discriminating fluid source regions in orogenic gold deposits using b isotopes abstract the genesis of orogenic gold deposits is commonly linked to hydrothermal ore fluids derived from metamorphic devolatilization reactions however there is considerable debate as to the ultimate source of these fluids and the metals they transport tourmaline is a common gangue mineral in orogenic gold deposits it is stable over a very wide p t range demonstrates limited volume diffusion of major and trace elements and is the main host of b in most rock types we have used texturally resolved b isotope analysis by secondary ion mass spectrometry sims to identify multiple fluid sources within a single orogenic gold ore district the loulo mining district in mali west africa hosts several large orogenic gold ore bodies with complex fluid chemistry associated with widespread pre ore na and multi stage b metasomatism the gara deposit as well as several smaller satellites formed through partial mixing between a dilute aqueous carbonic fluid and a hypersaline brine hydrothermal tourmaline occurs as a pre ore phase in the matrix of tourmalinite units which host mineralization in several ore bodies clasts of these tourmalinites occur in mineralized breccias disseminated hydrothermal and vein hosted tourmaline occur in textural sites which suggest growth during and after ore formation tourmalines show a large range in 11 b values from 3 5 to 19 8 which record a change in fluid source between paragenetic stages of tourmaline growth pre mineralization tourmaline crystals show heavy 11 b values 8 19 8 and high x site occupancy na ca 0 69 1 apfu suggesting a marine evaporite source for hydrothermal fluids syn mineralization and replacement phases show lighter 11 b values 3 5 to 15 1 and lower x site occupancy 0 62 0 88 apfu suggesting a subsequent influx of more dilute fluids derived from devolatilization of marine carbonates and clastic metasediments the large overlapping range in isotopic compositions and a skew toward the opposing population in the 11 b data for both tourmaline groups reflects continual tourmaline growth throughout mineralization which records the process of fluid mixing a peak in 11 b values at 8 largely controlled by tourmalines of syn to post ore timing represents a mixture of the two isotopically distinct fluids this paper demonstrates that b isotopes in tourmaline can be instrumental in interpreting complex and dynamic hydrothermal systems the importance of b as an integral constituent of orogenic ore forming fluids and as a gangue phase in orogenic gold deposits makes b isotope analysis a powerful tool for testing the level of source region variability in these fluids and by extension that of metal sources

paleoproterozoic volcano sedimentary series in the ca 2 27 1 96 ga birimian orogen of the southeastern west african craton abstract the ca 2270 1960 ma accretionary collisional birimian orogen in the southern west african craton swac contain a number of volcano sedimentary series which were deposited in basins that developed during the regional eburnean collisional orogeny ca 2160 1960 ma understanding their spatiotemporal distribution is thus crucial for constraining the tectonic evolution of the orogenic system this study presents new la icpms data on detrital zircon from volcano sedimentary series in southeastern swac which indicate maximum depositional ages between ca 2160 2130 ma the detrital zircon population is predominantly composed of grains with ages compatible with derivation from local birimian crust but older paleoproterozoic and archean components are also present these may have been derived from crustal domains now present in south american cratons and terranes and which would have been located to the present east or southeast of the swac the deposition of the volcano sedimentary series also coincided with the onset of the eburnean collisional orogeny in eastern swac which suggest a dynamic coupling between compression on one hand and extension on the other where the latter controlled the establishment of the depocentres in which the series were deposited the data in this study support previous observations that sedimentation across swac was diachronous during the course of the eburnean orogeny and that an early pulse occurred primarily in the east between ca 2165 2125 ma while a later pulse occurred in the west around ca 2115 2080 ma followed by limited volcanism and sedimentation at ca 2065 ma this diachroneity is also reflected in the lithostratigraphy with areally extensive shallow water deposits being more frequent in the east whereas carbonates are only present in the younger successions to the west this may reflect a hinterland foreland style relationship between eastern and western swac respectively where sedimentation occurred progressively during convergence related to the regional and diachronous eburnean orogeny

rapid flooding of the sunda shelf a late glacial sea level record the increase in sea level from the last glacial maximum has been derived from a siliciclastic system on the tectonically stable sunda shelf in southeast asia the time from 21 to 14 thousand calendar years before the present has been poorly covered in other records the record generally confirms sea level reconstructions from coral reefs the rise of sea level during meltwater pulse 1a was as much as 16 meters within 300 years 14 6 to 14 3 thousand years ago

problems and possible solutions concerning radiocarbon dating of surface marine sediments ross sea antarctica abstract radiocarbon accelerator mass spectrometric ams dates on the acid insoluble fraction from 38 core tops from the western ross sea antarctica are used to address these questions 1 what are the apparent ages of sediments at or close to the present sediment water interface 2 is there a statistically significant pattern to the spatial distribution of core top ages and 3 is there a correction factor that can be applied to these age determinations to obtain the best possible holocene downcore chronologies ages of core top sediments range from 2000 to 21 000 14c yr b p some old core top dates are from piston cores and probably represent the loss of sediment during the coring process but some core top samples 6000 14c yr b p may represent little or no holocene deposition four possible sources of variability in dates 6000 14c yr b p n 28 are associated with 1 different sample preparation methods 2 different sediment recovery systems 3 different geographic regions and 4 within sample lateral age variability statistical analysis on an a posteriori design indicates that geographic area is the major cause of variability there is a difference in mean surface sediment age of nearly 2000 yr between sites in the western ross sea and sites east of ross bank in south central ross sea the systematic variability in surface age between areas may be attributed to a variable sediment accumulation rates sar surface age is inversely related to sar b differences in the percentage of reworked dead carbon between each area and or c differences in the co2 exchange between the ocean and the atmosphere

fluid evolution and chemical controls in the fazenda maria preta fmp gold deposit rio itapicuru greenstone belt bahia brazil the fazenda maria preta fmp gold deposit is confined to two regional scale sinistral oblique brittle ductile shear zones which are located in a greenschist facies metamorphosed volcano sedimentary sequence in the northern sector of the lower proterozic rio itapicuru greenstone belt ne brazil alteration is pervasive throughout the shear zones and characterized by carbonatization and sericitization of intermediate volcanic volcaniclastic and sub volcanic rocks the orebodies are mainly shear veins hosted by or directly associated with highly deformed carbonaceous volcaniclastic rocks breccia and stockwork systems or disseminations in felsic and mafic sub volcanic bodies native gold appears as free grains finely disseminated in the quartz veins or closely related to arsenopyrite pyrite pyrrhotite sphalerite and fe chlorite in the carbonaceous wallrocks fluid inclusion studies by microthermometry and laser raman microspectroscopy revealed that the vein quartz is largely dominated by primary pseudosecondary and rarely secondary populations of co2 ch4 n2 inclusions type i whereas primary groups of low salinity 6 wt eq nacl h2o co2 ch4 n2 inclusions type ii comprise the dominant inclusion type in only a few veins the thco2 l v l data indicated a variation in the co2 density of 1 05 to 0 60 g cm3 for the type i inclusions and conversely a narrower range of 0 76 0 73 g cm3 for the type ii inclusions the broader variation of the co2 density for the co2 rich fluid could have been the result of trapping of the vein fluid under a variable pressure regime or by its re equilibration during continuous deformation within the shear zone domains and reduction of the overburden pressure during uplift the co2 rich fluid inclusions provide no evidence that they have been the result of h2o loss either during infiltration e g hydration reactions or after trapping of an original h2o co2 fluid accordingly the co2 ch4 n2 and the h2o co2 ch4 n2 fluids are considered as representatives of two distinct fluid regimes which were active during the gold mineralizing events within the shear zones both types of ore fluids are interpreted as part of a deep metamorphic magmatic hydrothermal system in which i a co2 rich fluid probably originated in the mantle was transported to and released in higher structural levels mainly by tonalitic granodioritic and alkaline magmas whereas ii a h2o co2 fluid generated by devolatilization reactions during the regional metamorphism was either directly channeled into favorable structural sites i e metamorphic fluid or absorbed and later exsolved by the crystallization of felsic magmas i e magmatic fluid sensu lato in this context gold deposition occurred between 320 c and 420 c and 2 1 to 4 4 kb and calculated values of fo2 between 10 26 5 and 10 32 4 bar reveal the relatively reducing nature of the mineralizing fluids the ore paragenesis constrains fs2 to the range of 10 10 1 10 6 5 bar for these fluids under such conditions the gold was transported mainly as bisulfide complexes by the h2o co2 fluid but in the case of the co2 rich fluid the role of thiocomplexes or some other types of ligands is obscured by the lack of high temperature experimental data deposition of the metal occurred in response to redox changes which accompanied fluid carbon interaction particularly a decrease in fo2 and fs2 of the fluid on a microscopic scale phisisorption and chemisorption processes at the fluid iron sulfide interface may have further enhanced the deposition of the gold particularly on the surfaces of precipitating pyrite and arsenopyrite

a biogeochemical influence on the secular distribution of orogenic gold orogenic gold deposits are unevenly distributed in geologic time the richest period of orogenic gold deposit formation was the phanerozoic which followed a 1 b y period of relatively meager deposit generation during the proterozoic this proterozoic low gold period developed despite continent building episodes which should have been appropriate for orogenic gold genesis here a brief review of the changes in global geochemistry brought on by evolving interactions between plate tectonics and the biosphere is conducted before the consequences for gold uptake into sedimentary pyrite are discussed it is suggested that the extensive oxygenation of the deep oceans during the second great oxidation event over the period 635 to 510 ma produced conditions where gold was soluble in the deep oceans from that point on regions where bacterial sulfate reduction drove formation of sedimentary pyrite also destabilized this soluble gold promoting its uptake into pyrite and allowing formation of metallogenically enhanced sedimentary sequences these sediments would have become ideal source rocks for gold deposits during subsequent collisional tectonics and metamorphism this biogenic influence on source region enhancement may explain the association during the phanerozoic eon between some of the world s most gold productive regions and sedimentary sequences that contain pyritic carbonaceous rocks

age constraints on tarkwaian palaeoplacer and lode gold formation in the tarkwa damang district sw ghana two major epigenetic gold forming events are recorded in the world class gold province of southwest ghana a pre tarkwaian event was the source of the world class tarkwa palaeoplacers whereas post birimian and tarkwaian deformation which was related to the eburnean orogeny gave rise to the world class e g prestea to giant e g obuasi orogenic gold deposits which have made the region famous for more than 2 500 years a maximum age of 2133 4 ma for tarkwaian sedimentation is provided by 71 of 111 concordant shrimp ii u pb dates from detrital zircons in tarkwaian clastic rocks from damang and bippo bin northeast of tarkwa the overall data distribution broadly overlaps the relatively poorly constrained ages of birimian volcanism and associated dixcove type granitoid emplacement indicating syntectonic development of the tarkwaian sedimentary basin these zircon ages argue against derivation of the palaeoplacer gold from an orogenic gold source related to the compressional phase of an orogeny significantly older than the eburnean orogeny instead they suggest that the gold source was either orogenic gold lodes related to an earlier compressional phase of a diachronous eburnean orogeny or ca 2200 2100 ma intrusion related gold lode the co2 rich fluid inclusions in associated vein quartz pebbles are permissive of either source at the damang deposit an epigenetic orogenic lode gold system clearly overprinted and sulphidised low grade palaeoplacer hematite magnetite gold occurrences in the banket series conglomerate within the tarkwaian sedimentary sequence gold mineralisation is demonstrably post peak metamorphism as gold related alteration assemblages overprint metamorphic assemblages in host rocks in alteration zones surrounding the dominant subhorizontal auriferous quartz veins there are rare occurrences of hydrothermal xenotime which give a shrimp u pb age of 2063 9 ma for gold mineralisation the similar structural timing of epigenetic gold mineralisation in tarkwaian host rocks at damang to that in mainly birimian host rocks elsewhere in southwest ghana particularly at obuasi suggests that 2063 9 ma is the best available age estimate for widespread orogenic gold mineralisation in the region argon argon ages of 2029 4 and 2034 4 ma for hydrothermal biotite from auriferous quartz veins appear to represent uplift and cooling of the region below about 300 c as estimates of the temperature of gold mineralisation are higher at around 400 c if peak metamorphism with temperatures of about 550 c is assumed to have occurred at about 2100 ma the biotite ages in combination with the xenotime age suggest a broadly constant uplift rate for the region of about 1 km per 10 million years from about 2100 to 2025 ma

physical chemical properties and metal budget of au transporting hydrothermal fluids in orogenic deposits we use a set of analytical techniques for fluid inclusion analysis to determine the bulk properties of the ore fluids from five orogenic gold deposits the most common ore fluid is a low salinity 0 4 6 5 wt nacl twoto three phase aqueous carbonic fluid its th total range is 200 400 8c and total homogenization occurs by bubble or liquid disappearance a less common aqueous fluid of similar bulk salinity and t h total is also documented microchemical data show that the ore fluid in the five deposits is chemically uniform and made of na subordinate k and b and minor to trace amounts of cu as li sr rb ba cs sb and au range 0 5 5 mg g thermodynamic modelling carried out for one of the studied deposits indicates that the ore fluid was in equilibrium with vein minerals at the time of gold deposition and that its vapour phase was distinctly enriched in au b as and sb the proposed mechanism of gold precipitation is a combination of fluid decompression and boiling combining these results with 3d geometrical reconstructions of one of the studied deposits sigma we estimate that a minimum ore fluid volume of c 0 1 1 0 km is sufficient to generate a world class orogenic deposit orogenic deposits provide about a third of the global gold production goldfarb et al 2001 frimmel 2008 these deposits are formed within active continental margins in orogenic belts where major tectonic boundaries juxtapose metamorphosed volcanic plutonic and sedimentary terranes the ore bodies consist of au mineralized vein networks filled with quartz carbonates and other silicates that in places crosscut the stacked terranes of the continental margins the veins are dilated portions of vertically extensive brittle ductile to ductile fracture networks that form due to the advection of aqueous hydrothermal fluids within the continental crust ridley et al 1996 groves et al 2003 these ore fluids are generated in still undefined source regions deep in the crust and transported to shallower levels 3 15 km depth through the vein networks along which they deposit gold a number of studies kerrich et al 2000 ridley diamond 2000 groves et al 2003 showed that although these networks do not occur systematically in all orogens they have a set of important geological features in common which include 1 formation during accretionary orogenic events 2 occurrence relatively close to regional tectonic lineaments especially along lower order splays of these structures 3 spatial association with granitoid intrusions 4 occurrence within some metamorphic belts with gold precipitation being synto postmetamorphic in age 5 relatively low sulphide abundance within the veins 6 carbonate sericitealbite sulphide hydrothermal alteration of host rocks postdating metamorphism 7 poorly developed mineral and metal zoning within the ore bodies and 8 genesis from low to moderate salinity ore fluids over a large temperature range 250 700 8c but mostly within 300 350 8c these common traits occur systematically in deposits from different terrains having distinct ages suggesting that a common geological process and au transporting fluid are responsible for their formation here we present a database of fluid inclusion properties from five typical orogenic deposits which show most of the geological characteristics listed above the database was constructed with the aim of determining as accurately as possible the properties of the au transporting fluid in this geological environment in particular laser ablation inductively coupled plasma mass spectrometry la icp ms of single fluid inclusions paragenetically coeval with vein gold provides the concentration of a suite of metals including au in this fluid this dataset when combined with 3d geometrical reconstructions of one of these deposits offers new constraints on the volume of the ore from garofalo p s ridley j r eds gold transporting hydrothermal fluids in the earth s crust geological society london special publications 402 http dx doi org 10 1144 sp402 8 the geological society of london 2014 publishing disclaimer www geolsoc org uk pub ethics by guest on april 3 2014 http sp lyellcollection org downloaded from

volume and routing of late glacial runoff from the southern laurentide ice sheet melting of the laurentide ice sheet during the last deglaciation added large volumes of water to many rivers and lakes of north america and to the world s oceans the volume and routing of this meltwater not only helped shape the land s surface but also played a role in the evolution of late glacial climate a computerized model was prepared to quantify meltwater generation from seven drainage areas along the southern side of the laurentide ice sheet at 500 yr time slices between 14 000 and 8000 yr b p nearly all waters reaching the oceans flowed through the st lawrence hudson or mississippi river valleys discharge through the mississippi river to the gulf of mexico during late glacial time varied by more than a factor of 5 ranging between 17 400 m3 sec 1 550 km3 yr 1 and 98 200 m3 sec 1 3200 km3 yr 1 discharge entering the north atlantic ocean through the st lawrence and hudson valleys ranged between 20 300 m3 sec 1 640 km3 yr 1 and 65 300 m3 sec 1 2060 km3 yr 1 with very abrupt twofold changes at about 11 000 10 000 and 9500 yr b p as a result of the rerouting of water from the lake agassiz basin as the areal extent and mass of the laurentide ice sheet diminished the total volume of meltwater plus runoff due to precipitation from its southern side declined from 3800 km3 yr 1 at about 14 000 yr b p to 2100 to 2600 km3 yr 1 between 11 500 and 8000 yr b p no meltwater entered the gulf of mexico after 9500 yr b p after the demise of the ice sheet over hudson bay about 8000 yr b p the modern continental drainage network was established and flows through the st lawrence declined to modern values of about 320 km3 yr 1

late pleistocene holocene marine conditions in the ross sea antarctica evidence from the diatom record statistical analyses of diatom assemblages from radiocarbon dated sediment cores were used to reconstruct changing palaeoceanographic conditions in the western and west central ross sea antarctica from c 12 14c kyr bp to the present data from three kasten cores support a north to south time transgressive glacial interglacial transition assemblages at the base of each core suggest that glacial processes affecting frustule preservation were significant during the late pleistocene increasing biogenic silic and decreasing per centages of reworked species suggest that upper water column productivity instead of preservational processes increasingly dominates diatom assemblages during grounding line recession c 12to c 6 14c kyr bp a warm ing during the middle to late holocene c 6to c 3 14c kyr bp may have resulted in a spring sea ice melting prior to the annual inception of the ross sea polynya in the west central ross sea and b increased plateletice delivery due to increased melting beneath t

an empirical oxygen and hydrogen isotope geothermometer for quartz tourmaline and tourmaline water abstract oxygen isotope equilibration temperatures of coexisting quartz muscovite illite chlorite and biotite from a variety of geologic environments have been used in conjunction with the 18o and d values of associated tourmaline to formulate empirical quartz tourmaline and tourmaline water isotope fractionation factors over temperatures from 200 to 600 c the fractionation factors determined using this method are as follows 1 1 1000 in quartz tourmaline 1 0 0 17 10 6 t 2 0 39 0 44 oxygen 2 2 1000 in tourmaline water 27 2 4 4 10 6 t 2 28 1 9 8 hydrogen 350 600 c the isotopic properties of tourmaline are nearly identical to those of muscovite and illite despite the significant differences in their crystal structures similarities in the oxygen and hydrogen isotope systematics of tourmalines and micas imply that the dominant controls on the isotopic properties of these minerals are the vibrational energies of the al si o and si o bonds however the bo3 groups in the tourmalines may have some effect on oxygen isotopic properties as there are minor differences between the 18o values of coexisting tourmalines and micas d values of coexisting tourmalines and micas are generally concordant to within 10 chemical compositions indicate that varying fe mg ratios in the tourmalines appear to have only a minor control on their d values there is a discontinuity in the 1000 in tourmaline water values for hydrogen at temperatures between 200 c and 300 c suggesting that the hydroxl groups in tourmaline are affected by hydrogen bonding and changes in the properties of water near its critical point as are some other hydrous minerals

deglaciation of a soft bedded laurentide ice sheet we present a series of numerical reconstructions of the laurentide ice sheet during the last deglaciation 18 7 14c ka that evaluates the sensitivity of ice sheet geometry to subglacial sediment deformation these reconstructions assume that the laurentide ice sheet flowed over extensive areas of water saturated deforming sediment soft beds corresponding to the st lawrence lowland the great lakes region the western prairies of the u s and canada and the hudson bay and hudson strait regions sediment rheology is based on a constitutive law that incorporates experimental results from late wisconsin till deposited by the laurentide ice sheet which suggest only mildly nonlinear viscoplastic behavior by varying the effective viscosity of till we produced four reconstructions for the ice sheet during the last glacial maximum 18 14c ka and two reconstructions each of the ice sheet at 14 13 12 11 and 10 14c ka we also produced one reconstruction for 9 8 4 8 and 7 14c ka reconstructions that assume a low effective viscosity for all areas of deforming sediment show a multidomed ice sheet with a large bowl shaped depression over hudson bay and thin ice 1000 m above modern sea level over the western and southern margins those reconstructions that assume a higher effective viscosity of till in the hudson bay region than for the western and southern margins also show a multidomed ice sheet but with considerably thicker ice over hudson bay and a more northerly position of the central ice divide these two different geometries may represent ice sheet orographic changes associated with a heinrich event further increases in effective viscosity of till approaching the effective viscosity of ice would result in a high monolithic ice dome centered over hudson bay reinforcing the notion that a multidomed ice sheet reflects the distribution of substrate geology modeled ice surface geometry at the last glacial maximum shows many of the same general features as previous reconstructions that incorporate deformable beds our reconstructions with higher effective till viscosities in hudson bay also agree with the ice 4g reconstructions peltier 1994 which are based on inversion of relative sea level data for the early part of the last deglaciation 18 13 14c ka but then depart significantly from ice 4g beginning at about 12 14c ka due to differing assumptions of the history of deglaciation modeled ice volume for the last glacial maximum suggests a glacioeustatic change of 50 55 m by a soft bedded laurentide ice sheet this would increase as the effective viscosity of till increases subsequent ice volume changes through the last deglaciation generally parallel the trend of eustatic rise recorded at barbados new guinea and tahiti but suggest that the laurentide ice sheet was not the source of meltwater pulse 1a

formation of gold deposits review and evaluation of the continuum model abstract the continuum model has been one of the more widely advocated genetic models for gold only deposits such as those found in archaean greenstone belts it postulates that hydrothermal gold deposits were formed throughout a 20 25 km vertical crustal profile from temperatures above 700 c to below 180 c and that the deposition occurred synchronous with the peak of metamorphism the continuum model is reviewed at this stage because we believe that it does not successfully account for many aspects of gold deposit formation the most obvious shortcoming is in considering ore deposits found in rocks of uppermost amphibolite and granulite facies domains where temperatures were appropriate for partial melting the main chemical condition that favours melting is access to h2o from either aqueous fluid or the breakdown of hydrous minerals a major gold forming hydrothermal event at the peak of high grade metamorphism as implied by the continuum model is incompatible with partial melting of wallrocks instead of forming a hydrothermal gold deposit an aqueous fluid introduced during partial melting would be consumed to produce further melt five gold deposits are documented from high metamorphic grade domains within four separate archaean cratons one is the type example used in the continuum model the other four have been significant producers partial melting has been recorded in the wallrocks adjacent to gold mineralisation at big bell hemlo challenger and renco gold deposits in the sulphide rich ore itself at challenger and hemlo deposits and as cross cutting dykes and migmatites at griffins find for these five deposits and indeed in general the continuum model does not easily account for aspects of the fluid source fluid composition gold transport and metal deposition the evidence that has been used to support deposit formation at the peak of high grade metamorphism as required by the continuum model is generally compatible with one or more alternative genetic models textural and structural siting of the gold high variance mineral assemblages proportion of various gold host rocks and relative element enrichments at high metamorphic grade can be accounted for by gold formation at greenschist facies conditions followed by progress of the deposit to high metamorphic grade metamorphic model moreover the type examples in the continuum model of deposits in sub greenschist facies domains appear to be incorrectly ascribed and are likely to be in greenschist facies domains the continuum model does not adequately explain archaean gold deposits

synmetamorphic carbon mobility and graphite enrichment in metaturbidites as a precursor to orogenic gold mineralisation otago schist new zealand the macraes orogenic gold deposit is hosted by a graphitic micaceous schist containing auriferous porphyroblastic sulphides the host rock resembles zones of unmineralised micaceous graphitic pyritic schists derived from argillaceous protoliths that occur locally in background pelitic otago schist metasediments this study was aimed at determining the relationship between these similar rock types and whether the relationship had implications for ore formation argillites in the protolith turbidites of the otago schist metamorphic belt contain minor amounts of detrital organic matter 50 million years of evolution of the metamorphic belt from development of sheared argillite in the jurassic to postmetamorphic ductile extension in the cretaceous introduced graphite is structurally controlled and occurs with metamorphic muscovite and chlorite as veins and slicken sided shears with some veins having 50 noncarbonate carbon graphitic foliation seams in low grade micaceous schist and metamorphic quartz veins contain equant graphite porphyroblasts up to 2 mm across that are composed of crystallographically homogeneous graphite crystals graphite reflectance is anisotropic and ranges from 1 to 8 green light texturally similar porphyroblastic pyrite has grown in micaceous schist up to 10 wt s metamorphic quartz veins and associated muscovite rich shears these pyritic schists are weakly enriched in arsenic up to 60 ppm the low grade metamorphic mobility and concentration of graphite in micaceous schists is interpreted to be a precursor process that structurally and geochemically prepared parts of the otago schist belt for later more restricted gold mineralisation economic amounts of gold and associated arsenic were subsequently introduced to carbonaceous sulphidic schists in the macraes gold deposit by a separate metamorphic fluid derived from high grade metaturbidites fluid flow at all stages in these processes occurred at metamorphic rates mm year and fluids were broadly in equilibrium with the rocks through which they were passing

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

meltwater pulse 1a from antarctica as a trigger of the bolling allerod warm interval meltwater pulse 1a mwp 1a was a prominent feature of the last deglaciation which led to a sea level rise of 20 meters in less than 500 years concurrent with mwp 1a was the onset of the bolling allerod interstadial event 14 600 years before the present which marked the termination of the last glacial period previous studies have been unable to reconcile a warm northern hemisphere with mwp 1a originating from the laurentide or fennoscandian ice sheets with the use of a climate model of intermediate complexity we demonstrate that with mwp 1a originating from the antarctic ice sheet consistent with recent sea level fingerprinting inferences the strength of north atlantic deep water nadw formation increases thereby warming the north atlantic region and providing an explanation for the onset of the bolling allerod warm interval the established mode of active nadw formation is then able to respond to subsequent freshwater forcing from the laurentide and fennoscandian ice sheets setting the stage for the younger dryas cold period the melting of continental ice sheets during the last deglaciation provided a freshwater source to the ocean that affected global sea level and the strength of the thermohaline circulation an exceptionally large melting event inferred from far field relative sea level records occurred

flash vaporization during earthquakes evidenced by gold deposits fluids flowing through cavities in earth s crust can deposit gold thermo mechanical modelling of a fluid filled cavity that expands suddenly during an earthquake shows that the drop in pressure would cause the fluid to vaporize and deposit the gold almost instantaneously

fluid inclusion and stable isotope o h c constraints on the genesis of the pedra branca gold deposit troia massif borborema province ne brazil an example of hypozonal orogenic gold mineralization abstract at the archean paleoproterozoic troia massif ne brazil two major paleoproterozoic greenstone belts are documented algodoes and serra das pipocas they share similar lithostratigraphic characteristics with other 2 2 2 1 ga greenstone belts of the surrounding cratonic domains e g guiana shield and west africa craton gold mineralization in the serra das pipocas greenstone belt is associated with a regional ne trending shear zone in the mineralized area the pedra branca gold deposit the main gold mineralization is found in association with quartz veins calc silicate alteration e g diopside k feldspar amphibole titanite biotite pyrite albite magnetite carbonates and albitization pyrrhotite is commonly found as inclusions in pyrite and late magnetite replaces pyrite suggesting progressive oxidation of the ore forming fluid free milling gold commonly precipitates at the late stages of alteration in close association with magnetite and tellurides the main tellurides found are hessite altaite sylvanite calaverite and tellurobismuthite two fluid inclusion assemblages were identified from mineralized quartz veins assemblage 1 is characterized by pseudo secondary trails that show the coexistence of co2 rich and low salinity 0 8 wt nacl equiv co2 h2o nacl and h2o nacl inclusions suggesting formation during phase separation fluid immiscibility assemblage 2 is represented by late secondary low temperature th

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

geochemistry of late metamorphic hydrothermal alteration and graphitisation of host rock macraes gold mine otago schist new zealand abstract the macraes gold deposit in the otago schist new zealand formed during late metamorphic fluid flow through a lower greenschist facies shear zone mineralisation occurred near to the brittle ductile transition at about 300 c large volumes of host rock in a shear zone up to 120 m thick have been hydrothermally altered by this fluid activity most alteration is not structurally controlled apart from proximity to the shear zone ductile and brittle microshears traverse the most mineralised rocks and some structural control of fluid flow occurred as well fluid flow was slow similar to that in metamorphic rocks mm year and diffusion through interconnected fluid was a significant chemical process localised extensional hydrofractures m scale are filled with mineralised quartz most alteration of the host rocks was isochemical with respect to the lithophile elements and mineralised rocks have been variably enriched in as au sb w mo and bi but not co or cd addition of sulphur has occurred to both host rocks and mineralised rocks up to 1 wt above a background of 0 1 wt host rock sulphur is mainly pyritic and is not structurally controlled mineralised rocks have pyrite and arsenopyrite along microshears pyrite chalcopyrite sphalerite and galena have formed from sulphidation of silicates with no addition of metals graphite has been added to mineralised rocks along microshears up to 3 wt locally above a background of 0 1 wt noncarbonate carbon graphite deposition may have occurred as a result of mixing of two fluids water methane and water carbon dioxide graphitisation and sulphidation reactions released low d water which accumulated in the slow moving mineralising fluid distinction between this low d reaction water and meteoric water incursion is difficult

north american glacial meltwater history for the past 2 3 m y oxygen isotope evidence from the gulf of mexico a high resolution 18 o record from the northeastern gulf of mexico ocean drilling program leg 100 hole 625b provides a history of surface waters for the past 5 35 m y negative 18 o anomalies between 0 3 and 3 0 in planktic foraminifera beginning at 2 30 ma can only have resulted from the discharge of isotopically light glacial meltwater from the mississippi river meltwater anomalies implicate either the periodic presence of north american mid latitude glacial ice since the late pliocene or a drainage system very different from that of the present

gold mineralization in the ashanti belt of ghana genetic constraints of the stable isotope geochemistry the ashanti belt of ghana is the key district of gold mineralization in the paleoproterozoic terrane of west africa the area considered in southwest ghana is covered by lithologies of the volcanic sedimentary birimian supergroup and the overlying elastic sedimentary tarkwaian group which were jointly folded and metamor phosed under greenschist facies conditions during the eburnean teetonothermal event at about 9 1 ga regional fotiation md subparallel shear zones hosting mesothermal gold mineralization developed during deformation coeval with metamorphism four major tsoes of primary gold mineralization are present in the ashanti belt 1 mesothermal generally steeply dipping quartz veins in shear zones mainly in birimian sedimentary rocks 9 sulfide ores with auriferous arsenopyrite and pyrite spatialty dosely associated with the quartz veins 3 sulfide disseminations and stockworks in granitoids and 4 paleoplaeers of the tarkwaian group this study concentrates on types 1 and 9 of the hydrotherlnal gold mineralization stable isotope analyses of host rock and ore components were performed with the ailn of obtaining parameters relevant to the origin and evolution of the fluids that produced gold mineralization carbonaeeous matter in the birimian metasediments displays 93c values ranging from 11 4 to 9 8 3 per mil relative to pdb indicating an organogenie origin carbonates display a unimodal distribution of 9ac values ranging from 9 9 to 17 0 per rail relative to pdb coz extracted from fluid inclusions in the auriferous quartz veins has 93c values ranging from 9 5 to 15 7 per mil relative to pdb it is proposed that these carbon isotope compositions of carbonates and coz reflect extensive interaction of the co 2 rieh hydrothermal fluids with reduced carbon in birimian sediments in the deeper parts of the hydrothermal systems carbonates and auriferous vein quartz have 6 so values ranging from 19 9 to 9 9 9 and 19 8 to 15 6 per mil relative to smow respectively carbonates and quartz were deposited in near isotopic equilibrium with respect to 6 so indicating fluid dominated conditions during ore formation from fluids of metamorphic or magmatie origin such an origin is corroborated bysd values of water extracted from fluid inclusions in vein quartz 37 to 53 relative to smoxv pyrite of synsedimentary diagenetie origin in birimian schists displays sulfur isotope compositions ranging from 7 3 to 9 0 9 per rail median ca 10 relative to cdt similar compositions and wide ranges are usually attributed to sulfide generation by bacterial sulfate reduction from seawater arsenopyrite and eogenetie pyrite from the sulfide ores generally have 534s values in the range 5 3 to 10 9 per rail relative to cdt the tight unimodal distribution of 534s values indicates a large homogeneous fluid reservoir the low 534s values are interpreted as source inherited not related to unusual ph eh temperature or depositional conditions sulfides in birimian sediments represent the most likely sulfur reservoir tapped by the fluid systems the c o h and s isotope compositions of ore related hydrothermal minerals and fluid inclusion compo nents indicate that the mineralizing fluids interacted extensively with the paleoproterozoie rocks especially birimian sediments at deeper crustal levels and at high temperatures the isotopic compositions are most compatible with the formation of fluids from devolatilization reactions invomng birimian strata during pro grade metamorphism at depth metamorphic fluids

proterozoic links between northeastern brazil and west africa a plate tectonic model based on gravity data abstract using the bullard predrift reconstruction for the southern atlantic analysis of the major gravimetric trends of the west african craton 2000 ma and the surrounding mobile belts allows us to delineate the upper proterozoic shield boundary and to put forward an evolutionary model for late precambrian time over the cratonic area gravimetric anomalies are related to a basement framework or to sedimentary basins whose geometry is linked to reactivation of ancient faults gravity anomalies and landsat imagery show a ne sw lineament in west africa which can be correlated with a variety of geologic features the largest being the late precambrian gourma aulacogen at the margins of the craton the deformation patterns and gravity signatures associated with the 600 ma orogeny were largely controlled by the geometry of the stable craton itself on the eastern edge pan african belt and brazilian belt east of the sao luis and amazonian cratons the distribution of the anomalies and deduced structures can be explained by a continental collision process on the western and southern edge brazilian belt between the sao luis and amazonian cratons rokelide belt mauritanide belt the gravity signature is different suggesting a large amount of intensely tectonized basic rocks a limited spreading process followed by an intense compressional episode is suggested

the growth and decay of the late weichselian ice sheet in western svalbard and adjacent areas based on provenance studies of marine sediments the history of the late weichselian northwestern barents shelf including western svalbard has been investigated by provenance sedimentologist studies of five cores from the continental shelf and slope west of svalbard the chronostratigraphy of the cores is based on ams 14c dates and oxygen isotope analyses interpretations of the cores suggest that the ice sheets of western svalbard and northwestern barents sea experienced advances and retreats in two steps the first significant ice advance beyond the present coastline occurred ca 22 000 14c yr b p and was followed by an ice advance to the shelf edge ca 18 000 14c yr b p ice recession from the outer shelf and the southwestern barents sea began 14 800 14c yr b p and was followed by a second ice recession between 13 000 and 12 000 14 c yr b p during which ice withdrew from the inner shelf a minor readvance of the ice sheet on the shelf west of svalbard occurred close to 12 400 14c yr b p the first deglaciation event was associated with release of icebergs containing ice rafted detritus while the later episode also included significant amounts of meltwater and fine grained sediment

stable isotope o h s c and n systematics of quartz vein systems in the turbidite hosted central and north deborah gold deposits of the bendigo gold field central victoria australia constraints on the origin of ore forming fluids the central and north deborah gold deposits are located in one of 15 mining centers of the bendigo gold field central victoria australia a major world class gold province developed in a turbidite terrane gold mineralization occurs primarily within domes and reverse fault systems in multiply deformed lower ordovician metaturbidites deformation and thrusting occurred between 450 and 420 ma coevally with regional metamorphism at zeolite to greenschist facies in the central and north deborah deposits six stages of quartz veining each with characteristic geometry and mineral paragenesis developed sequentially stages 1 2 and 4 are gold bearing whereas stages 3 5 and 6 are barren gold related quartz veins were deposited at temperatures of 350 25 c based on oxygen isotope geothermometry of silicate phases quartz muscovite and sulfur isotope compositions of coexisting sulfide mineral pairs sphalerite galena this result is consistent with temperatures inferred from fluid inclusions the 18o values of gold bearing vein quartz and coexisting hydrothermal muscovite are uniform within and between the six vein stages ranging from 15 9 to 17 2 per mil and 11 6 to 12 8 per mil respectively the d values of hydrothermal muscovites are also uniform at 68 to 55 per mil ore fluids have calculated 18o values of 8 to 11 per mil and d values of 37 to 17 per mil pyrite of synsedimentary diagenetic origin in slate and sandstone displays sulfur isotope compositions of 23 5 to 8 0 per mil and 0 1 to 11 9 per mil respectively sulfides from gold bearing quartz veins have a range of 7 4 to 8 1 per mil median ca 2 5 the relatively uniform vein pyrites indicate a large uniform sulfur source most likely from desulfidation reactions of sulfide minerals interaction of the ore forming fluid with depleted sulfides at the site of deposition may have shifted the sulfur isotope signatures of source sulfides to more negative values carbon isotope compositions of carbonates range from 5 1 to 9 0 per mil median ca 6 5 nitrogen concentrations and nitrogen isotope compositions of hydrothermal muscovite from quartz veins are 652 to 895 ppm and 2 84 to 4 49 per mil respectively collectively the data define a band in n content vegh rsus 15n coordinate space from higher n and lower 15n in the phanerozoic metasedimenary hosted central and north deborah gold deposits to lower n and higher 15n in predominantly meta igneous hosted neoarchean gold provinces collectively the h o s c and n isotope compositions of ore related hydrothermal minerals indicate that the dilute aqueous carbonic and n bearing ore forming fluids were generated from metamorphic dehydration reactions at deeper crustal levels and were variably influenced by exchange with wall rocks during transport to the site of deposition this genetic model for the central and north deborah gold deposits is consistent with other turbidite hosted lode gold deposits and counterparts in volcanic plutonic terranes in contrast these data are not consistent with either mantle derived fluids or magmatic fluids nor do they support a deeply circulated meteoric surface water model for vein formation

geochemistry and zircon geochronology of paleoproterozoic granitoids further evidence on the magmatic and crustal evolution of the sao luis cratonic fragment brazil abstract the tromai intrusive suite is the predominant exposed unit of the sao luis cratonic fragment in northern brazil the suite forms batholiths and stocks of granitoids that were emplaced between 2168 4 ma and 2149 4 ma and intruded a 2240 5 ma old metavolcano sedimentary sequence the batholiths are composed of a variety of petrographic types that have been grouped in three sub units based on the predominant petrographic type and named cavala tonalite bom jesus granodiorite and areal granite from the more primitive to the more evolved phases in addition to subordinate shallow felsic intrusions the tromai suite is an expanded magmatic association comprising minor mafic rocks to predominantly intermediate and felsic low to high k and metaluminous to weakly peraluminous granitoids that follow a na enriched calc alkaline trend combined rock association geochronology nd isotopes and geochemical signature indicate that the tromai suite formed from magmas derived from juvenile protoliths modified by fractional crystallization the juvenile protoliths included ocean plate mantle wedge and minor sediments the data also indicate an intra oceanic arc setting that possibly transitioned to a continental margin and that the tromai intrusive suite records the main accretionary stage of the rhyacian orogen ca 2 24 2 15 ma that culminated with a collision stage at about 2 1 ga and gave rise to the present day sao luis cratonic fragment this time interval is coincident with the main period of crustal growth in the south american platform and in the paleoproterozoic terranes of the west african craton the beginning of this period is also coincident with the end of a period in which only minor amounts of juvenile crust is found worldwide the negra velha granite is a distinct unit that forms a few stocks that intruded the granitoids of the tromai suite between 2076 and 2056 ma ago negra velha is an association of monzogranite and subordinate quartz monzonite and syenogranite with an alkaline signature that shows high rb sr ba enrichments resembling shoshonitic associations this granite represents the post orogenic phase of the rhyacian orogenesis

the role of carbonaceous indicator slates in the genesis of lode gold mineralization in the western lachlan orogen victoria southeastern australia indicator slates have long been considered to represent a useful exploration guide for turbidite hosted mesothermal lode gold mineralization in central victoria this assumption has been based on an apparent close spatial relationship between high gold grades and the proximity of thin commonly pyritic and carbon rich marker units detailed studies in a number of gold deposits throughout central victoria however reveal that highest gold grades do not necessarily coincide with the presence of carbonaceous units in many places where gold mineralization is associated with carbon rich matter the high c accumulations are the result of epigenetic remobilization during hydrothermal alteration and ore genesis petrographic geochemical and stable isotope c o s investigations into the origin and nature of the carbonaceous matter mainly amorphous bitumens and pyrobitumen of organic origin with biological fragments and rare graphite of both detrital and metamorphic origin demonstrate that black shales within the cambro ordovician succession in central victoria lacked the vital constituents to provide 1 a primary sink for precious metals and 2 whereas the presence of carbonaceous matter was likely to affect the evolution of epigenetic hydrothermal fluids and locally may have facilitated gold enrichment carbon rich sedimentary rocks were not crucial for ore genesis on the deposit scale instead the size of the hydrothermal cell physicochemical conditions of the ascending fluids and dynamic fault valve behavior played far more significant roles in controlling gold precipitation the importance of these processes has implications for exploration targeting sediment hosted lode and disseminated style gold mineralization in the western lachlan orogen and in slate belt provinces elsewhere

fluid immiscibility in natural processes use and misuse of fluid inclusion data ii interpretation of fluid inclusion data in terms of immiscibility abstract phase equilibrium analysis of the relevant systems together with the application of the principles of chemical equilibrium put severe constraints on the interpretation of fluid inclusion data in terms of immiscibility in part i following from that point the major limits on the accuracy and even the validity of fluid inclusion quantitative data and their interpretation in terms of fluid composition and density are briefly discussed the practical implications of the general constraints temperature pressure topology of the fluid systems are envisaged emphasis is laid on some important consequences such as the use of isochore intercepts and the possible resulting interpretation of fluid mixing rather than unmixing the case of highly saline inclusions the identification and interpretation of heterogeneous trapping the composition and density constraints on coexisting fluids are presented and illustrated by natural examples taking into account all the measurable parameters in fluid inclusions volume temperature and nature of phase transitions more or less complete individual spectroscopic analyses all the available experimental data and all the theoretical constraints may be long and difficult however it is most generally very informative and productive although part of these data is often sufficient to deny unmixing nevertheless a final example on metamorphic fluids demonstrates how such an approach can prove and also characterize a fluid unmixing during a geologic process

constraints on the timing of late eburnean metamorphism gold mineralisation and regional exhumation at damang mine ghana the damang gold deposit in southwest ghana is unique among known deposits in ghana comprising gold mineralisation in two distinct styles early gold hosted in a stratigraphically controlled auriferous quartz pebble conglomerate horizon is overprinted by later gold mineralisation contained in a sub horizontal fault fracture quartz vein array a multi system geochronological study is used to constrain the timing of igneous activity regional metamorphism gold mineralisation and the thermal history at damang u pb analysis of zircons from birimian volcaniclastic and intrusive rocks constrain volcanism and associated intrusive activity at 2178 0 9 3 ma and 2164 6 8 0 ma respectively which is consistent with previous studies the age of formation of staurolite grade amphibolite facies peak metamorphic mineral assemblages at 2005 26 ma is provided by u th total pb epma analysis of metamorphic monazite grains in the tarkwa phyllite measured 40ar 39ar biotite ages range between 1980 9 ma and 1898 11 ma argon diffusion modelling with the programme diffarg suggests that this age range could be achieved by a period of rapid cooling at a rate of approximately 17 c ma followed by a prolonged period of much slower cooling at a rate of 0 15 c ma the period of rapid cooling is interpreted to represent localised exhumation of the damang host rocks during the latest stage of the eburnean orogeny at the time of hydrothermal gold mineralisation given these age constraints hydrothermal gold mineralisation is inferred to have occurred between approximately 2030 ma and 1980 ma these ages constrain metamorphism fluid flow and gold mineralisation at damang and are the youngest currently recognised in the birimian of sw ghana

tree roots as a dominant agent of streambed habitat profile pattern and grade control abstract tree roots are a ubiquitous biotic agent in numerous fluvial settings but their role in controlling streambed habitat and profile patterns is understudied although their importance in resisting bank erosion is more widely reported tree roots also have the potential to span the channel bed particularly in narrower first to third order streams this enables tree roots to drive profile patterns and maintain important habitat elements such as steps that induce deep pools in both perennial and ephemeral systems in a study across two humid temperate ecoregions tree roots were associated with more abundant and larger steps than large wood lw forming 91 of the steps in the high gravel load setting and 40 of the steps in the steeper coarser low sediment supply setting when combined with lw a majority of the steps in both settings were formed by biotic agents 97 and 55 respectively furthermore 70 of the deepest pools across all 40 study reaches were associated with biotic controls with root steps being the most dominant driver 45 of the deepest pools in all study reaches including the maximum pool depths measured in both ecoregions 0 57 m and 0 38 m respectively many questions need further exploration for example what tree sizes and species are most likely to form root steps i e trunk diameters 40 cm and american sycamore platanus occidentalis in this study however even this somewhat limited study has clear implications to land use planning and stormwater management for example on land developments draining to root step dominated streams maintaining the integrity of the riparian zone would seem to be as important as ensuring hydrologic mimicry if channel integrity is to be preserved the study also has insights to the stream restoration industry where conventional practice typically involves large tree clearing operations to facilitate stream regrading

deglacial meltwater discharge north atlantic deep circulation and abrupt climate change high resolution paleogeochemical data from the north atlantic ocean indicate that in the interval 15 000 to 10 000 14c years before present b p north atlantic deep water nadw production was decreased or eliminated four times at about 14 500 and probably older 13 500 12 000 and 10 500 years b p each of these changes occurred at the same time as abrupt events of meltwater discharge to the surface ocean inferred from oxygen isotope studies of planktonic foraminifera and from glacial geological studies on land in addition each of these times may be associated with brief episodes of cooler climate in the north atlantic region the best example of which is the younger dryas cooling of 10 500 years ago these results support models linking meltwater discharge decreased nadw production and decreased north atlantic heat flux

magnitude and geographic variation of the radiocarbon content in antarctic marine life implications for reservoir corrections in radiocarbon dating abstract radiocarbon age estimates on marine derined materials from the antarctic appear old in comparison with the terrestrial radiocarbon timescale as a result of significantly depleted 14 c concentrations in antarctic seas we review measurements of 14 c concentrations in modern antarctic marine life from south georgia the shetland islands and antarctic peninsula and suggest correction factors required to relate radiocarbon measurements on fossil marine materials from these areas to the conventional radiocarbon timescale for dates based on shells seaweeds sealbones and penguin bones the minimum correction factor for south georgia is ca 750 years and for the south shetland islands and antarctic peninsula it may be up to 1250 1300 years for whalebones a minimum correction of 1000 years may apply over a wide geographic area for comparison we also review radiocarbon measurements on marine species and seawater from other sectors of the antarctic in high latitudes correction factors of ca 1400 years are generally applicable to shell seal and penguin samples

global dynamics of the antarctic ice sheet the total mass budget of the antarctic ice sheet is studied with a simple axi symmetrical model the ice sheet has a parabolic profile resting on a bed that slopes linearly downwards from the centre of the ice sheet into the ocean the mean ice velocity at the grounding line is assumed to be proportional to the water depth the accumulation rate is a linear function of the distance to the centre setting the total mass budget to zero yields a quadratic equation for the steady state ice sheet radius r analysis of the equilibrium states sheds light on the sensitivity of the ice sheet radius to changes in sea level s and precipitation with respect to the present state prel for model parameters obtained by matching the analytical model to the present state of the antarctic ice sheet the sensitivity values are dr ds 2400 and dr dprel 4000 m the model can also be used to study transient behaviour of the ice sheet the characteristic relaxation time e folding time scale is about 3500 years forcing the model with a sea level and accumulation history over the past few hundred thousands of years yields antarctic ice volume curves that are similar to those obtained by comprehensive numerical modelling the current imbalance predicted by the model corresponds to a sea level rise of 0 25 mm yr 1

the ross sea s contribution to eustatic sea level during meltwater pulse 1a abstract radiocarbon dates obtained from ross sea sediments combined with constraints on maximum ice extent based on sedimentological data and ice sheet modeling results indicate that the ross embayment antarctica is unlikely to have been a substantial contributor to meltwater pulse mwp 1a at 12 5 14c ka the ice sheet in the ross embayment during the last glacial maximum is estimated to have contained ice volume equivalent to 3 6 m eustatic sea level rise and does not show evidence of rapid retreat grounded ice advance occurred during initial late pleistocene eustatic rise and ice sheet retreat began coeval with or subsequent to mwp 1a chronological and sedimentological data from the ross embayment indicate that deglaciation in this region has occurred throughout the holocene and the maximum possible contribution of eustatic rise from the ross embayment is

a carbonaceous sedimentary source rock model for carlin type and orogenic gold deposits this paper presents evidence and arguments that carbonaceous sedimentary rocks were a source for au and as in sediment hosted orogenic and carlin type gold deposits and develops a corresponding genetic model in this two stage basin scale model gold and arsenic are introduced early into black shale and turbidite basins during sedimentation and diagenesis stage 1 and concentrated to ore grades by later hydrothermal structural or magmatic processes stage 2 in reduced continental margin basin settings organic matter sedimented under anoxic to euxinic conditions immobilizes and concentrates gold arsenic and a range of trace elements particularly v ni se ag zn mo cu u present in marine bottom waters into fine grained black mudstone and siltstone of slope and basin facies during early diagenesis gold and certain other trace elements ni se te ag mo cu pge are preferentially partitioned into arsenian pyrite that grows in the muds these processes produce regionally extensive black shale and turbidite sequences enriched in syngenetic gold and arsenic commonly from 5 to 100 ppb au and 10 to 200 ppm as rare organic and sulfide rich metalliferous black shales may contain up to 1 to 2 ppm au and over 1 000 ppm as present as refractory gold in arsenian pyrite and nanoparticles of free gold during late diagenesis and early metamorphism stage 2 the diagenetic arsenian pyrite is recrystallized to form coarser grained pyrite generations and the organic matter is cooked to bitumen under higher grade metamorphism lower greenschist facies and above arsenian pyrite in carbonaceous shales is converted to pyrrhotite these processes release gold arsenic sulfur and other elements sb te cu zn mo bi tl and pb from the source rocks to become concentrated by hydrothermal processes locally to produce gold ores in structural sites such as fold hinge zones shear or breccia zones within or above the black shale sequence la icp ms analyses of diagenetic pyrite in carbonaceous sediments both associated and not associated with gold deposits suggests that invisible gold contents of greater than 250 ppb in diagenetic pyrite are indicative of carbonaceous shale source rocks with the potential to produce economic gold deposits application of this sedimentary source rock model enables a systematic exploration approach for sediment hosted gold deposits based on the distribution composition and structure of carbonaceous shale sequences and their contained diagenetic pyrite

late weichselian deglacial history of the svyataya saint anna trough northern kara sea arctic russia abstract marine sediment core and seismic records from the svyataya saint anna trough provide new insight into the distribution of late weichselian glacial coverage ice retreat pattern and post glacial environments in the northern barents and kara seas these records indicate that the saint anna trough was filled with grounded glacier ice which likely reached the shelf edge during the late weichselian maximum several radiocarbon dates suggest early deglaciation of the deep axial part of the trough prior to 13 3 ka two sandy beds in the deglacial section of the cores imply distinct pulses of iceberg calving and or melting which were probably associated with stepwise retreat of the ice margin morainic ridges and glacial sole markings in the western part of the saint anna trough indicate that the northern central barents sea was a site of a large ice mass during deglaciation a smaller ice cap is inferred for the northern kara plateau at later stages ice retreat on the western flank of the trough was directed towards franz josef land and was presumably facilitated by a separation of the barents sea and novaya zemlya ice domes deglaciation of the saint anna trough was completed by ca 10 ka high post glacial sediment fluxes between 10 and 8 ka were probably related to sea floor coastal erosion and or siberian river discharge during the rising sea level

the last glacial maximum of svalbard and the barents sea area ice sheet extent and configuration abstract the timing extent and configuration of the late weichselian barents ice sheet has been debated for several decades this debate has arisen largely because of the limited or conflicting field evidence on which most models have been based in particular reconstruction of the marine parts of the former barents ice sheet has been controversial this paper aims to review the geological observations and interpretations regarding the size and timing of the late weichselian ice sheet combined with numerical modelling of its formation in order to produce a reconstruction of ice sheet extent and behaviour sub glacial till with overlying glacimarine deposits dated to the late weichselian is found over most of the barents sea floor and the continental shelf west of svalbard glacially induced debris flow deposits on the large bjonoya and isfjorden trough mouth fans strongly support the idea of ice sheet extension to the shelf edge during maximum glaciation isobase maps show a centre of post glacial uplift in the north central barents sea and glaciological and isostatic modelling suggest that the ice sheet was 2000 3000 m thick in this area the ice sheet was confluent with ice over the kara sea but the interaction between the barents and kara ice sheets is not yet fully understood the deglaciation of the barents ice sheet started ca 15 ka probably by calving within the deeper troughs by 12 ka most of the central barents sea was ice free and ice remained over the svalbard franz josef land and novaja zemlya archipelagos and adjacent shallow shelf areas the coasts and fjords of these islands were ice free by 10 ka

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

influence of structural setting on sulphur isotopes in archean orogenic gold deposits eastern goldfields province yilgarn western australia the published mean 34s values of ore related pyrites from orogenic gold deposits of the eastern goldfields province yilgarn craton lie between 4 and 4 as for orogenic gold deposits worldwide most deposits have positive means and a restricted range of 34s values but some have negative means and wider ranges of 34s values wall rock carbonation and back mixing of similar source fluids with different fluid pathways can explain some of the more negative 34s signatures however structural setting appears to be the most important factor controlling ore fluid oxidation state and hence the distribution of 34s values in gold related pyrites shear hosted deposits appear to have experienced fluid dominated processes such as phase separation whereas stockwork vein hosted or disseminated deposits formed under conditions of greater rock buffering at victory defiance in particular negative 34s values are more common in gently dipping dilational structures compared to more compressional steeply dipping structures it appears most likely that fluid pressure fluctuations during fault valve cycles establish different fluid flow regimes in structures with different orientations rapid fluid pressure fluctuations in dilational structures during seismic activity can cause partitioning of reduced gas phases from the ore fluid during extreme phase separation and hence are an effective method of ore fluid oxidation leading to large local fluctuations in oxidation state it is thus not necessary to invoke mixing with oxidised magmatic fluids to explain 34s signatures indicative of oxidation in any case available robust geochronology in the eastern goldfields province does not support the direct involvement of oxidised magmatic fluids from adjacent granitic intrusions in orogenic gold genesis thus negative mean 34s values and large variations in 34s values of ore related pyrites in world class orogenic gold deposits are interpreted to result from multiple mechanisms of gold precipitation from a single ubiquitous ore fluid in varying structural settings rather than from the involvement of oxidised ore fluids from a different source such signatures are indicative but not diagnostic of anomalously large orogenic gold systems

does gold in orogenic deposits come from pyrite in deeply buried carbon rich sediments insight from volatiles in fluid inclusions the origin of volatiles in fluid inclusions was reviewed for testing the involvement at depth of carbonaceous pyritic sedimentary rocks as the source for orogenic gold mineralization fluid inclusions from selected deposits were analyzed by solid probe mass spectrometry fluids are mostly aqueous carbonic with variable amounts of n 2 ch 4 c 2 h 6 ar h 2 s h 2 and he for fluids with ch 4 and c 2 h 6 their ratios c 1 c 2 range from 2 6 to 25 5 indicating that c 2 h 6 is sourced from thermally degraded organic matter proportions of co 2 ch 4 c 2 h 6 and h 2 are highly variable and can be explained by hydrothermal reactions where c 2 h 6 is degraded to co 2 by water consumption such reactions may account for the problematic co 2 rich h 2 o poor fluids associated with some of the richest gold districts conditions needed for c 2 h 6 degradation are also fundamental for forming gold deposits such as hs enriched fluids for carrying gold and local weakly oxidizing conditions for promoting gold precipitation the c 2 h 6 content is recorded in fluids from mesoarchean to cretaceous gold deposits providing support for a general model where fluids and gold were sourced from deeply buried carbon rich and pyrite gold bearing sedimentary rocks

insights into the genesis of orogenic gold deposits from the zhengchong gold field northeastern hunan province china abstract the zhengchong gold deposit is located in the central jiangnan orogen belt where gold ore occurs mainly as quartz veins altered slates and minor breccias hosted by shear zones in the lengjiaxi group based on its vein crosscutting relationships mineral assemblages and paragenetic sequences mineralization in this deposit is divided into four stages stage 1 quartz q1 pyrite stage stage 2 quartz q2 pyrite arsenopyrite stage stage 3 quartz q3 pyrite polymetallic sulfide mineral stage and stage 4 quartz q4 carbonate stage two types of fluid inclusions have been recognized in the quartz grains co2 bearing fluid inclusions c type and aqueous fluid inclusions w type fluid inclusions in hydrothermal quartz yield homogeneous temperatures and salinities of 312 406 c and 4 65 9 21 wt naclequiv stage 1 252 324 c and 2 20 8 10 wt naclequiv stage 2 193 296 c and 2 24 14 04 wt naclequiv stage 3 and 127 209 c and 2 57 6 59 wt naclequiv stage 4 the co occurrence of these inclusion types despite their variable fluid salinities can be explained by fluid boiling during stages 2 and 3 of mineralization possibly as a result of episodic pressure decreases linked to fault valve activity laser raman spectroscopic analysis results reveal that small quantities of ch4 and n2 are present in the stage 2 3 fluid inclusions fluid inclusion data and microthermometric results show that the ore fluids were low salinity and co2 rich yielding both co2 inclusions c type and aqueous inclusions w type the trapping pressures estimated from the c type inclusions are 80 180 mpa for stages 2 and 3 yielding gold mineralization depths of 7 8 km the mineralizing fluids had calculated 18oh2o values ranging from 3 7 to 8 2 and d values from 75 2 to 61 8 indicating derivation mainly from metamorphic fluids the sulfide minerals yield 34s values of 8 9 to 0 1 and 206pb 204pb 207pb 204pb and 208pb 204pb ratios of 17 83 18 12 15 67 15 76 and 38 03 38 62 respectively which differ significantly from the isotopic composition of the granodiorite 34s 2 4 to 1 0 206pb 204pb 18 57 19 28 207pb 204pb 15 69 15 76 208pb 204pb 38 97 40 34 however the s pb isotopic compositions of the sulfide minerals are similar to those of slates of the lengjiaxi group in the study area the zhengchong gold deposit probably originated from a metamorphic fluid system characterized by low salinity low density and co2 enrichment with limited admixture of magmatic fluids based on similarities to other orogenic gold deposits we infer that the zhengchong orebody represents an orogenic gold deposit formed during the late ordovician jiangnan orogeny caused by collision of the yangtze and cathaysia blocks

no mass independent sulfur isotope fractionation in auriferous fluids supports a magmatic origin for archean gold deposits the source of the auriferous fluids that formed archean greenstone gold deposits remains controversial metamorphic fluids fluids derived from felsic magmas mantle fluids and meteoric water have all been suggested we report the results of ion microprobe sensitive high resolution ion microprobe stable isotopes multiple sulfur isotope analyses 32 s 33 s 34 s conducted to constrain the source of sulfur in four archean gold deposits the analyses are of pyrite samples that are genetically related to the gold mineralization from the bellerophon victory and wallaby deposits in the eastern goldfield superterrane of the yilgarn craton western australia together with sulfide from black shale from the golden mile deposit and st ives gold camp ore associated pyrite from the beattie gold deposit of the abitibi greenstone belt of the superior craton in canada was measured to test whether the results obtained from the yilgarn craton apply to other archean gold deposits sulfur analyses from sulfide from shale show mass independent fractionation mif with 33 s as high as 1 8 whereas those from the 4 archean gold deposits are dominated by mass dependent fractionation with only a minor contribution of mif sulfur in this respect archean gold deposits contrast with archean nis deposits which are dominated by mif sulfur the absence of mif in the pyrite from the four archean gold deposits analyzed in this study is consistent with the auriferous fluid originating from a felsic magma or mantle source but inconsistent with the metamorphic and meteoritic fluid hypotheses

volume of antarctic ice at the last glacial maximum and its impact on global sea level change abstract the volume of antarctic ice at the last glacial maximum is a key factor for calculating the past contribution of melting ice sheets to late pleistocene global sea level change at present there are large uncertainties in our knowledge of the extent and thickness of the formerly expanded antarctic ice sheets and in the timing of their release as meltwater into the world s oceans this paper reviews the four main approaches to determining former antarctic ice volume namely glacial geology glacio isostatic studies glaciological modelling and ice core analysis and attempts to reconcile these to give a best estimate for ice volume in the ross sea there was a major expansion of grounded ice at the last glacial maximum accounting for 2 3 3 2 m of global sea level at some time in the weddell sea a large grounded ice sheet corresponding to c 2 7 m of global sea level extended to the shelf break however this ice expansion has not yet been confidently dated and may not relate to the last glacial maximum around east antarctica there was thickening and advance offshore of ice in coastal regions ice core evidence suggests that the interior of east antarctica was either close to its present elevation or thinner during the last glacial so the effect of east antarctica on sea level depends on the net balance between marginal thickening and interior thinning suggested east antarctic contributions vary from a 3 5 5 m lowering to a 0 64 m rise in global sea level the antarctic peninsula ice sheet thickened and extended offshore at the last glacial maximum with a sea level equivalent contribution of c 1 7 m thus the antarctic ice sheets accounted for between 6 1 and 13 1 m of global sea level fall at the last glacial maximum this is substantially less than has been suggested by most previous studies but the maximum figure matches well with one modelling estimate the timing of antarctic deglaciation is not well known in the ross sea terrestrial evidence suggests deglaciation may have begun at c 13 000 yr bp 1 but that grounded ice persisted until c 6 500 yr bp marine evidence suggests the western ross sea was deglaciated by c 11 500 yr bp deglaciation of the weddell sea is poorly constrained grounded ice in the northern antarctic peninsula had retreated by c 13 000 yr bp and further south deglaciation occurred sometime prior to c 6 000 yr bp many parts of coastal east antarctica apparently escaped glaciation at the lgm but in those areas that were ice covered deglaciation was underway by 10 000 yr bp with existing data the timing of deglaciation shows no firm relation to northern hemisphere driven sea level rise this is probably due partly to lack of antarctic dating evidence but also to the combined influence of several forcing mechanisms acting during deglaciation

preservation efficiencies and accumulation rates for biogenic silica and organic c n and p in high latitude sediments the ross sea rates of biogenic sediment accumulation biogenic silica and organic c n and p and pore water flux have been established for a variety of depositional environments in the ross sea on the basis of 14c measurements in kasten cores sediment accumulation rates ranged from 250 cm kyr 1 in the coastal basin of granite harbor to 1 2 cm kyr 1 in the shelf and slope environments of the northern and eastern ross sea burial of biogenic material was most rapid in the southwestern ross sea where biogenic silica accumulation rates ranged from 2 to 31 g cm 2 kyr 1 and organic carbon accumulation rates ranged from 0 05 to 1 4 g cm 2 kyr 1 in the northern and eastern ross sea biogenic silica and organic carbon accumulation rates typically equaled 0 02 0 03 g sio2 cm 2 kyr 1 and 0 002 0 004 g c cm 2 kyr 1 flux core measurements were used to estimate seabed regeneration rates for biogenic silica organic carbon and phosphorus pore water fluxes in general showed much less variability across the ross sea than did the biogenic accumulation rates pore water silicate fluxes in the study area ranged from 0 6 to 5 3 g sio2 cm 2 kyr 1 whereas carbon fluxes ranged from 0 1 to 1 2 g c cm 2 kyr 1 and phosphate fluxes varied from 0 006 to 0 012 g p cm 2 kyr 1 seabed preservation efficiencies were calculated for biogenic silica organic carbon and phosphorus by combining the solid phase and pore water data the seabed preservation efficiencies for biogenic silica 1 86 were greater than for organic carbon 1 71 at all nine stations examined the preferential preservation of biogenic silica relative to organic carbon also was apparent in ross sea sediments because the biogenic silica organic carbon ratio in the material buried in the seabed generally was 2 times greater than the ratio in sediment particles arriving at the sediment water interface sediment accumulation rate correlated strongly with both the biogenic silica and the organic carbon preservation efficiency data p preservation efficiencies remained relatively high 24 65 even when the accumulation rates were low 1 2 cm kyr 1 because of the near zero phosphate fluxes out of the seabed of the total p in the seabed 0 04 0 09 wt approximately 25 existed in the form of organic p the total amount of biogenic silica accumulating on the ross sea shelf is 2 3 1012 g sio2 yr 1 which is approximately an order of magnitude less than the ledford hoffman et al 1986 estimate that was based on 210pb chronologies biogenic silica accumulation rates in the southern central and western ross sea increased during the mid and late holocene reaching their maximum values during the past 500 to 1000 years

the compositional limits of fluid immiscibility in the system h2onaclco2 as determined with the use of synthetic fluid inclusions in conjunction with mass spectrometry abstract the compositional limits of fluid immiscibility in the system nacl h2o co2 were investigated from 500 to 700 c at pressures of 1 2 and 3 kbar synthetic fluid inclusions formed in quartz prisms were equilibrated with high temperature high pressure fluids in hydrothermal pressure vessels the inclusions were analyzed optically by mass spectrometry and by microthermometry the fields of immiscibility were defined as a function of temperature and pressure mass spectrometry was used to validate the existence of the immiscible fluids and to determine the co 2 h 2 o ratios of the two coexisting phases microthermometry i e the measurement of the melting temperature of solid sodium chloride was used in conjunction with the results of the mass spectrometry measurements to define the tie lines within the two phase fields isochores and the location of a portion of the isopleth were determined for composition nacl8 9 h2o76 1 co215 0

fluid sources and the role of abiogenic ch4 in archean gold mineralization constraints from noble gases and halogens abstract the st ives goldfield of western australia hosts high grade gold mineralization in a variety of lithologies within 2 7 ga greenstone belts the low salinity co 2 bearing fluids responsible for gold mineralization could have been generated either by metamorphic processes within the greenstones or from external magmatic sources and independently sourced abiogenic ch 4 has been suggested as an important control on gold mineralization in order to better constrain possible fluid sources we applied a combination of he ne ar kr xe isotope and cl br i analyses to quartz and carbonate veins containing h 2 o co 2 fluid inclusions pyrite magnetite hematite and quartz veins containing ch 4 rich fluid inclusions pyrrhotite pyrite samples containing h 2 o co 2 fluid inclusions have higher br cl and i cl values and less radiogenic noble gas isotope signatures than suggested for ch 4 rich fluid inclusions the samples containing ch 4 fluid inclusions have maximum 40 ar 36 ar of 50 000 20 ne 22 ne of 9 8 and maximum 21 ne 22 ne of 0 56 in contrast h 2 o co 2 fluid inclusions have maximum 40 ar 36 ar of only 20 000 and maximum 21 ne 22 ne of 0 14 the use of 40 ar 39 ar methodology to measure k simultaneously with 40 ar 36 ar enables us to show the highest 40 ar 36 ar ratios are representative of the fluids trapped at 2 65 ga and have not been significantly modified by post entrapment 40 ar r production 40 ar r radiogenic 40 ar furthermore the 40 ar 36 ar ratio is correlated with the 21 ne 22 ne and 136 xe 130 xe ratios demonstrating that other heavy noble gas isotope ratios are also close to their original composition the highly radiogenic noble gas isotope signatures are unlikely to have been generated within the greenstone host rocks because seawater altered volcanic rocks have high abundances of non radiogenic atmospheric noble gas isotopes in contrast the strongly radiogenic noble gas isotope signatures are likely to have been generated in ancient basement rocks underlying the greenstones at 2 65 ga the combined noble gas and halogen data are consistent with models in which redox reactions triggered by interaction of independently and distally sourced h 2 o co 2 and ch 4 localized high grade gold mineralization we suggest that the fluids were intimately associated with crustal magmatism in the st ives goldfield and that abiogenic ch 4 could be an important moderator of redox processes in other crustal environments

sea level fingerprinting as a direct test for the source of global meltwater pulse ia the ice reservoir that served as the source for the meltwater pulse ia remains enigmatic and controversial we show that each of the melting scenarios that have been proposed for the event produces a distinct variation or fingerprint in the global distribution of meltwater we compare sea level fingerprints associated with various melting scenarios to existing sea level records from barbados and the sunda shelf and conclude that the southern laurentide ice sheet could not have been the sole source of the meltwater pulse whereas a substantial contribution from the antarctic ice sheet is consistent with these records

age constraints on gold mineralization and paleoproterozoic crustal evolution in the ashanti belt of southern ghana abstract u pb isotopic dating in the region of the ashanti belt in ghana has been carried out to improve understanding of crustal evolution and gold mineralization in this paleoproterozoic terrane a u pb zircon age of ca 2174 2 ma for a belt type granitoid from sekondi accords with previous ages on belt type plutons in ghana but metamorphic titanite within this granitoid gives an age of 2092 3 ma coeval with intrusion of the late basin type granitoids the within belt granitoid at banso village has an age close to 2097 3 ma which falls into the age range typical for basin type granitoids 2116 2088 ma these results indicate that widespread plutonism in the birimian sedimentary basins also produced magmatic and metamorphic effects within the volcanic belts most dated detrital zircons from birimian metasediment samples at the ashanti gold mine agree in age at 2155 2 ma with others that are slightly older this gives a maximum estimate for the depositional age of the sediments and for gold mineralization at the ashanti gold mine hydrothermally altered and auriferous granitoid plutons along the western flank of the ashanti belt at anyankyerim nhyiaso yaomensakrom and esuajah ayanfuri all have intrusion ages within error of 2105 2 ma this is also the age of metamorphic titanite in an unmineralized basin type granitoid at kwapia rutiles from the hydrothermally altered yaomensakrom and the mineralized esuajah ayanfuri granitoid were dated at 2098 7 and 2086 4 ma respectively and probably provide minimum ages on gold mineralization these data indicate a close temporal association among auriferous granitoid plutons near the western margin of the ashanti belt at 2105 ma gold mineralization may be only slightly younger than magmatic emplacement of the plutons granitic plutonism in the sedimentary basins metamorphism and hydrothermal gold mineralization in southern ghana are all part of a major crust formation event the eburnean orogeny that can be constrained to the time range 2120 2080 ma

late pleistocene holocene sedimentary fluxes of organic carbon and biogenic silica in the northwestern ross sea antarctica eight sediment gravity cores collected from thejoides and drygalski basins were analysed in order to understand late pleistocene holocene biogenic dux changes in the ross sea driven by paleoenvironmental changes core lithologies and magnetic susceptibility depth profiles were used for core logging and stratigraphic corre lation nineteen ams radiocarbon dates of bulk organic matter were used to set chrono logical constraints and calculate sediment accumulation rates these rates which vary from 1 4 38 cm ka i were used to obtain the burial f1uxes of biogenic components the highest f uxes occur in the deepest parts o the basins toc 0 05 0 2 g cm 2 ka i biogenic silica 1 5 5 g cm 2 ka i whereas topographic highs show the lowest values toc 0 010 1 g cm 2 ka i biogenic silica 0 1 1 4 g cm 2 ka i dramatic changes in both physical properties and fluxes record the establishment of open marine sedimentation conditions which occurred first in thejoides basin and then with a delay of ca 6000 years in the drygalski basin both toc and biogenic silica ouxes increase through the holocene though slightly difterently the high f1uxes of both lobe and biogenic ba suggest that sedi ment accumulation at basin sites is strongly influenced by lateral transport

pre bomb radiocarbon and the reservoir correction for calcareous marine species in the southern ocean four antarctic marine mollusc shells which were collected alive between 1917 and 1940 were analyzed by accelerator mass spectrometry to provide the first pre bomb radiocarbon measurements of biogenic carbonates from the southern ocean after correcting for the impact of fossil fuel combustion suess effect radiocarbon activities of the pre bomb shells averaged 149 8 10 4 in contrast the 14c values for post bomb molluscs echinoderms brachiopods and foraminifera averaged 96 1 25 2 these biogenic carbonate 14c values are nearly identical to pre bomb estimates 148 to 152 and post bomb measurements 98 4 22 0 of the surface waters in the southern ocean average radiocarbon ages of the biogenic carbonates before and after 1950 1303 84 years and 811 205 years respectively along with those from seals and penguins indicate that the antarctic marine radiocarbon reservoir has decreased in age by nearly 500 years during the second half of the 20th century marine species and seawater measurements firmly place the radiocarbon reservoir correction at 1300 100 years for calcareous marine fossils which are widespread abundant and well preserved organic materials for interpreting ice sheet climate and sea level impacts on the antarctic marine ecosystem during the holocene

north american ice sheet reconstructions at the last glacial maximum abstract the areal extent of the last glacial maximum lgm ice sheets is well known in north america but there is no direct geological proxy for ice sheet thickness or volume uncertainties associated with glaciological and geophysical reconstructions give widely varying estimates of north american ice sheet nais volume at lgm in an effort to quantify the uncertainty associated with glaciological reconstructions we conducted a suite of 190 numerical simulations of the last glacial cycle in north america prescribing different climatic mass balance glaciologic and isostatic treatments for the least constrained model variables lgm ice sheet reconstructions were evaluated using the well established geologic record of ice sheet area and southern extent at lgm dyke and prest 1987 these constraints give a subset of 33 simulations that produce reasonable lgm ice cover in north america ice sheet dispositions and the associated parameter settings in this subset of tests provide insight into the plausible range of nais thickness form and mass balance regime at lgm ice volume in this subset of tests spans a range of 28 5 38 9 10 15 m 3 at lgm with a predominant cluster at 32 36 10 15 m 3 taking floating ice and displaced continental water into account this corresponds to 69 94 m eustatic sea level msl more than 75 of the accepted tests fall in the range 78 88 msl we argue that this is a plausible estimate of the volume of water locked up in the nais at lgm

origin of the first global meltwater pulse following the last glacial maximum well dated sea level records show that the glacioeustatic rise following the last glacial maximum was characterized by two or possibly three brief intervals of rapid sea level rise separating periods with much lower rates these very high rates of sea level rise indicate periods of exceptionally rapid deglaciation of remaining ice sheets the laurentide ice sheet is commonly targeted as the source of the first and largest of the meltwater pulses mwp ia between 14 200 12 200 14c years b p and 13 700 years ago 11 700 14c years b p in all oceanic records of deglaciation of the former northern hemisphere ice sheets that we review only those from the gulf of mexico and the bermuda rise show evidence of low 18o values at the time of mwp ia identifying the southern laurentide ice sheet as a potential source for mwp ia we question this source for mwp ia however because 1 ice sheet models suggest that this sector of the ice sheet contributed only a fraction 10 of the sea level needed for mwp ia 2 melting this sector of the ice sheet at the necessary rate to explain mwp ia is physically implausible and 3 ocean models predict a much stronger thermohaline response to the inferred freshwater pulse out of the mississippi river into the north atlantic than is recorded this leaves the antarctic ice sheet as the only other ice sheet capable of delivering enough sea level to explain mwp ia but there are currently no well dated high resolution records to document this hypothesis these conclusions suggest that reconstructions of the laurentide ice sheet in the ice 4g model which are constrained to match the sea level record may be too low for time periods younger than 15 000 years ago furthermore 18o records from the gulf of mexico show variable fluxes of meltwater from the southern margin of the laurentide ice sheet which can be traced to the opening and closing of eastward draining glacial lake outlets associated with surging ice sheet behavior these variable fluxes through eastern outlets were apparently sufficient to affect formation of north atlantic deep water thus underscoring the sensitivity of this process to changes in freshwater forcing

paleoproterozoic turbidite hosted gold deposits of the ashanti gold belt ghana west africa comparative analysis of turbidite hosted gold deposits and an updated genetic model abstract the paleoproterozoic gold deposits of the ashanti gold belt ghana occur mainly as auriferous quartz veins in graphitic shears proximal to contacts between deformed and metamorphosed turbidites and tholeiitic volcanics alternately block faulted tarkwaian sediments they are suggested in this article to belong to that class of gold deposits recognized as turbidite hosted hence evidence from previous investigations of turbidite hosted and turbidite associated gold deposits occurring in circumstances roughly similar to the ashanti deposits are used where relevant to produce an updated genetic model for the latter given the relatively significant amounts of graphite associated with the major ashanti deposits the role of non carbonate carbon is assessed in the light of conclusions reached by work on other turbidite hosted and turbidite associated gold belts conclusions as to the origins of mineralizing fluids and temporary repositories of gold auriferous interflow sediments reached by work on these other belts are also analysed a concept of the tectonic development of the ashanti belt kumasi basin area introduced in this article is deemed to be critical to the origin of the ashanti gold deposits it encompasses the role of non carbonate carbon turbidites and the primary sources and secondary repositories of au and as in the genesis of the major ashanti gold deposits it examines the significance of the intimate relationship between calc alkaline volcanism and the basin sediments the occurrences of felsic metasandstones in transition zone sediments and ttg granitic material and fragments of chemical sediments in turbidites all of which are incompatible with one or the other of existing concepts of tectonic development in the area

pyrite and pyrrhotite textures and composition in sediments laminated quartz veins and reefs at bendigo gold mine australia insights for ore genesis the various types textures and compositional zoning of pyrite in the gold bearing saddle reefs quartz veins and surrounding sedimentary rocks provide new information on the potential source and timing of gold and arsenic and related fluid processes responsible for mineralization at bendigo nodular diagenetic pyrite in the black shale tops to sandstone turbidites is enriched in invisible gold and arsenic with mean values of 0 61 ppm au 1 300 ppm as and au ag 1 based on la icpms analyses other elements enriched in the diagenetic pyrite within the organic rich shales are mn zn mo cu v ba ag cd tl co ni bi pb and te in contrast euhedral and growth zoned hydrothermal pyrite in the turbidites and bedding parallel laminated quartz veins contains lower contents of most trace elements but has higher contents of invisible au and as especially on the outermost rim of the pyrite the gold rich pyrite rims generally become thicker a few to hundreds of microns in proximity to the gold bearing saddle reefs pyrite in the reef commonly has the highest levels of invisible au and as and the lowest levels of other trace elements it is characterized by au ag 1 and au pb 0 01 in the deepest stratigraphic levels below known productive gold reefs diagenetic pyrite in the most carbonaceous shales has been replaced by pyrrhotite during metamorphism la icpms analyses reveal that the disseminated pyrrhotite contains similar levels of ni and co to the diagenetic pyrite but is strongly depleted in as and au the spatial relationships between organic rich shales folded bedding parallel laminated quartz veins and gold arsenic bearing saddle reefs combined with the consistent trends in the trace element composition of pyrite hosted by these three geological elements is interpreted to indicate that the black shales were an initial source of au and as and the laminated quartz veins acted as the initial pathways for hydrothermal fluid flow carrying au and as from the source shales to the saddle reefs maximum gold and arsenic input into the reefs principally as free gold plus arsenopyrite occurred late during deformation toward the end of the hydrothermal cycle and is expressed by the au as rich rims to hydrothermal pyrite in the sedimentary host rocks laminated quartz veins and reefs this corresponds with final fold lockup and the development of through going fault arrays linking adjacent anticlines the source of au and as for this final and most economically important fluid flow event is considered to be from carbonaceous shales deeper in the basin where original gold bearing diagenetic arsenian pyrite reacts with organic matter and is converted to pyrrhotite with release of au as and s to the metamorphic fluid

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

metallogenesis of the paleoproterozoic piaba orogenic gold deposit sao luis cratonic fragment brazil piaba is an orogenic gold deposit 3 5 moz of the sao luis cratonic fragment in north northeastern brazil the deposit is epizonal mesozonal and associated with the development of a subvertical strike slip fault that cut across rhyacian metavolcano sedimentary rocks and granitoids the metavolcano sedimentary sequence comprises carbon bearing schists andesite dacite ultramafic rocks and felsic tuffs formed at 2240 5 ma to 2227 33 ma that were intruded at 2214 3 ma by fine grained subvolcanic granophyric granodiorite whole rock geochemistry indicates that the hosting metavolcano sedimentary sequence formed in a subduction related arc back arc setting which has previously been interpreted as the early arc stage of an accretionary orogen 2240 2214 ma that was followed by voluminous subduction related calc alkaline arc magmatism 2168 2145 ma and by a collisional phase 2100 15 ma that produced several bodies of peraluminous granites the hydrothermal alteration of the host rocks produced early distal carbonization and hematitization and proximal ore related chlorite sericite carbonate sulfide alteration two styles of mineralization occurring in association are present 1 a network of thin quartz sulfide veinlets and subordinate narrow breccia veins with refractory to free milling gold dissemination in hydrothermally altered host rocks and 2 subordinate low sulfide au quartz veins with free milling gold fluid inclusions and stable isotopes characterize the mineralizing solution as a relatively reduced log fo2 30 to 35 neutral to slightly alkaline ph 5 to 6 2 low salinity average 5 nacl aqueous carbonic xco2 5 to 59 mol xh2o 40 to 93 mol xn2 and xch4 1 mol metamorphic fluid 18oh2o 5 5 to 8 9 dh2o 8 to 69 and suggest that gold mineralization occurred at 250 to 330 c and 1 25 to 2 8 kbar in response to phase separation and fluid rock interactions that produced sulfidation and minor carbonatization sulfur 34s 2 7 to 3 8 is from an undetermined source probably magmatic oxidized or sedimentary lead isotope compositions of gold ore related pyrite and k feldspar from regional rocks suggest that collision related granitoids of ca 2100 15 ma are the most likely sources for pb in the ore although mantle and lower crustal sources are also observed 40ar 39ar in hydrothermal sericite yielded a plateau age of 1980 13 ma which is similar to the pb pb model age of gold of 1959 ma these are considered the minimum age for gold mineralization whereas the maximum age of 2009 11 ma is given by a small gold hosting intrusion in the same region

late wisconsin and early holocene glacial history inner ross embayment antarctica abstract lateral drift sheets of outlet glaciers that pass through the transantarctic mountains constrain past changes of the huge ross ice drainage system of the antarctic ice sheet drift stratigraphy suggests correlation of reedy iii reedy glacier beardmore beardmore glacier britannia hatherton darwin glaciers ross sea mcmurdo sound and younger terra nova bay drifts radiocarbon dates place the outer limits of ross sea drift in late wisconsin time at 24 000 13 000 yr b p outlet glacier profiles from these drifts constrain late wisconsin ice sheet surface elevations within these constraints we give two extreme late wisconsin reconstructions of the ross ice drainage system both show little elevation change of the polar plateau coincident with extensive ice shelf grounding along the inner ross embayment however in the central ross embayment one reconstruction shows floating shelf ice whereas the other shows a grounded ice sheet massive late wisconsin holocene recession of grounded ice from the western ross embayment which was underway at 13 040 yr b p and completed by 6600 6020 yr b p was accompanied by little change in plateau ice levels inland of the transantarctic mountains sea level and basal melting probably controlled the extent of grounded ice in the ross embayment the interplay between the precipitation low late wisconsin and high holocene values and the influence of grounding on outlet glaciers late wisconsin thickening and late wisconsin holocene thinning with effects dying out inland probably controlled minor elevation changes of the polar plateau

textural evolution of synthetic fluid inclusions in quartz during reequilibration with applications to tectonic reconstruction experimental reequilibration of synthetic 10 wt nacl h2o inclusions in natural quartz reveals that reequilibration textures show distinct differences depending upon the p t path followed by the inclusion after formation these differences combined with other geological information may be used to determine whether the sample rock followed a dominantly isothermal or isobaricp t path following entrapment the intensity and style of inclusion reequilibration features is related to the direction and magnitude of the departure of thep t path from the original isochore for the inclusion thus fluid inclusion reequilibration textures not only permit inclusionists to determine whether the rocks followed an isothermal or isobaric retrogradep t path but also the magnitude of departure of this path from one that is isochoric

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

postglacial emergence of western franz josef land russian and retreat of the barents sea ice sheet uncertainty remains on the timing and extent of late weichselian glaciers over the qsr barents and kara sea shelves franz josef land russia is a critical area for deciphering the record of glacier fluctuations and postglacial relative sea level change coastal forelands on the archipelago are covered by a well preserved raised beach sequence containing abundant and pristine driftwood whalebone and shell 4c dating of these materials provides site specific emergence records for the archipelago and some of the oldest ages on deglaciation of 10 400 bp three apparent still stands in emergence are inferred from morphology of the raised beach sequence and 4c dating between 9900 and 6200 bp and at ca 5500 bp and 3500 bp the earliest sea level arrest after formation of the marine limit reflects a balance between glacio isostasy and eustasy concomitant with the final phase of global sea level rise the latter two sea level arrests during the middle to late holocene are less pronounced and may indicate increased height of the ocean surface with greater storminess and extended open water conditions raised beaches at 1 2 m a s l and j4c dated at 1000 to 2000 bp give contemporary emergence rates of 1 to 2 mm year the low current rates of emergence on franz josef land are similar to areas in fennoscandinavia that sustained modest late weichselian glacier loads 1500 m within a few 100s of km of the inferred ice sheet margin the pattern of postglacial emergence and strandline tilts indicates that past glacier loads were greater over the adjacent barents sea than franz josef land marine and terrestrial records from franz josef land place retreat of glaciers from coastal areas by 10 400 bp whereas the adjacent franz victoria trough was deglaciated by 13 000 bp retreat of the barents sea ice sheet was probably underway by 13 500 bp coincident with rising global sea level and elevated summer insolation we infer that iceberg calving through the deep troughs bordering the barents sea particularly with rapidly rising sea level ca 12 000 bp was an important process controlling the demise of the barents sea ice sheet copyright 1996 elsevier science ltd

ore fluids of orogenic gold deposits of the gurupi belt brazil a review of the physico chemical properties sources and mechanisms of au transport and deposition

fluid inclusion and stable isotope o h c and s constraints on the genesis of the serrinha gold deposit gurupi belt northern brazil the serrinha gold deposit of the gurupi belt northern brazil belongs to the class of orogenic gold deposits the deposit is hosted in highly strained graphitic schist belonging to a paleoproterozoic 2 160 ma metavolcano sedimentary sequence the ore zones are up to 11 m thick parallel to the regional nw se schistosity and characterized by quartz carbonate sulfide veinlets and minor disseminations textural and structural data indicate that mineralization was syn to late tectonic and post metamorphic fluid inclusion studies identified early co2 ch4 n2 and co2 ch4 n2 h2o nacl inclusions that show highly variable phase ratios co2 homogenization and total homogenization temperatures both to liquid and vapor interpreted as the product of fluid immiscibility under fluctuating pressure conditions more or less asso ciated with postentrapment modifications the ore bearing fluid typically has 18 33mol of co2 up to 4mol of n2 and less than 2mol of ch4 and displays moderate to high densities with salinity around 4 5wt nacl equiv mineralization occurred around 310 to 335 c and 1 3 to 3 0 kbar based on fluid inclusion homogenization temperatures and oxygen isotope thermometry with estimated oxygen fugacity indicating relatively reduced conditions stable isotope data on quartz carbonate and fluid inclusions suggest that veins formed from fluids with 18 oh2o and dh2o 310 335 c values of 6 2 to 8 4