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

the cenomanian turonian oceanic anoxic event in the south atlantic new insights from a geochemical study of dsdp site 530a abstract one of the key objectives of deep sea drilling project dsdp leg 75 was to shed light on the underlying causes of cretaceous oceanic anoxia in the south atlantic by addressing two major hypotheses productivity productivity driven anoxia vs enhanced ocean stratification leading to preservation of organic matter and black shale deposition here we present a detailed geochemical dataset from sediments deposited during the cenomanian turonian c t transition and the global oceanic anoxic event 2 oae 2 at dsdp site 530a located off shore namibia southeast angola basin north of walvis ridge to characterise the succession of alternating black and green shales at this site and to reconstruct the evolution of their paleoenvironmental setting we have combined data derived from investigations on bulk organic matter biomarkers and the inorganic fraction the location of the c t boundary itself is biostratigraphically not well constrained due to the carbonate poor but organic matter rich facies of these sediments the bulk 13corg record and compound specific 13c data in combination with published as well as new biostratigraphic data enabled us to locate more precisely the c t boundary at dsdp site 530a the compound specific 13c record is the first of this kind reported from c t black shales in the south atlantic it is employed for paleoenvironmental reconstructions and chemostratigraphic correlation to other c t sections in order to discuss the paleoceanographic aspects and implications of the observations at dsdp site 530a in a broader context e g with regard to the potential trigger mechanisms of oae 2 global changes in black shale deposition and climate on a stratigraphic level an approximation and monitoring of the syndepositional degree of oxygen depletion within the sediments bottom waters in comparison to the upper water column is achieved by comparing normalised concentrations of redox sensitive trace elements with the abundance of highly source specific molecular compounds these biomarkers are derived from photoautotrophic and simultaneously anoxygenic green sulphur bacteria chlorobiacea and are interpreted as paleoindicators for events of photic zone euxinia in contrast to a number of other oae 2 sections that are characterised by continuous black shale sequences dsdp site 530a represents a highly dynamic setting where newly deposited black shales were repeatedly exposed to conditions of subtle bottom water re oxidation presumably leading to their progressive alteration into green shales the frequent alternation between both facies and the related anoxic to slight oxygenated conditions can be best explained by variations in vertical extent of an oxygen minimum zone in response to changes in a highly productive western continental margin setting driven by upwelling

orbital forcing of organic carbon burial in the proto north atlantic during oceanic anoxic event 2 the cenomanian turonian c t intervals at dsdp sites 105 and 603b from the northern part of the proto north atlantic show high amplitude short term cyclic variations in total organic carbon toc content the more pronounced changes in toc are also reflected by changes in lithology from green claystones toc 1 although their depositional history was different the individual toc cycles at sites 105 and 603b can be correlated using stable carbon isotope stratigraphy sedimentation rates obtained from the isotope stratigraphy and spectral analyses indicate that these cycles were predominately precession controlled the coinciding variations in hi oi 13corg and the abundance of marine relative to terrestrial biomarkers as well as the low abundance of lignin pyrolysis products generated from the kerogen of the black claystones indicate that these cyclic variations reflect changes in the contribution of marine organic matter om the cooccurrence of lamination enrichment of redox sensitive trace metals and presence of molecular fossils of pigments from green sulfur bacteria indicate that the northern proto north atlantic ocean water column was periodically euxinic from the bottom to at least the base of the photic zone 150 m during the deposition of the black claystones in contrast the green claystones are bioturbated are enriched in mn do not show enrichments in redox sensitive trace metals and show biomarker distributions indicative of long oxygen exposure times indicating more oxic water conditions at the same time there is evidence e g abundance of biogenic silica and significant 13c enrichment for oc of phytoplanktic origin for enhanced primary productivity during the deposition of the black claystones we propose that increased primary productivity periodically overwhelmed the oxic om remineralisation potential of the bottom waters resulting in the deposition of om rich black claystones because the amount of oxygen used for om remineralisation exceeded the amount supplied by diffusion and deep water circulation the northern proto north atlantic became euxinic during these periods both sites 105 and 603b show trends of continually increasing toc contents and hi values of the black claystones up section which most likely resulted from both enhanced preservation due to increased anoxia and increased production of marine om during oceanic anoxic event 2 oae2

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

ages of protolith and neoproterozoic metamorphism of al p bearing quartzites of the veredas formation northern espinhaco brazil la icp ms age determinations on relict and recrystallized zircon and geodynamic consequences abstract the n s running espinhaco fold belt situated in the sao francisco craton in central eastern brazil separates this craton in a western and an eastern part along the paramirim corridor a brasiliano panafrican 650 500 ma deformation zone the meta sediments of the espinhaco basin consist of several synthems with dominantly siliciclastic deposits among them al and p rich lazulite bearing blue quartzite the amphibolite facies equivalent of a phosphatic sandstone of the veredas formation in the northern espinhaco form a conspicuous horizon zircon crystals from these quartzites have been dated using laser ablation inductively coupled plasma mass spectrometry the zircon crystals are both detrital and metamorphic in origin the latter exhibiting highly unusual textures produced by dissolution reprecipitation reaction and concomitant replacement of metamict areas ree y th u zircon fluid zircon xenotime detrital zircons reveal relict age groups of 3 1 3 3 ga 2 6 2 7 ga 2 0 2 3 ga and 1 5 ga the end of sedimentation and erosion of the magmatic and metamorphic rocks of the sao francisco craton can be estimated therefore as 1 5 ga the major metamorphic event recorded by reprecipitated zircon is neoproterozoic and documented by a lower intercept age of 634 19 ma which was calculated from analyses of individual spots on19 grains with a common relict orosirian rhyacian age of 2032 31 ma temporal equivalent to transamazonian the neoproterozoic age of the metamorphosed zircons of the vereda formation corresponds to the brasiliano panafrican remobilization of the sao francisco west congo craton along the eastern side of the espinhaco fold belt in the paramirim corridor

extreme euxinia just prior to the middle triassic biotic recovery from the latest permian mass extinction abstract the greatest mass extinction on earth occurred 252 million years ago during the latest permian complete biotic recovery characterized by a return to pre extinction diversity levels took an extraordinarily long time ca 5 10 6 yr probably because harsh conditions developed repeatedly during the early triassic here we show the recurrence of euxinic conditions during the early triassic and the development of especially severe such conditions during the late early triassic just prior to full biotic recovery from the mass extinction dibenzothiophenes and carotenoids accumulated in upper lower triassic sediments from south china located on the western margin of the paleotethys ocean among these chlorobactane a biomarker for green pigmented green sulfur bacteria and okenane a biomarker for purple sulfur photosynthetic bacteria were identified in upper early triassic samples both compounds were detected in limestone micrite and we therefore infer that planktonic green and purple sulfur bacteria were the sources of these carotenoids and that their presence indicates photic zone euxinia at the time of deposition concentrations of redox sensitive elements m v and u relative to that of al and the u th ratio progressively increased during the late early triassic the euxinic conditions represented by the carotenoids and the elements were likely caused by a combination of locally shallow restricted conditions and global environmental oscillations such as those related to global warming during the late early triassic the presence of these biomarkers together with the increase in the abundance of redox sensitive elements relative to al and the u th suggests that euxinic conditions could be one of the causes for the delayed recovery of marine communities in the paleotethys ocean after the latest permian mass extinction event

multi stage crustal accretion by magmatic flare up and quiescence intervals in the western margin of the sao francisco craton u pb hf and geochemical constraints from the almas terrane abstract the global plate tectonic regime in early paleoproterozoic times is highly debated the interval 2 45 2 2 ga is known for a minima in juvenile magmatism but this is not a global phenomenon new results of whole rock geochemistry and u pb hf analysis in igneous and detrital zircons allied with existing isotopic and geophysical data allow to identify and constrain the duration of magmatic flare up and quiescence events in the western sao francisco paleoplate igneous samples yield ages indicating three accretionary magmatic events an older with ages ca 2476 4 9 ma to 2462 13 ma an intermediate at 2390 14 ma and a younger from 2235 26 ma to 2201 5 ma all presenting magmatic arc geochemical signatures xenoliths of quartzite and volcanic tuff from the upper greenstone sequence morro do carneiro fm are hosted in the 2211 9 ma tonalite and the maximum depositional age of the morro do carneiro basin is dated 2234 12 ma indicating a syn orogenic setting for this basin detrital zircon u pb age distribution for quartzites of the greenstone sequence shows peaks at 2 65 2 47 2 39 2 27 and 2 23 ga altogether the studied rocks record an accretionary orogeny with four distinct episodes episode s1 2 52 2 46 ga hf t values from 0 57 to 6 36 episode s2 2 43 2 37 ga hf t values from 0 10 to 4 30 episode r1 2 32 2 26 ga hf t values from 1 61 to 7 23 from detrital zircons episode r2 2 24 2 20 ga hf t values from 0 39 to 2 73 these early paleoproterozoic accretionary orogenies mark the onset of amalgamation of the sao francisco continental paleoplate that surrounds the craton with accretions of an exotic micro block and continental magmatic arcs indicating evolution from dominant siderian juvenile magmatism to rhyacian crustal magmatism these patterns show striking similarities to the orogenies in the mineiro belt and north china craton

field and geochronological evidence for origin of the contendas mirante supracrustal belt sao francisco craton brazil as a paleoproterozoic foreland basin abstract the low grade contendas mirante meta volcano sedimentary belt cmb was previously interpreted as an archean greenstone belt with the uppermost formation deposited in the paleoproterozoic new zircon u pb la icp ms ages on sedimentary groups and granite sheets intrusive in the belt constrain the timing of basin development phyllites of the lower group and meta arenites of the upper group yielded similar young zircon populations at ca 2080 ma which are interpreted as the maximum deposition age for the entire sequence the unique exception is the 3273 3338 ma zircon bearing quartzite that occurs in spatial association with the 3300 ma rhyolite a granite sheet intrusive into metagraywacke of the lower group constrains the minimum deposition age of 2045 26 ma for the cmb altogether four main age clusters of detrital zircon grains were recognized namely 2075 2200 ma 2200 2440 ma 2500 2770 ma 3270 3380 ma these age intervals match the ages of igneous and metamorphic rocks in the different terranes that comprise the sao francisco craton maximum deposition age for both groups indicates that sedimentation occurred immediately prior the high grade metamorphism in the adjacent archean paleoproterozoic jequie block and itabuna salvador curaca orogen these observations along with coarsening upwards in the sediments indicate that the cmb represents a foreland basin developed along the eastern margin of the gaviao block during plate convergence paleoarchean basement rocks were reactivated and emplaced into the cmb during basin inversion and is timely correlated with the granitic intrusion recognition of older supracrustal fragments is ambiguous one main example is the 3273 3338 ma zircon bearing quartzite which correlates with the northern jacobina basin along the contendas mirante lineament e g bahia gold belt and may represent part of an archean intracontinental rift sequence

mass dependent and mass independent sulfur isotope fractionation δ34s and δ33s from brazilian archean and proterozoic sulfide deposits by laser ablation multi collector icp ms abstract mass independent sulfur isotope fractionation mif has been observed in rocks of the geological record older than about 2 45 ga a characteristic which is thought to be related to processes in the neoarchean atmosphere samples recording a mif effect therefore have to contain sulfur of the exogenic sulfur cycle while endogenic sulfur should not show this effect the sulfides analysed from six brazilian deposits represent either exogenic or endogenic sulfur sources with supposed ages ranging from about 1 9 to 2 7 ga sulfur isotopes were analysed by in situ laser ablation mc icp ms a range of experiments were conducted using international and in house isotope standards which were run under various conditions and set ups these include the reference materials iaea s1 iaea s3 nbs123 sphalerite nbs127 barite and in house standards bsb py pyrite and bsb cpy chalcopyrite during six days of analysis an internal precision of sulfide analyses of 0 10 0 15 1 s for 34 s and 0 40 0 60 1 s for 33 s and an accuracy of 0 30 for 34 s and 33 s was achieved the standard measurements define a 33 s 34 s relationship of 33 s 0 513 34 s 0 149 with r 0 9997 which is close to the theoretical relationship for mass dependent fractionation the by far best analytical errors were obtained for natural pyrite reaching a within run precision of about 0 05 0 15 for 34 s 1 s and 0 10 0 15 1 s for 33 s determinations including all precision and accuracy data we arrive at the following 1 s error limits to which sulfur isotope analyses for mif studies obtained with this method are reliable 0 32 0 34 and 0 46 for pyrite 34 s 33 s and 33 s determinations respectively 0 34 0 42 and 0 54 for pyrrhotite and 0 34 0 50 and 0 58 for chalcopyrite pyrites and pyrrhotites from 2200 ma old sediments and volcanics associated with the pilar greenstone belt goias have 34 s from 15 0 to 2 0 and 33 s from 8 0 to 1 0 with 33 s being slightly but not significantly positive pyrites from supposed ca 2060 ma old clastic sediments of the aguas claras formation carajas para have 34 s 9 5 12 5 and 33 s 5 0 6 5 with 33 s close to zero chalcopyrites from the salobo and sequerinha sulfide deposits also carajas are genetically related to 2570 ma old a type alkaline granites with endogenic sources they have 34 s 1 6 2 9 and 33 s 1 0 1 5 and 34 s 0 2 0 9 and 33 s 0 65 respectively with 33 s ranging from 0 15 to 0 56 the only 33 s values of this study which clearly suggest sulfur isotope mif come from the mineralized banded iron formations of the quadrilatero ferrifero minas gerais pyrites from the mina de cuiaba deposit 2700 ma have 34 s 3 8 5 1 and 33 s 2 9 4 8 with 33 s 0 9 2 2 while pyrites and chalcopyrites from sao bento 2720 ma have 34 s 1 6 3 2 and 33 s 1 5 2 8 with 33 s 0 2 1 5 the sulfides represent an epigenetic mineralization derived from supracrustal rocks making part of an exogenic sulfur cycle the study shows that in situ laser ablation icp ms is sufficiently sensitive for detecting mif effects in neoarchean rocks whether or not such samples show mif related 33 s values depends on the age and provenance of the sulfur

repeated anoxia extinction episodes progressing from slope to shelf during the latest cenomanian abstract oceanic anoxic event 2 oae 2 during the cenomanian turonian c t transition caused stepwise marine extinctions using organic compounds stable carbon and oxygen isotopes and foraminifera from three depth transect sections in northern spain this study revealed repeated anoxic euxinic events coinciding with warming and stepwise extinctions of planktonic and or benthic foraminifera within intermediate to surface waters in the proto north atlantic during the c t transition those short duration euxinic events occurred four times at 93 95 ma marked by the extinction of rotalipora greenhornensis at 93 90 ma marked by the extinction of rotalipora cushmani at the mid time maximum of the plateau of the 13 c of carbonates 93 70 ma and at the time of the c t boundary 93 55 ma furthermore the main benthic foraminiferal extinctions occurred during the first and second euxinic events in the upper slope during the second and third euxinic events in the outer to middle shelf and during the third and fourth events in the middle shelf the main euxinic events in each section also showed a progression to the shallow shelf the main anoxia extinction events occurred in the upper slope and outer shelf then moved to the middle shelf the shallowest section had relatively weak anoxia and a proportionally low extinction rate these new findings indicate that foraminiferal extinctions started from the intermediate water and the continental slope and then moved to the continental shelf this was the result of the repeated progression of euxinic anoxic water from the upper slope to the middle shelf on the eastern continental margin of the proto north atlantic four times during a 400 kyr period to the end of the cenomanian

no plate tectonic shutdown in the early paleoproterozoic constraints from the ca 2 4 ga granitoids in the quanji massif nw china abstract the global plate tectonic regime in the early paleoproterozoic period is highly debated granitoids bear key information to address such a debate petrological geochemical and geochronological studies are conducted on two post collisional granitoid plutons in the quanji massif northwestern china to investigate the tectonic regime during this period the granitoids are composed of syenogranite monzogranite and granodiorite with minor tonalite which intruded into the delingha and hudesheng regions at ca 2 39 2 37 ga these plutons are high k i type granitoids with variable ga al ratios showing some characteristics of a2 type granitoids they are characterized by enrichment of liles and lrees and depletion of sr p ti and eu they show depleted nd and hf isotope signatures with whole rock end t 0 7 to 4 8 and zircon ehf t 1 0 to 7 8 indicating a juvenile crustal growth event at ca 2 44 2 37 ga our new results together with other coeval post collisional granitoids in the quanji massif suggest that the protracted post collisional magmatism at ca 2 39 2 34 ga occurred just after a short interval of subduction and generation of juvenile magmas before or around ca 2 4 ga collectively the formation of these ca 2 4 2 3 ga granitoids in the quanji massif may correlate with coeval granitoids in the tarim and north china cratons and is broadly coeval with magmatism in several other cratons in the west african and canadian shields thus the globally well documented magmatism at early paleoproterozoic or siderian provides further information for filling up the age gap of the so called plate tectonic shutdown in the early paleoproterozoic period worldwide the geologic record therefore suggests no siderian shutdown of plate tectonics but instead continuous global subduction and generation of juvenile magmas from the archean through the paleoproterozoic

enhanced productivity led to increased organic carbon burial in the euxinic north atlantic basin during the late cenomanian oceanic anoxic event 1 three cenomanian turonian c t 93 5 ma black shale sections along a northeast southwest transect in the southern part of the proto north atlantic ocean were correlated by stable carbon isotope stratigraphy using the characteristic excursion in 13c values of both bulk organic matter om and molecular fossils of algal chlorophyll and steroids all three sites show an increase in marine organic carbon oc accumulation rates during the c t oceanic anoxic event oae the occurrence of molecular fossils of anoxygenic photosynthetic green sulfur bacteria lack of bioturbation and high abundance of redox sensitive trace metals indicate sulfidic conditions periodically reaching up into the photic zone before as well as during the c t oae during the c t oae there was a significant rise of the chemocline as indicated by the increase in concentrations of molecular fossils of green sulfur bacteria and mo al ratios the presence of molecular fossils of the green strain of green sulfur bacteria indicates that euxinic conditions periodically even occurred at very shallow water depths of 15 m or less during the c t oae however bottom water conditions did not dramatically change as indicated by more or less constant v al and zn al ratios at site 367 this suggests that the increase in oc burial rates resulted from enhanced primary productivity rather than increased anoxia which is supported by stable carbon isotopic evidence and a large increase in ba al ratios during the c t oae the occurrence of the productivity event during a period of globally enhanced organic carbon burial rates i e the c t oae points to a common cause possibly related to the formation of a deep water connection between north and south atlantic basins

new oxygen isotope evidence for long term cretaceous climatic change in the southern hemisphere a new composite 18 o record generated from calcareous fine fraction and bulk sediments from the exmouth plateau details long term cretaceous climatic change at mid latitudes in the southern hemisphere assessment of new and previously published 18 o data indicates that a mid cretaceous global climatic optimum was achieved sometime between the time of the cenomanian turonian boundary and the middle turonian when surface ocean paleotemperatures were the highest of the past 115 m y periods of cooling and warming that reversed the general patterns were superimposed on long term aptian turonian warming and turonian maastrichtian cooling trends respectively extrapolation of southern hemisphere paleotemperature trends to maastrichtian paleotemperature data from a low latitude pacific guyot implies that maximum mid cretaceous low latitude paleotemperatures could have been in excess of 33 c

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

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

planktonic foraminifers across the bonarelli event oae2 latest cenomanian the italian record abstract a high resolution quantitative study of the planktonic foraminiferal assemblages across the bonarelli level oae2 latest cenomanian of italy from different areas and geological settings has allowed the recognition of several events and biotic changes including acmes and crises of different genera it provides evidence of a progressive and rapid change of palaeoenvironmental conditions reaching a climax coincident with the bonarelli event and of the subsequent gradual although not complete resumption of previous conditions the observed pattern is illustrated by marked changes in the assemblages that indicate five discrete phases of environmental perturbation within the marine ecosystem though there are several similarities our analyses highlight that each section has its own peculiarities clearly indicating more or less extreme environmental conditions

faster spreading or greater ridge length in the archean dissipation of more heat from a hotter early earth by the plate tectonics mechanism would require subduction of on average younger lithosphere in principle this could be achieved equally well by faster spreading by having greater ridge length or some combination thereof present day evidence however suggests that younger oceanic lithosphere subducts more slowly than older if this applied in the archean then the greater ridge length alternative prevailed a quantitative expression is derived which indicates that oceanic heat loss is proportional to the cube root of ridge length this relationship means that if archean heat flow was 3 times that of the present 27 times as much ridge would have been required which would suggest that the archean earth was covered by many small plates moving slowly

magnetic fabrics and rock magnetism of archaean and proterozoic dike swarms in the southern sao francisco craton brazil abstract magnetic fabric and rock magnetism studies were performed on three mafic dike swarms total of 38 dikes from the southernmost part of the sao francisco craton sfc minas gerais state se brazil they cut archaean granite gneiss migmatite and paleoprototerozoic terranes these swarms are classified as basic noritic sm nd age 2 65 ga basic rb sr age 1 87 ga and metamorphic rb sr age 1 87 ga suites in which the second is the most important magnetic fabrics were determined by applying both anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm in most sites magnetic susceptibility is dominantly carried by ferromagnetic minerals however in some sites the paramagnetic contribution exceeds 70 of bulk susceptibility mainly coarse to fine grained ti poor titanomagnetite up to pure magnetite carry the magnetic fabrics three primary ams fabrics are recognized which are all coaxial with the aarm fabric normal ams fabric is dominant in the basic suite 16 of 20 analyzed dikes and occurs in 4 and 3 dikes from the basic noritic and metamorphic suites respectively this fabric is interpreted as a result of magma flow in which the analysis of k max inclination permitted to infer that the majority of dikes were fed by inclined flows 30 k max k max intermediate ams fabric was found in 50 of dikes from the basic noritic and metamorphic suites but in only 2 dikes from the basic suite it is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike inverse ams fabric is a minority 2 dikes from each suite the parallelism between ams and aarm tensors for dikes with abnormal fabrics suggests a primary origin for them gyroremanent magnetization grm effect was negligible for the majority of dikes but it was found in two dikes from the basic suite with normal ams fabric magnetic fabrics recognized for the three studied swarms do not depend on magnetic mineralogy geochemical composition dike strikes nor the age of the swarms since the same magnetic minerals and magnetic fabric types are found in dikes from all suites inclined and horizontal flows allow us to infer the relative position of at least three magma sources or magma chambers from which the dikes were fed

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

a euxinic southern north atlantic ocean during the cenomanian turonian oceanic anoxic event abstract during mid cretaceous times large amounts of organic carbon corg became sequestered in black shales possibly due to oceanic anoxic events characterized by the development of an extended oxygen minimum zone omz here we provide the first direct evidence for an open ocean omz in the cenomanian turonian c t southern north atlantic and in fact show that the base of the photic zone was euxinic as revealed by molecular fossils from photosynthetic green sulfur bacteria in c t black shales this together with evidence for bottom water anoxia and accumulation of redox sensitive trace metals and hydrogen rich organic matter indicates a continuously euxinic water column concurrent with the high corg accumulation rates which are 15 150 times greater in the southern than in the northern north atlantic and the low biological productivity this suggests that preservation controlled the accumulation of corg in c t black shales

geochemical and climatic effects of increased marine organic carbon burial at the cenomanian turonian boundary perhaps the most significant event in the cretaceous record of the carbon isotope composition of carbonate1 2 other than the 1 2 5 negative shift in the carbon isotope composition of calcareous plankton at the cretaceous tertiary boundary3 is the rapid global positive excursion of 2 13c enrichment which took place between 91 5 myr and 90 3 myr late cenomanian to earliest turonian c t boundary event 1 4 5 this excursion has been attributed to a change in the isotope composition of the marine total dissolved carbon tdc reservoir resulting from an increase in rate of burial of 13c depleted organic carbon which coincided with a major global rise in sea level5 during the so called c t oceanic anoxic event oae 6 here we present new data from nine localities which demonstrate that a positive excursion in the carbon isotope composition of organic carbon at or near the c t boundary7 8 is nearly synchronous with that for carbonate and is widespread throughout the tethys and atlantic basins fig 1 as well as in more high latitude epicontinental seas the postulated increase in the rate of burial of organic carbon may have had a significant effect on co2 and o2 concentrations in the oceans and atmosphere and consequent effects on global climate and sedimentary facies

cenomanian turonian carbonate and organic carbon isotope records biostratigraphy and provenance of a key section in ne sicily italy palaeoceanographic and palaeogeographic implications abstract in eastern sicily a series of highly organic rich black shales occur as exotic blocks 100 m across floating in tectonized sediments argille varicolori unit containing olistoliths of cretaceous palaeogene age a 19 metre section through one of these blocks near the town of novara di sicilia includes cyclically bedded black shales marlstones and claystones which have been dated using planktonic foraminiferal and nannofossil biostratigraphy on this basis the section is assigned to the latest cenomanian and clearly represents a manifestation of the oceanic anoxic event characteristic of that interval total organic carbon values range up to 23 and the relatively high hydrogen indices record the presence of marine organic matter of low thermal maturity high resolution carbonate and organic carbon isotope curves are comparable with those recorded elsewhere in indicating a significant positive excursion and confirm that in the novara di sicilia section the black shales are latest cenomanian in age by comparison with cenomanian turonian black shales exposed elsewhere in italy calabianca section western sicily livello bonarelli bottaccione gorge gubbio marche umbria the section of novara di sicilia is different in being more stratigraphically expanded however this section from eastern sicily does resemble extremely closely coeval sediments cropping out in tunisia and morocco this association is taken as evidence that the argille varicolori unit includes elements that were initially deposited on the north african shelf during cretaceous time

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

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

late archean initiation of manganese metallogenesis its significance and environmental controls abstract manganese metallogenesis was initiated in a modest scale during the late archean time selectively in india and brazil sedimentary deposits occur in both greenstone belts and high grade granulite terranes in shallow water shelf regimes only and are often intimately associated with stromatolites the changes in the rates of continental crustal growth mantle heat flux and outgassing along with the evolution of tectonic regimes sedimentary environments the state of the biosphere and finally the compositions of interacting atmosphere hydrosphere system from the early to the late archean time have been traced as interdependent parameters and interpreted to explain the first appearance of sedimentary manganese deposits promoted by possibly localized oxygenated environments compared to the spectacular manganese event represented by the giant paleoproterozoic kalahari deposit south africa which is interpreted to have been formed as a consequence of the rapid increase in atmospheric oxygen level ca 2 5 2 4 ga ago the modest initiation of the late archean manganese depositions was triggered by localized photosynthetic oxygen supply forming basin margin oxygen oases as a prologue for the succeeding stratified ocean system the dissolved manganese in the basin water that was oxidized to form shelf type shallow water deposits is suspected by analogy with contemporary banded iron formations bifs but not proved to be of hydrothermal ancestry the initiation of concentrated manganese sedimentation during the late archean by itself provides an evidence for an increase in the oxygen content in the contemporary atmosphere hydrosphere system compared to that in the earlier period supporting the geochemical indications from paleoweathering profiles

petrogenesis of the gross spitzkoppe topaz granite central western namibia a geochemical and nd sr pb isotope study abstract the early cretaceous gross spitzkoppe granite is one of several igneous complexes that formed in the damaraland of namibia during an episode of continental rifting and magmatism associated with opening of the south atlantic this is a classic setting of rift related anorogenic magmatism showing the typical association of mantle derived mafic and alkaline rocks along with silicic units the gross spitzkoppe granite is an important example of the latter because its peraluminous composition and crustal isotopic signature makes it well suited to characterize the crustal contribution to magmatism the gross spitzkoppe stock gss is composed of three texturally distinct topaz bearing biotite monzogranites average 76 wt sio 2 30 wt rb sr and 0 5 wt f iron rich biotite is the only mafic silicate in proximity to the stock are numerous felsic dikes rhyodacites to rhyolites the elevated high field strength elements hfse contents and high ga al ratios of the granites and felsic dikes are typical of a type granites the granites and dikes show similar geochemical differentiation trends and the compositions of the more evolved dikes overlaps with the granites this and similar initial isotope ratios suggest that the dikes and granites have a common origin major and trace element modeling confirms that the composition of the gross spitzkoppe granite can be produced by fractional crystallization from magma with the average felsic dike composition rb sr isotope data for nine whole rock samples of the gss yield an age for emplacement of the complex at 125 1 ma initial 87 sr 86 sr ratios vary from 0 710 to 0 716 neodymium isotope compositions of the granites and dikes yield initial e nd 125 values from 5 6 to 6 4 the pb isotopic ratios k feldspar or age corrected whole rock fall in the range 206 pb 204 pb 18 70 18 82 207 pb 204 pb 15 66 15 71 and 208 pb 204 pb 38 57 39 07 these geochemical and isotopic characteristics are consistent with derivation of the gross spitzkoppe granitic magma from a granulite facies metasedimentary source batch melting calculations suggest that low 10 20 degree partial melting of a felsic granulitic source is capable of producing melt compositions similar to those of the felsic dikes with further fractional crystallization being required to produce the granites the source rocks are thought to be damara belt metasediments that were melt depleted in the early paleozoic by extraction of the s type damara granites the most likely scenario for crustal melting is by basaltic underplating of the source region in the lower crust the range in initial nd and sr values is attributed to minor mixing of basaltic magma with the crustal melts although sr was also affected by minor contamination from the radiogenic damara granites near the level of emplacement

periodicity in extinction and the problem of catastrophism in the history of life the hypothesis that extinction events have recurred periodically over the last quarter billion years is greatly strengthened by new data on the stratigraphic ranges of marine animal genera in the interval from the permian to recent these data encompass some 13 000 generic extinctions providing a more sensitive indicator of species level extinctions than previously used familial data extinction time series computed from the generic data display nine strong peaks that are nearly uniformly spaced at 26 ma intervals over the last 270 ma most of these peaks correspond to extinction events recognized in more detailed if limited biostratigraphic studies these new data weaken or negate most arguments against periodicity which have involved criticisms of the taxonomic data base sampling intervals chronometric time scales and statistical methods used in previous analyses the criticisms are reviewed in some detail and various new calculations and simulations including one assessing the effects of paraphyletic taxa are presented although the new data strengthen the case for periodicity they offer little new insight into the driving mechanism behind the pattern however they do suggest that many of the periodic events may not have been catastrophic occurring instead over several stratigraphic stages or substages

warm tropical ocean surface and global anoxia during the mid cretaceous period the middle of the cretaceous period about 120 to 80 myr ago was a time of unusually warm polar temperatures1 repeated reef drowning in the tropics2 and a series of oceanic anoxic events oaes that promoted both the widespread deposition of organic carbon rich marine sediments and high biological turnover3 4 5 6 7 8 the cause of the warm temperatures is unproven but widely attributed to high levels of atmospheric greenhouse gases such as carbon dioxide7 8 9 10 11 12 in contrast there is no consensus on the climatic causes and effects of the oaes with both high biological productivity and ocean stagnation being invoked as the cause of ocean anoxia3 4 5 6 7 8 here we show using stable isotope records from multiple species of well preserved foraminifera that the thermal structure of surface waters in the western tropical atlantic ocean underwent pronounced variability about 100 myr ago with maximum sea surface temperatures 3 5 c warmer than today this variability culminated in a collapse of upper ocean stratification during oae 1d the breistroffer event a globally significant period of organic carbon burial that we show to have fundamental stratigraphically valuable geochemical similarities to the main oaes of the mesozoic era our records are consistent with greenhouse forcing being responsible for the warm temperatures but are inconsistent both with explanations for oaes based on ocean stagnation and with the traditional view reviewed in ref 12 that past warm periods were more stable than today s climate

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

identification of aryl isoprenoids in source rocks and crude oils biological markers for the green sulphur bacteria abstract a series of c13 to c31 aryl isoprenoids 1 alkyl 2 3 6 trimethylbenzenes have been identified in reef hosted oils and their source rocks from the middle and upper silurian of the michigan basin and middle devonian of the alberta basin canada their structure has been confirmed by unambiguous synthesis of the c14 member of the series their structure and isotopic composition indicate that they are derived from isorenieratene from the chlorobiaceae family of sulphur bacteria these results are consistent with geological and geochemical studies that show that the source rocks were deposited under metahaline to hypersaline sulphate and sulphide rich water columns the distribution of other biomarkers in these oils and source rocks indicates that a diverse biota contributed organic matter to the source environment in conjunction with the aryl isoprenoids they show that there is a remarkable similarity in composition between the two sets of oils and source rocks despite their great temporal and geographic separation this reflects the similarity of their environments and emphasizes the importance of sedimentary facies in controlling the composition of organic matter in source rocks and their derived oils

short term euxinia coinciding with rotaliporid extinctions during the cenomanian turonian transition in the middle neritic eastern north atlantic inferred from organic compounds oceanic anoxic event 2 oae2 which occurred during the cenomanian turonian c t transition and lasted 1 m y is characterized by a positive global carbon isotopic excursion and stepwise extinctions in marine biota to examine temporal variations in the dissolved oxygen content of the water column shallow marine c t sediments from northern spain were analyzed for concentrations of dibenzothiophenes which are indicators of euxinic depositional environments and 2 3 6 trimethylarylisoprenoids which probably indicate photic zone euxinia the positive excursion in 13 c values of carbonates is accompanied by short term 10 3 10 4 yr and long term 10 5 yr increases in dibenzothiophene and 2 3 6 trimethylarylisoprenoid concentrations suggesting that the bottom water and photic zone of the eastern marginal sea of the north atlantic ocean were euxinic two of the short term increases in organic compound concentrations took place just after the last occurrence of the planktonic foraminifers rotalipora greenhornensis and r cushmani these transient maxima indicate that the extinction of both planktonic foraminifers was due to short term oaes lasting 10 3 10 4 yr

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

planktonic and benthic foraminiferal extinction events during the last 100 m y abstract five major planktonic and benthic foraminiferal extinction events occurred during the past 100 m y at the cenomanian turonian c t boundary at the cretaceous tertiary k t boundary in the latest paleocene in the late eocene and in the early middle miocene to clarify the character of the five events i estimated 1 paleo waterdepths and paleoenvironments where foraminifera were affected 2 the duration of each extinction event and 3 evaluated possible causes for each extinction event on the basis of recent literature these extinction events can be classified into three fundamentally different types 1 the mass extinction event at the k t boundary was abrupt and concentrated among surface dwelling species 2 extinctions at the c t boundary and during the latest paleocene were abrupt and mainly affected intermediate and deep water taxa over short periods 3 the late eocene and early middle miocene events affected surface and deep water species and involved stepwise or gradual extinctions over a long period 4 m y i attributed the extinction events to 1 a bolide impact for the k t event 2 a decrease in dissolved oxygen in intermediate and deep waters due to climatic warming for the c t and latest paleocene events and 3 long term global cooling for the late eocene and early middle miocene events the following relationships exist between the causes and types of extinction events 1 large extraterrestrial impacts strongly affected surface dwelling organisms in short term extinction events 2 intermediate and deep water low oxygen conditions induced by climatic warming have severely affected intermediate and deep water organisms over medium and short terms

metal source and fluid rock interaction in the archean bif hosted lamego gold mineralization microthermometric and la icp ms analyses of fluid inclusions in quartz veins rio das velhas greenstone belt brazil the lamego orogenic gold deposit 440 742 oz gold measured reserves and 2 4 million t measured resources with an average grade of 5 71 g t au and a cut off grade of 2 15 g t au anglogold ashanti corrego do sitio mineracao s a aga personal communication 2014 is located in the 5 km long trend that includes the world class cuiaba deposit it is hosted in the neoarchean metavolcano sedimentary rocks of the rio das velhas greenstone belt quadrilatero ferrifero brazil mineralization is associated mainly with metachert banded iron formation bif and carbonaceous phyllites in the reclined lamego fold in which the cabeca de pedra orebody represents the hinge zone mineralization is concentrated in silicification zones and their quartz veins as well as in sulfide minerals product of bif sulfidation hydrothermal alteration varies according to host rock with abundant sulfide carbonate in bif and sericite chlorite in carbonaceous phyllite quartz vein classification according to structural relationships and host rocks identified three vein systems the v1 system mainly composed of smoky quartz qtz i and pyrite is extensional crosscuts the bedding plane s0 of bif and is parallel to the fold axis the v2 system of the same composition is represented by veins that are parallel to the s1 2 foliation and s0 this system is also characterized by silicification zones in the bif carbonaceous phyllite contact that has its maximum expression in the hinge zone of folds the v3 system has milky quartz qtz ii veins which result from the recrystallization of smoky quartz located mainly in shear zones and faults these veins form structures en echelon and vein arrays the most common ore minerals are pyrite as pyrite and arsenopyrite fluid inclusion fi trapped in all quartz veins present composition in the h2o co2 ch4 nacl system fluid evolution can be interpreted in two stages i aqueous carbonic fluid trapped in qtz i of low salinity 2 equiv wt nacl and ii carbonic aqueous fluid of moderate salinity average 9 eq wt nacl hosted in qtz ii both stages are characterized by decrepitation temperatures in the range of 200 to 300 c and suggest a fluid of metamorphic origin applying an arsenopyrite geothermometer the calculated formation temperature for the cabeca de pedra orebody is 300 to 375 c the vertical intersection of the isochors allows a minimum pressure calculation of 2 6 kbar the composition of individual fis of this orebody obtained by la icp ms analyses compared with results of fis for the carvoaria velha deposit corrego do sitio lineament highlights a standard composition typical of metamorphic fluids with na k ca mg which increase or decrease in concentration as a function of salinity in both deposits trace elements vary according to fluid rock reactions and are directly related to the host rock composition the comparison of data sets of the two deposits shows that the cabeca de pedra fis have a higher enrichment in zn while cu as and sb are richer in carvoaria velha suggesting influence of the host rock geochemistry the suggested mechanisms for gold precipitation at the cabeca de pedra orebody lamego gold deposit are i hydrolysis of the carbonaceous matter of phyllite and bif affecting fo2 destabilizing sulfur complexes and enhancing gold precipitation ii replacement of bif iron carbonates by sulfides and iii continuous pressure changes that lead to silica precipitation and free gold other than playing the long recognized role of the carbonaceous phyllites as a fluid barrier the data highlight their importance as a source of metals

dividing the south american continent to fit a gondwana reconstruction a model based on continental geology abstract the south american continental plate has been affected by intraplate deformation since the start of west gondwana disruption in the lower cretaceous about 140 ma that the present shape of south america is not precisely the same as its shape in reassembled gondwana partly explains the imperfect fits of the conjugate margins of the south atlantic proposed since the first reconstruction models of the early 20th century several attempts at defining subplates within south america have been published but not all take account of existing knowledge of its continental geology here a subdivision into eight rigid subplates is proposed based primarily on geological and tectonic evidence our model is tested against three published models of a multi subplate africa as re shaped to the pre breakup outline of that continent by visual fitting and the use of piercing points the south america blocks were rotated and the euler poles calculated interactively in reconstruction software all three proposed fits had overlapping block margins within south america indicating post breakup rifting except for the transbrasiliano lineament this nne ssw crustal scale shear zone was used as a boundary for seven of the eight blocks it is clearly the main intraplate accommodation zone in south america and an important piercing point in relation to the kandi lineament in west africa the other block boundaries correspond to narrow zones with sedimentary basins and or dyke swarms that have developed since south atlantic opening each fit required a different configuration of the south america subplates since the pre rift disposition of the african subplates also differ from each other contributing to the uncertainty in any detailed reassembly

tropical warming and intermittent cooling during the cenomanian turonian oceanic anoxic event 2 sea surface temperature records from the equatorial atlantic oceanic anoxic event 2 oae 2 occurring during the cenomanian turonian c t transition is evident from a globally recognized positive stable carbon isotopic excursion and is thought to represent one of the most extreme carbon cycle perturbations of the last 100 myr however the impact of this major perturbation on and interaction with global climate remains unclear here we report new high resolution records of sea surface temperature sst based on tex86 and 18o of excellently preserved planktic foraminifera and stable organic carbon isotopes across the c t transition from black shales located offshore suriname french guiana demerara rise ocean drilling program leg 207 site 1260 and offshore senegal cape verde basin deep sea drilling project leg 41 site 367 at site 1260 where both sst proxy records can be determined a good match between conservative sst estimates from tex86 and 18o is observed we find that late cenomanian ssts in the equatorial atlantic ocean 33 c were substantially warmer than today 27 29 c and that the onset of oae 2 coincided with a rapid shift to an even warmer 35 36 c regime within the early stages of the oae a marked 4 c cooling to temperatures lower than pre oae conditions is observed however well before the termination of oae 2 the warm regime was reestablished and persisted into the turonian our findings corroborate the view that the c t transition represents the onset of the interval of peak cretaceous warmth more importantly they are consistent with the hypotheses that mid cretaceous warmth can be attributed to high levels of atmospheric carbon dioxide co2 and that major oaes were capable of triggering global cooling through the negative feedback effect of organic carbon burial led co2 sequestration evidently however the factors that gave rise to the observed shift to a warmer climate regime at the onset of oae 2 were sufficiently powerful that they were only briefly counterbalanced by the high rates of carbon burial attained during even the most extreme interval of organic carbon burial in the last 100 myr

the ratios of dibenzothiophene to phenanthrene and pristane to phytane as indicators of depositional environment and lithology of petroleum source rocks abstract the ratio of dibenzothiophene to phenanthrene and the ratio of pristane to phytane when coupled together provide a novel and convenient way to infer crude oil source rock depositional environments and lithologies such knowledge can significantly assist in identifying the source formation s in a basin thereby providing valuable guidance for further exploration the ability to infer this information from analysis of a crude oil is especially valuable as frequently the earliest samples in a new area may be shows and or drill stem test samples from exploratory wells which are characteristically drilled on structural highs stratigraphically remote from the source formation s a cross plot of dibenzothiophene phenanthrene versus the pristane phytane ratios measured on seventy five crude oils from forty one known source rocks ranging in age from ordovician to miocene consistently classified the oils into the following environment lithology groups marine carbonate marine carbonate mixed and lacustrine sulfate rich lacustrine sulfate poor marine and lacustrine shale and fluvial deltaic carbonaceous shale and coal the dibenzothiophene phenanthrene ratio alone is an excellent indicator of source rock lithology with carbonates having ratios 1 and shales having ratios the dibenzothiophene to phenanthrene and the pristane to phytane ratios can also be used to classify source rock paleodepositional environments the classification scheme is based on the premise that these ratios reflect the different eh ph regimes resulting from the significant microbiological and chemical processes occurring during deposition and early diagenesis of sediments the dibenzothiophene phenanthrene ratio assesses the availability of reduced sulfur for incorporation into organic matter and the pristane phytane ratio assesses the redox conditions within the depositional environment interpretation of these ratios has been aided by quantitative biomarker analysis and by carbon isotope data for pristane and phytane obtained by gas chromatography isotope ratio mass spectrometry

u pb geochronology of the 2 0 ga itapecerica graphite rich supracrustal succession in the sao francisco craton tectonic matches with the north china craton and paleogeographic inferences abstract la icpms u pb isotopic studies are conducted for key samples from the itapecerica graphite rich supracrustal succession and the crystalline basement in the southern sao francisco craton sfc the detrital zircon analyses in paragneiss indicated a maximum deposition age of 2080 ma the age spectra in the relative probability diagrams suggested the precursor basin was essentially fed with detritus from the accretionary complexes of the mineiro belt 2470 2000 ma which is the most preserved segment of the minas orogen the basement rocks yielded zircon u pb crystallization ages between 2753 81 and 2679 61 ma whereas the metamorphic overprint gave 2069 84 ma one nearby charnockitic gneiss yielded a sm nd garnet whole rock linear array with an age of 2086 19 ma the youngest ages estimate the timing of the regional prograde granulite facies metamorphism that also overprinted the itapecerica rocks given our upper intercept zircon u pb ages of 1997 20 ma and 1971 40 ma in paragneiss and quartzite respectively these ages also constrained the timing of the conversion of carbonaceous material into graphite in the itapecerica mine the second largest graphite mine in brazil the high grade metamorphism was associated with the final docking of the mineiro belt against the neoarchean foreland the minas orogen has geologic matches with the jiao liao ji orogen located in the present eastern margin of the north china craton where khondalites uht granulites anatectic charnockites as well as large graphite deposits are documented such as in the 1 97 1 88 ga korean arc the tectonic similarities between the two cratons support a relatively close position at the time frame the dynamic scenario considers broadly coeval accretionary collisional belts along the active margins of archean blocks the progressive closure of the paleoproterozoic ocean and final docking of the sao francisco congo paleocontinent against the active continental margin of the north china craton the work model allows new insights into the paleogeography of the core of supercontinent columbia at 1 90 ga and the subsequent 1 78 1 50 ga intraplate activity

early to late ediacaran conglomeratic wedges from a complete foreland basin cycle in the southwest sao francisco craton bambui group brazil abstract stratigraphic isotopic and geochronological data from two late neoproterozoic aged conglomerate wedges in the southwest sao francisco craton support the interpretation that the ediacaran bambui group in east central brazil was deposited in a foreland basin the sambura formation forms the base of the bambui group in the southwestern part of the bambui basin and was deposited synchronously with the brasilia orogeny it is interpreted to be a sedimentary product of a retrogradational coastal alluvial fan system deposited in an underfilled flexural foredeep during the early stages of foreland basin development sometime between 630 and 560 ma the basal sete lagoas formation carbonates were deposited towards the cratonic margin on the forebulge which provided an ideal environment for carbonate production the lateral relationship between the sambura and sete lagoas formations further implies that an unconformity was generated by foreland flexure and that this unconformity separates an early ediacaran phase of the foreland basin from a late ediacaran phase the lagoa formosa formation was deposited in the latter phase after peak orogenesis with a provenance that includes post orogenic granites and zircons as young as 560 ma it records a prograding turbidite fan system in the lagoa formosa formation that was deposited during orogenic unroofing and basin wide shallowing in a filled stage of the foreland basin a shift from highly enriched 13 c values towards global like carbon isotopes values in carbonates within the lagoa formation in conjunction with the occurrence of banded iron formation may suggest deposition in a basin with anoxic and ferruginous deep waters in the bambui basin in the latest ediacaran

deep sea paleotemperature record of extreme warmth during the cretaceous oxygen isotope analyses of well preserved foraminifera from blake nose 30 n paleolatitude north atlantic and globally distributed deep sea sites provide a long term paleotemperature record for the late albian maastrichtian interval that is difficult to reconcile with the existence of significant cretaceous ice sheets given reasonable assumptions about the isotopic composition of cretaceous seawater our results suggest that middle bathyal water temperatures at blake nose increased from 12 c in the late albian through middle cenomanian to a maximum of 20 c during the latest cenomanian and earliest turonian bottom waters were again 12 c during the middle campanian and cooled to a minimum of 9 c during the maastrichtian correlative middle bathyal foraminifera from other ocean basins yield paleotemperature estimates that are very similar to those from blake nose comparison of global bottom water temperatures and latitudinal thermal gradients suggests that global climate changed from a warm greenhouse state during the late albian through late cenomanian to a hot greenhouse phase during the latest cenomanian through early campanian then to cool greenhouse conditions during the mid campanian through maastrichtian

crustal origin of early proterozoic syenites in the congo craton ntem complex south cameroon two petrographically different syenite suites can be distinguished in the mostly archean ntem complex of cameroon a suite of two pyroxene syenites and a suite of clinopyroxene syenites the syenites occur as discrete intrusions surrounded by archean gneisses based on structural relationships the syenites are of late to post orogenic origin the pb pb zircon evaporation ages indicate that the syenites intruded at ca 2 3 ga and thus postdate significantly the archean episode of crust formation and differentiation that generated the surrounding gneiss terrane the isotope signatures and the trace elements provide unambiguous evidence that the two syenite suites are of crustal origin the syenite suites do not show simple chemical trends have highly variable trace element contents and display only a small variation in their major element contents particularly in their sio2 contents the isotope characteristics including 143nd 144nd 87sr 86sr and common pb point to a heterogeneous source or multiple sources for the syenites the sr isotope compositions calculated for the time of intrusion vary over a wide range from an unrealistically low 87sr 86sr ratio of 0 690 to high values of 0 713 in contrast the initial 143nd 144nd ratios are more homogenous with end 2320 from 5 8 to 7 5 the pb isotopes from leached k feldspars indicate that the sources of the syenites had a history of u depletion and th u enrichment the syenites show the same isotope characteristics as the surrounding gneiss terrane thus the deep continental crust of the ntem complex itself is the most likely source for the syenite melts this conclusion is also supported by the heterogeneity of the trace elements in the syenites that exclude a homogenous source in addition inherited zircons from the syenites yield pb pb evaporation ages that are consistent with ages obtained for zircons extracted from the surrounding gneisses in contrast to many other syenites worldwide the syenites in the ntem complex do not contain juvenile mantle material but are the sole products of crustal reworking thus the syenites did not contribute to the crustal growth in the ntem complex however this conclusion may not preclude the possibility that the syenites were generated in the deep crust during a time of addition of mantle material to the continental crust the cause for the generation of these large syenite bodies in the ntem complex is most likely an episode of deep seated thermal activity that was so far not known from this part of the congo craton

cenomanian turonian organic sedimentation in north west africa a comparison between the tarfaya morocco and senegal basins abstract the cenomanian turonian oceanic anoxic event was recognised in north western africa in various depositional settings from abyssal areas to continental shelves to derive information on environmental conditions in these different settings and define a depositional model a petrographical and geochemical study of the organic matter was performed on sediments from the tarfaya morocco and senegal basins the results obtained for these two locations were compared to those of previous studies namely from dsdp wells petrographic and geochemical data allow the differentiation of two main organofacies a shallow depositional facies continental shelf is characterised by low total organic carbon toc contents the second organofacies deposited in the deep marine environment slope and abyssal shows a high toc content 7 the predominance of fluorescing amorphous om combined with high hi values suggests good preservation conditions the s c ratio 0 36 and abundance of organic sulphur compounds support this interpretation and indicate a development of anoxic conditions to explain the organic contrast between both environments a depositional model has been developed which is based on limited water exchange between both depositional settings the main factor which has determined black shale sedimentation is the restricted water circulation related to the presence of isolated depositional environment during atlantic ocean opening compared to present upwelling zones the palaeoproductivity in the studied area was relatively moderate during cenomanian turonian and seems not to be the only determining factor of organic matter accumulation

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 and pb pb age constraints on paleoproterozoic magmatism deformation and gold mineralization in the omai area guyana shield abstract the omai intrusion centred au quartz vein system the largest au producer presently operating in the guyana shield was sampled for detailed u pb and pb pb geochronology and petrological investigation the age of a metavolcanic sub volcanic unit in the host rock sequence is 2120 2 ma zircon analyses from the main body dioritic rocks give u pb ages of 2094 6 2092 11 and 2096 11 10 ma magmatic titanite and apatite that grew in hornblende rich peripheral phases of the intrusion define a consistent pb pb age of 2094 1 ma in agreement with the zircon data colourless titanite and rutile from strongly altered phases of the intrusion along with low u apatite and feldspar define a significantly younger pb pb isochron age of 2002 5 ma the igneous ages agree with data from similar units in the guyana shield and west africa showing that 2100 ma was a time of significant intrusive activity the ages obtained for the deformed metavolcanic and undeformed intrusion at omai define a 26 2 ma bracket for trans amazonian deformation in central guyana previous fluid inclusion studies indicate that the mineralizing solutions at omai were too co 2 rich to form titanite and the titanite bearing sample is unmineralized suggesting that it was not altered by gold bearing solutions therefore the 2002 5 ma age is interpreted as a late hydrothermal overprint that formed titanite and reset rutile zircon and baddeleyite from a thick gabbro dyke of the avanavero suite which cuts the omai pluton define an age of 1794 4 ma ruling out the dyke as a source for the late thermal effects the hydrothermal age may record the passage of fluids released by deep crustal metamorphism due to late stage tectonic underplating as previously proposed for the superior province

migrating cretaceous eocene magmatismin the serra do mar alkaline province se brazil melts from the deflected trindade mantle plume department of geological sciences university of durham south road durham dh1 3le uk department of earth sciences university of cambridge downing street cambridge cb2 3eq uk department of physics the university newcastle upon tyne ne1 7ru uk department of geology mcmaster university 1280 main street west hamilton ont canada l8s 4m1 departmento de geoquimica e recursos minerais instituto de geociencias universidade de brasilia

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

low latitude sea surface temperatures for the mid cretaceous and the evolution of planktic foraminifera isotopic data from exceptionally well preserved foraminifera show that the subtropical north atlantic had sea surface temperatures ssts between 30 and 31 c during the late albian and early cenomanian these temperatures were warmer than average modern temperatures in the tropical western atlantic and support previous inferences based on diagenetically suspect material of albian tropical ssts of 30 c this finding is consistent with basic theories of oceanic thermal response in the tropics to the high atmospheric carbon dioxide levels during the middle cretaceous greenhouse however our data do not support the concept of a super tethyan region of hypersaline waters in the western north atlantic during late albian and early cenomanian time because seawater salinities substantially higher than modern ones would require significantly higher ssts than those reported here isotopic analysis of mid cretaceous planktic foraminifera shows that planomalina ticinella and biticinella grew in surface waters and provide isotopic proxies for sst whereas deeper growing species include hedbergella rotalipora and praeglobotruncana none of these taxa display isotopic indications of photosymbiosis suggesting that this ecology did not evolve in planktic foraminifera before the late cretaceous contrary to previous hypotheses none of these species increased their depth habitat during their life cycles unlike many extant planktic foraminifera the rotaliporids may have been preferentially susceptible to extinction during oceanic anoxic events because they lived within or below the thermocline for most of their life cycles

molecular isotopic evidence of environmental and ecological changes across the cenomanian turonian boundary in the levant platform of central jordan abstract we evaluated the structure of planktonic communities and paleoenvironmental conditions throughout the cenomanian turonian oceanic anoxic event oae2 by studying bulk geochemical properties and the molecular isotopic composition of source specific hydrocarbons from organic rich sediments deposited in an intrashelf basin at the levant platform central jordan high concentrations of desmethyl and 4 methylsteranes as well as dinosteranes indicate that marine algae including dinoflagellates were the main primary producing organisms the presence of 2 methylhopanes and 13 c enriched hopanes in addition to isotopically enriched aryl isoprenoids evidenced the contribution of cyanobacteria and green sulfur bacteria respectively additionally variable but fairly low 15 n values during oae2 suggest the occurrence of diazotrophy as a likely important process fueling primary production during oae2 in this stratified anoxic continental platform variations in the relative contribution of biomarkers revealed changes in planktonic communities associated with sea level change and water column stratification oae2 was characterized by strong stratification anoxic bottom waters and a deep chemocline as evidenced by high gammacerane and homohopane indices and the absence of photic zone euxinia pze markers respectively however the presence of isorenieratane and its derivatives in post oae black shales points to a shoaling of the chemocline and to pze this interval was also characterized by an exceptionally high abundance of chlorophyll derived pristane and phytane up to 2 mg g 1 toc likely as a result of highly enhanced primary production and organic matter preservation remarkably this high productivity event co occurs with an exceptionally high abundance of calcispheres reported elsewhere to be part of a global bio event

the cenomanian turonian oceanic anoxic event i stratigraphy and distribution of organic carbon rich beds and the marine δ13c excursion marine strata deposited during late cenomanian and early turonian time display lithological faunal and geochemical characteristics which indicate that significant parts of the world ocean were periodically oxygen deficient at or very close to the cenomanian turonian boundary between 90 5 and 91 5ma ago oxygen deficiencies were particularly marked over a period of 1ma this short lived episode of oceanic oxygen deficiency has been termed the cenomanian turonian oceanic anoxic event oae the widespread distribution of anoxic sediments deposited synchronously during such a short lived event indicates that such sediments are not simply the product of coincidental local climatic or basinal water mass characteristics but are the result of a global expansion and intensification of the cenomanian turonian oxygen minimum zone related to feedback between sea level rise and regional palaeoceanography the palaeoceanography of the cenomanian turonian oae is discussed in detail in a companion paper by arthur et al 1987 from authors

age and paleoenvironment of the cenomanian turonian global stratotype section and point at pueblo colorado biostratigraphy and stable isotopes indicate that the global stratotype section and point gssp at pueblo contains an essentially complete sedimentary record across the global ocean anoxic event oae 2 and the cenomanian turonian boundary the oae 2 d 13 c shift occurred over a period of about 90 ky and was accompanied by a major sea level transgression which at its peak was marked by an incursion of oxygen rich waters creating a benthic oxic zone that lasted about 100 ky a mid cenomanian d 13 c shift sea level transgression and faunal turnover occurred about 2 my before oae 2 d 18 o values of the planktic foraminifer hedbergella planispira and its relative abundance changes reveal cyclic variations in surface salinity due to alternating freshwater influx and marine incursions whereas dominance by the low oxygen tolerant heterohelix species indicates a well developed oxygen minimum zone omz for most of the middle to late cenomanian and early turonian profound faunal changes accompanied these oceanographic events including the extinction of 30 of the species assemblage and an equal gain in evolutionary diversification though the overall combined relative abundances of outgoing and incoming species were less than 2 and 4 respectively of the total assemblages the faunal turnover began with the sea level transgression and rapid increase in d 13 c values and accelerated with the influx of oxygen rich deep water increased water mass stratification and competition during the benthic oxic zone the incursion of oxygen rich deep waters at this time was also observed in morocco and may represent a global event of a still unknown source d 2003 elsevier b v all rights reserved

manganese mineralization in archean greenstone belt joda noamundi sector noamundi basin east indian shield abstract a mesoarchean greenstone belt 3 5 3 0 ga in the western part of the east indian shield comprising the iron ore group of the noamundi basin contains economic resources of both iron and manganese ores in the nne plunging regional synclinorium manganese mineralization in the central and eastern parts of this synclinorium particularly in joda noamundi sector has taken place in multiple cycles starting from syngenetic sedimentary and exhalative type through mobilization and remobilization in different stages of tectonism deformation and hydrothermal activities to latest lateritic or supergene type a relatively high temperature metamorphic jacobsite hausmannite bixbyite braunite assemblage low temperature hydrothermal pyrolusite psilomelane hollandite assemblage and supergene pyrolusite manganomelane groutite polianite assemblage are present and were formed by recycling of manganese in different stages of mineralization a detailed structural study of the manganese ore bodies as well as their ore petrographic and mineralogical characteristics with mineral chemistry has revealed systematic mineralization and their relation to deformational phases such recycling of manganese and its structural control of mineralization in different phases is unique of its kind in comparison with other archean manganese deposits in the world

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

oxygen and neodymium isotope evidence for source diversity in cretaceous anorogenic granites from namibia and implications for a type granite genesis abstract many of the early cretaceous intrusive complexes in the damaraland of west central namibia are silicic in composition although all have trace element characteristics typical for the so called a type granites major differences in alkali aluminum ratios and isotopic compositions require diverse magma sources this paper presents nd and o isotope data from the five largest silicic complexes paresis erongo brandberg cape cross gross spitzkoppe that provide new constraints on the nature of crustal and mantle sources involved and their relative proportions the paresis complex has an isotopic signature 18o 9 end130 ma 21 indicating a crustal component similar to mesoproterozoic gneisses of the angola craton the other complexes have isotope variations 18o from 8 1 to 10 7 and end130 ma from 1 to 9 that can be explained by a binary mixing model between a mantle and crustal component more importantly this same mixing line also fits the nd o isotope variations reported from the mafic okenyenya and messum complexes and from rhyodacites in the southern etendeka volcanic sequence the uniformity of the crustal component implied by this mixing model suggests lower crustal material in contrast to the geologic complexity of the neoproterozoic damara belt presently exposed at the surface this is consistent with the isotopic data and we interpret the crustal component to be lower crustal metametasediments that were dehydrated and perhaps melt depleted by generation of the s type granites which are widespread in the damara belt the mantle component is interpreted to be dominated by the tristan mantle plume but some involvement of depleted mantle material is needed to explain all of the isotope data the data rule out any significant role for enriched subcontinental mantle lithosphere all silicic damaraland complexes as well as the etendeka rhyodacites classify as a type granites despite their proven source diversity this means that the a type geochemical characteristics of these and by analogy other anorogenic granites does not depend on a particular magma source the unifying feature of the diverse granites in the damaraland is likely to be related to the high temperatures involved which allow partial melting of dehydrated and possibly melt depleted lower crustal lithologies

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

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

black shale deposition on the northwest african shelf during the cenomanian turonian oceanic anoxic event climate coupling and global organic carbon burial high resolution geochemical records from a depth transect through the cenomanian turonian c t tarfaya basin northwest african shelf reveal high amplitude fluctuations in accumulation rates of organic carbon oc redox sensitive and sulphide forming trace metals and biomarkers indicative of photic zone euxinia these fluctuations are in general coeval and thus imply a strong relationship of oc burial and water column redox conditions the pacing and regularity of the records and the absence of a prominent continental signature suggest a dynamic depositional setting linked to orbital and higher frequency forcing determining the dominant frequency depends on the definition of the most pronounced oceanic anoxic event oae2 and its duration we propose that eccentricity is the main forcing factor at tarfaya and controlled fluctuations in wind driven upwelling of nutrient rich oxygen depleted intermediate waters from the adjacent atlantic and the periodic development of photic zone and bottom water euxinia on the mid cretaceous northwest african shelf accumulation records clearly identify the basin center as the primary site of sediment deposition with highest temporal variability and an up to six fold increase in oc burial from similar to2 g m 2 yr prior to the oae2 to similar to12 g m 2 yr during the oae2 photic zone and bottom water euxinia alternated with periods of greater oxygenation of the water column in response to climate forcing mass balance calculations imply that similar to2 of the overall global excess oc burial associated with the oae2 was deposited in the tarfaya basin an area that represented only similar to0 05 of the total global c t ocean floor in fact the lateral extent of similar black shales along the african continental margin indicates that this part of the ocean contributed significantly to the global increase in organic carbon burial during the oae2

fluid inclusion and sulfur and oxygen isotope studies on quartz carbonate sulfide veins of the carvoaria velha deposit corrego do sitio gold lineament quadrilatero ferrifero minas gerais brazil abstract the corrego do sitio lineament is defined as a 16 km long ne sw trending ductile shear zones which controls fourteen gold deposits including the carvoaria velha deposit the dominant lithotypes of this deposit are metagrewackes with subordinate carbonaceous phyllites lenses of the archean rio das velhas greenstone belt which host gold mineralization metamafic dikes and sills are parallel and crosscut the host metasedimentary sequence all these rocks exhibit alteration to quartz carbonate and sericite besides sulfides and sulfosalts the main gold mineralization styles at the carvoaria velha deposit and at corrego do sitio as a whole are quartz carbonate sulfide sulfosalt veins of varied distribution the veins are classified as v1 veins quartz ankerite pyrite berthierite gold parallel to the main regional foliation s n v2 veins quartz ankerite pyrite developed at extensional crenulation cleavage s n 1 and rarely gold mineralized v3 veins quartz ankerite filling s n 3 fractures usually free of sulfides and sulfosalts v4 veins quartz calcite of restricted occurrence in metamafic dikes and sills the latter crosscut the metasedimentary sequence are extensional and display no preferential orientation the most common ore minerals in all vein types are arsenopyrite pyrite berthierite and pyrrhotite microprobe analyses reveal the presence of metallic phases rich in sb pb and co such as stibnite ullmanite tetrahedrite galena cobaltite which commonly overgrow the sulfides fluid inclusion studies trapped in quartz from v1 v2 and v4 veins have identified a h 2 o co 2 ch 4 nacl mineralizing fluid that may contain kcl mgcl 2 e fecl 2 the presence of ch 4 in fluid inclusions of the v1 and v2 veins reflect interaction of the fluid with the corrego do sitio and santa quiteria carbon bearing metapelitic host units based on the microthermometric data the hydrothermal fluid is interpreted to have evolved in at least two stages i an early fluid stage trapped in smoky quartz of moderate salinity 8 5 eq wt nacl and minimal trapping temperature of 330 17 c and ii a late stage fluid trapped in recrystallized quartz with lower salinity 4 6 eq wt nacl and a minimal trapping temperature of 365 33 c isotopic values of 18 o fluid 7 9 and 13 0 33 s 0 3 to 3 5 and 34 s 2 9 to 6 1 suggest that indeed the hydrothermal fluid responsible for the gold mineralization at the carvoaria velha deposit must have had a metamorphic origin and interacted with metasedimentary sequences during its ascending path the addition of ch 4 during fluid rock interaction may have caused some decrease in o 2 of the fluid which as a consequence destabilized gold bearing sulfur complexes liberating s 2 for the formation of fe sulfides and sulfosalts arsenopyrite pyrite pyrrothite and berthierite etc and outcome gold deposition

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

end cenomanian benthic foraminiferal extinctions and oceanic dysoxic events in the northwestern pacific ocean abstract the second largest cretaceous extinction event coincided with an oceanic anoxic event at the cenomanian turonian c t boundary this high resolution analysis of upper bathyal benthic foraminifera in central hokkaido japan indicates that the extinction event occurred in the latest cenomanian over a period of 0 2 m y 93 6 93 4 ma analysis of the dissolved oxygen index based on calcareous benthic foraminiferal wall thickness size and morphology shows that low oxygen 0 1 1 5 ml l conditions developed during a period of 0 15 m y before the c t boundary and 1 5 m y after the minimum oxygen condition 0 1 1 ml l prevailed twice during this interval similarly benthic foraminiferal extinctions occurred twice in concert with initiation of the two minimum oxygen levels coincidence of the two extinction horizons with developemt of low oxygen conditions suggests that the main cause of the extinctions was formation of dysoxic intermediate water this report suggests that the global c t boundary extinction event occurred in a stepwise fashion during the latest cenomanian 0 2 0 5 m y due to development of widespread oceanic anoxic dysoxic conditions within intermediate water masses

petrology geochemistry and sm nd systematics of the paleoproterozoic itaguara retroeclogite from sao francisco congo craton one of the oldest records of the modern style plate tectonics abstract paleoproterozoic retrogressed eclogite retroeclogite occurs in the itaguara sequence included in the suture zone formed by collision between the archean divinopolis and campo belo bonfim complexes in the southern sao francisco craton which represents the south american counterpart of the african congo craton the itaguara retroeclogite contains scarce omphacite and phengite but abundant garnet porphyroblasts embedded in a fine grained amphibole biotite and quartz bearing matrix the 2 20 0 05 ga eclogitization event garnet and whole rock sm nd isochronic age of the e morb protolith tdm 2 47 ga is recorded by omphacite formation during high pressure prograde stage in amphibole eclogite facies due to 70 km depth subduction process amphibole eclogite facies metamorphic peak stage of 17 20 kbar and 600 700 c occurred during 2 1 ga continental collision tectonic exhumation related decompression during collision probably triggered partial melting of the eclogitic rock finally decompression late stage estimated between 5 and 8 kbar and 550 650 c under amphibolite facies overprint during orogenic collapse was responsible for appearance of kelyphitic reaction rims symplectite around garnet crystals as its paleoproterozoic contemporary analogues from congo craton the itaguara retroeclogite is one of the oldest records of the modern style plate tectonics

enhanced productivity rather than enhanced preservation led to increased organic carbon burial in the euxinic southern proto north atlantic ocean during the cenomanian turonian oceanic anoxic event three cenomanian turonian c t 93 5 ma black shale sections along a northeast southwest transect in the southern part of the proto north atlantic ocean were correlated by stable carbon isotope stratigraphy using the characteristic excursion in c values of both bulk organic matter om and molecular fossils of algal chlorophyll and steroids all three sites show an increase in marine organic carbon oc accumulation rates during the c t oceanic anoxic event oae the occurrence of molecular fossils of anoxygenic photosynthetic green sulfur bacteria lack of bioturbation and high abundance of redox sensitive trace metals indicate sulfidic conditions periodically reaching up into the photic zone before as well as during the c t oae during the c t oae there was a significant rise of the chemocline as indicated by the increase in concentrations of molecular fossils of green sulfur bacteria and mo al ratios the presence of molecular fossils of the green strain of green sulfur bacteria indicates that euxinic conditions periodically even occurred at very shallow water depths of 15 m or less during the c t oae however bottom water conditions did not dramatically change as indicated by more or less constant v al and zn al ratios at site 367 this suggests that the increase in oc burial rates resulted from enhanced primary productivity rather than increased anoxia which is supported by stable carbon isotopic evidence and a large increase in ba al ratios during the c t oae the occurrence of the productivity event during a period of globally enhanced organic carbon burial rates i e the c t oae points to a common cause possibly related to the formation of a deep water connection between north and south atlantic basins