the use of quartz grain microtextures in the study of the origin of sand terrains in western australia the origin of extensive sand terrains which lie inland from the coastal margin of western australia is contentious with the debate centering around an in situ vs an aeolian origin to resolve this debate the shape and surface features of sand grains are reported for sandplains sand dunes and bedrock for the victoria plateau sandplain which lies in the central west coastal region of western australia all three components of the victoria plateau are similar in grain shape and microtexture quartz grains show a combination of chemical dissolution and precipitation micromorphology characteristics of dissolution include etch patterning triangular shaped etch pits and solution features precipitation forms include edge rounding silica veneers and plate and sheet like structures features such as peeling plates complex precipitation forms projecting from grain surfaces adhering particles and preserved grain contact faces indicate stability rather than a transport dominated environmental history the combination of preserved dissolution and precipitation forms a dearth of mechanically derived features and a correspondence between grains in rock and overlying sands points to local siliceous sedimentary rocks as the source of the sands

cenozoic ice rafting history of the central arctic ocean terrigenous sands on the lomonosov ridge the cenozoic ice rafted debris ird history of the central arctic is reconstructed utilizing the terrigenous coarse sand fraction in iodp 302 cores from 0 to 273 meters composite depth this holocene middle eocene quantitative record of terrigenous sand accumulation on the lomonosov ridge along with qualitative information on grain texture and composition confirms the interpretation that ice initiation sea ice and glacial ice occurred 46 ma in the arctic and provides a long term pattern of arctic ice expansion and decay since the middle eocene ird mass accumulation rates range from 0 to 0 13 g cm2 ka in the middle eocene and from 0 to 0 36 g cm2 ka in the neogene ird mass accumulation rate mar maxima in the miocene and pliocene cooccur with either glacial initiation or intensification in the sub arctic the 46 25 ma ird onset in the central arctic slightly precedes the earliest evidence of ice in the antarctic and compares in timing with a 1000 ppm decrease in atmospheric concentrations of co2 the decline of pco2 in the middle eocene may have driven both poles across the temperature threshold that enabled the nucleation of glaciers on land and partial freezing of the surface arctic ocean especially during times of low insolation

re os and pb pb geochronology of the archean salobo iron oxide copper gold deposit carajas mineral province northern brazil rhenium osmium ages were determined for two molybdenite samples and a pb pb age was derived from bornite chalcopyrite magnetite at the salobo iron oxide copper gold deposit to determine the timing of mineralization and its relation to the nearby old salobo granite rhenium osmium dating of molybdenite spatially associated with copper sulfide minerals yields ages with weighted means of 2576 8 and 2562 8 ma removing the error multiplier introduced by the decay constant uncertainty appropriate for comparing ages from the same isotopic system these data convincingly argue for two temporally separated pulses of molybdenite deposition at 2576 1 1 4 ma n 2 and 2561 7 3 1 ma n 3 the 2576 8 ma age coincides with a previously published u pb age of 2573 2 ma for the old salobo granite suggesting that main stage ore formation may have been contemporaneous with granite magmatism the slightly younger 2562 ma age most likely represents new molybdenite precipitation associated with the development or reactivation of local shear zones lead lead stepwise leaching of copper sulfide minerals yields a less precise isochron age of 2579 71 ma and supports an archean age for the salobo ores this is the first documentation of an archean iron oxide copper gold deposit and the re os and pb pb geochronology herein support 2580 2550 ma estimates for basement reactivation and regional granite magmatism associated with the development of brittle ductile shear zones

trace elements in ilmenite a way to discriminate provenance or age in coastal sands trace elements and ti percentage in ilmenite grains magnetically separated from modern and late pleistocene coastal sands of southeastern virginia and northwestern north carolina were used to distinguish different deposits multivariate analysis of ilmenite composition ti mn mg cr v ni and cu from coastal deposits and potential source rivers enabled the identification of dominant source rivers using the trace element content of one mineral instead of heavy mineral suites eliminated most of the hydraulic sorting selective weathering and intrastratal solution problems that often obscure heavy mineral provenance determinations most ilmenite grains lacked exsolution or twinning which are common to ilmenite however there were no significant optical differences between river and coastal deposits and thus weathering effects were considered to be negligible in provenance determinations based on ilmenite composition owing to the dynamic mixing of beach sands during deposition they contained more homogeneous ilmenite trace element values than did river or bay sands late pleistocene and modern beach deposits were compositionally similar but different from associated bay sands bay sands were more similar to different source river deposits than were beach sands despite a similar primary or distal provenance subtle differences in the mixture of proximal sources were revealed between the ilmenite composition of samples from a modern and a late pleistocene beach deposit besides aiding in provenance determination ilmenite trace element content thus might be used for distinguishing beach deposits of different ages and for subsurface correlation of discontinuous segments from a barrier island chain or other similarly well mixed sand deposit

surface microtextures of ice rafted quartz grains revealing glacial ice in the cenozoic arctic abstract ice rafted grains of the central arctic ocean sediments provide a reliable proxy for arctic palaeoenvironments and climatic conditions that prevailed at the time of their deposition to understand ice involvement and implications for environmental advancement it has proven necessary to ascertain whether these detrital grains were rafted on glacial ice or sea ice surface microtexture identification allows grain differentiation based on the processes and conditions in its environmental provenance here quartz sand grain surface microtextures are used to detect glacially crushed and reworked grains from the lomonosov ridge sediments obtained by the integrated ocean drilling program expedition 302 arctic coring expedition the data show that the most frequent distributions of glacial grains were deposited at the middle late miocene and during the pliocene and early pleistocene especially the middle miocene climatic cooling and the presence of glacial ice are well indicated at 13 5 ma sediment grains earliest finding of the high frequency occurrence of glacigenic microtextures represents the age of 56 ma this first sedimentological evidence for the late palaeocene glacial ice supports the existing hypothesis of ephemeral polar glaciations during times of marked global warmth however due to lack of uninterrupted sediment recovery and non continuous nature of sampling the glacially influenced samples recovered here should be treated as an estimate of the minimum frequency of glacier appearance in the arctic

regional scale pressure shadow controlled mineralization in the principe orogenic gold deposit central brazil abstract the principe gold deposit situated in the natividade gold district resulted from establishment of the regional transbrasiliano strike slip system during the late neoproterozoic brasiliano event the host rocks however were formed in the paleoproterozoic with sm nd model ages from 2250 to 2500 ma therefore the principe gold deposit is epigenetic the local lithological association is a tonalitic granodioritic suite formed in an island arc environment that later experienced continental collision tectonics leading to the intrusion of the peraluminous principe granite with an age of 2169 42 ma by zircon u pb during the neoproterozoic these units underwent deformational greenschist facies metamorphism and hydrothermal alteration forming metamorphic tectonites inside the principe granite and hydrothermally altered rocks at the northern termination of the granite where the principe gold deposit is located a strong structural control is identified in the deposit with n10 w n20 e trending brittle ductile shear zones that are visible at all scales from thin sections to regional satellite images these shear zones played an important role in the formation of the deposit acting as pathways that promoted a channelized fluid flow to metal precipitation sites hydrothermal alteration processes of different grades occur along these structures mainly characterized by saussuritization biotitization chloritization sericitization carbonatization silicification and sulfidation reactions producing a neoformed mineral assemblage the principe granite acted as a regional scale porphyroclast for migration of the hydrothermal solutions as well as deformation forming two pressure shadows that acted as structural traps for the ore as stress release zones these structures concentrated hydrothermal solutions during seismic fault movement events under a transpressive regime they also changed the physicochemical conditions in the fluid by abruptly reducing the pressure and increasing the fluid rock interaction rate producing a structural framework characterized by a pervasively interconnected system of discontinuous and anastomosed quartz carbonate sulfide gold veins fieldwork petrographic analysis and mineral chemistry characterization suggest that the fluid had an aqueous carbonic nature and low salinity values thermometric studies suggest that gold mineralization occurred between 250 c and 400 c stable isotopes of s c and o indicate a metamorphic source for the fluid whole rock geochemistry and microprobe studies highlight hydrothermal alteration processes involving fe ca mg k na au and ag metasomatism the principe gold deposit belongs to the orogenic class but it is distinguished from other orogenic type occurrences in the natividade gold district and from most orogenic deposits of the world for not being located along a shear zone instead it is situated within a regional scale auriferous pressure shadow that was brought about by a short lived pressure drop during the brasiliano cycle

sulfur cycling at the mid atlantic ridge a multiple sulfur isotope approach abstract the role of sulfur in two hydrothermal vent systems the logatchev hydrothermal field at 14 45 n 44 58 w and several different vent sites along the southern mid atlantic ridge smar between 4 48 s and 9 33 s and between 12 22 w and 13 12 w is examined by utilizing multiple sulfur isotope and sulfur concentration data isotope compositions for sulfide minerals and vent h 2 s from different smar sites range from 1 5 to 8 9 in 34 s and from 0 001 to 0 051 in 33 s these data indicate mixing of mantle sulfur with sulfur from seawater sulfate combined 34 s and 33 s systematics reveal that vent sulfide from smar is characterized by a sulfur contribution from seawater sulfate between 25 and 33 this higher contribution compared with epr sulfide indicates increased seawater sulfate reduction at mar because of a deeper seated magma chamber and longer fluid upflow path length and points to fundamental differences with respect to subsurface structures and fluid evolution at slow and fast spreading mid ocean ridges additionally isotope data uncover non equilibrium isotopic exchange between dissolved sulfide and sulfate in an anhydrite bearing zone below the vent systems at fluid temperatures between 335 and 400 c 34 s values between 0 2 to 8 8 for dissolved and precipitated sulfide from logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at smar 34 s values between 24 5 and 6 5 and 33 s values between 0 001 and 0 125 for sulfide bearing sediments and mafic ultramafic host rocks from drill cores taken in the region of logatchev indicate a clear contribution of biogenic sulfides formed via bacterial sulfate reduction basalts and basaltic glass from smar sites with 33 s 0 008 reveal lower 33 s lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials we conclude that the combined use of both 34 s and 33 s provides a more detailed picture of the sulfur cycling in hydrothermal systems at the mid atlantic ridge and uncovers systematic differences to hydrothermal sites at different mid ocean ridge sites multiple sulfur isotope measurements allow identification of incomplete isotope exchange in addition to isotope mixing as a second important factor influencing the isotopic composition of dissolved sulfide during fluid upflow furthermore based on 33 s we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when 34 s were identical

nature and origin of the bif hosted sao bento gold deposit quadrilatero ferrifero brazil with special emphasis on structural controls abstract the orogenic banded iron formation bif hosted au mineralization at sao bento is a structurally controlled hydrothermal deposit hosted by archean rocks of the rio das velhas greenstone belt quadrilatero ferrifero region brazil the deposit has reserves of 14 3 t au and historical underground production of 44 6 t au between 1987 and 2001 the oxide facies sao bento bif is mineralized at its lower portion where in contact with carbonaceous pelitic schists particularly in the proximity of sulfide bearing quartz veins shear related au deposition is associated with the pervasive hydrothermal sulfidation mainly arsenopyrite of the fe rich bands of the sao bento bif auriferous sulfide and quartz rich zones represent proximal alteration zones they are enveloped by ankerite dominated haloes which reflect progressive substitution of siderite and magnetite within the bif by ankerite and pyrrhotite respectively the sao bento bif was intensely and extensively deformed first into open upright folds that evolved into tight asymmetric isoclinal folds the inverse limb of these folds attenuated and gave way to sheath folds and the establishment of ductile thrusts mineralized horizons at sao bento result from early structural modifications imposed by major transcurrent and thrusts faults comprising the conceicao barao de cocais and sao bento shear zones dextral movement on the sw ne directed conceicao shear zone may have generated splays at a compressional side stepping zone such as the sao bento shear zone which is the structural locus for the sao bento gold mineralization relaxation of the conceicao shear zone under more brittle conditions resulted in the development of dilatational zones where gold sulfide quartz veins formed these structures are considered to have been generated in the archean geochronological data are scarce with pb pb analyses of refractory arsenopyrite and pyrite from bedded and remobilized ore plotting on a single stage growth curve at 2 65 ga a later compressional ductile deformation of unknown age overprinted rotated and flattened the original n60e directed structure of the whole rock succession with development of planar and linear fabrics that appear similar to proterozoic aged structures fluid inclusion studies indicate low salinity aqueous fluids with or without co 2 and or ch 4 with extremely variable co 2 ch 4 ratios of probable metamorphic origin fluid evolution shows a paragenetic decrease in the carbonic phase from 10 15 to 5 and increase in the h 2 o co 2 ch 4 and co 2 ch 4 ratios suggesting important interaction with carbonaceous sediment trapping conditions indicate a temperature of 300 c at 3 2 kbar

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

uv induced mass independent sulfur isotope fractionation in stratospheric volcanic sulfate 1 sulfuric acid aerosols produced in the stratosphere following massive volcanic eruptions possess a mass independent sulfur isotopic signature acquired when volcanic so2 experiences uv photooxidation the volcanic data are consistent with laboratory so2 photooxidation experiments using uv light at 248 nm maximum absorption of ozone whereas sulfur isotopic anomalies previously observed in archean samples are consistent with photodissociation at 190 220 nm a mechanism of so2 photooxidation occurring in the early stage of a stratospheric volcanic plume in the range of 220 320 nm weak band absorption of so2 is also proposed since mass independent sulfur isotope anomalies in stratospheric volcanic sulfate appear to depend on the exposure of so2 to uv radiation their measurements might therefore offer the possibility to determine the degree of uv penetration in the ozone absorption window for the present and past atmospheres they can also be used to determine the stratospheric or tropospheric nature of volcanic eruptions preserved in glaciological records offering the possibility to reassess the climatic impact of past volcanic eruptions

in situ sulfur isotope analysis of sulfide minerals by sims precision and accuracy with application to thermometry of 3 5 ga pilbara cherts abstract secondary ion mass spectrometry sims measurement of sulfur isotope ratios is a potentially powerful technique for in situ studies in many areas of earth and planetary science tests were performed to evaluate the accuracy and precision of sulfur isotope analysis by sims in a set of seven well characterized isotopically homogeneous natural sulfide standards the spot to spot and grain to grain precision for 34 s is 0 3 for chalcopyrite and pyrrhotite and 0 2 for pyrite 2sd using a 1 6 na primary beam that was focused to 10 m diameter with a gaussian beam density distribution likewise multiple 34 s measurements within single grains of sphalerite are within 0 3 however between individual sphalerite grains 34 s varies by up to 3 4 and the grain to grain precision is poor 1 7 n 20 measured values of 34 s correspond with analysis pit microstructures ranging from smooth surfaces for grains with high 34 s values to pronounced ripples and terraces in analysis pits from grains featuring low 34 s values electron backscatter diffraction ebsd shows that individual sphalerite grains are single crystals whereas crystal orientation varies from grain to grain the 3 4 variation in measured 34 s between individual grains of sphalerite is attributed to changes in instrumental bias caused by different crystal orientations with respect to the incident primary cs beam high 34 s values in sphalerite correlate to when the cs beam is parallel to the set of directions uuw from 111 to 110 which are preferred directions for channeling and focusing in diamond centered cubic crystals crystal orientation effects on instrumental bias were further detected in galena however as a result of the perfect cleavage along 100 crushed chips of galena are typically cube shaped and likely to be preferentially oriented thus crystal orientation effects on instrumental bias may be obscured test were made to improve the analytical precision of 34 s in sphalerite and the best results were achieved by either reducing the depth of the analysis pits using a kohler illuminated primary beam or by lowering the total impact energy from 20 kev to 13 kev the resulting grain to grain precision in 34 s improves from 1 7 to better than 0 6 2sd in both procedures with careful use of appropriate analytical conditions the accuracy of sims analysis for 34 s approaches 0 3 2sd for chalcopyrite pyrite and pyrrhotite and 0 6 for sphalerite measurements of 34 s in sub 20 m grains of pyrite and sphalerite in 3 5 ga cherts from the pilbara craton western australia show that this analytical technique is suitable for in situ sulfur isotope thermometry with 50 c accuracy in appropriate samples however sulfides are not isotopically equilibrated in analyzed samples

multiple sulfur isotopes from paleoproterozoic huronian interglacial sediments and the rise of atmospheric oxygen mass dependently fractionated mdf sulfur isotopes in sedimentary sulfides and sulfates can provide information on the past activity of microbial metabolisms and serve as a proxy for the concentration of seawater sulfate in conjunction with these proxies mass independently fractionated mif sulfur isotopes can be used to track the rise of atmospheric oxygen the anoxic oxic transition of the surface environment in the paleoproterozoic has been constrained by mif sulfur in 2 47 ga sulfides from western australian banded iron formations and the absence of mif in diagenetic sulfides from 2 32 ga black shales in the transvaal supergroup south africa we report new multicollector ion microprobe data for individual sulfides from water lain sedimentary units in the 2 45 2 22 ga huronian supergroup ontario canada the authigenecity of sulfides in these units is interpreted on the basis of host lithology mode of distribution sulfide chemistry and mdf sulfur isotopes sulfides interpreted to have an authigenicsedimentary origin from the mckim fm have a range of 0 7 in 33 s values and those in the pecors fm preserve small magnitude mif 33 s values up to 0 88 which we propose represents the last remnants of an anoxic atmosphere high 34 s values up to 31 2 and near zero 33 s values in authigenic sulfides from the stratigraphically higher espanola and gordon lake fms suggest local variations of seawater sulfate concentrations and or 34 ssulfate consistent with increased atmospheric o2 levels mif sulfur is absent from the marine record after the second huronian glaciation which is analogous to the record of the transvaal supergroup where mif is absent from the upper part of the interglacial sequence our results suggest that the possible simultaneous demise of mif sulfur isotopes at these two localities may serve as a useful global geochemical marker to correlate paleoproterozoic supergroups we propose a new biogeochemical model where enhanced weathering rates during paleoproterozoic post glacial thawing served as a critical stimulus for interglacial blooms of oxygenic photosynthesis the demise of methane and ultimately to the irreversible rise in atmospheric oxygen

in situ multiple sulfur isotope analysis by sims of pyrite chalcopyrite pyrrhotite and pentlandite to refine magmatic ore genetic models with growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic magmatic hydrothermal and hydrothermal ore systems these materials include sierra pyrite fes2 nifty b chalcopyrite cufes2 alexo pyrrhotite fe 1 x s and vmso pentlandite fe ni 9s8 that have been chemically characterized by electron microprobe analysis isotopically characterized for 33s 34s and 36s by fluorination gas source mass spectrometry and tested for homogeneity at the micro scale by secondary ion mass spectrometry beam sample interaction as a function of crystallographic orientation is determined to have no effect on 34s and 33s isotopic measurements of pentlandite these new findings provided the basis for a case study on the genesis of the long victor nickel sulfide deposit located in the world class kambalda nickel camp in the southern kalgoorlie terrane of western australia results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite pyrrhotite and chalcopyrite can better constrain genetic models related to ore forming processes data indicate that pentlandite pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar 33s values 0 2 this isotopic equilibrium unequivocally fingerprints the isotopic signature of the magmatic assemblage the three sulfide phases show slightly variable 34s values 34schalcopyrite 2 9 0 3 34spentlandite 3 1 0 2 and 34spyrrhotite 3 9 0 5 which are indicative of natural fractionation careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage which may help resolve the nature of the ore forming process hence this sims based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic and alteration related sulfides which may not be resolved during bulk rock fluorination analysis the results indicate that unlike the giant dunite hosted komatiite systems that thermo mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative 33s values the kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales

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

distribution geometry and origin of heavy mineral placer deposits on oregon beaches abstract beach segments 0 5 3 km in length enriched in heavy minerals 20 100 by weight abundance are located south of six prominent headlands that interrupt the 175 km coastline between cape lookout and cape perpetua oregon beach face retreat in the winter of 1982 83 exposed underlying placers containing the economic minerals garnet zircon ilmenite and chromite in each case the highest concentration of placer minerals occurs in an area of maximum shoreline curvature at a distance south of the north bounding headland that is proportional to seaward extent of the headland cross sections and magnetic surveys of one 8 km coastal section otter rock to yaquina head define a small placer body heavy minerals 50 some 0 75 km in length and 80 103 m3 in volume that comprises 10 of the total beach sand between the bounding headlands mineral grain size analyses of winter beach samples along this coastal section confirm earlier models of mineral segregation by entrainment processes the light minerals are more susceptible to fluid shear stress than the heavy minerals maximum segregation and heavy mineral enrichment occur in the backshore and at shoreline inflection points where decreasing flow velocities produce gradients of fl id shear stress allowing light minerals to be winnowed from more stable heavy minerals development of placer deposits is controlled by shoreline configuration as the first order factor and seasonal variations of wave climate and longshore currents as second order factors that delineate shape and concentration of heavy minerals in placer bodies

sulfur isotopic composition of fe ni sulfide grains in ci and cm carbonaceous chondrites in situ secondary ion mass spectrometry analyses of 32 s 33 s and 34 s in iron nickel sulfide grains in two ci1 chondrites and six cm chondrites were performed the results show a wider range of both enrichment and depletion in 34 s relative to troilite from the canyon diablo meteorite cdt than has been observed in previous studies all data points lie within error of a single mass dependent fractionation line sulfides from ci1 chondrites show 34 scdt from 0 7 to 6 8 while sulfide grains in the cm1 chondrite are generally depleted in heavy sulfur relative to cdt 34 s from 2 9 to 1 8 cm2 chondrites contain sulfide grains that show enrichment and depletion in 34 s 34 scdt from 7 0 to 6 8 sulfates forming from sulfide grains during aqueous alteration on the chondrite parent body are suggested to concentrate light sulfur leaving the remaining sulfide grains enriched in the heavy isotopes of sulfur the average degree of enrichment in 34 s in cm chondrite sulfides is broadly consistent with previously suggested alteration sequences

geology and shrimp u pb geochronology of the igarape bahia deposit carajas copper gold belt brazil an archean 2 57 ga example of iron oxide cu au u ree mineralization a striking feature of the carajas region brazil is the clustering of a variety of different types of cu au deposits the most abundant in the belt are the 200 million metric tons mt of fe oxide cu au u ree deposits which despite the variety of host rocks and different orebody morphologies share a number of diagnostic features including 1 intense fe metasomatism leading to the formation of grunerite fayalite and or fe oxides magnetite and or hematite 2 intense carbonate alteration mainly siderite 3 sulfur poor ore mineralogy chalcopyrite and bornite 4 quartz deficient gangue 5 extreme low ree enrichment and 6 enrichment in u and co the igarape bahia deposit is perhaps the best documented fe oxide cu au u ree deposit of the belt containing about 219 mt at 1 4 percent cu and 0 86 g t au the cu au ore consists of steeply dipping breccia bodies that are hosted by hydrothermally altered metavolcano sedimentary rocks shrimp ii zircon dating of the host metavolcanic rocks gives a 207pb 206pb age of 2748 34 ma this suggests a correlation between the igarape bahia volcano sedimentary sequence and the grao para volcanic rocks which have published ages of ca 2 75 ga shrimp dating of monazite from the matrix of ore bearing magnetite breccias gives a 207pb 206pb age of 2575 12 ma confirming the epigenetic nature of the mineralization and placing it 175 m y after accumulation of the host volcano sedimentary sequence the 2575 12 ma shrimp age of hydrothermal monazite from the igarape bahia mineralization is indistinguishable from published conventional 207pb 206pb ages for zircons from the archean a type granites of the carajas belt indicating that mineralization processes at igarape bahia were temporally related to these a type archean granites the wide range of highly radiogenic 87sr 86sr ratios 0 714 0 755 of carbonates from the igarape bahia deposit suggests multiple crustal sources consistent with a magmatic hydrothermal origin shrimp dating of zircon xenocrysts recovered from crosscutting diabase dikes indicates a maximum 207pb 206pb age of 2670 ma consistent with field evidence and the age of host rocks but does not unequivocably constrain the age of the ores the styles of hydrothermal alteration mineralogy and geochemistry of the igarape bahia ore as well as published fluid inclusion and stable isotope data support its classification as a member of the world class olympic dam type fe oxide cu au u ree group of deposits as previously argued by several authors the shrimp age of 2575 12 ma for hydrothermal monazite indicates that igarape bahia is an archean example of this deposit group

metasomatic diamond growth a multi isotope study 13c 15n 33s 34s of sulphide inclusions and their host diamonds from jwaneng botswana abstract the formation of diamond through metasomatic events from volatiles enriched fluids melts brought into pre existing mantle rocks has been suggested from a series of independent studies however the link between these hypothetical volatile rich fluids and deep seated mineral inclusions mostly silicate and sulphides entrapped in diamonds remains unclear yet the relationship between these two species may provide the key to our understanding of diamond crystallization in order to address the relationship between the origin and formation of diamonds and their mineral inclusions we carried out the first coupled stable isotopic study 13 c 15 n and multiple s isotopes as 34 s 33 s 33 s of diamonds containing sulphide inclusions using samples from the jwaneng kimberlite as our model system sulphides extracted from the present collection of 55 diamonds belong to either eclogitic e type 52 diamonds or peridotitic p type 3 diamonds paragenetic suites as attested by their cr and ni content cr 0 18 wt and ni 14 wt for p types sulphur isotopic compositions have been measured in situ by multicollector secondary ion mass spectrometry and reveal that peridotitic sulphide inclusions in diamonds n 4 lie on the terrestrial fractionation line whereas mass independent sulphur isotope fractionations mif are preserved inside eclogitic sulphides inclusions 0 5 33 s n 33 such large mif values extended here to the negative values recorded in archean sediments are thought to be produced through photochemical reactions in the archean atmosphere which implies that sulphides inclusions contain an archean sedimentary sulphur component that was transferred to the earth s mantle in contrast to this isotopic dichotomy between e type and p type sulphides the geochemical characteristics of their host diamonds 13 c 15 n n content n speciation are largely homogeneous throughout the population and differ from silicate bearing diamonds previously studied in the same locality sulphide bearing diamonds appear to define a distinct population which might reflect a different crystallization process characterized by carbon and nitrogen isotopic compositions falling within the range of unfractionated mantle values the lack of a distinct isotopic signature of recycled sedimentary nitrogen and carbon in sulphide bearing diamonds leads us to infer that sulphide bearing jwaneng diamonds were not derived from the same chemical reservoir as their inclusions one possible formation mechanism that is compatible with our observations is diamond crystallization from a mantle derived carbon bearing fluid associated with pre existing sedimentary sulphide minerals this model has important implications for the interpretation of diamond ages obtained from sulphide inclusions using re os systematics here we suggest that the incorporation of a significant amount of mantle related sulphur during a diamond growth event into the original sulphide could lead to a re equilibration of the re os isotope composition in any inclusion and accordingly account for some scatter on re os isochrons

sulfur isotopic compositions of sulfides along the southwest indian ridge implications for mineralization in ultramafic rocks the recently explored tianzuo hydrothermal field in serpentinized ultramafic rocks of the amagmatic segment of the ultraslow spreading southwest indian ridge displays high temperature sulfide mineralization isocubanite sphalerite and minor pyrrhotite and low temperature pyrite and covellite phases pyrite can be subdivided into pyrite i and ii with the former generally having a pseudomorphic texture after pyrrhotite and the latter typically growing around isocubanite sphalerite and pyrite i or occurring as individual grains in quartz veinlets the sulfide minerals have the greatest range of 34s values 23 8 to 14 1 found so far among modern sediment starved ridges with distinct 34s values for low and high temperature mineral phases the high 34s values of isocubanite 9 6 to 12 2 and sphalerite 9 1 to 14 1 suggest that sulfate which precipitated from seawater during an early low temperature phase of hydrothermal circulation was the main sulfur source for these sulfides pyrite ii has the lowest and most variable 34s values 23 8 to 3 6 suggesting microbial sulfate reduction pyrite i has variable and generally positive 34s values 0 1 to 12 0 with sulfur being inherited from pyrrhotite from the original thermochemical reduction of sulfate mixed with volcanogenic sulfur intermittent magmatism represented by gabbroic intrusions and high permeability caused by well developed fractures associated with detachment faults contributed to the formation of sulfides in the tianzuo hydrothermal field these factors possibly control sulfide mineralization in amagmatic segments of ultraslow spreading ridges

sulfur isotope and metal variations in sulfides in the bif hosted orogenic cuiaba gold deposit brazil implications for the hydrothermal fluid evolution abstract textural chemical and multiple sulfur isotope analyses of sulfides from the archean world class cuiaba orogenic au deposit quadrilatero ferrifero district brazil were conducted to track distinct trace element signatures from syngenetic to epigenetic hydrothermal pyrite types in different host units and to deduce the nature and source of the mineralizing fluids gold mineralization in cuiaba is hosted in greenschist facies volcanic and sedimentary host rocks belonging to the archean rio das velhas supergroup especially in association with quartz veins and sulfide minerals the bif hosted fonte grande sul orebody is controlled by quartz veins and disseminated gold bearing pyrite pyrrhotite and arsenopyrite in the wall rocks five pyrite types are classified based on textural relations as spongy syngenetic py1 porous early py2 smooth main py3 and smooth isolated and overgrown late stage py4 and py5 types variably present in metamorphosed carbonaceous pelites bif and andesite besides pyrite pyrrhotite and arsenopyrite are also defined according to textural and paragenetic differences and they display significant trace element contents trace element abundance maps and la icp ms analyses display trace element incorporations in py1 present only in carbonaceous pelite py2 and py3 in all host rocks and py4 and py5 only in bif py1 yields high as co ni pb and ag concentrations whereas py2 formed by agglomeration of py1 maintains high trace element concentrations with slightly less of those elements trace element incorporation in py3 is similar to py1 in carbonaceous pelite early stage py2 and main stage py3 in bif and andesite have increased co and ni but have less au and as late stage py4 is characterized by lower trace element concentrations whereas py5 is further enriched in as bi co ni and pb the hydrothermal alteration is divided into early main and late stage which correlate to the sulfide hydrothermal evolution the study shows that carbonaceous pelite is pre enriched in co ni and pb whereas certain elements like ag au bi and as are only hydrothermally concentrated either during the early main or late stage in bif and andesite this geochemical pattern supports a syngenetic versus hydrothermal origin of distinct elements including some trace elements now in the ore zones multiple sulfur isotope data suggest that mineralizing fluids at the cuiaba gold deposit reflect a complex mixing of sulfur evolved from three possible sources seawater mantle and reduced elemental sulfur spongy syngenetic py1 yields 33s values ranging from 2 28 to 0 25 separated into two ranges the first from 2 28 to 1 97 and the second from 0 96 to 0 25 the first range suggests py1 in the carbonaceous pelite was deposited in a seawater environment these pyrites probably mixed with later mantle sulfur carrying a near zero 33s signature which is indicated by the second 33s range or could have mixed with fluids sourced from sedimentary rocks at depth carrying a positive 33s signature the second scenario is more common in sedimentary and diagenetic pyrites an unequivocal contrast is shown by early main and late stage pyrite types py2 py3 py4 and py5 in carbonaceous pelite bif and andesite which present a continuous process of crustal assimilation values towards increasingly positive 34s and 33s values this confirms evidence for a sedimentary derived sulfur source as also indicated by fluid inclusion data of the andesite hosted orebody at the cuiaba deposit

clay mineral fluctuations and surface textural analysis of quartz grains in pliocene quaternary marine sediments from wilkes land continental rise east antarctica paleoenvironmental significance abstract pliocene quaternary marine sediments from wilkes land continental rise were investigated using clay mineral analysis and quartz grain surface texture observations the studied piston cores were collected during a geophysical and geological survey in the framework of the italian and australian wega project the cores were collected along two transects crossing the channel mound system that characterizes the wilkes land continental rise massive and laminated facies were recognized in these sedimentary sequences the clay mineral assemblages are dominated by illite smectite and chlorite occur in intermediate amounts whereas kaolinite is scarce illite derives from the hinterland of the mertz and ninnis glaciers smectite and kaolinite derive from the ross sea area and they are transported by westward marine bottom currents chlorite is derived both from the adjacent continent and from the victoria land different concentrations of individual clay minerals in the massive and laminated facies are controlled by the interplay between different source areas and the main sedimentary processes glacial and eolian quartz grains prevail in the massive units indicating short transportation and rapid deposition from melting ices with no further significant transport action by marine currents in the laminated facies the high amount of quartz grains affected by high energy subaqueous transport features are related both to turbiditic processes and to bottom current transport

mid to late pleistocene ice drift in the western arctic ocean evidence for a different circulation in the past the provenance of ice rafted debris ird in four arctic sediment cores implies that icebergs from the northwestern laurentide ice sheets drifted across the western arctic ocean along the 180 0 meridian toward fram strait during mid to late pleistocene deglaciations within the last 700 000 years this iceberg drift was different from the present day beaufort gyre circulation and resembled a dislocated transpolar drift tpd sea ice mainly followed the iceberg trajectories but also frequently drifted from the russian shelves eastward into the amerasian basin

sulphur isotope evidence for an oxic archaean atmosphere the presence of mass independently fractionated sulphur isotopes mif s in many sedimentary rocks older than approximately 2 4 billion years gyr and the absence of mif s in younger rocks has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2 4 gyr ago this is because the only mechanism known to produce mif s has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen poor atmosphere here we report the absence of mif s throughout approximately 100 m sections of 2 76 gyr old lake sediments and 2 92 gyr old marine shales in the pilbara craton western australia we propose three possible interpretations of the mif s geologic record 1 the level of atmospheric oxygen fluctuated greatly during the archaean era 2 the atmosphere has remained oxic since approximately 3 8 gyr ago and mif s in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere or 3 mif s in rocks was mostly created by non photochemical reactions during sediment diagenesis and thus is not linked to atmospheric chemistry

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

a new understanding of the provinces of the amazon craton based on integration of field mapping and u pb and sm nd geochronology abstract new conventional and sensitive high resolution ion microprobe zircon u pb dating has led to a new understanding of the subdivision and evolution of the amazon craton during precambrian time with major improvements and changes made to the previous rb sr based model the interpretation of u pb and sm nd isotopic data identifies eight main precambrian tectonic provinces in the craton with ages ranging from 3 1 to 0 99 ga some of the provinces were generated by accretional arc related processes carajas transamazonic tapajos parima and rondonia juruena and others by recycling of continental crust central amazon rio negro and sunsas the exposed archean crust is restricted to the east carajas and south amapa in brazil and north imataca in venezuela of the craton indicating that the amazon craton is largely a proterozoic crust the carajas imataca 3 10 2 53 ga and transamazonian 2 25 2 00 ga provinces are composed predominantly of granite greenstone terranes the tapajos parima 2 10 1 87 ga and rondonia juruena 1 75 1 47 ga provinces represent new crust added as orogenic belts while the rio negro 1 86 1 52 ga and sunsas 1 33 0 99 ga provinces originated mainly by magmatic tectonic recycling of the above two orogenic belts the only zone with a prominent northeast trend is the poorly known k mudku shear belt characterized by a 1 20 ga shear zone which deforms the rocks of at least three different provinces rio negro tapajos parima and transamazonic the central amazon province comprises mostly orosirian volcano plutonic rocks uatuma magmatism and is a terrane in which the exposed crustal structure and deformation are pluton related the sm nd tdm model ages and nd suggest that the central amazon province was generated by the partial melting of archean continental crust carajas province perhaps related to underplating that began at the end of the tapajos parima orogeny 1 88 1 86 ga

in situ sulfur isotope analysis by laser ablation mc icpms and a case study of the erlihe zn pb ore deposit qinling orogenic belt central china abstract sulfur isotopic composition is useful for tracing sources of ore forming materials in situ sulfur isotope analysis by laser ablation coupled with multi collector inductively coupled plasma mass spectrometry la mc icpms has an advantage over traditional bulk analysis in tracing sources of sulfur in an ore deposit that has complex mineral types and multiple ore forming processes however in using this method matrix effects exist widely for sulfides from ore deposits in this work matrix effects of different sulfides were examined by matrix unmatched tests in which the 34s values of different types of sulfides were calibrated against a laboratory pyrite standard the results show that the 34s values of pyrrhotite chalcopyrite molybdenite sphalerite and arsenopyrite are in agreement with the reference values suggesting that the matrix effect was negligible however a significant matrix effect occurred in the analyses of ag2s and galena leading to significant deviations in 34s values of up to 1 2 and 3 2 respectively the matrix effect occurred not only during the laser ablation stage but also during the transition of ions into the mass spectrometer resulting in significant space charge effects caused by heavy mass elements thus pyrite can be used as an external bracketing standard for sulfides that do not have a heavy mass matrix based on our matrix effect study the following available external standards were selected for the in situ sulfur isotope analysis of complex type minerals from the erlihe zn pb deposit the coarse grained sphalerite grains sph1 and euhedral to subhedral pyrite grains py1 from massive ores have 34s values ranging from 9 2 to 18 1 such heavy sulfur isotopic compositions indicate the derivation of sulfur from the deposits of seawater sulfate related to a thermochemical reducing process fine grained sphalerite sph2 and pyrite py2 from vein and disseminated ores coexist with pyrrhotite and chalcopyrite and have relatively light sulfur isotopic compositions with 34s values of about 7 the slightly positive 34s values are similar to those of sulfides from sulfide rich quartz veins which contain 34s values of about 4 and granodiorite rocks with 34s values of about 6 based on previous studies the light sulfur of the late stage sulfides may have resulted from a metamorphic or magmatic hydrothermal event during the indosinian orogeny in situ sulfur analysis by la mc icpms provides new evidence for sources of sulfur from each stage of mineralization in the erlihe deposit suggesting that this is a promising method for studying complex ore deposits

analysis of barrier island surface sediments by scanning electron microscopy abstract the analysis of quartz grain surface microtextures from a sedimentary environment where grain transport and depositional pathways were closely interconnected allowed insight into subtle microfeature alteration during movement between subenvironments four fire island new york and one montauk point beach sample were analysed principally through use of a checklist approach using 36 common surface features beach overwash floodtidal delta dune and inlet grains displayed similar surface features only the latter two samples displaying distinctive microtextural patterns the pattern of development of inlet grains was related to the application of higher shear stresses on the grains during transport in the shallow channel the results from dune grains testify to the effectiveness of aeolian transport mechanisms in rapidly altering existing features glacially derived microtextures altered by subsequent subaqueous action were prevalent but whether these glacial microfeatures are derived from the headland region montauk point or from a closer offshore glacial lobe source is not determined comparison of offshore lobe and adjacent sediments revealed contrasting surface and morphological differences statistical treatment of checklist data in binary form 1 0 using stepwise discriminant analysis proved workable but more rigorous testing is required to prove its relevance and use in deciphering raw textural data

simultaneous measurement of sulfur and lead isotopes in sulfides using nanosecond laser ablation coupled with two multi collector inductively coupled plasma mass spectrometers abstract we herein report the coupling of a nanosecond laser ablation system with a large scale multi collector inductively coupled plasma mass spectrometer nu1700 mc icpms np 1700 and a conventional nu plasma ii mc icpms np ii for the simultaneous laser ablation and determination of in situ s and pb isotopic compositions of sulfide minerals we found that the required aerosol distribution between the two spectrometers depended on the pb content of the sample for example for a sulfide containing 100 3000 ppm pb the aerosol was distributed between the np 1700 and the np ii spectrometers in a 1 1 ratio while for lead contents 3000 and

a comparative sem study on the micromorphology of glacial and nonglacial clasts with varying age and lithology preliminary research on striated clasts from a variety of depositional environments suggests that scanning electron microscopy sem of striated clasts of varying lithology in diamictons may prove useful in interpreting a glacial origin evaluating whether sem analysis of clasts from diamictons is an applicable technique to define a glacial origin requires a better understanding of the microfeatures occurring on glacial and nonglacial clasts we describe microtextures and surface characteristics for samples of quartzite granite limestone basalt chert pillow basalt and quartz pebbles collected from a variety of depositional environments our study suggests that it is possible to differentiate between glacial and nonglacial deposits based on frequency and morphology of diagnostic surface microfeatures observed on entrained clasts these microfeatures are best preserved and identified on competent and monomineralic samples e g quartzite chert limestone and poorly preserved on less resistant and p

magnetic anomaly map of north america data presented on the magnetic anomaly map in the continental areas have largely been derived from published map sources however these maps for the most part have been digitized and subjected to various processing steps in preparation for compositing the marine areas of the map consist mostly of digital data track except in the gulf of mexico caribbean sea and the continental shelf areas of alaska where the data were obtained from digitized maps

sources and provinces of late pleistocene and holocene sand and silt on the mid atlantic continental shelf abstract the shapes and surface textures of quartz sand and silt grains from the mid atlantic continental shelf were measured and analyzed with the fourier grain shape technique and examined with the scanning electron microscope in order to determine the sources and distribution of late pleistocene and holocene sediment on this shelf the study indicates that there are two distinct sources of sediment for the mid atlantic shelf the atlantic coastal plain and the appalachian highlands the sediment contribution of the atlantic coastal plain is represented by abundant spherical rounded quartz grains with surface textures created by the weathering of the quartz grains in organic rich soils the sediment contribution of the appalachian highlands is represented by abundant nonspherical angular quartz grains with crystalline nodes grain embayments fractures and euhedral quartz overgrowths created by the crystallization recrystallization and diagenesis of quartz the distribution of coastal plain and appalachian sand on the mid atlantic shelf is controlled to a great extent by the late quaternary paleogeography the highest percentages of appalachian sand are generally found within the estuaries and on the surfaces of the shelf valleys and deltas of the hudson great egg delaware susquehanna and roanoke river systems which drain the appalachian highlands the highest percentages of coastal plain sand are generally found in areas most removed from the depocenters of the appalachian river systems and in areas that are proximal to exposures of coastal plain strata on the other hand the distribution of coastal plain and appalachian silt is controlled to a great extent by the prevailing holocene coastal and shelf currents these currents are ery capable of transporting silt across the shelf and have effectively mixed appalachian silt from the depocenters of the appalachian river systems with coastal plain silt from the intervalley areas five sedimentary petrologic provinces are defined for the mid atlantic shelf on the basis of areal variations in the abundances of coastal plain and appalachian sand three provinces hudson delaware and hatteras are the deposits of the large appalachian river systems of the mid atlantic seaboard the two remaining provinces southern new jersey and southern delmarva are deposits of fluvial and littoral sediments that were eroded from local outcrops of coastal plain strata

u pb geochronology of archean magmatism and basement reactivation in the carajas area amazon shield brazil abstract the archean carajas subprovince is a metallogenic district bif bauxite cu ni mn au located in the easternmost amazonian craton in brazil the main conclusions from a u pb study of the itacaiunas supergroup and a locality in the xingu complex are the following the xingu basement complex the oldest unit underwent migmatization at 2859 2 ma and also grew new zircon at 2851 4 ma the luanga mafic intrusion is 2763 6 ma old and coeval with the 2759 2 ma felsic volcanics of the grao para group amphibolites from the salobo group contain 2851 ma zircons were metamorphosed 2761 3 ma and intruded by granitic veins ca 2758 ma at the time of grao para magmatism and also grew new zircon at 2732 2742 ma the data indicate that the salobo group is part of the xingu complex and not of the itacaiunas supergroup as heretofore believed the pojuca group possibly coeval with the salobo group was metamorphosed at 2732 3 ma a late archean reactivation event of regional extent is marked by titanite growth at 2581 5 ma in the mirim area intrusion of the old salobo granite at 2573 2 ma new zircon growth in salobo group amphibolites at 2555 4 3 ma and monazite growth in a metamorphosed iron formation at 2551 2 ma in addition k metasomatism associated with shear zones formed titanite in salobo group amphibolites at 2497 5 ma and new titanite at 2519 5 ma in the xingu complex events in the 2581 2497 ma interval are probably associated with the latest activity of a wnw ese sinistral shear system which could also be responsible for the establishment of a transtensional basin filled by the volcano sedimentary sequence of the grao para bahia and rio fresco groups finally undeformed proterozoic granitoid intrusions were dated at 1883 2 ma cigano 1883 5 2 ma musa 1880 2 ma carajas and 1874 2 ma pojuca

character of the alleghanian orogeny in the southern appalachians part iii regional tectonic relations geological and geochronological studies in the eastern piedmont of the southern appalachians have documented a polyphase late paleozoic deformational chronology d 2 d 4 that is contemporaneous with the alleghanian orogeny recorded in the western appalachian foreland the scale and vergence of d 3 anticlinoria in the eastern piedmont are similar to those of anticlinoria in the western appalachians which are related to ramping of underlying thrust surfaces we suggest that the appalachian decollement extends southeastward beneath the entire piedmont province and that all of the piedmont rocks were displaced at least 175 km northwestward relative to north america during the alleghanian orogeny late paleozoic deformational effects in the eastern piedmont thus are considered cogenetic with alleghanian foreland deformation we interpret the alleghanian orogeny to be the result of oblique dextral convergence and collision of gondwana and laurentia

numerical simulation of thermally induced gyres in lake biwa the formation process of three gyres in lake biwa as observed using an acoustic doppler current profiler is simulated using three different sgs models smagorinsky model dynamic mixed model dmm and mixed scaling formulation model msfm based on very large eddy simulation vles the driving force of gyre formation is assumed to be differential heating due to heat volume differences between the middle lake and shoreline areas this condition is created in the simulation by setting the initial temperature distribution in the lake based on field observation data the significant patterns of gyre formation are calculated both by dnm and msfm owing to the huge disproportion of horizontal and vertical grid spacing the isotropic subgrid scale sgs model such as the smagorinsky model does not perform well when using normal grid spacing according to the comparison between field observation data and numerical simulation it is concluded that the surface heat transfer process is one of the main driving forces for gyre formation in lake biwa copyright 2003 john wiley sons ltd

geology and pyrite sulfur isotopes of the suoluogou gold deposit implication for crustal continuum model of orogenic gold deposit in northwestern margin of yangtze craton sw china abstract since the crustal continuum model was proposed mainly based on the variation of mineralization and alteration features of hypozonal mesozonal to epizonal deposits genetic relationship between the different types in one ore belt still requires research this issue was addressed by the research on the suoluogou mesozonal orogenic gold deposit and its comparison with the hypozonal orogenic gold deposit in northwestern margin of yangtze craton gold mineralization in suoluogou is controlled by the bends of roughly ew trending brittle fault developed in the late triassic strata and it is associated with quartz sericite chlorite carbonate and sulfide alteration pyrite and arsenopyrite are the predominant sulfide mineral with minor amounts of chalcopyrite sphalerite and galena four paragenetic stages are divided based on crosscutting relationship and mineral assemblage including the pre mineralization stage stage the hydrothermal quartz sulfide stage stage and carbonate quartz sulfide stage stage iii and the post mineralization carbonate pyrite stage stage the 34s values for pyrite in stage vary from 6 01 to 8 59 and those in stage iii are from 0 96 to 4 62 the decrease of 34s value from stage to iii is explained to the result of the isotope fractionation during separation of oxidised sulfur species in the brecciated space the sulfide 34s values in the initial gold mineralization stage of the suoluogou deposit are similar to those of regional hypozonal orogenic deposit with derivation from metasomatized mantle in the craton margin this similarity suggests that the mesozonal and hypozonal gold deposits formed in different depths of a crustal continuum model for the deposits sharing similar subcrustal source

heavy mineral beach placers in southeastern australia their nature and genesis southeastern australia has been one of the world s most productive heavy mineral sand provinces concentrations of detrital rutile zircon ilmenite and minor monazite occur as beach placers and in dunes in high stand barriers of quaternary age along the present coast although individual mineral grains are of terrigeneous origin the heavy mineral deposits themselves are not linked to specific source rocks in the hinterland rather they arise under the action of waves during the building of coastal sand barriers a number of fractionating mechanisms that concentrate heavy minerals operate during marine transgressions and under highstand conditions to produce three different types of beach placers type a deposits occur along erosional discontinuities in leaky embayments or between barriers of different age here repeated storm reworking has produced beach face concentrations on drift aligned sectors of coast by a process termed littoral bypassing fractionation type b heavy mineral deposits are found in back barrier washover facies and sometimes as condensed sections at the rear of swash aligned barriers in coastal embayments these were produced by a process of transgressive barrier fractionation that reworked heavy minerals from shelves undergoing erosion during marine transgressions type c heavy mineral deposits occur in prograded barriers and represent episodic influxes of remobilized heavy mineral rich sand that originated elsewhere through the two fractionation mechanisms mentioned above necessary environmental conditions for these fractionating mechanisms to operate include low rates of clastic sediment supply and long periods of weathering and abrasion to create a mature heavy mineral suite an energetic swell wave climate driving large sand fluxes onshore and alongshore and changing sea levels especially marine transgressions that have the effect of moving heavy minerals from the shelf onto the present coast because of the last mentioned effect the mineral prospectivity for the southeastern australian continental shelf is not high

multiple sulfur isotopes and the evolution of earth s surface sulfur cycle abstract the distribution of sulfur isotopes in geological materials reveals information about earth history and biogeochemical processes research during the last several decades has used sulfur isotope geochemistry as a tool to better understand microbial processes harrison and thode 1958 kaplan 1975 monster et al 1979 peck 1959 peck 1962 rees 1973 and sediment diagenesis berner 1969 berner 1982 canfield et al 1993b earth historians also realized this potential as there exists a rich record of environmental change within the sedimentary records canfield and teske 1996 claypool et al 1980 goodwin et al 1976 habicht et al 2002 kah et al 2004 monster et al 1979 shen et al 2001 strauss 1993 thode and goodwin 1983 these applications have championed the use of the two most abundant sulfur isotopes 32s and 34s and provide a rich introduction to what the sulfur isotope record has to offer see canfield 2004 canfield and raiswell 1999 within the last decade this information has been supplemented by new data derived from the less abundant isotopes 33s and 36s the measurement of all four stable sulfur isotopes multiple sulfur isotope geochemistry has expanded our understanding of biological evolution and activity atmospheric chemistry and transport crustal recycling and many more fields related to earth surface processes see farquhar and wing 2003 here i present a review of recent works in multiple sulfur isotope geochemistry with a focus on results that inform our understanding of biogeochemical processes and earth surface evolution

new insights into archean sulfur cycle from mass independent sulfur isotope records from the hamersley basin australia we have measured multiple sulfur isotope ratios 34 s 33 s 32 s for sulfide sulfur in shale and carbonate lithofacies from the hamersley basin western australia the v 33 s values v 33 swn 33 s30 515un 34 s shift from 31 9 to 6 9x over a 22 m core section of the lower mount mcrae shale v2 5 ga likewise sulfide sulfur analyses of the jeerinah formation v2 7 ga yield v 33 s values of 30 1 to 8 1x over a 50 m section of core despite wide variations in v 33 s and n 34 s these two shale units yield a similar positive correlation between v 33 s and n 34 s in contrast pyrite sulfur analyses of the carawine dolomite v2 6 ga yield a broad range in n 34 s 3 2 to 16 2x but a relatively small variation and negative values in v 33 s 32 5 to 31 1x the stratigraphic distribution of n 33 s n 34 s and v 33 s in western australia allows us to speculate on the sulfur isotopic composition of archean sulfur reservoirs and to trace pathways in the archean sulfur cycle our data are explained by a combination of massindependent fractionation mif in the atmosphere and biological mass dependent fractionation in the ocean in the archean volcanic sulfur bearing gas species were photolysed by solar ultraviolet uv radiation in an oxygen free atmosphere resulting in mif of sulfur isotopes aerosols of s8 with v 33 ss 0 and sulfuric acid with v 33 s6 0 formed from the products of uv photolysis and carried mass independently fractionated sulfur into the hydrosphere the signatures of atmospheric photolysis were preserved by precipitation of pyrite in sediments pyrite precipitation was mediated by microbial enzymatic catalysis that superimposed mass dependent fractionation on mass independent atmospheric effects multiple sulfur isotope analyses provide new insights into the early evolution of the atmosphere and the evolution and distribution of early sulfur metabolizing organisms a 2003 elsevier science b v all rights reserved

numerical simulation of coastal upwelling and inerfacial instability of a rotaion and stratified fluid the evolution of the coastal upwelling and interfacial instability of a stratified and rotating fluid is studied numerically by using large eddy simulation upwelling is generated near the sidewall of a rotating annulus by the shear at the top the fluid initially consists of a stably stratified two layer structure with a narrow interface separating the two layers the large scale motion of the flow is simulated by solving the time dependent non hydrostatic incompressible navier stokes and scalar transport equations while the small scale motion is represented by a dynamic subgrid scale model the upwelling process contains both stable and unstable stratification the vertical structure of upwelling consists of a persistent primary front a trailing mixing zone on the shore side of the front and a temporary secondary front which leads a top inversion layer the longshore velocity profile has two maxima which occur at the edge of the sidewall boundary layer and at the density front the upwelled density front is unstable to azimuthal perturbations and baroclinic waves develop and grow to large amplitude pairs of cyclonic and anticyclonic waves appear at the front which form jet streams the secondary front is unstable to azimuthal perturbations its instability and the associated drop of the top inversion layer take the form of radial bands which subsequently break up into isolated patches and eventually sink the computed values of various upwelling time and length scales are compared to and are in good agreement with past experimental data

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

rare earth mineral potential in the southeastern u s coastal plain from integrated geophysical geochemical and geological approaches we combined geophysical geochemical mineralogical and geological data to evaluate the regional presence of rare earth element ree bearing minerals in heavy mineral sand deposits of the southeastern u s coastal plain we also analyzed regional differences in these data to determine probable sedimentary provenance analyses of heavy mineral separates covering the region show strong correlations between thorium monazite and xenotime suggesting that radiometric equivalent thorium eth can be used as a geophysical proxy for those ree bearing minerals airborne radiometric data collected during the national uranium resource evaluation nure program cover the southeastern united states with line spacing varying from 2 to 10 km these data show eth highs over cretaceous and tertiary coastal plain sediments from the cape fear arch in north carolina to eastern alabama these highs decrease with distance from the piedmont quaternary sediments along the modern coasts show weaker eth anomalies except near coast parallel ridges from south carolina to northern florida prominent eth anomalies are also observed over large riverbeds and their floodplains even north of the cape fear arch where surrounding areas are relatively low these variations were verified using ground geophysical measurements and sample analyses indicating that radiometric methods are a useful exploration tool at varying scales further analyses of heavy mineral separates showed regional differences not only in concentrations of monazite but also of rutile and staurolite and in magnetic susceptibility the combined properties suggest the presence of subregions where heavy mineral sediments are primarily sourced from high grade metamorphic low grade metamorphic or igneous terrains or where they represent a mixing of these sources comparisons between interpreted sources of heavy mineral sands near the fall line and igneous and metamorphic piedmont and blue ridge units showed a strong correspondence with rocks closest to the fall line and poor correspondence with rocks farther inland this strongly suggests that the primary source of those heavy minerals especially monazite is the rocks that formed the rocky coast that was present during opening of the atlantic ocean which in turn indicates the importance of coastal processes in forming heavy mineral sand concentrations furthermore narrow radiometric eth and k anomalies are associated with major rivers indicating limited spatial influence of fluvial processes later coastal plain sediment deposition appears to have involved reworking of sediments providing an inheritance of the rocky coast composition that persists for some distance from the fall line however this inheritance is reduced with distance and sediments within 100 km of the coast in georgia and florida exhibit properties indicative of mixing from multiple sources

the crixas gold deposit brazil thrust related postpeak metamorphic gold mineralization of possible brasilino cycle age the crixas gold deposit is located in central goias state approximately 400 km northwest of the capital brasilia it is owned in joint venture by inco gold ltd and anglo american ltd the indicated reserves are 7 million metric tons with a grade of 10 to 12 g metric ton the discovery was made in the late 1970s with mining commencing in the fall of 1988 the hosting crixas greenstone belt is underlain by a typical archcan metamorphosed volcanic sedimentary sequence and is surrounded by granite granite gneiss terrane the crixas gold deposit is located at the contact of the metavolcanic and metasedimentary sequences gold ore occurs within an upper and lower ore zone the upper ore zone consists of an outer shell of ferroan dolomite chlorite biotite schist an inner shell of sericite chlorite schist and magnetite chlorite schist which in turn surrounds a core of massive arsenopyrite pyrrhotite chalcopyrite the lower ore zone consists of a quartz cemented graphitic pelite breccia the upper and lower ore zones are interpreted to represent metasomatically altered parent lithologies metasomatism postdates a maximum regional metamorphism of epidote amphibolite facies or biotite garnet grade carbonatization sericitization base leaching silicification and sulfidization are indicated to have operated during the development of the ore deposit gold deposition is believed to have occurred as the result of reduction of f sub s 2 through the formation of fe rich carbonate silicate and sulfide phases the style of deformation within the deposit is typical of a high strain environment regional structural data provide strong evidence that the high strain is related to low angle thrusting the age of the thrusting or the time of mineralization is not known however low angle thrusting is related to a brasiliano cycle event 550 450 ma

a u pb zircon paleoproterozoic age for the metasedimentary host rocks and gold mineralization of the crixas greenstone belt goias central brazil abstract the crixas greenstone belt central brazil consists of a greenschist to low amphibolite facies metamorphic volcanosedimentary sequence that contains the sixth largest gold reserve 70 t au in the average 12 g t of brazil and the largest gold only deposit of central brazil the gold orebodies are hosted by low angle thrust faults within the stratigraphically upper metasedimentary rocks and are crosscut by a set of post mineralization mafic dykes for decades the sequence has been interpreted as a typical archean greenstone belt of the brazilian shield and the gold mineralization considered either archean or neoproterozoic this study shows the results of in situ u pb shrimp and la icp ms dating of detrital zircon grains from graywacke and mafic dyke samples as well as pb and nd isotopic signatures of the ores the oldest detrital zircon grains of the graywacke samples yielded u pb ages of 3354 40 ma and the youngest between 2209 28 ma and 2222 30 ma which indicates a mixture of archean and paleoproterozoic rhyacian source area and that the maximum depositional age is of ca 2 2 ga magmatic zircon of the post mineralization mafic dykes yielded the u pb age of 2170 17 ma which suggests that the basin stage its deformation metamorphism and mineralization were restricted to the rhyacian and took place during a time span of about 30 40 m y the paper thus provides a new insight into the age of the crixas greenstone belt rocks and of its gold only deposits and reveals that mid paleoproterozoic was a more widespread metallogenetic epoch related to greenstone belts of the brazilian precambrian platform then previously interpreted the observed pb and nd isotopic signatures of the ores suggest that metabasalts carbonaceous schist and metagraywackes could provide the adequate sources for the ore bodies and that the initiation of fluid circulation may have resulted by the emplacement of a high level granite

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

mercury and in situ sulfur isotopes as constraints on the metal and sulfur sources for the world s largest sb deposit at xikuangshan southern china it has been well established that metal and sulfur sources in mineral deposits can be very difficult to identify especially for ore deposits hosted in sedimentary rocks using the world s largest sb deposit at xikuangshan in southern china as a case study this study combined hg isotopes and in situ sulfur isotope measurements to constrain the sources of sb and sulfur a variation of 1 1 in 202hg 0 04 to 1 15 was observed in stibnite ore samples suggesting that mass dependent fractionation of hg isotopes occurred during the formation of the deposit significant mass independent fractionation of hg isotopes with 199hg ranging from 0 03 to 0 17 was also observed in the ore samples suggesting that hg transported by the ore fluids was inherited from proterozoic basement metamorphic rocks as these rocks show similar 199hg signatures 0 03 to 0 07 in situ sulfur isotope measurements yielded 34s values that cluster in the range of 6 8 to 10 2 providing evidence that sulfur contained in ore fluids may also have been dominantly derived from underlying proterozoic basement metamorphic rocks 34s 5 6 to 11 5 using the new results from hg and s isotopes we proposed that deep circulated meteoric water mobilized sb hg and s from the proterozoic metamorphic basement ascended along deep faults and subsequently deposited sb at favorable structural zones as a result of boiling of the hydrothermal fluids generating the world class xikuangshan sb deposit this study also highlights the combined use of hg s isotopes as a novel method to provide new and additional insights into the source regions of ore materials for sedimentary hosted sb deposits

mass independent isotope effects in archean 2 5 to 3 8 ga sedimentary sulfides determined by ion microprobe analysis we report sulfur isotope anomalies with 33 s the deviation from a mass dependent fractionation line for the three isotope system 34 s 32 s vs 33 s 32 s ranging up to 2 within individual archean sedimentary sulfides from a variety of localities our measurements which are made in situ by multicollector secondary ion mass spectrometry unequivocally corroborate prior bulk measurements of mass independent fractionations mif in sulfur and provide additional evidence for an anoxic atmosphere on the earth before 2 ga this technique also offers new opportunities for exploring ancient sulfur metabolisms preserved in the rock record the presence of mif sulfur in sulfides from a 3 8 ga fe rich quartzite from akilia island west greenland is consistent with a marine sedimentary origin for this rock copyright 2003 elsevier science ltd

in situ sulfur isotope analysis by laser ablation mc icpms abstract a new method for in situ s isotopic analysis was tested using a laser ablation system together with a multi collector mc icpms the method was tested for the analysis of pyrite pyrrhotite chalcopyrite and pentlandite using a large pyrite crystal as an in house standard repeated measurements of py pn po and cpy provide an average internal precision of less than 0 1 2 the method was also applied to a pyrite bearing orogenic au deposit to display the ability of the method to resolve minor variations in 34s across growth zoning in pyrite

from the roots to the roof an integrated model for the neoarchean carajas iocg system brazil abstract the carajas mineral province located in northern brazil comprises the oldest iron oxide copper gold mineralizing system on earth and is one of the world s most significant cluster of large tonnage iocg deposits based on a critical assessment of the literature a new integrated model for the carajas neoarchean iocg deposits the model considers the hydrothermal alteration footprint and the rock textures the geometry of the mineralization and the ore assemblage and the isotopic and geochronological signature the majority of the deposits form in the hypozonal domain pervasive and well developed sodic magnesian and calcic iron hydrothermal alteration halos form through magmatic derived hydrothermal fluids the occurrence of hydrothermal orthopyroxene marialite in the ni rich gt 34 and salobo deposits extend further the p t boundaries to temperatures of up to 700 c and 5 kbars thus co is proposedrresponding to the root zone of the system epizonal deposits conversely are characterized by low temperature potassic biotite phlogopite propylitic and chloritic hydrolytic hydrothermal assemblages towards shallower crustal levels although a mantle contribution to the hydrothermal fluids is recognized the o b stable isotopic signature reveals the participation of surface derived basinal brines in the ore forming process the occurrence of polymetallic ooids in the alemao orebody igarape bahia deposit hosting identical hydrothermal alteration mineralogy to the chalcopyrite breccias of other iocg deposits in the carajas mineral province suggests that the mineralizing fluids may have reached the paleo surface of the carajas basin in the neoarchean representing the roof of the iocg system conversely the ore mineralogy is consistent throughout the vertical scheme magnetite is the only iron oxide formed and precipitate from iron rich hydrothermal alteration accompanying the calcic alteration similarly chalcopyrite represents the cu sulfide there is no transition to more oxidized assemblages towards shallower zones supporting that the overall system was relatively reduced since more oxidizing phases hematite bornite are not observed as part of the neoarchean iocg system thus the neoarchean carajas iocg system represents a crustal scale continuum of a vertical paleo hydrothermal system formed coevally to the voluminous bimodal magmatism and the formation of the world class banded iron formation hosted iron ore

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

evidence for glaciation in the northern hemisphere back to 44 ma from ice rafted debris in the greenland sea the widely accepted age estimate for the onset of glaciation in the northern hemisphere ranges between 2 and 15 million years ago ma however recent studies indicate the date for glacial onset may be significantly older we report the presence of ice rafted debris ird in 44 to 30 ma sediments from the greenland sea evidence for glaciation in the north atlantic during the middle eocene to early oligocene detailed sedimentological evidence indicates that glaciers extended to sea level in the region allowing icebergs to be produced ird may have been sourced from tidewater glaciers small ice caps and or a continental ice sheet

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

downscaling model resolution to illuminate the internal wave field in a small stratified lake this paper presents the application of hydrostatic and nonhydrostatic three dimensional hydrodynamic models to a stratified lake focus was given to the multiscale response of the internal wave field to strong wind gusts exceeding 20 m s 1 simulations were performed using different horizontal grid resolutions with uniform grid sizes varying from 100 100 to 10 10 m results of the hydrostatic models were used to investigate the large scale features of the internal wave motion with the intent of investigating the high frequency waves observed results of these simulations were used as initial conditions for nonhydrostatic simulations using smaller grids wavelength of the high frequency waves decreased with grid resolution however none of the uniform grids were sufficiently fine to capture the waves of the highest frequency simulations performed using a nonuniform grid produced internal waves of similar frequency of the waves observed in the field the simulations showed that these waves were shear u

multiple sulfur isotopes of sulfides from sediments in the aftermath of paleoproterozoic glaciations geochemical evidence reported from paleoproterozoic sediments has long been used to evaluate the transition from the anoxic archean atmosphere to an oxygenated atmosphere sulfur isotopes 32s 33s 34s and 36s in sedimentary sulfides and sulfates are an especially sensitive means to monitor this transition such that the timing of the paleoproterozoic great oxidation event can be investigated using mass independently fractionated mif sulfur isotope systematics expressed as 33s here we report data from 83 individual analyses of pyrite pyrrhotite and chalcopyrite on a new suite of 30 different samples from finland south africa wyoming and ontario that span 600 my and follow one or several snowball earth events in the paleoproterozoic the samples were measured using a high resolution secondary ion mass spectrometry technique in multicollection mode that investigates multiple sulfur isotopes in microdomains 30 m within individual sulfide grains while preserving petrographic context we focused on sediments deposited in the aftermath of the paleoproterozoic glaciations between 1 9 and 2 2 ga to trace fluctuations in atmospheric o2 concentrations that were likely affected by an interplay of o2 sinks in the atmosphere and the upper ocean and continental crust and by the emergence and diversification of aerobic organisms our results demonstrate that mif sulfur isotopes are absent in sediments deposited after the period of protracted global cooling in the paleoproterozoic and independently confirm observations that mif ceased during this time we interpret our results by integrating 33s and 34s data in sulfides 13c data in carbonates and the estimated timing of glaciation events in the paleoproterozoic data strongly hint at the presence of microbial sulfate reduction and fluctuations in the concentration of dissolved seawater sulfate and or in 34ssulfate in the aftermath of glaciations and likely were affected by changing erosion rates and nutrient delivery to the oceans these changes modulated the population of primary producers especially oxygenic photosynthesizers and led to fluctuations in the abundance of atmospheric o2 co2 and ch4 our results support the interpretation that the world wide 13ccarb excursion observed between 2 25 and 2 05 ga karhu and holland 1996 was a period of significant accumulation of o2 in the atmosphere

shipboard magnetic field noise reveals shallow heavy mineral sediment concentrations in chesapeake bay abstract shipboard magnetic field data collected over chesapeake bay exhibit low amplitude short wavelength anomalies that most likely indicate shallow concentrations of heavy mineral sediments piston core layers and black sand beach samples exhibit enhanced magnetic susceptibilities and carry remanent magnetization with mineralogical analyses indicating ilmenite and trace magnetite and or maghemite and hematite the anomalies are subtle and would be filtered as noise using traditional approaches but can instead be highlighted using spectral methods thus providing nearly continuous coverage along survey tracks the distribution of the anomalies provides constraints on relevant sorting mechanisms comparisons to sonar data and previous grab samples show that two of three areas surveyed exhibit short wavelength anomalies that are clustered over sand covered areas suggesting initial sorting through settling mechanisms this is supported by a correlation between core magnetic susceptibility and grain size near the choptank river where sediment resuspension is wave dominated anomalies show a sharp decrease with seafloor depth that cannot be explained by signal attenuation alone in pocomoke sound where both tidal currents and wave action impact sediment resuspension anomalies show a more gradual decrease with depth near the mouth of the bay where there is a higher influx of sediments from the continental shelf short wavelength anomalies are isolated and do not appear to represent heavy mineral sand concentrations these combined observations suggest the importance of further sorting by erosional processes in certain parts of the bay additionally comparisons of these data to cores sampling pre holocene sediments suggest that the sorting of heavy minerals in higher energy shallow water environments provides a mechanism for correlations between core magnetic susceptibility and sea level changes

spatial and temporal zoning of hydrothermal alteration and mineralization in the sossego iron oxide copper gold deposit carajas mineral province brazil paragenesis and stable isotope constraints the sossego iron oxide copper gold deposit 245 mt 1 1 cu 0 28 g t au in the carajas mineral province of brazil consists of two major groups of orebodies pista sequeirinho baiano and sossego curral with distinct alteration assemblages that are separated from each other by a major high angle fault the deposit is located along a regional wnw ese striking shear zone that defines the contact between metavolcano sedimentary units of the 2 76 ga itacaiunas supergroup and tonalitic to trondhjemitic gneisses and migmatites of the 2 8 ga xingu complex the deposit is hosted by granite granophyric granite gabbro and felsic metavolcanic rocks the pista sequeirinho baiano orebodies have undergone regional sodic albite hematite alteration and later sodic calcic actinolite rich alteration associated with the formation of massive magnetite apatite bodies both these alteration assemblages display ductile to ductile brittle fabrics they are cut by spatially restricted zones of potassic biotite and potassium feldspar alteration that grades outward to chlorite rich assemblages the sossego curral orebodies contain weakly developed early albitic alteration and very poorly developed subsequent calcic sodic alteration these orebodies contain well developed potassic alteration assemblages that were formed during brittle deformation that resulted in the formation of breccia bodies breccia matrix commonly displays coarse mineral infill suggestive of growth into open space sulfides in both groups of deposits were precipitated first with potassic alteration and more importantly with a later assemblage of calcite quartz epidote chlorite in the sequeirinho orebodies sulfides range from undeformed to deformed sulfides in the sossego curral orebodies are undeformed very late weakly mineralized hydrolytic alteration is present in the sossego currral orebodies the sulfide assemblage is dominated by chalcopyrite with subsidiary siegenite and millerite pyrrhotite and pyrite are minor constituents of ore in the sequerinho orebodies while pyrite is relatively abundant in the sossego curral bodies oxygen isotope partitioning between mineral pairs constrains temperatures in the deposit spatially and through time in the sequeirinho orebody the early sodic calcic alteration stage was characterized by temperatures exceeding 500 c and delta text 18 text o text h text 2 text o values for the alteration fluid of 6 9 0 9 temperature declines outward and upward from the zone of most intense alteration paragenetically later copper gold mineralization displays markedly lower temperatures 300 c and was characterized by the introduction of 18o depleted hydrothermal fluids 1 8 3 4 the calculated dh2o and delta text 18 text o text h text 2 text o values suggest that the fluids that formed the early calcic sodic alteration assemblage were of formational metamorphic or magmatic origin the decrease of delta text 18 text o text h text 2 text o values through time may reflect influx of surficially derived waters during later alteration and mineralization events influx of such fluids could be related to episodic fluid overpressure resulting in dilution and cooling of the metalliferous fluid causing deposition of metals transported as metal chloride complexes

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

precision analysis of multisulfur isotopes in sulfides by femtosecond laser ablation gc irms at high spatial resolution abstract a new laser ablation method has been developed for measurements of multiple sulfur isotopes in sulfides using a femtosecond laser ablation system in combination with a reactor for sulfide aerosol conversion into sf6 gas a cryogenic and chromatographic purification system and an isotope ratio mass spectrometer fsla gc irms in this method femtosecond laser generated aerosol sulfide particles were transferred by the helium carrier gas from the sample chamber into the reactor to convert them to sf6 via reaction with brf5 cryogenically and chromatographically purified sf6 was transferred by helium carrier gas to the interface that was developed for introducing sf6 into the ion source of a mass spectrometer running in high vacuum operation mode for the routine analysis of sulfide a precision of 0 1 0 2 1 for 34s and higher than 0 03 1 for 33s values has been obtained with an ablation crater size of 80 m diameter and 40 m depth the method was tested using reference materials iaea s 1 iaea s 2 iaea s 3 and nbs 123 as well as some natural pyrite sphalerite and galena samples the 34s values obtained by our method are in good agreement within analytical precision with the results obtained by the conventional dual inlet method the fsla cg irms method was applied to study sulfur isotope ratios in archean rocks of the cheremshanskaya formation sharyzhalgai uplift in the southeastern siberian craton we found that 33s values in sulfides from these rocks ranged 2 30 to 2 68 the reliability of the results obtained here was confirmed by comparative analyses of isotope ratios in sulfides from rocks younger than 2 4 ga in which the anomaly of the 33s isotope could be unlikely to occur the calculated 34s ln 34s 1000 1 1000 and 33s ln 33s 1000 1 1000 values of this set of samples yielded a straight line passing through the origin with a slope of 0 5152 0 002 on a conventional sulfur three isotope plot that is in good agreement with a slope of 0 515 which was theoretically calculated for the mass dependent fractionation of sulfur isotopes

trace element and multi isotope geochemistry of late archean black shales in the carajas iron ore district brazil abstract the 250 300 m thick carajas formation in the carajas mineral province northern brazil consists of banded iron formation including giant high grade iron ore deposits and minor black shale overlying a thick pile 2 3 km of about 2 75 ga old metabasalt carbonaceous shale with pyrite and locally pyrrhotite rich patches from drillcore of the serra sul exploration project has up to 29 ppm mo iron speciation analysis indicates essentially ferruginous and for some samples likely euxinic depositional conditions positive 34 s isotope ratios of tris are between 0 3 to 10 7 with heavy data restricted to pyrrhotite free samples the data suggest microbial sulfate reduction under at least partially sulfate limiting conditions with later overprint by migrating solutions the black shale is affected by pronounced low temperature potassium metasomatism k 2 o na 2 o 100 up to 10 wt k 2 o as adularia related to diagenetic processes at 98 95 mo for continental input t dm of 2 8 3 1 ga according to nd isotope data and 0 9 98 95 mo for a hydrogenous component black shale samples from the bottom of the bif sequence have heavy mo isotope composition of up to 1 8 98 95 mo the significant mo isotope fractionation is either the result of an early and transient whiff of oxygen at 2 7 ga or the result of hydrothermal fluid overprint

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

multiple sulfur isotopes and the evolution of the atmosphere interest in multiple sulfur isotope analyses has been fueled by recent reports of mass independent sulfur isotope signatures in the geologic record a non zero multiple isotopic signature of sulfur 33s and 36s is produced primarily through photochemical reactions and it is an almost perfect tracer of the source of sulfur once the signature is passed on to a given sulfur reservoir it will be preserved unless there is addition of sulfur with a different 33s or 36s in the geological record this signature has been used to study the evolution of the earth s atmosphere and to trace movement of sulfur through geological systems early in earth s history recently small but significant mass independent signatures have been reported for some younger samples raising the possibility of additional applications of multiple isotope studies the purpose of this review is to introduce and discuss the implications of multiple isotope studies and to focus attention on these anomalous but not uncommon isotopic signatures

iron and sulphur isotopes from the carajas mining province para brazil implications for the oxidation of the ocean and the atmosphere across the archaean proterozoic transition abstract mineralogical analyses and major and trace element geochemistry combined with iron and sulfur isotopes were applied to banded iron formations bifs and pyrite bearing sandstones from the carajas mineral province brazil the bifs belong to the 2 7 ga old grao para group and show a rather simple mineralogy hematite and quartz or cryptocrystalline silica their ree y patterns suggest deposition under marine sub oxic conditions fe isotopes show highly positive values 57 fe irmm 14 2 06 0 34 in average which can be explained by the partial oxidation of fe 2 aq in the surface ocean subsequent to the upwelling of deep anoxic seawater the resulting iron oxy hydroxides likely formed in the shallow photic zone as a seasonal process and acquired highly positive iron isotope signature values consistent with a rayleigh distillation model the pyrite bearing sandstones belong to the aguas claras formation of palaeoproterozoic younger age and were cored at the base of the giant azul manganese deposit diagenetic pyrite is characterized by positive 57 fe irmm 14 1 02 0 2 and 34 s vctd 11 97 0 12 average values that are different from values of modern marine biogenic pyrite results likely indicate that dissolved sulfate was the limiting factor controlling biogenic pyrite formation at that time near zero 33 s values 0 013 0 003 indicate minimal mass independent sulfur isotope fractionation mif s consistent with the 2 1 ga age previously obtained from the aguas claras pyrites this result suggests deposition at a time when the oxygen partial pressure in the atmosphere was already high enough 10 5 to 10 2 pal or present atmospheric level to hinder mif s by photochemical reactions incidentally this also confirms that the giant mn deposit of carajas is in the same age range 2 1 0 2 ga as other mn ores in gabon and south africa pointing to the importance of mn precipitation in paleoproterozoic times

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

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event

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

continental ice in greenland during the eocene and oligocene the eocene and oligocene epochs approximately 55 to 23 million years ago comprise a critical phase in earth history an array of geological records supported by climate modelling indicates a profound shift in global climate during this interval from a state that was largely free of polar ice caps to one in which ice sheets on antarctica approached their modern size however the early glaciation history of the northern hemisphere is a subject of controversy here we report stratigraphically extensive ice rafted debris including macroscopic dropstones in late eocene to early oligocene sediments from the norwegian greenland sea that were deposited between about 38 and 30 million years ago our data indicate sediment rafting by glacial ice rather than sea ice and point to east greenland as the likely source records of this type from one site alone cannot be used to determine the extent of ice involved however our data suggest the existence of at least isolated glaciers on greenland about 20 million years earlier than previously documented at a time when temperatures and atmospheric carbon dioxide concentrations were substantially higher