomalies which may be interpreted as evidence for the oldest oxygenation pulse documented in brazil to date 2 86 2 78 ga such anomalies endorse the presence of episodic periods of seawater oxygenation before the great oxidation event

geochemistry and pb pb geochronology of the neoarchean nkout west metamorphosed banded iron formation southern cameroon the nkout west iron deposit of the northern edge of the congo craton in southern cameroon forms part of the northwest plunging isoclinal mega fold composed of metamorphosed banded iron formations bifs which have been deformed and metamorphosed resulting in recrystallized magnetite quartz and silicates here we present whole rock geochemistry together with pb pb isotopic compositions and dating of metamorphosed bif with the aim to constrain the genesis and elucidate the metamorphic resetting age based on the mineral composition two types of bif magnetite bif garnet bif have been distinguished the magnetite bif shows high sio2 and fe2o3 and low al2o3 tio2 hfse and ree contents similar to detritus free chemical sediments in contrast their high concentrations in garnet bif indicate clay rich contaminated input the co zn ratios are consistent with trace metals of hydrothermal sources 0 14 for magnetite bif or with minor hydrogenous component 0 27 for garnet bif the paas normalized ree y patterns of the studied metamorphosed bifs show lree depletion relative to hree and positive eu anomalies the magnetite bif exhibits positive y anomaly y y paas 1 3 which is the signature inherited from sea water but its y ho 39 93 ratio suggests a mixture of hydrothermal solution with sea water the garnet bif shows positive eu negative y anomalies and chondritic y ho ratio 27 49 which is consistent with hydrothermal solution with significant clastic contamination whole rock pb concentration is roughly on average of 2 0 6 0 ppm suggesting a hydrothermal pb source the pb pb dating yielded an age of 2679 ma interpreted as the neoarchean metamorphic event in the ntem complex

iron isotope and ree y composition of the caue banded iron formation and related iron ores of the quadrilatero ferrifero brazil the minas supergroup banded iron formations bifs of the brazilian quadrilatero ferrifero qf mineral province experienced multiple deformational events synchronous with hypogene mineralization which resulted in the metamorphism of bifs to itabirites and their upgrade to high grade iron ore here we present rare earth element and yttrium ree y compositions together with iron isotope ratios of itabirites and their host iron orebodies from 10 iron deposits to constrain environmental conditions during bif deposition and the effects of hypogene iron enrichment the ree y characteristics of itabirites positive eu anomaly and lree depletion indicate hydrothermal iron contribution to the minas basin iron isotope data and ce anomalies suggest bifs were precipitated by a combination of anoxic biological mediated ferrous iron oxidation and abiotic oxidation in an environment with free oxygen such as an oxygen oasis perhaps related to increase in oxygen concentrations before the great oxidation event goe the similarity of the ree y composition of the itabirites from the different qf deformational domains as well as to other superior type bifs indicates that the metamorphism and synchronous hydrothermal mineralization did not significantly affect the geochemical signature of the original bifs however iron isotope compositions of iron ore vary systematically between deformational domains of the qf likely reflecting the specific mineralization features in each domain

the ancestry and magmatic evolution of archaean ttg rocks of the quadrilatero ferrifero province southeast brazil abstract combined la icp ms and shrimp u pb ages from quadrilatero ferrifero qf province reveals for the first time three main periods of ttg magmatism in the southern part of the sao francisco craton brazil these periods described here as the santa barbara sb rio das velhas i rvi and rio das velhas ii rvii events embody a significant part of a protracted tectonomagmatic history of the craton spanning from 3220 to 2770 ma the initial stages of ttg magmatism the sb event led to formation of a core of palaeoarchaean ttg crust via magmatic additions of juvenile felsic rocks into mafic ultramafic rocks at ca 3212 3210 ma the following event rvi event saw the growth of this palaeoarchaean core into a more composite polydefomed continental segment through magmatic additions of juvenile ttg rocks and tectonic accretion of mafic ultramafic greenstone belt terrains our u pb data and regional constraints suggest that much of the ttg crust exposed in the qf region today was produced during the rvi event between 2930 and 2900 ma the final stages of ttg crust production rv ii event were marked by convergence related magmatism affecting an extensive palaeo to mesoarchaean continental block the rvi block the timing and duration of the latest ttg event coincides with the felsic volcanism and deposition of turbiditic wackes of the main greenstone belt sequence

the geology of the morro velho gold deposit in the archean rio das velhas greenstone belt quadrilatero ferrifero brazil abstract the morro velho gold deposit quadrilatero ferrifero region minas gerais brazil is hosted by rocks at the base of the archean rio das velhas greenstone belt the deposit occurs within a thick carbonaceous phyllite package containing intercalations of felsic and intermediate volcaniclastic rocks and dolomites considering the temporal and spatial association of the deposit with the rio das velhas orogeny and location in close proximity to a major nnw trending fault zone it can be classified as an orogenic gold deposit hydrothermal activity was characterized by intense enrichment in alteration zones of carbonates sulfides chlorite white mica biotite albite and quartz as described in other archean lode type gold ores two types of ore occur in the deposit dark gray quartz veins and sulfide rich gold orebodies the sulfide rich orebodies range from disseminated concentrations of sulfide minerals to massive sulfide bodies the sulfide assemblage comprises by volume on average 74 pyrrhotite 17 arsenopyrite 8 pyrite and 1 chalcopyrite the orebodies have a long axis parallel to the local stretching lineation with continuity down the plunge of fold axis for at least 4 8 km the group of rocks hosting the morro velho gold mineralization is locally referred to as lapa seca these were isoclinally folded and metamorphosed prior to gold mineralization the lapa seca and the orebodies it hosts are distributed in five main tight folds related to f1 the best examples are the x main and south orebodies in level 25 which are disrupted by ne to e striking shear zones textural features indicate that the sulfide mineralization postdated regional peak metamorphism and that the massive sulfide ore has subsequently been neither metamorphosed nor deformed lead isotope ratios indicate a model age of 2 82 0 05 ga for both sulfide and gold mineralization the lapa seca are interpreted as the results of a pre gold alteration process and may be divided into carbonatic micaceous and quartzose types the carbonatic lapa seca is subdivided into gray and brown subtypes non mineralized gray carbonatic lapa seca forms the hanging wall to the orebodies and is interpreted as the product of extreme co 2 metasomatism during hydrothermal alteration this dolomitic lapa seca ranges in composition from relatively pure limestone and dolomite to silty limestone and dolomite the brown carbonatic and micaceous lapa secas are the host rocks to gold these units are interpreted to correspond to the sheared and hydrothermal products of metamorphosed volcaniclastic and or volcanic rocks of varying composition from dacitic to andesitic forming various types of schists and phyllites the high grade massive sulfide orebodies occur at the base of the gray carbonatic lapa seca both disseminated mineralization and quartz veins are hosted by micaceous lapa seca the data are consistent with a model of epigenetic mineralization for the lapa seca from a hydrothermal fluid derived in part from the archean basement or older crust material

mineralogical characterization and origin of uranium mineralization in witwatersrand like metaconglomerate of the moeda formation quadrilatero ferrifero brazil abstract the moeda formation is the basal unit of the minas supergroup in the quadrilatero ferrifero brazil and has quartz pebble metaconglomerate that hosts au u mineralization commonly assumed to be detrital in origin the au u mineralization occurs immediately above an unconformity with the underlying archean rio das velhas greenstone sequence in this work we focus on the moeda metaconglomerate in its u bearing portions in the gandarela syncline the southern sector near the syncline axis and the western limb near the palmital gold deposit the metaconglomerate has mica seams that host zircon xenotime rutile nodules of organic matter and sulfide minerals the sulfide assemblage includes four generations of pyrite along with fe ni cu co zn pb mo sulfides a sb as sulfosalt assemblage of gersdorffite stibnite and ullmannite is late in relation to pyrite and other sulfides gold is argentiferous with minor hg uranium minerals include uraninite coffinite brannerite and uranyl arsenates an early generation of uraninite and brannerite occurs as inclusions in organic matter nodules mainly in the southern sector a late generation of uraninite formed due to a low temperature hydrothermal overprint coffinite and late stage brannerite are products of hydrothermal alteration of zircon and rutile respectively uranyl arsenates have ni as the prevalent cation they formed as replacement of arsenic minerals such as gersdorffite by oxidized fluids containing u microstructural data and mineral chemistry favor a hydrothermal hypothesis analogous to an unconformity related setting rather than a detrital placer deposit model for the occurrence of u in the moeda metaconglomerate

experimental study of the compaction of phyllosilicate bearing sand at elevated temperature and with controlled pore water pressure relative to the time dependent compaction behavior of clean quartz sands little experimental work has been carried out on phyllosilicate rich siliciclastic rocks mudstones compaction is expected to be produced by intracrystalline plastic deformation of the phyllosilicate matrix coupled with pressure solution of embedded quartz grains and with silica reprecipitation in the adjacent pore spaces or loss from the system via an advective flux of pore water time dependence of compaction also arises in nature owing to the generation of excess pore pressure which lowers effective stress and hence compaction rate until such pressure excess becomes dissipated by permeation a suite of experiments was performed on synthetic mixtures of quartz sand in a matrix of fine grained illite muscovite hydrostatically compacted at a range of effective pressures between 10 mpa and 210 mpa with a constant pore water pressure of 70 mpa and at temperatures ranging between 300 and 450 c a small adjacent volume of porous quartz sand was provided as a precipitation sink for silica dissolved from the phyllosilicate quartz mixture volume strains were measured by a direct measurement of sample length and b measurement of amount of pore water expelled at constant pore pressure and interpreted with the aid of quantitative petrography on compacted samples control experiments were performed on i dry clean sand ii dry sand phyllosilicate and iii wet clean sand dry clean sand showed almost no compaction wet clean sand showed a degree of compaction similar to dry phyllosilicate sand but wet phyllosilicate sand showed dramatically more compaction compaction rate for the wet mixed phyllosilicate sand samples is described by formula 1 in which is effective stress mpa is porosity r is the gas constant and t is temperature k petrographic study showed interpenetration of grain contacts formation of quartz overgrowths a decrease in the amount of quartz embedded in the phyllosilicate matrix and an increase in mean grain size of quartz with progressive compaction these effects are attributed to solution and redeposition of quartz the empirical compaction law was integrated over a natural burial history the porosity evolution is extremely sensitive to the evolution of pore fluid pressure and relatively less to temperature gradient and burial rate there is reasonable qualitative agreement when compared with porosity depth relations reported in the literature 1 embed graphic 1 gif

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

rate laws for water assisted compaction and stress induced water rock interaction in sandstones mineral water interactions under conditions of nonhydrostatic stress play a role in subjects as diverse as ductile creep in fault zones phase relations in metamorphic rocks mass redistribution and replacement reactions during diagenesis and loss of porosity in deep sedimentary basins as a step toward understanding the fundamental geochemical processes involved using naturally rounded st peter sand we have investigated the kinetics of pore volume loss and quartz water reactions under nonhydrostatic hydrothermal conditions in flow through reactors rate laws for creep and mineral water reaction are derived from the time rate of change of pore volume sand water dissolution kinetics and flow rate independent steady state silica concentrations and reveal functional dependencies of rates on grain size volume strain temperature effective pressure confining minus pore pressure and specific surface areas together the mechanical and chemical rate laws form a self consistent model for coupled deformation and water rock interaction of porous sands under nonhydrostatic conditions microstructural evidence shows a progressive widening of nominally circular and nominally flat grain grain contacts with increasing strain or equivalently porosity loss and small quartz overgrowths occurring at grain contact peripheries the mechanical and chemical data suggest that the dominant creep mechanism is due to removal of mass from grain contacts termed pressure solution or solution transfer with a lesser component of time dependent crack growth and healing the magnitude of a stress dependent concentration increase is too large to be accounted for by elastic or dislocation strain energy induced supersaturations favoring instead the normal stress dependence of molar gibbs free energy associated with grain grain interfaces

chemostratigraphy of carbonates from the minas supergroup quadrilatero ferryifero iron quadrangle brazil a stratigraphic record of early proterozoic atmospheric biogeochemical and climactic change the ca 2 42 ga old gandarela formation overlies the finely laminated caue banded iron formation bif of the itabira group minas supergroup brazil and consists of red carbonate facies bif grading upsection into buff dolomites and limestones which are locally stromatolitic carbonates of the gandarela formation have 1 3 c values ranging narrowly from 1 6 to 0 4 permil versus pdb n 93 the cercadinho formation at the base of the overlying piracicaba group is a thick arenaceous succession with rare thin carbonates that have 1 3 c values ranging from 3 3 to 5 4 permil n 12 stromatolitic dolomites of the overlying fecho do funil formation have pb pb carbonate age 2 11 0 11 ga babinski and others 1995 that provide the minimum age for this unit and 1 3 c values ranging from 5 6 to 7 4 permil n 69 at present we find no evidence in brazil for the any of the three early paleoproterozoic glacial events recognized in 2 45 to 2 22 ga sedimentary successions in north america given the radiometric constraints however it is possible that geological and geochemical traces of these ice ages are missing along the unconformity between the gandarela and cercadinho formations if correct carbonates of the gandarela formation may provide a record of the carbon isotope composition of the seawater prior to any of the paleoproterozoic ice ages combined with other available 1 3 c data results from the gandarela formation suggest that seawater preceding the ice ages was not significantly enriched in 1 3 c on the other hand the carbon isotopic signature of the cercadinho carbonates and chemical composition of the cercadinho arenites are consistent with deposition after the end of the glacial epoch and likely during the early stage of ca 2 22 to 2 1 ga carbon isotope excursion the 1 3 c values of marine carbonates of the fecho do funil formation are consistent with carbon isotope record of contemporaneous carbonate successions worldwide providing additional evidence for a global biogeochemical anomaly at ca 2 22 to 2 1 ga these compositional and isotopic changes allow integration of the minas supergroup into the global record of the paleoproterozoic evolution as well as correlation with other successions of similar age

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

direct dating of stromatolitic carbonates from the schmidtsdrif formation transvaal dolomite south africa with implications on the age of the ventersdorp supergroup pb isotopic analyses of stromatolitic limestones from the schmidtsdrif formation transvaal dolomite of south africa give a late archean isochron age of 2 557 plus minus 49 2 sigma ma the new date significantly changes the position of the archean proterozoic boundary in the kaapvaal province and suggests that a possible division of the transvaal supergroup is necessary it also resolves the long standing controversy about the age of the ventersdorp supergroup whose zircon u pb data of ca 2 7 ga must be more acceptable than the rb sr dates of about 2 3 ga the study provides a new example that demonstrates the great potential of employing the u pb systems of carbonate rocks for direct dating of sedimentary sequences

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

sm nd rb sr pb pb dating of carbonates from the early archaean barberton greenstone belt south africa evidence for post depositional isotopic resetting at low temperature abstract two silicic carbonate sequences occurring in the onverwacht group og and the overlying fig tree group ftg of the early archaean barberton greenstone belt south africa contain silicified dolomites with primary structures and textures they occur with volcanic rocks dated by single zircons the associated onverwacht felsic volcanics have an age of 3440 3416 ma whereas the fig tree volcanics are 3225 ma old therefore the depositional age of both sequences is between 3440 and 3225 ma pb isotopic data for different parts of a single og whole rock sample form a linear array corresponding to a pb pb age of 2632 99 ma sm nd and rb sr isotopes were analysed in four whole rock samples of the ftg silicified carbonates and form an sm nd array corresponding to an age of 2612 84 ma with initial end 8 1 0 5 and an rb sr array of 2743 36 ma with initial 87 sr 86 sr 0 7013 0 0003 these whole rock samples have rb sr ratios of 1 and unusually high rb concentrations for carbonates 100 ppm which derive from processes accompanying silicification all three isotopic systems of whole rocks indicate ages of 2 7 ga much younger than the depositional age of the successions the agreement provides strong evidence that an alteration event reset all of the isotopic systems sm nd as well as rb sr and u pb we suggest that the ages represent the timing of silicification and sericitization of the carbonates by large scale fluid movement at an apparent temperature of 200 c

dating the itabira iron formation quadrilatero ferrifero of minas gerais brazil at 2 65 ga depositional u pb age of zircon from a metavolcanic layer abstract uranium pb dating of zircon from a metavolcanic layer within the itabira iron formation or caue itabirite quadrilatero ferrifero of minas gerais brazil indicates that its deposition occurred at 2 65 ga i e considerably earlier than proposed by previous geochronological studies which suggested deposition between 2 58 ga detrital zircon age from the underlying moeda formation and 2 42 ga pb pb isochron age from dolomitic rocks of the immediately overlying gandarela formation the new age has major implications for the geology of the quadrilatero ferrifero and connects it with the neoarchaean iron formation of the carajas mineral province

proterozoic first order sedimentary sequences of the sao francisco craton eastern brazil abstract the sao francisco craton in eastern brazil hosts sedimentary sequences deposited between the paleoarchean 3300 ma and late neoproterozoic 580 ma proterozoic successions occurring in this region comprise five 1st order sedimentary sequences which besides episodes of global significance record major basin forming events the ca 8000 m thick minas itacolomi 1st order sequence exposed in the brazilian mining district of the quadrilatero ferrifero and containing as marker bed the lake superior type caue banded iron formation tracks the operation of a wilson cycle in the paleoproterozoic era the quartz arenite dominated espinhaco i and ii sequences record at least two major rift sag basin forming events which affected the host continent of the sao francisco craton at around 1 75 ga and 1 57 ga the macaubas sequence and its correlatives in the extracratonic domains witness the individualization of a sao francisco congo plate in synchronicity with the break up of rodinia in the cryogenian period the sao francisco congo plate together with various fragments derived from rodinia reassembled to form gondwana in the ediacaran period in the course of the gondwana amalgamation orogenic belts developed along the margins of the craton its interior converted into foreland basins received the shallow water carbonates and pelites of the bambui 1st order sequence and its correlatives

the pb pb age of the minas supergroup carbonate rocks quadrilatero ferrifero brazil abstract pb pb isochron ages determined on carbonate rocks from the minas supergroup quadrilatero ferrifero qf show that the deposition of the gandarela formation the intermediate unit of the minas supergroup took place at 2420 19 ma the entire sedimentary sequence and underlying basement rocks have been deformed at a regional scale and data from greenschist facies carbonate rocks from the piracicaba group indicate that the pb isotopes were reset at about 2100 ma this age is in agreement with whole rock rb sr and u pb titanite ages determined by many authors for metamorphism of the surrounding archean granite gneiss terrane

intergranular pressure solution in four quartzose sandstones abstract cathodoluminescence petrography and scanning electron microscopy reveal that several geologic variables significantly influenced intergranular pressure solution and quartz cementation in the bromide ordovician oklahoma upper minnelusa permian wyoming nugget jurassic utah and tuscarora silurian pennsylvania sandstones a linear relationship exists between mean grain size and intergranular pressure solution with finer grained samples having experienced more intergranular pressure solution than coarser grained ones samples that display pervasive illite grain coatings have also experienced more intergranular pressure solution than those with no or poorly developed grain coatings sorting and limited framework grain compositional variability have exerted little influence o intergranular pressure solution silica budgets indicate that in the bromide upper minnelusa and nugget sandstones more quartz has been dissolved by intergranular pressure solution than is present as cement these sandstones have exported silica during burial diagenesis in the tuscarora the amount of quartz dissolved by intergranular pressure solution is about equal to the amount of quartz cement present implying approximate mass balance however petrographic evidence suggests that most of the quartz cement in at least three of these sandstones predates most intergranular pressure solution implying that they acted as silica importers during shallow diagenesis and silica exporters during deeper diagenesis intergranular pressure solution has played varying roles in determining the ultimate porosity of these four sandstones in the nugget intergranular pressure solution was the single most important diagenetic process that influenced porosity evolution in the bromide intergranular pressure solution was slightly more important than mechanical compaction and considerably more important than cementation in the tuscarora intergranular pressure solution was slightly more important than mechanical compaction and of about equal importance to quartz cementation in the upper minnelusa intergranular pressure solution was less important than both mechanical compaction and gypsum anhydrite cementation

effect of water on the strength and microstructure of carrara marble axially compressed at high temperature in this study we aimed to i answer the question if water affects the strength of marble deforming at high temperature and ii establish criteria that allow assessment of the role of water in microstructure development in marble samples of carrara marble were axially compressed at temperatures ranging 600 1000 8c and strain rate w10 k5 s k1 with or without addition of 0 4 2 1 wt water water added samples were found to be slightly weaker than dry samples microstructurally all samples showed grain flattening and twinning at 600 8c and dynamic recrystallization dominated by grain boundary migration at higher temperature grain boundaries in wet samples showed isolated or semi continuous remnants of fluid pockets the average grain roundness r and fractal dimension d relating grain diameter d and grain perimeter p pfd d for wet recrystallized samples were systematically lower than for dry samples at 600 8c high pore fluid pressure probably enhanced dilatation of wet samples and helped grain boundary sliding gbs thereby lowering the strength at higher temperature fluid enhanced grain boundary migration served as a recovery process reducing the contribution of gbs to creep overall water has little effect on strength and microstructure of marble experimentally deformed at high temperature q 2005 elsevier ltd all rights reserved

episodic negative anomalies of cerium at the depositional onset of the 2 65 ga itabira iron formation quadrilatero ferrifero of minas gerais brazil abstract magnetite bands in sericitic phyllite of the batatal formation mark the transition to the overlying 2 65 ga itabira iron formation or caue itabirite in the quadrilatero ferrifero of minas gerais brazil the magnetite bands have low non crustal th u ratios suprachondritic y ho ratios and sporadic anomalies of ce that are characterised as truly negative the data point to mild oxygenation of sea water at the depositional onset of the itabira iron formation some 200 ma before the great oxidation event

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

stylolites a review abstract stylolites are ubiquitous geo patterns observed in rocks in the upper crust from geological reservoirs in sedimentary rocks to deformation zones in folds faults and shear zones these rough surfaces play a major role in the dissolution of rocks around stressed contacts the transport of dissolved material and the precipitation in surrounding pores consequently they play an active role in the evolution of rock microstructures and rheological properties in the earth s crust they are observed individually or in networks in proximity to fractures and joints and in numerous geological settings this review article deals with their geometrical and compositional characteristics and the factors leading to their genesis the main questions this review focuses on are the following how do they form how can they be used to measure strain and formation stress how do they control fluid flow in the upper crust geometrically stylolites have fractal roughness with fractal geometrical properties exhibiting typically three scaling regimes a self affine scaling with hurst exponent 1 1 0 1 at small scale up to tens or hundreds of microns another one with hurst exponent around 0 5 to 0 6 at intermediate scale up to millimeters or centimeters and in the case of sedimentary stylolites a flat scaling at large scale more complicated anisotropic scaling scaling laws depending of the direction of the profile considered is found in the case of tectonic stylolites we report models based on first principles from physical chemistry and statistical physics including a mechanical component for the free energy associated with stress concentrations and a precise tracking of the influence of grain scale heterogeneities and disorder on the resulting micro structures experimental efforts to reproduce stylolites in the laboratory are also reviewed we show that although micrometer size stylolite teeth are obtained in laboratory experiments teeth deforming numerous grains have not yet been obtained experimentally which is understandable given the very long formation time of such geometries finally the applications of stylolites as strain and stress markers to determine paleostress magnitude are reviewed we show that the scalings in stylolite heights and the crossover scale between these scalings can be used to determine the stress magnitude its scalar value perpendicular to the stylolite surface during the stylolite formation and that the stress anisotropy in the stylolite plane can be determined for the case of tectonic stylolites we also show that the crossover between medium millimetric scales and large pluricentimetric scales in the case of sedimentary stylolites provides a good marker for the total amount of dissolution which is still valid even when the largest teeth start to dissolve which leads to the loss of information since the total deformation is not anymore recorded in a single marker structure we discuss the impact of the stylolites on the evolution of the transport properties of the hosting rock and show that they promote a permeability increase parallel to the stylolites whereas their effect on the permeability transverse to the stylolite can be negligible or may reduce the permeability depending on the development of the stylolite

an image mapping approach to u pb la icp ms carbonate dating and applications to direct dating of carbonate sedimentation we present a new approach to laser ablation inductively coupled plasma mass spectrometry la icp ms u pb dating of carbonates based on selection and pooling of pixels from 2 d elemental and isotopic ratio maps this image mapping technique is particularly useful for targeting subdomains in samples with complex geological histories key major and trace elements that are sensitive to detrital components postformational fluid ingress mineralogical changes or diagenetic overprinting are measured along with the pb and u isotopic data laser sampling is undertaken along successive linear rasters that are compiled into maps using the monocle add on for iolite with one pixel in the map corresponding to one time slice of the time resolved signal these element element ratio and isotope ratio maps can be overlain over photomicrographs or scanning electron microscopy images to spatially link compositional data to textural and structural features the pixels corresponding to likely homogeneous age domains can be isolated by applying appropriate selection criteria e g th 0 3 ppm mg ca 0 004 and pooled into pseudo analyses using a proxy for the parent daughter ratio e g 207pb 235u 238u 208pb to retrieve the largest possible spread of the data points on isochron diagrams the approach is best suited for analytical setups capable of rapidly or simultaneously scanning over a large mass range and can yield a precision of 1 or better on quadrupole instruments depending on u concentration 238u 204pb and age of the sample the sample specific filtering criteria for selection and rejection of data and their rationale can be reported resulting in more transparency with regard to data processing

mineralogy and trace element geochemistry of the high grade iron ores of the aguas claras mine and comparison with the capao xavier and tamandua iron ore deposits quadrilatero ferrifero brazil several major iron deposits occur in the quadrilatero ferrifero qf southeastern region of brazil where metamorphosed and heterogeneously deformed banded iron formation bif of the caue formation regionally called itabirite was transformed into high fe 64 and low grade 30 fe 64 hematite ores based on their mineralogical composition three major types of itabirites occur in the qf siliceous dolomitic and amphibolitic itabirite unlike other mines in the qf the aguas claras mine contained mainly high grade ores hosted within dolomitic itabirite two distinct types of high grade ore occurred at the mine soft and hard the soft ore was the most abundant and represented more than 85 of the total ore mined until it was mined out in 2002 soft and hard ores consist essentially of hematite occurring as martite anhedral to granular tabular hematite and locally specularite gangue minerals are rare consisting of dolomite sericite chlorite and apatite in the hard and soft ores and mn oxides and ferrihydrite in the soft ore where they are concentrated within porous bands chemical analyses show that hard and soft ores consist almost entirely of fe2o3 with a higher amount of detrimental impurities especially mno in the soft ore both hard and soft ores are depleted in trace elements the high grade ores at the aguas claras mine have at least a dual origin involving hypogene and supergene processes the occurrence of the hard massive high grade ore within fresh dolomitic itabirite is evidence of its hypogene origin despite the contention about the origin of the dolomitic itabirite if this rock is a carbonate rich facies of the caue formation or a hematite carbonate precursor of the soft high grade ore mineralogical and geochemical features of the soft high grade ore indicate that it was formed by leaching of dolomite from the dolomitic itabirite by meteoric water the comparison of the aguas claras capao xavier and tamandua orebodies shows that the original composition of the itabiritic protore plays a major role in the genesis of high and low grade soft ores in the qf under the same weathering and structural conditions the dolomitic itabirite is the more favorable to form high grade deposits than siliceous itabirite field relations at the aguas claras and capao xavier deposits suggest that it is not possible to form huge soft high grade supergene deposits from siliceous itabirite unless another control such as impermeable barriers had played an important role the occurrence in the tamandua mine of a large soft high grade orebody formed from siliceous itabirite and closely associated with hypogene hard ore suggests that large soft high grade orebodies of the quadrilatero ferrifero which occur within siliceous itabirite have a hypogene contribution in their formation

u pb dating of geodic calcite new insights on western europe major tectonic events and associated diagenetic fluids this study presents the first application of the u pb dating method to highly pb depleted diagenetic geodic calcites of the jurassic formations of the paris basin that leads to a reappraisal of the palaeohydrological history of this region composite u pb ages from multiple geodes combined with 18 o analyses reveal two main phases of diagenetic fluid circulations linked with major regional tectonic events dogger formations recorded a first diagenetic fluid episode at 147 8 3 8 ma i e at the very beginning of the emersion of the basin during the tithonian period and 30 ma earlier than previously assumed 18 o results confirmed that most of the calcitic cement phases that closed the porosity of these formations precipitated at the beginning of the cretaceous period oxfordian formations recorded another major meteoric fluid circulation at 33 5 2 8 ma related to the initial stage of the european cenozoic rift system ecris consequently the porosity of the mesozoic formations of the studied area may have been closed sooner than previously thought before the main ecris rifting phase of oligocene age this study shows that u pb dating of secondary geodic calcite offers a new powerful way for reconstructing the coupled palaeohydrological and diagenetic history of sedimentary basins supplementary material pb pb diagrams 238 u and 235 u series isochron diagrams and effect of blank correction on u and pb isotopic compositions and on ages calculation are available at https doi org 10 6084 m9 figshare c 3886885

stylolites in the caballos novaculite west texas stylolites within the caballos novaculite developed by pressure solution in response to lithostatic pressure after the original sediment had been converted to a novaculite estimates of minimum stratigraphic thinning of the novaculite are fairly uniform across the marathon basin they range from 3 5 to 5 6 on the basis of stylolite column amplitude

effects of phosphate ions on intergranular pressure solution in calcite an experimental study abstract intergranular pressure solution ips is a coupled chemical mechanical process of widespread importance that occurs during diagenesis and low temperature deformation of sedimentary rocks laboratory experiments on ips in halite quartz and calcite have largely concentrated on the mechanical aspects of the process in this study we report the effects of pore fluid chemistry specifically varying phosphate ion concentration on the mechanical compaction by ips of fine grained calcite powders at room temperature and 1 to 4 mpa applied effective stress phosphate was investigated because of its importance as a biogenic constituent of sea and pore waters increasing the pore fluid phosphate concentration from 0 to 10 3 mol l systematically reduced compaction strain rates by up to two orders of magnitude the sensitivity of the compaction strain rate to phosphate concentration was the same as the sensitivity of calcite precipitation rates to the addition of phosphate ions reported in the literature suggesting that the rate of ips in phosphate bearing samples was controlled by calcite precipitation on pore walls the results imply that ips and associated porosity permeability reduction rates in calcite sediments may be strongly reduced when pore fluids are enriched in phosphates for example through high biologic productivity or a seawater origin future modeling of ips related processes in carbonates must therefore take into account the effects of pore fluid chemistry specifically the inhibition of interfacial reactions

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

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

crystallographic fabric development along a folded polycrystalline hematite abstract detailed analyses of microstructures and crystallographic preferred orientations cpos of hematite rocks were conducted in samples of polycrystalline hematite around a tight fold in the quadrilatero ferrifero region southeastern brazil grain size is dominantly very fine in all samples 15 30 m with a slight increase toward the hinge grain aspect ratio increases substantially toward the hinge from an average of 2 in the limb to 9 in the hinge distribution patterns of 0001 axes and poles of planes 10 1 0 11 2 0 and 10 1 4 suggest that intracrystalline slip operate on the basal plane along the direction the distribution of the poles of prismatic planes parallel to the foliation of reference frame indicate that the all the symmetric slip directions of hematite crystals were equally efficient during activation of basal intracrystalline slip increasing aspect ratio is accompanied by cpo intensification toward the hinge where intensification was aided during deformation by bulk rotation of the hematitic plates and possibly by some grain boundary sliding such cpo intensification with the increase of aspect ratio toward the hinge is used to infer that the fold developed by flexural flow

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

sedimentary manganese metallogenesis in response to the evolution of the earth system abstract the concentration of manganese in solution and its precipitation in inorganic systems are primarily redox controlled guided by several earth processes most of which were tectonically induced the early archean atmosphere hydrosphere system was extremely o2 deficient thus the very high mantle heat flux producing superplumes severe outgassing and high temperature hydrothermal activity introduced substantial mn2 in anoxic oceans but prevented its precipitation during the late archean centered at ca 2 75 ga the introduction of photosystem ii and decrease of the oxygen sinks led to a limited buildup of surface o2 content locally initiating modest deposition of manganese in shallow basin margin oxygenated niches e g deposits in india and brazil rapid burial of organic matter decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the archean proterozoic transition concurrently a massive drawdown of atmospheric co2 owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused paleoproterozoic glaciations 2 45 2 22 ga the spectacular sedimentary manganese deposits at ca 2 4 ga of transvaal supergroup south africa were formed by oxidation of hydrothermally derived mn2 transferred from a stratified ocean to the continental shelf by transgression episodes of increased burial rate of organic matter during ca 2 4 and 2 06 ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere black shale hosted mn carbonate deposits in the birimian sequence ca 2 3 2 0 ga west africa its equivalents in south america and those in the francevillian sequence ca 2 2 2 1 ga gabon are correlatable to this period tectonically forced doming up attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric co2 drawdown causing glaciation on the neoproterozoic rodinia supercontinent tectonic rifting and mantle outgassing led to deglaciation dissolved mn2 and fe2 concentrated earlier in highly saline stagnant seawater below the ice cover were exported to shallow shelves by transgression during deglaciation during the sturtian glacial interglacial event ca 750 700 ma interstratified mn oxide and bif deposits of damara sequence namibia was formed the varangian marinoan ca 600 ma cryogenic event produced mn oxide and bif deposits at urucum jacadigo group brazil the datangpo interglacial sequence south china liantuo nantuo varangian event contains black shale hosted mn carbonate deposits the early paleozoic witnessed several glacioeustatic sea level changes producing small mn carbonate deposits of tiantaishan early cambrian and taojiang mid ordovician in black shale sequences china and the major mn oxide carbonate deposits of karadzhal type central kazakhstan late devonian the mesozoic period of intense plate movements and volcanism produced greenhouse climate and stratified oceans during the early jurassic oae organic rich sediments host many mn carbonate deposits in europe e g urkut hungary in black shale sequences the late jurassic giant mn carbonate deposit at molango mexico was also genetically related to sea level change mn carbonates were always derived from mn oxyhydroxides during early diagenesis large mn oxide deposits of cretaceous age at groote eylandt australia and imini tasdremt morocco were also formed during transgression regression in greenhouse climate the early oligocene giant mn oxide carbonate deposit of chiatura georgia and nikopol ukraine were developed in a similar situation thereafter manganese sedimentation was entirely shifted to the deep seafloor and since ca 15 ma b p was climatically controlled glaciation deglaciation assisted by oxygenated polar bottom currents aabw nadw the changes in climate and the sea level were mainly tectonically forced

pb pb dating of young marbles from taiwan direct determination of the depositional age of a sedimentary sequence usually relies on isotopic analyses of authigenic minerals such as glauconite1 and fine grained clay minerals2 4 moorbath et al 5 recently reported the first direct dating of sedimentary carbonates using the whole rock pb pb method the success of this method depends on the chemical fractionation of u from pb and the consequent variation in 206pb 204pb and 207pb 204pb ratios the high values and large variation of u pb ratios in sedimentary carbonates long known to marine geochemists have however not been exploited by geochronologists until recently5 6 here i report a pb pb isochron age for a relatively young carbonate sequence in taiwan and demonstrate that the long lived u pb decay scheme can be used to determine events as young as mesozoic age the new results also lead to the identification of an early jurassic 170 myr bp thermal event which has never before been recognized in the polymetamorphic schist belt the tananao schist of taiwan but is widespread in southeastern continental china and other parts of eastern asia7 10

regional sulfate hematite sulfide zoning in the auriferous mariana anticline quadrilatero ferrifero of minas gerais brazil the distribution of mineral deposits characterised as barite deposits hematite rich auriferous deposits and auriferous tourmaline sulfide deposits displays a regional sulfate hematite sulfide zoning along the thrust delineated limbs of the mariana anticline in the south eastern part of the quadrilatero ferrifero of minas gerais brazil cross cut relationships of barite veins and sulfide lodes indicate that sulfidation occurred in a late tectonic context which is here attributed to the collapse of the 0 6 ga brasiliano thrust front reconnaissance s isotopic data from barite and pyrite antonio pereira barite deposit and its adjacent gold deposit respectively and arsenopyrite passagem de mariana gold deposit suggest a new interpretation for the hydrothermal fluid overprint in the mariana anticline the antonio pereira barite has 33s values that are near zero constraining the sulfate source to rocks younger than 2 45 ga the barite 34s values are between 19 6 and 20 8 the passagem arsenopyrite and tourmaline have co ni ratios that define a positive linear trend with the antonio pereira pyrite the latter has homogenous 34s values between 8 8 and 8 9 which are compatible with thermochemical reduction of aqueous sulfate with the s isotopic composition of the antonio pereira barite

nature and evolution of fluids associated with specularite bearing fe and au pge jacutinga mineralization during the brasiliano orogeny in the eastern sao francisco craton minas gerais brazil abstract hydrothermal fluids related to the development of the aracuai orogen during the brasiliano cycle have affected paleoproterozoic banded iron formation bearing sequences along the eastern border of the sao francisco craton controversy surround the origin of specularite in high grade schistose iron ore bodies in the serpentina range serpentina type ore versus specularite rich s free au pge quartz veins jacutinga type that occur at the eastern border of the quadrilatero ferrifero mining district chemical and thermobarometric characteristics of fluid inclusions from serpentina type quartz specularite veins show that metamorphic aqueous saline fluids of low to moderate temperature i e salinity between 6 3 and 13 7 wt nacl eqv and homogenization temperatures from 91 to 228 c were responsible for silica leaching and fe enrichment along shear zones in the statherian serra da serpentina group these serpentina type fluids present a distinct pattern with na ca k the fluids are depleted in li na ba ca k cu and mg and enriched in as and sr in comparison to the jacutinga type fluids in contrast other two distinct fluids and chemical processes were involved to precipitate fe oxides with au pge in jacutinga type veins the first type is an aqueous carbonic fluid from the catas altas and itabira deposits these jacutinga type fluids present a pattern with na k ca mg and k li ba sr the fluids are interpreted to be of brasiliano age and are typically associated with anatectic pegmatites from santa maria de itabira the high salinity of these brines 23 2 25 3 wt nacl eqv from the itabira ore likely originated from their percolation through evaporitic layers from salt concentration during deformation or from fluid rock interactions mixing between high temperature aqueous carbonic pegmatite forming fluids and low temperature aqueous metamorphic brines could have induced an abrupt change in the physical chemical conditions of the system which led to au and pge precipitation in the itabira jacutinga type deposit a second jacutinga type fluid is observed at the gongo soco and barao de cocais deposits this fluid is interpreted as an aqueous saline metamorphic fluid with the pattern na ca k mg low to moderate salinity 4 3 16 4 wt nacl eqv and temperatures of 77 to 305 c it is relatively enriched in b depleted in li ba na cu zn and k and follows the trend k li ba in both types of fluids responsible for jacutinga type mineralization gold was transported as chlorine complexes under acidic and oxidizing conditions the metal precipitation may have occurred due to a rapid depressurization during failure of shear zones operating in the upper levels of the crust or variations in the ph and redox conditions during the interaction with the host dolomitic itabirite

paleomagnetic correlation of columbia river basalt flows using secular variation a statistical method is developed to evaluate stratigraphic correlations that are based on the similarity of paleomagnetic directions it involves comparison of the probabilities that such directions could have arisen by either simultaneous or random samplings of the ancient magnetic field to calculate these probabilities one must estimate the effects of paleomagnetic errors and paleosecular variation respectively the method is tested successfully on sections of 8 10 grande ronde basalt columbia river basalt group flows at sentinel gap and umtanum ridge which are separated by only 25 km and contain two flows thought to be the same on geological grounds more significantly for widely separated sections at sentinel gap and butler canyon the analysis again shows that simultaneous magnetization is a much more likely explanation of the general similarity of paleomagnetic directions than chance agreement of ancient field directions the simplest interpretation of this result is that many of the flows at the two sites are the same implying that series of grande ronde flows extended over distances of several hundred kilometers

compaction creep of wet granular calcite by pressure solution at 28 c to 150 c 1 uniaxial compaction experiments have been carried out on wet calcite powders prepared from milled limestone analytical grade calcite and superpure calcite the tests were performed at 28 c 150 c effective stresses of 20 47 mpa and a pore pressure of 20 mpa using presaturated caco3 solution as the pore fluid sample grain sizes ranged from 12 to 86 m the aim was to determine if creep occurs by intergranular pressure solution ips under these conditions and to identify the rate controlling process the wet samples showed significant creep while dry and oil flooded samples did not wet samples were also characterized by microstructures such as sutured grain contacts grain indentations and overgrowths suggesting the operation of ips the behavior of the wet samples at low strains 4 5 agreed well with the predictions of standard models for diffusion controlled pressure solution however at higher strains 5 10 creep slowed down compared with the model predictions transient flow through tests performed in this regime led to an acceleration of creep consistent with models for precipitation controlled ips and demonstrated an increase in the impurity content of the pore fluid toward higher strains since some of these impurities notably mg2 retard precipitation in calcite we suggest that creep occurred by diffusion controlled ips at low strain giving way to precipitation control at larger strains as impurities accumulated fitting of the diffusion controlled ips model to the low strain data yielded values of the grain boundary diffusion product ds in calcite of 10 19 m3 s 1 at 150 c

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

geologic evidence for a mantle superplume event at 1 9 ga 1 both preserved and restored areal distributions of proterozoic marine intracratonic passive margin and platform sediments show a prominent peak at 1 9 ga indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time the chemical index of alteration in shales deposited at this time was high suggesting warm climates possibly due to enhanced co2 levels in the atmosphere high sea level and warms climate may also explain an abundance of black shale banded iron formations and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1 9 ga all of these observations are consistent with a 1 9 ga superplume event the occurrence of only a minor positive carbon isotope shift in marine carbonates at 1 9 ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present slightly low 87sr 86sr isotopic ratios in seawater at 1 9 ga reflect increased mantle input of sr from the proposed superplume event whereas higher ratios at 1 85 1 75 ga may reflect increased input of continental sr from a growing supercontinent the first massive sulfate evaporites in the geologic record at 1 8 1 6 ga follow the possible 1 9 ga superplume event this may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume related volcanism wanes

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

effects of interfacial energy on compaction creep by intergranular pressure solution theory versus experiments on a rock analog nano3 groundwater systems are increasingly used for seasonal aquifer thermal energy storage sates for periodic heating and cooling of buildings its use is hampered in contaminated aquifers because of the potential environmental risks associated with the spreading of contaminated groundwater but positive side effects such as enhanced contaminant remediation might also occur a first reactive transport study is presented to assess the effect of sates on the fate of chlorinated solvents by means of scenario modeling with emphasis on the effects of transient sates pumping and applicable kinetic degradation regime temperature effects on physical chemical and biological reactions were excluded as calculations and initial simulations showed that the small temperature range commonly involved t 15 c only caused minor effects the results show that a significant decrease of the contaminant mass and eventually plume volume occurs when degradation is described as sediment limited with a constant rate in space and time provided that dense non aqueous phase liquid dnapl is absent however in the presence of dnapl dissolution particularly when the dissolved contaminant reaches sates wells a considerably larger contaminant plume is created depending on the balance between dnapl dissolution and mass removal by degradation under conditions where degradation is contaminant limited and degradation rates depend on contaminant concentrations in the aquifer a sates system does not result in enhanced remediation of a contaminant plume although field data are lacking and existing regulatory constraints do not yet permit the application of sates in contaminated aquifers reactive transport modeling provides a means of assessing the risks of sates application in contaminated aquifers the results from this study are considered to be a first step in identifying the subsurface conditions under which sates can be applied in a safe or even beneficial manner

sedimentation patterns during the precambrian a unique record abstract although the similarities between depositional processes and products as well as the analogous controls on basin filling and evolution appear to have enjoyed great uniformity throughout the sedimentary rock record a noticeable distinction exists in the rates and intensities of a broad range of geological processes in the precambrian epoch this paper searches for distinctiveness in the precambrian sedimentary record both siliciclastic and carbonate through an extensive though not exhaustive review of the relevant literature augmented by new observations while differences in precambrian deltaic aeolian glacial and possibly also lacustrine deposits and settings appear to have been small their large scale development was controlled largely by a combination of temporal and geodynamic influences essentially of global compass in this regard the onset of the supercontinent cycle and major perturbations in palaeo atmospheric composition appear to have been significant marine environments provide a poor platform for precambrian phanerozoic comparisons of sedimentation patterns as those from the former period are preserved almost exclusively in epeiric settings an environment essentially lacking on modern earth for the shallow marine carbonates biological mediation of chemical sediment deposition changed radically from dominance by microbial biota in the precambrian to a combination of metazons protozoans and algae for the skeletal carbonates of the phanerozoic despite it being widely recognized that precambrian channel systems were braided in all environments deltaic tidal alluvial fluvial as a consequence of the lack of vegetation and poor development of soils the fluvial setting has some enigmatic aspects amongst these is evidence for ponding of muddy detritus in apparently sandstone bed load dominated braided systems with effects on local palaeoslopes which have resulted in unusual palaeohydraulic parameters for precambrian fluvial systems this is perhaps a field of research which holds greater promise when investigating sedimentation patterns prior to the phanerozoic

talc mineralisation associated with soft hematite ore gongo soco deposit minas gerais brazil petrography mineral chemistry and boron isotope composition of tourmaline talc mineralisation occurs as hematite talc schist between soft hematite ore and dolomitic itabirite at gongo soco quadrilatero ferrifero of minas gerais brazil the hematite talc schist and soft hematite have a prominent tectonic foliation of tabular hematite tabular hematite without preferential orientation is superimposed on the tectonic foliation the talcose schist is enriched in f and has a constant fe s ratio electron microprobe analyses indicate trace amounts of s in different generations of hematite the whole rock fe s ratio possibly represents sulfate s from hematite hosted fluid inclusions fluid inclusions in foliation overprinting hematite and chlorite geothermometry from talcose rocks suggest respectively temperatures from 200 c to 300 c tourmaline a rarely observed mineral in the hematite talc schist belongs to the alkali group and falls in the dravite compositional field boron isotope determinations of tourmaline crystals using secondary ion mass spectrometry vary from 20 to 12 11b this compositional isotopic range and the tourmaline chemical composition suggest a meta evaporitic origin a non marine evaporitic setting is the most likely source of acidic highly oxidising fluids which resulted in the abundant f bearing talc and the presence of otherwise immobile ti in hematite oxidising brines were channelled along shear zones and converted dolomitic itabirite into the gongo soco soft hematite and the talc mineralisation the latter is envisaged as the hydrothermal wall rock alteration of dolomitic itabirite which gave rise to the soft hematite ore

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

age constraints on the neoproterozoic baikal group from combined sr isotopes and pb pb dating of carbonates from the baikal type section southeastern siberia abstract depositional ages for the middle part of the neoproterozoic sedimentary succession exposed along lake baikal in the southern margin of the siberian platform have been constrained by the application of sr isotope chemostratigraphy and pb pb dating of carbonate rocks the thick siliciclastic carbonate baikal group is the type section of the regional siberian succession although many of the baikal group carbonate rocks have undergone pervasive dolomitization by mixed meteoric marine fluids oolitic limestones in the 800 m thick carbonate member of the uluntui formation preserve primary textures and high sr contents up to 3200 ppm suggesting that aragonite might have been the original precursor the 87sr 86sr in the least altered high sr limestones mg ca 8 pdb range within 0 70842 0 70876 values we interpret as representing the best proxy for coeval seawater stepwise dissolution using 0 5 n hbr was applied to enrich samples in primary carbonate from the uluntui formation and nine data points from the five least altered high sr limestones yielded a pb pb isochron age of 560 30 ma with mswd 0 6 and probability 0 8 this age matches that inferred from sr isotope stratigraphy 580 550 ma and further supports the inference that the baikal paleobasin was connected to the contemporary oceans our sr chemostratigraphy and pb pb isochron age of the uluntui limestones cast doubt on previously proposed correlations of the baikal group as late mesoproterozoic to cryogenian instead the data indicate that the entire siliciclastic carbonate succession of the baikal group was most likely deposited during the ediacaran or vendian of the russian stratigraphic scale

stabilization of the southern portion of the sao francisco craton se brazil through a long lived period of potassic magmatism abstract although the southern portion of the sao francisco craton in southeast brazil encompasses one of the largest segments of meso to neoarchaean crust in the south american plate there is little understanding of the processes leading to cratonization of this region our combined field and u pb age data show that the crust experienced massive intrusions of potassic k rich granitoids between 2750 and 2600 ma and that this magmatic event marked the beginning of a fundamental change in the tectonics of the terrain with the ensuing billion years being dominated by the deposition of a series of thick clastic and chemical sedimentary successions thus deep crustal partial melting and subsequent transport and intrusion of these granites in the upper crust is a likely mechanism by which this segment of the sao francisco crust attained buoyancy to survive recycling regional mapping indicates that the potassic granitoids are distributed over an area of 25 000 km 2 and are related to one of the most prolific periods of potassic magmatism in the craton la icp ms and shrimp u pb data from 16 samples of potassic granitoids together with published tims ages suggest a long lived period of magmatism from 2750 to 2700 ma and a small pulse of granite crystallisation at ca 2612 ma the u pb shrimp and la icp ms data also indicate that the archaean granitoids do not record evidence of u pb zircon resetting our interpretation is that partial melting and transport of granitoid melts rich in heat producing elements to the upper crust gave rise to a thermally stable lower crust and that this lower crustal segment in the craton area became sufficiently refractory and resistant to further partial melting

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

depositional history of the upper vindhyan succession central india time constraints from pb pb isochron ages of its carbonate components abstract age of the upper sedimentary units rewa and bhander groups of the largest proterozoic vindhyan basin in central india has so far been considered to be close to 650 ma late neoproterozoic based on collective proxy dates in the absence of direct isotopic dates a drastic upward revision of this age by more than 300 ma has very recently been proposed from the similarity of the paleomagnetic directions in these units with those of the 1070 ma old majhgawan kimberlite intrusion into their basal parts and the presence of detrital zircons not younger than 1020 ma we report now pb pb isochron ages of 908 72 ma 866 180 ma and 1073 210 ma for the three distinct carbonate horizons bhander balwan and lakheri limestones respectively as independent and direct evidence on the age of the upper vindhyan succession the consistency of the three ages close to 900 ma provides a strong support for the proposed shift on the timing and duration of sedimentation in the upper vindhyan sequence