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

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

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

brittle microstructures in gold nuggets a descriptive study using backscattered electron imaging abstract gold nuggets from specular hematite rich sulphide free auriferous veins jacutinga at gongo soco an iron ore deposit in the quadrilatero ferrifero of minas gerais brazil have inclusions of palladium arseno antimonides isomertieite and mertieite ii and hematite both isomertieite and mertieite ii are commonly fractured and their fragments are displaced in relation to each other without any optically recognisable brittle failure of the host gold more rarely isomertieite has sub parallel s shaped intragranular microfractures terminating in angular edges hematite shows microfracturing and displacement as well as microgouge formation and slumping it occasionally exhibits deformation twins microcracking also occurs in gold itself goethite seals microcracks in gold with or without angular fragments of goethite crack sealing in nugget gold by hg pd enriched gold hematite kaolinite infill indicates recurrent fracturing and mineralisation although the microstructures are enigmatic two hypotheses are suggested for their formation i fluid pressure increase and its sudden seismogenic release could fracture erstwhile ductile gold ii the ductile gold transferred tectonic stresses to the harder mineral inclusions which deformed in a brittle manner

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

polymetamorphic relations in iron ores from the iron quadrangle brazil the correlation of oxygen isotope variations with deformation history this study deals with the oxygen isotope composition of hematite rich ore bodies in the iron quadrangle brazil the area studied can be divided into two different regions a western w zone of greenschist facies assemblages and an eastern e zone of amphibolite facies with transitions into granulite facies assemblages

the archean bif hosted cuiaba gold deposit quadrilatero ferrifero minas gerais brazil abstract the cuiaba gold deposit is located in the northern part of the quadrilatero ferrifero minas gerais state brazil the region constitutes an archean granite greenstone terrane composed of a basement complex ca 3 2 ga the rio das velhas supergroup greenstone sequence and related granitoids 3 0 2 7 ga which are overlain by the proterozoic supracrustal sequences of the minas the stratigraphy of the cuiaba area is part of the nova lima group which forms the lower part of the rio das velhas supergroup the lithological succession of the mine area comprises from bottom to top lower mafic metavolcanics intercalated with carbonaceous metasedimentary rocks the gold bearing cuiaba banded iron formation bif upper mafic metavolcanics and volcanoclastics and metasedimentary rocks the metamorphism reached the greenschist facies tectonic structures of the deposit area are genetically related to deformation phases d 1 d 2 d 3 which took place under crustal compression representing one progressive deformational event e n the bulk of the economic grade gold mineralization is related to six main ore shoots contained within the cuiaba bif horizon which range in thickness between 1 and 6 m the bif hosted gold orebodies 4 ppm au represent sulfide rich segments of the cuiaba bif which grade laterally into non economic mineralized or barren iron formation transitions from sulfide rich to sulfide poor bif are indicated by decreasing gold grades from over 60 ppm to values below the fire assay detection limit in sulfide poor portions the deposit is gold only and shows a characteristic association of au with ag as sb and low base metal contents the gold is fine grained up to 60 m and is generally associated with sulfide layers occurring as inclusions in fractures or along grain boundaries of pyrite the predominant sulfide mineral 90 vol gold is characterized by an average fineness of 0 840 and a large range of fineness 0 759 to 0 941 the country rocks to the mineralized bif show strong sericite carbonate and chlorite alteration typical of greenschist facies metamorphic conditions textures observed on microscopic to mine scales indicate that the mineralized cuiaba bif is the result of sulfidation involving pervasive replacement of fe carbonates siderite ankerite by fe sulfides gold mineralization at cuiaba shows various features reported for archean gold lode deposits including the 1 association of gold mineralization with fe rich host rocks 2 strong structural control of the gold orebodies showing remarkable down plunge continuity 3 km relative to strike length and width up to 20 m 3 epigenetic nature of the mineralization with sulfidation as the major wall rock alteration and directly associated with gold deposition 4 geochemical signature with mineralization showing consistent metal associations au ag as sb and low base metal which is compatible with metamorphic fluids

zircon saturation and zr diffusion in rhyolitic melts and zircon growth geospeedometer zircon is a ubiquitous accessory mineral in silicic igneous rocks we have carried out new zircon dissolution experiments to refine our understanding of zr diffusion and zircon solubility in several rhyolitic melts zr diffusivity depends strongly on temperature and h 2 o content and weakly on pressure and anhydrous melt composition the diffusion data for each individual melt follows the arrhenius relation the dependence of zr diffusivity on temperature pressure and melt composition including h 2 o content is modeled for different rhyolitic melts in this study and for the combined literature and our data our data on zr concentration at zircon saturation in silicic melts show strong dependence on temperature and weak dependence on pressure and melt composition and are somewhat off the trend based on existing zircon solubility models the dissolution or growth rate of a freely falling zircon crystal in a specific hydrous rhyolitic melt is modeled the controlling factors are mostly the temperature and zr concentration in the melt typical zircon growth rate in wet rhyolitic melt is 0 01 to 1 0 m yr the size of zircon crystals can be used to place limit on the cooling rate of its hosting magma the presence of large indigenous zircon crystals in bishop tuff requires slow cooling of the bishop tuff magma chamber

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

depositional and erosional bedforms in late pleistocene holocene pro delta deposits of the gulf of patti southern tyrrhenian margin italy abstract multibeam bathymetry high resolution seismic profiles and seafloor samples have been analyzed to characterize depositional and erosional dynamics recorded pro deltaic deposits and outer shelf sediments along of a sector of the ne sicilian margin southern tyrrhenian sea the deltaic deposits cover an area of ca 15 km 2 in front of the mazzarra river and are morphologically characterized by waveforms trending overall along strike and incised cross strike gullies of variable length the gullies are shallow and characterized by small coaxial erosive scours in the inner middle shelf whereas they become larger and deeper in the outer shelf upper slope in relation to the marked increase of slope gradients at the shelf break here the wider gullies are characterized by a frame of crescent shaped bedforms interpreted as cyclic steps indicating the occurrence of sedimentary gravity flows in supercritical regime prodelta waveforms are widespread between 50 and 120 m with wave lengths of 34 110 m and wave heights of 0 5 3 m morphometric characterization and spatial distribution of the waveforms suggests a main role in their genesis played by hyperpycnal flows although we cannot discard the possible effect of internal waves or slow deformation processes i e creep depositional and erosional features similar to those observed on the seafloor are evident in the subsurface stratigraphy revealing the onset and growth of the mazzarra delta since the last glacial maximum the post glacial sea level rise caused lateral shifts of the mazzarra river mouth controlling migration of depositional lobes and intensity of seafloor incision and sediment reworking ultimately resulting in the observed wavy bedforms extending from the inner shelf to the upper slope

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

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

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

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

microstructures in a banded iron formation gua mine india this paper provides a detailed documentation of microstructures developed in the banded iron formation bif of gua mine located in the bonai synclinorium eastern india where the rocks have been subjected to three deformations d 1 to d 3 folded iron ores quartz strain fringes around rigid core objects and folded iron ore layers and refracted quartz veins are described from samples taken from d 2 folds in the banded iron formation orientations of microstructures are compared with mesoscopic structures to interpret the generations of ore minerals planar structures and the time relationship between deformation and development of different microstructures the mechanism of d 2 folding is worked out and its bearing on microstructure development is discussed the d 2 folds are inferred to have developed by a combination of tangential longitudinal strain in the competent layer flexural flow in the incompetent layers and flexural slip at the interface between layers of differing competence homogeneous flattening strain superposed the earlier strain which led to modification of the folds in the competent layer from class 1b to 1c this strain is quantified and is found to be higher in the limb than the hinge of a fold diffusive mass transfer by solution and bulging dynamic recrystallization in quartz are inferred as the dominant deformation processes during folding moreover based on comparison with published deformation microstructure maps the microstructures of the present study are estimated to have developed between 300 and 350 c temperatures at a strain rate of 10 14 10 12 s 1 which are geologically realistic conditions for naturally deformed rocks

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

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

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

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

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

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