global distribution of beryllium isotopes in deep ocean water as derived from femn crusts the direct measurement of the ratio of cosmogenic 10be t12 1 5 ma to stable terrigenously sourced 9be in deep seawater or marine deposits can be used to trace water mass movements and to quantify the incorporation of trace metals into the deep sea in this study a sims based technique has been used to determine the 10be9be ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans 10be9be ratios time corrected for radioactive decay of cosmogenic 10be using 234u238u are in good agreement with ams measurements of modern deep seawater ratios are relatively low in the north and equatorial atlantic samples 0 4 0 5 10 7 in the southwest atlantic ratios increase up to 1 10 7 they vary between 0 7 and 1 0 10 7 in indian ocean samples and have a near constant value of 1 1 0 2 10 7 for all pacific samples if the residence time of 10be 10be in deep water is constant globally then the observed variations in 10be9be ratios could be caused by accumulation of 10be in deep water as it flows and ages along the conveyor following a transient depletion upon its formation in the northern atlantic in this view both 10be and 9be reach local steady state concentration in pacific deep water and the global 10be 600 a an alternative possibility is that the be isotope abundances are controlled by local scavenging for this scenario 10be would vary according to local particle concentration and would 600 a in the central pacific but 10be 230 a in the atlantic mass balance considerations indicate that hydrothermal additions of 9be to the oceans are negligible and that the dissolved riverine source is also small furthermore aeolian dust input of 9be appears insufficient to provide the dissolved 9be inventory the dissolution of only a small proportion 2 of river derived particulates could in principle supply the observed seawater 9be content if true ocean margins would be the sites for 9be addition due to the particle reactive nature of be these would also be the primary sites of be removal a possible net result of horizontal water masses passing through these marginal areas might be a decrease in seawater 10be9be and establishment of a relatively constant 9be concentration as 10be 600 a is less than the apparent age of deep water in the pacific 1500 a the pacific record of 10be9be is not expected to show secular variations due to changes in deep water flow despite the large variations in 10be9be between different water masses because of this insensitivity to deep water flow however it is suggested that the 10be9be ratio determined in the authigenic phase of marine sediments or hydrogenetic precipitates should be a suitable tool for monitoring changes in continental input or cosmic ray intensity on longer time scales

growth rate variations of manganese nodules and crusts induced by paleoceanographic events time markers at about 3 ma 6 5 ma and 14 ma are common to 16 manganese nodules and crusts from different localities in the world oceans these time markers correspond to variations in the growth rates in the inner structure and in the chemistry of the samples the observed time markers could be related to 1 changes of the bottom water velocity and 2 variations in the manganese and iron contents in the deep ocean the time markers were determined by high resolution 10be dating applying the accelerator mass spectrometry facility at the eidgenossische technische hochschule zurich

simple x ray diffraction techniques to identify mg calcite dolomite and magnesite in tropical coralline algae and assess peak asymmetry abstract understanding of dolomite development in reef environments has been frustrated by the failure to identify dolomite forming in abundant tropical reef algae the analytical and sampling techniques presented here will allow fundamental questions about dolomite development in reef environments to be addressed using the physical evidence available but not previously identifiable in this paper we describe techniques to determine the mineralogy of living tropical coralline algae and specifically to identify dolomite and magnesite methods based on x ray diffraction peak position have attracted criticism in the past because the resulting mg contents in tropical coralline algae were in some cases lower than those based on chemical analyses of the bulk sample in solution the recent discovery of dolomite and magnesite in living coralline algae skeletons explains this issue and this study demonstrates that the use of x ray diffraction xrd is a reliable quick and affordable method for the study of the mineral make up of coralline algae building on existing xrd methods to develop simple sampling and analytical methods to identify the presence of dolomite and magnesite this study presents techniques to numerically assess 104 peak asymmetry caused by the overlapping reflections of calcite dolomite and magnesite this technique allows differences between many samples and representative types to be found quickly and accurately without relying on visual descriptions of peak shapes or mineral phase quantification the information on the additional mineral phases dolomite and magnesite can be used to study intracellular calcification and or mineral alteration processes the results illustrate that xrd can be an integral part of any mineralogical analysis of coralline algae skeletons which may be composed of not only mg calcite ca0 8 0 9mg0 2 0 1 co3 but also dolomite ca0 5mg0 5 co3 magnesite mgco3 and aragonite caco3 furthermore the xrd subsampling and analytical techniques could be utilized to reanalyze carbonate reef cores to improve our understanding of dolomite distribution in reefs and transition from primary biomineralized dolomite to fabrics observed in ancient dolomites

age of the camaguey gold silver district cuba tectonic evolution and preservation of epithermal mineralization in volcanic arcs epithermal precious metal deposits commonly form in middle to late cenozoic volcanic arcs several kilometers above the level at which batholiths are emplaced this relationship has led to the guideline that epithermal deposits should not be sought in volcanic arcs where batholiths are widely exposed the camaguey district in central cuba violates this tenet and provides important insights into arc forming processes that create and expose epithermal deposits the camaguey district hosts the jacinto and florencia low sulfidation vein deposits and the deseada and golden hill high sulfidation prospects all of which are within a few kilometers of the large camaguey batholith some of the deposits such as jacinto lie immediately below a local paleosurface that exposes the batholith and probably formed when rhyolite domes were emplaced onto this surface a second group of deposits such as the deseada high sulfidation system is hosted by volcanic rocks that overlie the camaguey batholith and stand above the paleosurface new ar ar age measurements indicate that jacinto type deposits and prospects cooled through ar ar closure temperatures for adularia and muscovite at about 71 ma and are slightly younger than the widespread granodiorite facies of the camaguey batholith in contrast deseada type deposits and mineralized zones cooled through roughly similar closure temperatures for sericite illite and alunite at about 80 ma which is older than the age of the immediately underlying camaguey batholith these ages are best explained if the camaguey batholith and surrounding volcanic rocks underwent rapid uplift probably by tectonic unroofing to form the paleosurface below which jacinto type deposits formed and if the older deseada type deposits were thrust into place over this surface this tectonic unroofing probably took place in an extensional regime generated during oblique convergence of cuba with yucatan as the greater antilles arc moved north into the caribbean region during late cretaceous time thrusting probably accompanied late stages of this convergence and or later collision with the bahamas bank in eocene time all of the epithermal deposits were covered by late cretaceous eocene sedimentary rocks that have been removed only recently by erosion thus accounting for the preservation of these unusually old epithermal deposits extensional events causing tectonic unroofing are probably common in arcs that undergo oblique convergence and could be an important agent for formation of epithermal deposits in other parts of the world preservation of these deposits might be aided by deposition and later erosion of sedimentary cover related to later collisions

nd isotopic structure of the pacific ocean 40 10 ma and evidence for the reorganization of deep north pacific ocean circulation between 36 and 25 ma abstract early paleogene neodymium nd isotopic reconstructions of ocean circulation in the pacific suggest a bipolar mode of overturning circulation characterized by convection of deep water masses from 70 to at least 40 ma this mode is fundamentally different from the sluggish diffusive mode that characterizes the modern north pacific therefore major reorganization of pacific circulation must have occurred between 40 ma and the modern to investigate the potential timing of such a reorganization we present new nd isotope data from drill sites spanning 40 to 10 ma here we use nd isotopic analyses to identify the sources of dissolved nd to the pacific in order to evaluate the application of the nd proxy for ancient water mass composition we combine analyses of fossil fish debris sequentially leached oxide coatings and leach residues to further refine our understanding of nd marine geochemical cycling and to contribute new constraints on the paleoceanographic history of the pacific overall these data support previous interpretations of the convection of deep water in the pacific sector of the southern ocean and surface ventilated north pacific deep water formation from 70 to 40 ma the new nd isotope data suggest a gradual transition between 36 and 25 ma a time period characterized by global cooling this transition consists of a change from north pacific nd isotopic signatures controlled by convection and water mass mixing to values heavily influenced by overprinting from other sources of nd to the deep pacific such a transition is consistent with a change in overturning circulation from a paleogene mode to a mode dominated by sluggish diffusive mixing after 25 ma the data indicate a significant contribution of radiogenic dissolved nd to the deep waters of the north pacific the most likely source of radiogenic dissolved nd would have been labile volcanic ash that altered the composition of the water mass as it slowly advected through the north pacific a slowing of overturning circulation in the north pacific could have resulted in diminished poleward heat transport increasing equator to pole thermal gradients and contributing to a cooling global climate

the behavior of rare earth elements in seawater precise determination of variations in the north pacific water column abstract we present new highly precise and accurate data for ten rare earth elements rees in two water column profiles and in bottom waters from three additional localities in the north pacific our results presented here and in an earlier publication indicate that the water column in the western north pacific is stratified with respect to the isotopic composition of nd as well as nd la sm nd and er la both the nd isotopic and the ree data show that ree transport is strongly influenced by horizontal advection even in the sluggish circulation of the pacific in general rees increase in concentration with depth at mid latitude they reach a maximum concentration at about 3000 m and then decrease to the bottom at high latitude heavy rees reach a maximum concentration near 3000 m whereas light rees continue to increase in concentration with depth the deep water maximum and bottom water depletion is interpreted as an advective feature resulting from the flow of a lower concentration of rees in antarctic bottom water under a higher concentration of rees in the north pacific concentrations of rees do not show a linear relationship with those of si indicating that the ree cycle is not directly coupled to that of the si cycle ree element ratios i e nd la correlate with hydrographie properties which suggests that major features of the ree patterns may be conserved during lateral transport comparison of our ree data to those in previously published reports on pacific seawater shows that the identification of ree distributions affected by advection is only made possible by the high precision analysis we developed for this study the ce concentration we observe is relatively constant over 200 4500 m depth at both sites where we have complete profiles this unique constancy of ce is in contrast to the contiguous rees la and nd which show substantial concentration variations with depth

the isotopic composition of neodymium in the north pacific abstract we have determined the isotopic composition of neodymium in two water column profiles in the western north pacific and in bottom waters from three other locations in the north pacific the vertical profiles reveal that the water column is stratified with respect to isotopic composition mid depths associated with pacific deep water are uniform having nd 0 3 shallow waters are variable but deep waters are less radiogenic having md 0 4 4 to 5 in the western basin the more negative bottom waters in the western north pacific provide evidence for the advective transport of rare earth elements from antarctic sources to this region bottom waters in the eastern north pacific are more radiogenic nd 0 3 3 to 3 8 indicating very little transport of ree from antarctic sources to this basin overall the isotopic composition of nd in pacific bottom waters appears to be conservative when compared to the silica distribution in spite of evidence for nd removal the findings are generally compatible with other parameters such as temperature salinity and nutrient distributions the presence of nd isotopic signatures associated with antarctic sources in these waters provides evidence for a small component of rare earth elements of north atlantic origin in the bottom waters of the north pacific this is the first tracer evidence to corroborate the suggestion of reid and lynn 1971 that some of the temperature and salinity properties of the deep waters of the north pacific originated in the norwegian and greenland seas

tectonics and climate of the southern central andes the history of the southern central andes including the world s second largest plateau and adjacent intermontane basins and ranges of the eastern cordillera and the northern sierras pampeanas of argentina and bolivia impressively documents the effects of tectonics and topography on atmospheric circulation patterns the development of orographic barriers and their influence on erosion and landscape evolution at various timescales protracted aridity in the orogen interior has facilitated the creation and maintenance of the puna altiplano plateau contraction and range uplift filling of basins and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior which caused the eastward migration of deformation into the foreland and successive aridification the uplift of the andean orogen has also had a far reaching influence on atmospheric and moisture transport patterns in south america this is documented by the onset of humid climate conditions on the eastern side of the andes in late miocene time which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen and changes in tectonic processes in the andean orogenic wedge

comparative diagenesis and rare earth element variation in miocene invertebrate and vertebrate fossils from panama abstractthe incorporation of rare earth elements rees into the mineral lattices of skeletonized fossils has been used particularly in vertebrates to understand diagenesis and other postmortem paleoenvironmental parameters little is known about similar processes in invertebrates invertebrate and vertebrate fossils were analyzed for ree concentrations and patterns from five miocene formations in central panama 20 6 m yr ago these formations culebra cucaracha gatun chucunaque and chagres contain sediments interpreted to range from freshwater to shallow marine environments depth of 300 m we also compared different compositional types of fossils including hydroxylapatite bone and aragonite otoliths in vertebrates and calcite and aragonite in invertebrates rees from modern invertebrates indicate that as previously reported for vertebrates invertebrates do not incorporate significant amounts of rees into their mineral lattices during life therefore uptake must primarily occur during diagenesis comparisons of reen concentrations ree concentrations normalized to post archean australian shale of the fossils from panama indicate differences in diagenesis within our largest sample from the gatun formation there are significance differences in reen concentrations and patterns within different compositional types for example shark teeth have the highest reen concentrations and otoliths the lowest with these differences likely a result of physical chemical and mineralogical factors calcitic and aragonitic fossils from the gatun formation show reen patterns indicating differential uptake of individual elements during diagenesis the cerium anomaly previously used to indicate relative paleobathymetry did not discriminate water depth in the marine formations comparative analysis of rees in vertebrates and invertebrates provides a powerful tool to understand diagenetic processes as demonstrated in the five formations and different compositional types studied from the miocene of panama

strontium and neodymium isotopes in hot springs on the east pacific rise and guaymas basin we have determined the concentrations and isotopic compositions of sr and nd in hydrothermal fluids from 21 n east pacific rise and guaymas basin gulf of california the purest solutions analyzed from 21 n exhibit a small range in sr concentration between individual vents from 5 8 to 8 7 ppm close to normal seawater sr concentrations they exhibit a small range in 87 sr 86 sr from e sr 0 13 4 to 17 7 corresponding to e sr 0 18 2 in the pure hydrothermal end member these results indicate extensive but not complete isotopic exchange with sr in the depleted oceanic crust e sr 0 31 8 and suggest that sr concentrations in these solutions are buffered in contrast the concentration and isotopic composition of nd in solutions show large variations between vents the concentration of nd ranges from 20 to 336 pg g 6 100 times seawater nd concentrations the isotopic composition ranges from e nd 0 3 6 similar to pacific seawater to 7 9 many samples show substantial contributions from morb but all have e nd 0 well below morb at this locality e nd 0 9 7 in spite of very large enrichments in nd concentrations while complete isotopic exchange with water rock 2 or exchange with anomalous oceanic crust can explain the sr data the nd data require exchange with a reservoir having e nd 0 pacific seawater low temperature reactions with metalliferous sediments on the ridge flanks may provide such a source both sr and nd in the guaymas basin solution are very different from21 n e sr 0 11 0 and e nd 0 11 4 and are consistent with the fluid exchanging sr and nd with heated sediments having a substantial component of old continental detritus some irregularities in the nd isotopic data reported here indicate that there must be a problem of contamination for some ultra low level trace elements during sample collection and processing which requires further attention using a simple box model the estimates for hydrothermal nd fluxes are compared with fluxes which would be required to maintain the relatively radiogenic value of e nd 0 3 in the pacific against the influx of more negative antarctic waters e nd 0 9 it is shown that the hydrothermal flux of nd from mid ocean ridges falls far short of that necessary to maintain the isotopic balance this indicates that weathered material from volcanic terranes e nd 0 7 is the most reasonable major source of radiogenic nd in the pacific

isotopic composition of neodymium in waters from the drake passage the isotopic composition of neodymium has been determined in seawaters from the drake passage the antarctic circumpolar current which controls interocean mixing flows through this passage the parameter e nd 0 which is a function of the ratio of neodymium 143 to neodymium 144 is found to be uniform with depth at two stations with a value which is intermediate between the values for the atlantic and the pacific and indicates that the antarctic circumpolar current consists of about 70 percent atlantic water cold bottom water from a site in the south central pacific has the neodymium isotopic signature of the waters in the drake passage by using a box model to describe the exchange of water between the southern ocean and the ocean basins to the north together with the isotopic results an upper limit of approximately 33 million cubic meters per second is calculated for the rate of exchange between the pacific and the southern ocean concentrations of samarium and neodymium were also determined and found to increase approximately linearly with depth these results suggest that neodymium may be a valuable tracer in oceanography and may be useful in paleo oceanographic studies

distribution and sources of pre anthropogenic lead isotopes in deep ocean water from fe mn crusts abstract the lead isotope composition of ocean water is not well constrained due to contamination by anthropogenic lead here the global distribution of lead isotopes in deep ocean water is presented as derived from dated ca 100 ka surface layers of hydrogenetic fe mn crusts the results indicate that the radiogenic lead in north atlantic deep water is probably supplied from the continents by river particulates and that lead in pacific deep water is similar to that characteristic of island and continental volcanic arcs despite a short residence time in deep water 80 100 a the isotopes of lead appear to be exceedingly well mixed in the pacific basin there is no evidence for the import of north atlantic deep water derived lead into the pacific ocean nor into the north indian ocean this implies that the short residence time of lead in deep water prohibits advection over such long distances consequently any climate induced changes in deep water flow are not expected to result in major changes in the seawater pb isotope record of the pacific ocean

coral reef sedimentation on rodrigues and the western indian ocean and its impact on the carbon cycle coral reefs in the southwest indian ocean cover an area of ca 18530 km2 compared with a global reef area of nearly 300000 km2 these regions are important as fishing grounds tourist attractions and as a significant component of the global carbon cycle the mass of calcium carbonate stored within holocene neritic sediments is a number that we are only now beginning to quantify with any confidence in stark contrast to the mass and sedimentation rates associated with pelagic calcium carbonate which have been relatively well defined for decades we report new data that demonstrate that the reefs at rodrigues like those at reunion and mauritius only reached a mature state reached sea level by 2 3 ka thousands of years later than most of the reefs in the australasian region yet field observations show that the large lagoon at rodrigues is already completely full of carbonate detritus typical lagoon depth less than 1 m at low spring tide the presence of aeolian dunes at rodrigues indicates periodic exposure of past lagoons throughout the pleistocene the absence of elevated pleistocene reef deposits on the island indicates that the island has not been uplifted most holocene reefs are between 15 and 20 m in thickness and those in the southwest indian ocean appear to be consistent with this observation we support the view that the co2 flux associated with coral reef growth acts as a climate change amplifier during deglaciation adding co2 to a warming world southwest indian ocean reefs could have added 7 10 to this global flux during the holocene

age determinations and growth rates of pacific ferromanganese deposits using strontium isotopes abstract 87 sr 86 sr ratios trace element and ree compositions and textural characteristics were determined for three hydrogenetic fe mn crusts one hydrothermal deposit and two mixed hydrothermalhydrogenetic crusts from the pacific the sr isotope data are compared to the sr seawater curve for the cenozoic to determine the ages and growth rates of the crusts the 87 sr 86 sr in the crusts does not increase monotonically with depth as expected if the sr were solely derived from seawater and perfectly preserved since deposition this indicates post depositional exchange of sr or heterogeneous sources for the sr originally contained in the crusts textures of hydrogenetic crusts generally correlate with sr isotopic variations the highest porosity intervals commonly exhibit the highest 87 sr 86 sr ratios indicating exchange with younger seawater intervals with the lowest porosity commonly have lower 87 sr 86 sr and may preserve the original sr isotopic ratios minimum ages of crust growth inception were calculated from dense low porosity intervals growth of the hydrogenetic crusts began at or after 23 ma although their substrates are cretaceous estimated average growth rates of the three hydrogenetic crusts vary between 0 9 and 2 7 mm ma consistent with published rates determined by other techniques within the marshall islands crust growth rates for individual layers varied greatly between 1 0 and 5 4 mm ma for one crust very low 87 sr 86 sr ratios occurred in detrital rich intervals hydrothermal fe mn oxide from the active lau basin back arc spreading axis valu fa ridge has an 87 sr 86 sr ratio with a predominantly seawater signature 87 sr 86 sr 0 709196 indicating a maximum age of 0 9 ma one crust from an off axis seamount west of gorda ridge may have begun precipitating hydrogenetically at 0 5 ma 0 709211 and had increasing hydrothermal or volcanic input in the top half of the crust indicated by a significantly lower 87 sr 86 sr ratio 0 709052

ti diffusion in quartz inclusions implications for metamorphic time scales quartz inclusions in garnet from samples collected from the staurolite zone in central new england are zoned in cathodoluminescence cl the cl intensity is interpreted to be a proxy for ti concentration and the zoning attributed to ti diffusion into the quartz grains driven by ti exchange between quartz and enclosing garnet as a function of changing temperature the cl zoning has been interpreted using a numerical diffusion model to constrain the time scales over which the diffusion has occurred temperature time histories are sensitive to the presumed peak temperature but not to other model parameters the total time of the metamorphic heating and cooling cycle from around 450 c to the peak temperature 550 600 c back to 450 c is surprisingly short and encompasses only 0 2 2 million years for peak temperatures of 600 550 c the metamorphism was accompanied by large scale nappe and dome formation and it is suggested that this occurred as a consequence of in sequence thrusting resulting in a mid crustal ductile duplex structure

porphyry epithermal transition maricunga belt northern chile the refugio aldebaran and la pepa districts in the maricunga belt of northern chile contain advanced argillic alteration zones that locally host high sulfidation epithermal gold deposits in proximity to porphyry gold copper deposits the spatial association suggests a genetic link mineralized zones are characterized by four main vein types that formed at different times and have specific zonal relationships a veinlets are the earliest and deepest vein type they are restricted to potassic alteration zones in intrusive rocks a veinlets contain variable amounts of quartz magnetite biotite and chalcopyrite and locally have k feldspar halos they have nonmatching irregular vein walls and lack internal symmetry hypersaline liquid rich inclusions coexisting with vapor rich inclusions in a veinlets indicate temperatures as high as nearly 700 c and pressures between 200 and 400 bars assuming a lithostatic load depths of 0 8 to 1 6 km are inferred zones of abundant a veinlets contain mostly 1 ppm gold and 0 1 to 0 4 percent hypogene copper banded quartz veinlets occur mostly above a veinlets and cut a veinlets where they overlap dark gray bands the color resulting from a high density of vapor rich fluid inclusions and micron sized grains of magnetite commonly occur as symmetric pairs near the vein walls vein walls are parallel and slightly wavy vuggy vein centers are common and alteration envelopes are absent data from rare liquid rich inclusions in banded quartz veinlets indicate temperatures 350 c at pressures between 20 and 150 bars assuming a hydrostatic load depths of 0 2 to 1 5 km are inferred zones of abundant banded quartz veinlets generally contain 0 5 to 2 ppm gold and 0 1 percent hypogene copper d veins are pyrite veins with quartz sericite pyrite halos they are widespread and crosscut a veinlets and banded quartz veinlets the brittle nature of d veins and limited fluid inclusion data suggest temperatures 400 c d veins serve as important time lines they are nowhere truncated or crosscut by intrusions a veinlets or banded quartz veinlets quartz alunite replacement veins referred to as ledges in this paper are typical of the high sulfidation epithermal environment they are mostly limited to overlying volcanic rocks they contain local core zones of vuggy residual quartz that can contain enargite or at higher elevations barite of the three districts studied only la pepa has mineable quartz alunite ledges which contain an average gold grade of about 20 ppm a spectrum of porphyry style deposits exists cerro casale at aldebaran shares many characteristics of porphyry copper deposits worldwide whereas verde at refugio is a true porphyry gold deposit potassic alteration zones and a veinlets are strongly developed at cerro casale whereas they are absent at verde banded quartz veinlets predominate at verde whereas they occur only at the upper levels of cerro casale the pancho deposit at refugio and the cavancha deposit at la pepa are telescoped systems in which banded quartz veinlets overprint potassic alteration zones and a veinlets a veinlets and banded quartz veinlets cut and are cut by intrusions indicating multiple cycles of intrusion potassic alteration a veinlets banded quartz veinlets during formation of porphyry style mineralization banded quartz veinlets are thought to have formed by flashing of magmatic fluids during episodic transitions from lithostatic to hydrostatic pressure loss of sulfur to the vapor phase during flashing inhibited formation of copper sulfides in banded quartz veinlets and therefore resulted in high gold copper ratios where rising magmatic vapors condensed into overlying meteoric water along faults barren quartz alunite ledges formed this conclusion is supported by equivalent 40ar 39ar dates on hydrothermal biotite associated with porphyry style ore and alunite from barren ledges at aldebaran 40ar 39ar dates at la pepa indicate alunite formed at least 140 000 years to as long as 900 000 years after hydrothermal biotite within the high sulfidation epithermal environment the development of ore depends on the ability of late moderate salinity magmatic fluids to reach the surface without condensing a brine upon ascent cooling and boiling of the moderate salinity fluid below its critical temperature results in the formation of sericite at depth and alunite near the surface that is essentially synchronous with high sulfidation ore formation the timing of the switch from lithostatic pressures to brittle hydrostatic conditions relative to the life of the hydrothermal system might determine how much porphyry style ore forms relative to high sulfidation epithermal ore

zircon u pb hf o and molybdenite re os isotopic constraints on porphyry gold mineralization in the bilihe deposit ne china abstract the bilihe gold deposit in inner mongolia is situated in the central asian metallogenic domain its major orebody ii is a porphyry type body spatially and temporally associated with granodiorite porphyry and granite aplite in this study the timing of gold mineralization is precisely constrained by using the zircon u pb dating for pre mineralization intrusions and the molybdenite re os dating for later molybdenite veins furthermore zircon hf o isotope analyses have also been carried out to decipher the nature of primary magma zircon u pb dating shows that the granodiorite porphyry and the granite aplite were emplaced at 269 2 ma and 270 2 ma respectively indicating the gold mineralization occurring no earlier than 269 ma meanwhile molybdenite veins are developed within the fractures and commonly cut across the auriferous veins thus combined with a molybdenite re os isochron age of 268 1 ma the gold mineralization in the bilihe deposit can be precisely restricted to ca 269 ma zircon ehf t values are mostly positive 1 6 11 3 along with high 18o values of 6 20 7 63 suggesting a mixed source between mantle materials and ancient continental crust such as the bainaimiao group for the bilihe magma it is also supported by the presence of the captured detrital zircons in these intrusions given a universally metallogenic environment for porphyry gold deposits a thicken crustal setting related to the collisional intermission of the paleo asian ocean is favored to interpret the formation setting of the bilihe deposit

microbially catalyzed dolomite formation from near surface to burial abstract over the past century a number of models have been developed to explain the vast stratigraphic distribution of authigenic dolomite these models include hydrogeological geochemical and geomicrobiological interpretations that shed light on kinetic constraints on low temperature dolomite formation and in particular how certain microbes can overcome such limitations in this review we focus on the direct and indirect mechanisms by which microbes facilitate the nucleation and shallow burial diagenetic stabilization of dolomite in marine sediments we also consider the likely effects of local and global scale biogeochemical perturbations over the distribution and activities of subsurface microbes and thus potentially over the stratigraphic distribution of dolomite considering the many advances that have been made in the fields of geomicrobiology and biogeochemistry during the past 20 years investigating the important role microbes play in sedimentary systems will undoubtedly contribute to a more complete interpretation of dolomite formation in the rock record

sequential leaching of marine ferromanganese precipitates genetic implications abstract sequential leaching experiments were carried out on twenty one hydrogenetic crust samples from different locations in the central pacific and the results are compared with four crust and nodule samples of different genetic origin fe vernadite mno2 is the most important minor and trace metal bearing mineral phase and shows high concentrations of co ni cd ti ba zn and cu which are present in seawater mainly as hydrated and labile complexed cations elements forming carbonate and hydroxide complexes and oxyanions in seawater like pb mo v and minor proportions of cu and zn are bound in the fe00h fraction the ca carbonate phase and the residual fraction which consists of aluminosilicates and crystalline oxides show only minor heavy metal associations e g cu and zn only a small part of the high ti concentrations in hydrogenetic crusts is of detrital origin ti mainly forms a hydrogenetic phase probably consisting of ti02 2h20 intergrown with the amorphous fe00h phase analyses of different sample types and crust layers show that aging and diagenetic effects can change the phase associations of some elements the most notable change was observed for pb which during phosphatization of crusts is transferred from the mn fe oxide fraction to the apatite fraction a colloid chemical model for the hydrogenetic precipitation of ferromanganese crusts on seamounts is proposed in the first stage mn2 rich water from the oxygen minimum zone is mixed with oxygen rich deep water and oxidized mn iv and other metals like fe ti al and si form oxide and hydroxide colloids these form mixed colloidal phases and scavenge trace metals by sorptive processes which are dominated by coulombic and chemical interactions between colloid surfaces and dissolved metal species in stage two the colloidal phases precipitate on the substrate rocks of the seamounts as ferromanganese oxide encrustations incorporating the sorbed heavy metals into the mineral phases

a lead isotopic study of circum antarctic manganese nodules abstract lead isotope ratios in manganese nodules from the circum antarctic ocean display systematic geographical variations eastward in the direction of current movement from south of australia they progressively decrease in the pacific sector and into the western atlantic sector until 30 w from this longitude lead isotope ratios increase in the eastern atlantic sector and reach their highest values in the indian sector near 110 e south of western australia they abruptly decrease these two lead isotope domains correspond closely to the circum antarctic neodymium isotope domains previously recognized by albarede and goldstein 1992 however the intraocean isotopic variations are much clearer for pb than nd reflecting the shorter oceanic residence time of pb 102 y compared with nd 103 y and confirming that pb is a much more sensitive marine tracer throughout the circum antarctic ocean the lead isotopic changes follow the pattern of present day oceanic circulation the pb shows an overall continental signature which is strongest in the indian sector the most probable candidate as the source of the continental signal is north atlantic deep water nadw which enters the circum antarctic in the south atlantic as nadw mixes with circumpolar water lead isotope ratios increase eastward in the indian sector reflecting progressive loss of the pacific derived pb signal the abrupt decrease of lead isotope ratios between the indian and pacific sectors appears to be a consequence of addition of pacific seawater through the pacific lead isotope ratios gradually decrease eastward approaching values of east pacific rise metalliferous sediments ocean ridge basalts and south pacific mn nodules this trend results from an increasing influence of pacific ocean water containing a component derived from pacific volcanism the pacific signature persists into the argentine basin in the southwest atlantic ocean reflecting significant input of water from the drake passage northward into the atlantic the pacific pb component is seen in the atlantic and indian oceans near southern africa as well as in the atlantic and indian sectors of the circum antarctic lead and neodymium isotopes correlate in the atlantic indian sectors but not in the pacific sector reflecting differences in residence times and modes of water mass mixing the renewal of circum antarctic water occurs on a timescale which is short compared to the residence time of nd but long compared to pb as a result dissolved pb is subject to considerable changes over the timescale of circumpolar current movement while nd is less easily affected nadw is added locally in the south atlantic and moves eastward as a water mass as nadw mixes with circumpolar water cpw through the indian sector changes affecting nd pb in nadw and cpw occur in tandem and neodymium and lead isotopes are correlated through the pacific sector gradual addition of pacific seawater occurs over a large region and nd pb relationships are not coupled because simple isotopic mixing systematics are not conserved there neodymium and lead isotopes vary independently the consistency of lead isotope variations with present day ocean bottom water circulation is remarkable considering that the mn nodules as sampled reflect the integrated flux of pb over 105 106 y the oceanic mixing time is 103 y the residence time for pb is 102 y and the circulation time of the circumantarctic ocean is 30 y processes occurring over such different timescales can show such a relationship only if the system is stable over the longest time interval thus on average element sources and ocean circulation through the pleistocene have been similar to the present day despite significant perturbations during glacial periods

maturation of neogene dolomite from xuande atoll of xisha archipelago the south china sea abstract dolomitization is a remarkable diagenetic process occurring in the subsurface carbonate beds at xuande atoll of xisha islands in the south china sea dolomite crystals initially appear at the depth of 180 m in holocene pleistocene carbonate with relatively small concentrations whereas extensive dolomitization has developed at the minimum burial depth of 290 m in pliocene strata of the zk 1 well petrographically zk 1 dolomites have undergone from shallow to deep depth a progressive maturation process indicated by fabric evolvement of dolomitized rocks rock lithification induced by textural coarsening and continued growth of cements increase in stoichiometry and cation ordering of dolomite downcore and alteration of metastable dolomite to stable one involving preferential dissolution and subsequent reprecipitation process under cathodoluminescence dolomite cements are mostly dull red to nonluminescent or zoned whereas the replacive matrix dolomite exhibits a homogeneous greyish red cathodoluminescence whole rock elemental analyses show that dolomite rock is characterized by low concentrations of mn

closure of the central american isthmus and its effect on deep water formation in the north atlantic modern ocean thermohaline driven circulation influences global climate by transporting heat to high latitudes1 2 and by affecting the exchange of co2 between ocean and atmosphere3 north atlantic deep water nadw plays a key role in this circulation and quaternary climate cycles have been linked to changes in nadw flow4 general circulation model simulations indicate that before closure some 3 4 million years ago of the central american isthmus the narrow strip of land linking north and south america the direct flow of low salinity water from the pacific to the atlantic ocean would have led to a smaller nadw flow5 6 sedimentation patterns7 and nutrient proxies8 11 support these model results by indicating an increase in nadw flow around the time of isthmus closure but these records do not allow changes in different nadw sources to be distinguished and the overall effect of closure on global ocean circulation is poorly known here we present nd pb and sr isotope records preserved by a hydrogenous ferromanganese crust from the nadw flow path in the western north atlantic ocean these records indicate that the isotopic signal associated with nadw strengthened around 3 4 million years ago showing that deep water that formed in the labrador sea made a gradually increasing contribution to nadw flow these data taken together with those from the central pacific ocean12 indicate an increased nadw flow since isthmus closure and suggest that the closure established today s general pattern of ocean circulation

lead concentrations in the northeast pacific evidence for global anthropogenic perturbations lead concentrations were determined by isotope dilution mass spectrometry in 34 surface and deep water samples collected in the northeast pacific between hawaii and california and off the california coast using a deep water sampler protected against fouling by contamination from the ship and hydrowire measured concentrations lie more than one order of magnitude below previously published open ocean values and they show that in most cases 90 or more of the total lead is in a dissolved form lead concentrations are about 10 fold higher in surface and thermocline waters than in deep waters values drop as low as 1 ng kg 5 pmol kg below 3500 m depth lead profiles thus appear different compared to those of most trace metals which show enrichment in deep waters compared to surface concentrations lead concentrations in surface waters increase from 5 to 15 ng kg 25 to 75 pmol kg along a transect starting from a location 200 km off the california coast and continuing towards the center of the north pacific gyre this increase is congruent with that observed for 210 pb concentrations in the same waters sampled at the same time lead is supplied to the open north pacific largely from the atmosphere at a rate of about 60 ng cm 2 yr which exceeds the prehistoric oceanic output flux of authigenic lead recorded in pelagic sediments about tenfold this excess originates from emissions from smelters and combustion of leaded gasoline overwhelming natural lead inputs that entered the ocean during prehistoric time probably largely through rivers vertical lead concentration profiles below the surface mixed layer are probably not in steady state there concentrations must be increasing in response to the increase of anthropogenic inputs because the estimated lead residence times are more than 20 years in the thermocline and about 80 years in deep waters based on an estimated 10 fold input and concentration increase since the mid eighteenth century in the surface mixed layer in the central northeast pacific it is calculated that the 10 ng kg average concentration between 100 and 900 m is 2 to 5 times larger and the 1 8 ng kg average concentration between 900 and 5000 m depth is about 2 times larger than it was in the mid eighteenth century profiles of lead concentrations in the north atlantic are expected to be shifted generally to larger values by a factor of 2 to 3 compared to those in the north pacific because of the effects of greater industrial lead contamination while lead concentration profiles in the south pacific are expected to be shifted generally to lower values compared to the north pacific by about this same factor because of the effects of lesser industrial lead contamination

timing of porphyry cu mo and base metal zn pb ag cu mineralisation in a magmatic hydrothermal system morococha district peru the morococha district in central peru is characterised by economically important cordilleran polymetallic zn pb ag cu vein and replacement bodies and the large toromocho porphyry cu mo deposit in its centre u pb re os and 40ar 39ar geochronology data for various porphyry related hydrothermal mineralisation styles record a 3 5 ma multi stage history of magmatic hydrothermal activity in the district in the late miocene three individual magmatic hydrothermal centres were active the codiciada toromocho and ticlio centres each separated in time and space the codiciada centre is the oldest magmatic hydrothermal system in the district and consists of a composite porphyry stock associated with anhydrous skarn and quartz molybdenite veins the hydrothermal events are recorded by a titanite u pb age at 9 3 0 2 ma and a molybdenite re os age at 9 26 0 03 ma these ages are indistinguishable from zircon u pb ages for porphyry intrusions of the composite stock and indicate a time span of 0 2 ma for magmatic hydrothermal activity the small ticlio magmatic hydrothermal centre in the west of the district has a maximum duration of 0 3 ma ranging from porphyry emplacement to porphyry mineralisation at 8 04 0 14 ma 40ar 39ar muscovite cooling age the toromocho magmatic hydrothermal centre has a minimum of five recorded porphyry intrusions that span a total of 1 3 ma and is responsible for the formation of the giant toromocho cu mo deposit at least two hydrothermal pulses are identified post dating a first pulse of molybdenite mineralisation wide spread hydrous skarn covers an area of over 6 km2 and is recorded by five 40ar 39ar cooling ages at 7 2 6 8 ma these ages mark the end of the slowly cooling and long lived toromocho magmatic hydrothermal centre soon after last magmatic activity at 7 26 0 02 ma district wide 50 km2 cordilleran base metal vein and replacement bodies post date the youngest recorded porphyry mineralisation event at toromocho by more than 0 5 ma polymetallic veins 5 78 0 10 and 5 72 0 18 ma 40ar 39ar ages and the manto italia polymetallic replacement bodies 6 23 0 12 and 6 0 0 2 ma 40ar 39ar ages are interpreted to have been formed by a single hydrothermal pulse hydrothermal activity ceased after the formation of the base metal vein and replacement bodies overlapping monazite u pb 8 26 0 18 ma and muscovite 40ar 39ar ages 8 1 0 5 ma from the early base metal stage of one cordilleran vein sample in the sulfurosa area provide evidence that a discrete hydrothermal pulse was responsible for polymetallic vein formation 2 6 ma prior to the district wide polymetallic veins these ages pre date those of toromocho porphyry cu mo formation and show that zn pb ag cu mineralisation formed during several discrete magmatic hydrothermal pulses in the same district

sr and nd isotopic variations in ferromanganese crusts from the central pacific implications for age and source provenance abstract isotopic analyses of two hydrogenetic ferromanganese fe mn crusts from volcanic edifices in the central pacific ocean reveal systematic variations in 87 sr 86 sr and 143 nd 144 nd with both ratios decreasing as a function of depth into the fe mn crusts leaching experiments suggest that sr in the crusts is contained in at least two discrete sites a loosely bound sr dominated by modern marine sr is removed by leaching with a 10 percent acetic acid solution the 87 sr 86 sr ratio of the residue is significantly less than the 87 sr 86 sr ratio of the unleached material the sr isotope ratios of leached samples are compared with the temporal variation in seawater to provide ages for layers within the fe mn crusts these data suggest that the oldest crust layers began to accrete in the early to middle miocene correlated to the 87 sr 86 sr variations 143 nd 144 nd ratios suggest that the nd isotope composition of central pacific ocean seawater also changed systematically over this time interval or that the fe mn crusts simply incorporated nd from various parts of an isotopieally heterogeneous ocean as the crusts were carried along with the oceanic plate in contrast to the layered ferromanganese crust the phosphatized volcaniclastic substrates have sr and nd isotope compositions that are consistent with their volcanic origin

porphyry gold deposits of the refugio district maricunga belt northern chile the porphyry gold deposits of the refugio district and similar deposits in the maricunga belt contain the lowest known copper to gold ratios cu ppm au 0 03 of any porphyry type deposit the gold deposits are associated with subvolcanic andesitic to dacitic intrusions emplaced into coeval volcanic rocks both the verde and pancho deposits are zoned in space from a deeper zone of banded quartz veinlets associated with chlorite magnetite albite and or pyrite albite clay alteration to a shallow zone of pyrite albite clay and local quartz alunite ledges pancho contains an additional deepest porphyry copperlike zone with quartz veinlets a veinlets and potassic alteration relative to verde pancho is telescoped with all three zones present within a 400 m vertical interval the porphyry copperlike zone at pancho is characterized by a veinlets and pervasive potassic alteration both restricted to intrusive rocks a veinlets range from hairline streaks of magnetite biotite with minor quartz and chalcopyrite and k feldspar alteration envelopes to sugary quartz veinlets 1 cm in width with magnetite and chalcopyrite and no alteration envelopes hypersaline liquid inclusions coexisting with vapor rich inclusions indicate temperatures above 600 c and salinities as high as 84 wt percent nacl equiv a pressure estimate of 250 bars indicates a depth of 1 000 m assuming lithostatic pressure potassic alteration consists of a central zone of magnetite k feldspar oligoclase that changes outward to a biotite rich zone total sulfide content predominately as chalcopyrite is generally 1 vol percent whereas magnetite content is 2 to 5 percent where a veinlets and potassic alteration predominate grades are typically 0 1 wt percent hypogene copper and 0 5 to 1 ppm gold banded quartz veinlets are present at both verde and pancho where they occur mostly above a veinlets and cut a veinlets where they overlap they are less than 2 cm in thickness and lack alteration envelopes dark gray bands whose color is due to abundant vapor rich fluid inclusions and micrometer sized grains of magnetite commonly occur as symmetric pairs near the vein walls the bands are commonly botryoidal and are continuous through quartz grains suggesting that the quartz recrystallized from a silica gel rare liquid rich fluid inclusions in quartz indicate temperatures 350 c and salinities 35 wt percent nacl equiv estimated pressures are 200 bars suggesting depths of 190 to 1 500 m under hydrostatic pressure gold occurs both in the dark bands with magnetite and outside the dark bands with pyrite chlorite illite and k feldspar banded veinlets occupy steeply dipping radial and shallowly dipping concentric fractures zones of abundant banded veinlets without early a veinlets generally contain 0 5 to 2 ppm gold and 0 05 wt percent hypogene copper most of the differences between porphyry gold deposits at refugio and porphyry copper deposits can be attributed to shallower depths of formation less than 1 km compared to 1 5 to 4 km that is typical for porphyry copper deposits shallower depths resulted in lower sulfide concentrations local garnet veinlets widespread albite bearing alteration and most importantly banded quartz veinlets which are unique to porphyry gold deposits banded quartz veinlets are a direct result of episodic intrusion of magma to within 1 km of the surface and exposure of high temperature magmatic fluids to hydrostatic pressures episodic rupturing of a brittle ductile boundary surrounding the intrusive centers at verde and pancho led to flashing of magmatic fluids loss of sulfur to vapor and low sulfide gold ratios in ore

cenozoic sediment accumulation on drifts in the northern north atlantic the northern north atlantic sediment drifts have a much greater areal extent than has previously been indicated the northern north atlantic extends from the east greenland to european continental shelves and from the charlie gibbs fracture zone to the greenland scotland ridge within this region are seven major sediment drifts containing some of the best clues for pre quaternary bottom water circulation in the north atlantic feni drift is the oldest in the region originating near the eocene oligocene boundary it was followed by accumulation of bjorn gardar hatton and snorri drifts from the late early to middle miocene eirik drift may have started to accumulate in the late miocene and gloria drift in the early pliocene through analysis of the cenozoic sediment mass age distribution and the change in area of seafloor for the study area through time it can be concluded that there is a deficit of oligocene to miocene sediment in the region this could be explained by sediment cycling the erosion of older sediment to produce young sediment the mass of pliocene to quaternary sediment is much larger than that of the older sediment and may have been derived primarily through the erosion of eocene and oligocene sediment by deepwater currents the sediment drift portion of the total regional sediment mass increased significantly three times during the cenozoic each of these drift growth phases lasted between 3 and 4 m y the first two growth phases from the late eocene to early oligocene and from the early to middle miocene were accompanied by northward shifts of depocenters to the newly forming sediment drifts the third and largest growth phase occurred between 7 and 3 ma and may have been preceded by the initiation of accumulation on eirik drift 8 7 ma this most recent growth phase was accompanied by high apparent accumulation rates over southern gardar drift as it expanded to the east erosion rates over reykjanes ridge may have been higher during the mid pliocene than in the quaternary there are two main sites where dense water could have formed and flowed south through the rockall trough to start the accumulation of feni drift if the atmosphere were cool enough dense water could have formed in the norwegian sea or on the faeroe shelf dense water may also have formed on the rockall plateau due to salinity increase through evaporation in an arid climate as an alternative to the hypothesis of arctic ocean water overflowing the iceland faeroe ridge in the early middle miocene it is suggested that dense water formed along shallow segments of the iceland faeroe ridge and then flowed into the south iceland basin to begin accumulation of bjorn gardar and snorri drifts the initiation of sediment drift formation in the south iceland basin may have been caused by a decreased rate of deep water production combined with a higher rate of detrital input due to a middle miocene uplift event along the iceland faeroe ridge a second uplift event along the greenland scotland ridge and a decreased production of northern component water in the early pliocene may be responsible for increased sediment accumulation in bjorn gardar and eirik drifts

neogene stratigraphy paleoceanography and paleobiogeography in northwest south america and the evolution of the panama seaway abstract the atrato basin in the pacific coastal region of nw south america colombia exhibits stratigraphic and biostratigraphic characteristics similar to other neogene pacific coastal basins of southern central america and northern south america an evaluation of the neogene stratigraphy and foraminiferal biostratigraphy with emphasis on paleoceanographic and paleobiogeographic phenomena indicate the following 1 previous to the middle miocene well aerated deep and open oceanic conditions and free and active water circulation prevailed along the steep continental margins of nw south america 2 during the early middle miocene right after the neogene hiatus nh 2 15 2 ma tectonic disturbances triggered the initial uplift of the panama sill bottom water circulation foraminiferal and sedimentation changes are observed in both the pacific and caribbean coastal areas of nw south america 3 during the middle miocene coinciding with the neogene hiatus nh 3 12 9 11 8 ma an abrupt paleobathymetric change from lower to middle bathyal depths indicates a major uplift of the panama sill to about 1000 m reflecting the middle miocene tectonic disturbances in nw south america 4 during the late middle miocene immediately following this uplift a distinctive pacific benthic foraminiferal fauna abruptly appeared and extended from ecuador to california because these assemblages do not occur in the adjacent caribbean region a circulation barrier between the atlantic and pacific oceans as a result of intensification of the cool marginal california current is proposed anoxic middle to upper bathyal environments prevailed under well aerated cool surface waters in the pacific coastal areas of nw south america 5 during the latest miocene immediately following the neogene hiatus nh 6 7 0 6 3 ma surface water circulation between the caribbean and the pacific was re established and the previous influence of the california current disappeared a rapid filling and shallowing of the basin to mostly neritic depths is indicated along the pacific and caribbean coastal basins of colombia as well as a progressive increase in similarity between the coastal pacific and caribbean benthic biotas this shallowing in the early late miocene also appears to have been associated with the earliest terrestrial interchange of ground sloths to north america and raccoons and their allies to south america 6 immediately following early pliocene time the panamanian isthmus became completely emergent providing a terrestrial environment favourable for intermingling of terrestrial faunas and floras between north and south america i correlate this pliocene uplift with the neogene hiatus nh 8 3 7 3 1 ma four regional unconformities associated with tectonic disturbances in the nw corner of south america have also been recognized and correlated with neogene hiatuses recorded from the oceans early middle miocene middle miocene late miocene and early pliocene

a relationship between d104 value and composition in the calcite disordered dolomite solid solution series x ray diffraction has been widely used in analyzing ca mg carbonates compositions of biogenic and inorganic ca mg co3 crystals are often calculated by comparing their d 104 values with published empirical curves however previous studies suggested that these curves do not apply to very high mg calcite and disordered dolomite based on synthesized high mg calcite and disordered dolomite a new empirical curve between values of magnesian calcite d 104 and mgco3 content in the calcite disordered dolomite solid solution series is constructed this new curve is consistent with the significant cell parameter changes accompanying the mg ca cation disorder in dolomite and it can help the characterization of the mgco3 content of both natural and synthetic magnesian calcite and disordered dolomite especially for the mineral mixtures that are not suitable for other analysis methods

10 be dating of a manganese crust from central north pacific and implications for ocean palaeocirculation one of the most promising applications of the new 10be detection technique using accelerator mass spectrometry is the determination of growth rates of mn nodules and crusts1 5 because the half life of cosmogenic 10be 1 5 myr is of the same order as the time needed for a few millimetres of nodule material to grow 10be is a useful tool for unravelling such evolution during the late tertiary when most nodules are assumed to have started their growth6 mass spectrometric measurement of a 10be profile across the manganese crust va 13 2 from the central pacific using a tandem van de gr a ff accelerator yields growth rates of 2 7 and 4 8 mm myr 1 for the layers of the crust accumulated between recent and 6 myr bp and between 6 and 11 myr respectively using these measurements as well as 230th dating we have been able to distinguish boundaries between zones in the crust with different structures and chemical compositions and to assign ages to them these ages are 0 12 2 9 3 4 5 7 6 7 7 9 10 12 and 13 16 myr bp all of these boundaries apparently coincide with the times reported for quaternary and late tertiary palaeoclimatic events suggesting that the crust growth has been strongly influenced by palaeoclimate

physiographic and tectonic settings of high sulfidation epithermal gold silver deposits of the andes and their controls on mineralizing processes abstract gold and silver ores in the vast majority of andean high sulfidation epithermal au ag deposits occur at high present day elevations and typically 200 500 m below low relief landforms situated at 3500 to 5200 m a s l most deposits are middle miocene and younger and include el indio tambo pascua lama veladero el indio belt chile argentina cerro de pasco central peru pierina lagunas norte yanacocha northern peru quimsacocha ecuador and the california vetas mining district santander colombia jointly accounting for 130 moz au resources slightly older examples are only preserved in the atacama desert and include the middle eocene el guanaco and el hueso and the late oligocene early miocene la coipa deposits the absence of paleocene and older high sulfidation epithermal deposits can be explained by limited preservation potential imposed by transpressional tectonics within overall contractile episodes and surface uplift these conditions prevailed predominantly in segments of shallow angle subduction of the nazca or caribbean plate below the south american continent a tectonic setting also common for porphyry style cu au mo deposits stratovolcanoes are uncommon ore hosts and volcanic rocks coincident with mineralization are in most cases volumetrically restricted or absent recording the terminal stages of local arc magmatism however dacitic domes are important at e g yanacocha and la coipa at lagunas norte a small stratovolcano largely pre dating but temporally overlapping with mineralization occurs immediately east of the deposit and volcanic sector collapse may have occurred during hydrothermal activity mineralization is typically located near the backscarp of pediments or the heads of valleys incising now high elevation low relief surfaces in the california vetas mining district and el indio belt hydrothermal alunite ages become generally younger upstream along the incising valleys indicating that hydrothermal activity and by inference ore deposition were facilitated by erosion the lowering of the water table and reduction of hydrostatic and lithostatic pressure at these sites of high local relief are believed to have enhanced both boiling and mixing of magmatic with meteoric fluids ultimately enhancing ore deposition the host rock composition permeability and location of the water table control the distribution of alteration zones and ore intermediate volcanic rocks are the most common ore hosts but they typically pre date mineralization by several ma however high sulfidation epithermal mineralization can be hosted in any conceivable rock type including high grade metamorphic rocks california vetas mining district significantly older plutonic rocks pascua lama or quartzites lagunas norte large vuggy quartz alteration zones and commonly oxidized low grade large tonnage mineralization are best developed in relatively permeable volcaniclastic rocks or hydrothermal breccia bodies whereas coherent volcanic plutonic or metamorphic rocks may host fault and breccia controlled ores the near surface steam heated zone can attain a thickness of several hundred meters in dry climates e g veladero pascua lama tambo but is typically poorly developed and less than 20 m thick in humid climatic zones the physiographic and tectonic settings of high sulfidation epithermal deposits are distinct from low sulfidation epithermal districts such as those of patagonia el penon chile or fruta del norte ecuador the latter range to significantly older ages jurassic to early eocene occur at mainly lower elevations and were emplaced in extensional settings a temporal coincidence between uplift erosion and mineralizing processes as well as a spatial and temporal association with porphyry style mineralization is not evident for these low sulfidation districts

mg fractionation in crustose coralline algae geochemical biological and sedimentological implications of secular variation in the mg ca ratio of seawater the mg ca ratio of seawater has varied significantly throughout the phanerozoic eon primarily as a function of the rate of ocean crust production specimens of the crustose coralline alga neogoniolithon sp were grown in artificial seawaters encompassing the range of mg ca ratios shown to have existed throughout the phanerozoic significantly the coralline algae s skeletal mg ca ratio varied in lockstep with the mg ca ratio of the artificial seawater specimens grown in seawater treatments formulated with identical mg ca ratios but differing absolute concentrations of mg and ca exhibited no significant differences in skeletal mg ca ratios thereby emphasizing the importance of the ambient mg ca ratio and not the absolute concentration of mg in determining the mg ca ratio of coralline algal calcite specimens grown in seawater of the lowest molar mg ca ratio mmg ca 1 0 actually changed their skeletal mineralogy from high mg skeletal mmg ca 0 04 to low mg calcite skeletal mmg ca 0 04 suggesting that ancient calcitic red algae which exhibit morphologies and modes of calcification comparable to neogoniolithon sp would have produced low mg calcite from the middle cambrian to middle mississippian and during the middle to late cretaceous when oceanic mmg ca approached unity by influencing the original mg content of carbonate facies in which these algae have been ubiquitous this condition has significant implications for the geochemistry and diagenesis of algal limestones throughout most of the phanerozoic the crustose coralline algae s precipitation of high mg calcite from seawater that favors the abiotic precipitation of aragonite indicates that these algae dictate the precipitation of the calcitic polymorph of caco3 however the algae s nearly abiotic pattern of mg fractionation in their skeletal calcite suggests that their biomineralogical control is limited to polymorph specification and is generally ineffectual in the regulation of skeletal mg incorporation therefore the mg ca ratio of well preserved fossils of crustose coralline algae when corrected for the effect of seawater temperature may be an archive of oceanic mg ca throughout the phanerozoic magnesium fractionation algorithms that model algal skeletal mg ca as a function of seawater mg ca and temperature are presented herein the results of this study support the empirical fossil evidence that secular variation of oceanic mg ca has caused the mineralogy and skeletal chemistry of many calcifying marine organisms to change significantly over geologic time

special paper major gold deposits and belts of the north and south american cordillera distribution tectonomagmatic settings and metallogenic considerations a compilation of economically viable gold concentrations containing 10 moz in the north and south american cordillera reveals the existence of 22 discrete belts in addition to five major isolated deposits most formed over the last 150 m y the gold concentrations are attributed to eight widely recognized deposit types of which porphyry sediment hosted and high sulfidation epithermal are economically the most important individual gold belts are typically several tens to hundreds of kilometers long dominated by single deposit types and metallogenically active for relatively brief periods 5 20 m y many of the gold belts and major isolated deposits were generated under extensional or transtensional tectonic conditions in either arc or back arc settings nevertheless the two main high sulfidation epithermal gold belts were generated in thickening or already thickened crust during low angle subduction eight other gold belts or districts also accompanied compression or transpression with two of them the main orogenic gold belts occupying fore arc sites there is a strong suggestion that the preeminent cordilleran gold concentrations formed either during or immediately following prolonged contraction the major gold deposits and belts occur along the craton edge as well as in adjoining accreted terranes but almost all are of postaccretionary timing many of the gold belts and isolated deposits were localized by crustal scale faults or lineaments which may be either parallel or transverse to the cordilleran margin the gold concentrations accumulated during active subduction commonly in close spatial and temporal association with intermediate to felsic medium to high k calc alkaline igneous rocks by contrast the low sulfidation epithermal gold deposits accompany bimodal volcanic pulses of calc alkaline tholeiitic or alkaline affinity however a gold alkaline rock association is uncommon a genetic link between gold mineralization and coeval magmatism is widely accepted for most of the deposit types the exceptions being the sediment hosted and orogenic gold deposits notwithstanding the small cumulative extent of the gold concentrations relative to the entire cordilleran margin there is a marked tendency for two or more belts or isolated deposits of different ages and genetic types to occur in close proximity within relatively restricted arc including fore and back arc segments in the case of the western united states for example six belts and four isolated major deposits make up a particularly prominent cluster if fortuity is discounted this clustering or pairing of gold concentrations must imply a predisposition of certain arc segments to gold mineralization an analogous situation is evident for other metals particularly copper and tin the reason for the recurrent generation of major deposits and belts dominated by one or more metals remains uncertain although heterogeneously distributed metal preconcentrations favorable redox conditions or other parameters somewhere above the subducted slab between the mantle wedge and upper crust are widely contemplated possibilities elucidation of the reason s for this metallogenic inheritance at the scale of limited arc segments poses an important and challenging series of research questions as well as being critical to the planning of potentially successful greenfield exploration programs

nd sr isotope evidence for dramatic changes of paleocurrents in the atlantic ocean during the past 80 m y nd isotopic variations in atlantic seawater during the past 80 m y as deduced from both nd and sr isotopic compositions of marine deposits are strongly related to major plate motions involving openings of the south atlantic and the drake passage and closure of the tethys seaway changes in the proportions of two main fluxes to the atlantic the admixture of pacific seawater with radiogenic neodymium and continental components with nonradiogenic nd average river flux north atlantic bottom water controlled the secular variation of nd

neodymium isotopic variations in north pacific modern silicate sediment and the insignificance of detrital ree contributions to seawater the neodymium isotopic composition of the silicate fraction of holocene pelagic sediments from the north pacific define two provinces a central north pacific province characterized by unradiogenic and remarkably homogeneous end 10 2 0 5 and a narrow circum pacific marginal province characterized by more radiogenic and variable end 4 2 3 8 the silicate fraction in the central north pacific is exclusively eolian based on prevailing wind patterns meteorological data and neodymium isotopic data the only significant sediment source is chinese loess leaching experiments on chinese loess confirm that leachable nd is isotopically indistinguishable from bulk and residual silicate nd silicates in the circum north pacific marginal province comprise eolian loess volcanic ash and hemipelagic sediments derived from volcanic arcs a compilation of pacific seawater and mn nodule end data shows no clear spatial variation except for a general decrease from surface to deep waters from 3 to 4 and slightly lower end in bottom waters along the western north pacific due to the incursion of antarctic bottom water the relative homogeneity of bottom water end which contrasts sharply with the distinctive variation in sediment end plus the large difference between the average end of bottom waters and the central north pacific eolian silicates 4 vs 10 suggests that any contribution of ree to seawater from eolian materials is insignificant furthermore leaching of ree from eolian particles as they sink though the water column must be insignificant because nd in shallow waters is more radiogenic than nd in deeper waters that there is no contrast in the nd isotopic composition of bottom waters that overlie the central and marginal sediment provinces suggests that the ash and hemipelagic sediments derived from pacific rim volcanic arcs also contribute minimal ree to seawater the elimination of eolian ash and hemipelagic sediments leaves only near shore riverine particulates as a possibly significant particulate source of ree to seawater

the veladero high sulfidation epithermal gold deposit el indio pascua belt argentina geochronology of alunite and jarosite the world class veladero high sulfidation epithermal gold deposit is located near the northern end of the el indio pascua metallogenic belt in argentina new 40ar 39ar alunite and jarosite ages were obtained at four mineralized centers in the veladero district the amable and filo federico orebodies that compose the veladero deposit and the argenta and fabiana satellite centers these ages indicate that magmatic hydrothermal alteration and related gold mineralization were episodic and activity in the veladero district occurred over a period of approximately 3 m y the earliest magmatic hydrothermal alteration in the veladero district is recorded by alunite from argenta that formed at 13 12 0 18 ma at the fabiana prospect alunite is dated at 10 09 0 08 ma at the veladero deposit the alteration and mineralization at the filo federico and amable orebodies formed at different times despite their close proximity previously available data indicate that alunite at amable formed at 11 93 0 09 ma whereas filo federico alunite samples are younger and yield 40ar 39ar ages of 11 05 0 12 and 10 34 0 11 ma the oldest postmineralization jarosite alteration at amable is dated at 11 8 0 3 ma and the youngest is dated at 8 58 0 17 ma the age constraints suggest that the spatially overlapping amable and filo federico orebodies formed approximately 1 m y apart at about 11 9 ma at amable and after 11 1 ma at filo federico the results demonstrate that the veladero district is host to the oldest centers of significantly mineralized magmatic hydrothermal alteration within the el indio pascua metallogenic belt magmatic hydrothermal alteration and gold mineralization at the veladero deposit broadly overlapped with isolated centers of middle to late miocene volcanic activity in the region dated at 12 7 0 9 to 11 0 0 2 ma in contrast to veladero the other high sulfidation deposits in the belt are significantly younger with reported 40ar 39ar alunite alteration ages of other major deposits in the belt ranging from 9 4 the lama deposit at pascua lama to 6 2 ma the el indio deposit these deposits formed during volcanic quiescence caused by shallowing of the nazca plate below south america during the late miocene whereas veladero formed prior to the cessation of volcanism fertile magmas of roughly 13 to 10 ma were previously thought to be restricted to the maricunga metallogenic belt approximately 200 km north of veladero but the present study demonstrates that mineralization in this age range occurred as far south as veladero these results are significant for regional exploration in particular in the area between the maricunga and el indio pascua metallogenic belts

improving confidence in ferromanganese crust age models a composite geochemical approach abstract accurate age models for marine ferromanganese fe mn crusts are essential to understand paleoceanographic changes and variations in local environmental factors affecting crust growth rate and their lateral continuity however no absolute method exists for dating these deposits beyond the age of 10 myr which requires the combination of a number of approaches here we present a composite age model for a 15 cm thick fe mn crust sample obtained by unique core drilling using a remotely operated vehicle at a water depth of 1130 m on the summit of tropic seamount in the north east atlantic the age model is based on cross validation of laser ablation u pb dating co chronometry and os isotopes these enable robust calibration of the age depth model using the bayesian statistical modelling of markov chain monte carlo mcmc simulations the results show that this fe mn crust commenced growth in the late cretaceous between 73 and 77 ma and grew at a rate between 1 and 24 mm myr averaging 4 mm myr the phosphatised carbonate substrate capping tropic seamount and underlying most of the fe mn crusts yields a u pb age of 84 4 myr and provides the upper age limit for the model less radiogenic excursions of 188os 187os in the vertical profile through the crust permit the identification of key inflection points in the os isotope seawater curve at the eocene oligocene and cretaceous paleogene transitions growth rates estimated from the empirical co chronometer are combined with the age envelope defined by the os data and used to validate the mcmc simulations the model identifies five hiatuses that occurred during the pliocene 2 5 1 9 5 3 1 7 ma early miocene 16 1 27 2 ma oligocene 29 2 32 1 ma eocene 41 2 52 0 6 ma and the late paleocene 55 1 59 1 4 ma a major phosphatisation event affecting the fe mn core can be dated to the late eocene 38 1 2 ma which coincides with a recorded change in the global oceanic system from warm and sluggish circulation to cold and vigorous thermohaline driven meridional overturn at the onset of antarctic glaciation

low magnesium calcite produced by coralline algae in seawater of late cretaceous composition abstract shifts in the mg ca ratio of seawater driven by changes in midocean ridge spreading rates have produced oscillations in the mineralogy of nonskeletal carbonate precipitates from seawater on time scales of 108 years since cambrian time skeletal mineralogies of anatomically simple organisms functioning as major reef builders or producers of shallow marine limestones have generally corresponded in mineral composition to nonskeletal precipitates here we report on experiments showing that the ambient mg ca ratio actually governs the skeletal mineralogy of some simple organisms in modern seas coralline algae produce skeletons of high mg calcite 4 mol mgco3 we grew three species of these algae in artificial seawaters having three different mg ca ratios all of the species incorporated amounts of mg into their skeletons in proportion to the ambient mg ca ratio mimicking the pattern for nonskeletal precipitation thus the algae calcified as if they were simply inducing precipitation from seawater through their consumption of co2 for photosynthesis presumably organic templates specify the calcite crystal structure of their skeletons in artificial seawater with the low mg ca ratio of late cretaceous seas the algae in our experiments produced low mg calcite 4 mol mgco3 the carbonate mineral formed by nonskeletal precipitation in those ancient seas our results suggest that many taxa that produce high mg calcite today produced low mg calcite in late cretaceous seas

evolution of nd and pb isotopes in central pacific seawater from ferromanganese crusts abstract hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts by analysing nd pb and be isotope profiles within crusts it is possible to reconstruct seawater tracer histories depth profiles of 10be 9be ratios in three pacific ferromanganese crusts have been used to obtain growth rates which are between 1 4 and 3 8 mm ma nd and pb isotopes provide intact records of isotopic variations in pacific seawater over the last 20 ma or more there were only small changes in pb isotope composition in the last 20 ma this indicates a constant pb composition for the erosional sources and suggests further that erosional nd inputs may have been uniform too end values vary considerably with time and most probably reflect changes in ocean circulation the end values of the crusts not only vary as a function of age but also as a function of water depth from 25 to 0 ma crust va13 2 from 4 8 km water depth has a similar pattern of end variation to the two shallower crusts from 1 8 and 2 3 km but about 1 0 to 1 5 units more negative this suggests that end stratification in pacific seawater as demonstrated for the present day has been maintained for at least 20 ma each crust shows a decrease in end from 3 5 ma to the present which is interpreted in terms of an increase in the nadw component present in the pacific from 10 to 3 5 ma ago the crusts show an increase in end this suggests a decreasing role for a deep water source with end less than circum pacific sources in this regard the panamanian gateway restriction from 10 ma with final closure at 3 5 ma may have played an important role in reducing access of atlantic derived nd to the pacific

reconnaissance k ar geochronology of the maricunga gold silver belt northern chile the maricunga metallogenic belt in northern chile is defined by the presence of both intrusion related porphyry type and volcanic hosted high sulfidation acid sulfate epithermal deposits containing gold and or silver the nine principal mineralized centers in the belt including the la coipa la pepa marte and lobo deposits were dated radiometrically using the k ar method hypogene alunite from advanced argillic assemblages was dated from eight of the deposits the 18 new radiometric ages permit subdivision of the maricunga belt into two overlapping longitudinal subbelts the western of early miocene including latest oligocene age 25 20 ma and the eastern of middle miocene age 14 12 5 ma this spatial and temporal subdivision of the alteration and mineralization accords well with that defined previously for volcanic rocks in the region and is substantiated further by regional structural and volcanic relationships the migration of the magmatic and metallogenic activity from the western to the eastern subbelt may be attributed to shallowing of the subduction zone which was initiated at 18 to 16 ma the western and eastern maricunga subbelts are broadly correlative with two discrete volcanic alteration events in the el indio precious metal belt 300 km to the south the maricunga subbelts are also integral parts of an eastward younging sequence of gold bearing magmatic metallogenic subbelts of paleocene to late miocene age which span an east west distance of 380 km in this transect 26 degrees 28 degrees s of the central andes

predicting gold rich epithermal and porphyry systems in the central andes with a continental scale metallogenic gis brgm s gis andes a comprehensive continental scale metallogenic information system for the entire andes cordillera is based on original syntheses structured into thematic layers the aim of developing the gis was to produce an integrated tool to understand ore deposit localization in the andes a fundamental question arising at the outset was whether the tool would be suitable for producing predictive mineral resource maps at the continental scale considering that previous predictive studies focus only on the regional scale the continental scale synthesis implied working with heterogeneous data in terms of distribution quantity quality and in particular accuracy the benefit is the ability to include uncommon parameters linked to the geodynamic evolution of the active margin and significant only at the continental scale in view of the particularities of the gis dataset an expert guided data driven approach was adopted for the multicriteria processing an approach that combined expert knowledge and the use of elementary statistics allowing to provide a link between the tectonic development of the whole andean margin and the spatial and temporal distribution of individual mining districts this study was purposely restricted to assessing the distribution of neogene gold in the central andes between lat 3 and 33 s thus a incorporating well constrained data on the present morphology of the convergent margin and b avoiding ambiguities in the less well constrained older history of the complex evolution of the andean margin five regional parameters were selected and were considered to have a significant influence on the neogene magmatic hydrothermal ore formation at continental scale the five parameters i host rock lithostratigraphy ii lithostratigraphic contacts iii structural discontinuities and iv depth and v dip of the wadati benioff zone modeled from seismic data had assigned favorability scores from 0 to 2 based on their associated metal content with respect to a set of identified neogene gold deposits the next step was to calculate favorability maps for each criterion that were combined to create an overall cumulative favorability map or predictive gold map verifying the predictive map against known gold deposits it was found that the cumulative favorability score of 4 out of 10 maximum located about two thirds of the known gold bearing epithermal and porphyry deposits and 95 of the metal content a cumulative favorability score of 5 reduced these figures to 50 of the deposits and 71 of the metal content and that of 6 relocated 24 of the deposits and 51 of the metal content in addition to verifying the method the predictive map outlines new potentially favorable gold areas and even indicates that some known districts could well host yet undiscovered mineralization

geological development and mineralization in the atacama segment of the south american andes northern chile 26 15 27 25 s between the late jurassic and the middle miocene widespread magmatism tectonic events and hydrothermal mineralization characterized the geological evolution of the atacama segment of the south american andes a characteristic feature of this zone is the coincidence in time and space between subduction generated igneous activity crustal deformation and mineralization in the magmatic arcs which formed longitudinal belts migrating eastward

links between topography erosion rheological heterogeneity and deformation in contractional settings insights from the central andes abstract orogenic structure in the central andes 15 s 34 s systematically varies with mean annual precipitation suggesting that erosional processes may be coupled to tectonic processes we explore the range of possible interactions between deformation erosional processes changing geodynamic conditions and preexisting geologic structures to assess the relative importance of each our review and synthesis indicates that the pre orogenic geologic history and changing geodynamic conditions leave a lasting imprint on the modern structure of the mountain belt in contrast changes in precipitation that result from regional scale atmospheric and oceanic circulation interact with local topography to control the climatic zoning across the central andes changes in erosion which likely correspond to changes in climate rock uplift rate and the exposure of different lithologies may in turn affect the tectonic development of the region at the scale of each morphotectonic province in this view geologic and geodynamic factors set the stage for the nature and strength of coupling between erosional processes tectonic deformation and mountain belt structure in the different parts of the central andes

isotopic paleoceanography of the caribbean and east pacific role of panama uplift in late neogene time comparisons of carbon isotopic data on benthic foraminifera from deep sea drilling project sites 502 western caribbean and 503 eastern pacific indicate that the difference between the atlantic and the pacific in the per mil enrichment in carbon 13 of total dissolved carbon dioxide increased about 6 million years ago and again 3 million years ago when the difference reached the modern level 1 per mil comparisons of planktonic foraminiferal oxygen isotopic data for the caribbean and the pacific suggest that the salinity of caribbean surface waters began increasing 4 million years ago possibly in response to shoaling of the panama isthmus these results suggest that modern circulation patterns in the caribbean and eastern pacific developed by 3 million years ago in concert with changing tectonic climatic and biogeographic patterns

rare earth elements and neodymium isotopes in ferromanganese oxide coatings of cenozoic foraminifera from the atlantic ocean abstract an evaluation has been made of the method of establishing the ree contents and patterns and nd isotopic compositions of sea water over cenozoic time from their record in the femn oxide coatings of foraminiferal calcite using 0 60 ma samples from the rio grande rise dsdp site 357 it has been established that the ree contents of the coatings are generally similar to those of recent samples however in the cenozoic samples the surface coatings have been diagenetically modified under suboxic conditions resulting in a distinctly different ree pattern although the original 143 nd 144 nd ratios appear to have been preserved the nd isotopic curve for cenozoic sea water in the s atlantic shows clear temporal trends although these are not so extreme as to show 143 nd 144 nd ratios outside the range observed in modem sea water with the principal exception of the oldest samples there is an approximate inverse relationship between the nd and sr isotopic compositions of the foraminifera it is suggested that the changes reflect both global changes in the relative proportions of nd and sr derived from continental input and from the weathering of volcanic debris together with short term and local variations to which the sr curve is insensitive reflecting the different response times of the two elements to changes in oceanic input functions the nd isotope curve appears to be a potentially useful tracer of ocean palaeochemistry

the geochemical balance of the rare earth elements and neodymium isotopes in the oceans profiles of ree concentrations and neodymium isotope measurements in the western indian ocean are consistent with published data from the other major oceans ree concentrations show a strong hydrographic control the influence of intermediate water masses is particularly significant surface water concentrations vary with salinity paniculate rees are less than 5 of dissolved concentration except for ce where up to 20 of total rees are present as 0 4 m particles in contrast to dissolved profiles concentration profiles of paniculate rees including ce all increase with depth ree concentrations correlate with silica over all but the deepest waters although in detail correlations are complex the neodymium isotopic composition of indian ocean seawater avg nd 0 8 3 is intermediate between that for the atlantic and pacific oceans comparison of ree data from the major oceans shows that the hrees behave more like si than do the lrees sm nd ratios are near constant for all oceans whereas ernd ratios vary between the atlantic indian and pacific oceans the fractionation of the rees in surface seawaters can be explained by fractionation factors relative to silica from 0 12 for la to 0 05 for er use of a seven box model shows that the interoceanic variations in neodymium isotopic compositions and nd concentrations cannot be reconciled unless particle water exchange is invoked the degree of exchange required is consistent with particle residence times of 1 10 yr

miocene deepwater oceanography a global synthesis of miocene benthic foraminiferal carbon and oxygen isotopic and faunal abundance data indicates that miocene thermohaline circulation evolved through three regimes corresponding approximately to early middle and late miocene times there is evidence for major qualitative differences between the circulation of the modern ocean and the miocene ocean prior to 11 ma the 13c 12c ratios of the benthic foraminifera cibicidoides are interpreted in terms of water mass aging i e the progressive depletion of dissolved o2 and lowering of 13c values as the result of oxidation of organic matter as water flows further from its sources at the surface of the oceans both isotopic and faunal data indicate that the early miocene regime from 22 to 15 ma was the most different from today s during that interval intermediate and deep waters of both the atlantic and the pacific oceans aged in a northward direction and the intermediate waters of the indian the south atlantic and the south pacific oceans were consistently the youngest in the global ocean we speculate that early miocene global thermohaline circulation may have been strongly influenced by the influx of warm saline water tethyan indian saline water from the tethys into the northern indian ocean the isotopic and faunal data suggest that flow from the tethyan region into the indian ocean diminished or terminated at about 14 ma isotopic and faunal data give no evidence for north atlantic deep water nadw formation prior to about 14 5 ma with the exception of a brief episode in the early miocene from 14 5 to 11 ma nadw formation was weak and circumpolar and antarctic water flooded the deep south atlantic and south pacific as the antarctic ice cap grew from about 10 ma to the end of the miocene thermohaline circulation resembled the modern circulation in many ways in latest miocene time 6 to 5 ma circulation patterns were very similar to today s except that nadw formation was greatly diminished the distribution pattern of siliceous oozes in miocene sediments is consistent with our proposed reconstruction of thermohaline circulation major changes which occurred in circulation during the middle miocene were probably related to the closing of the tethys and may have contributed to rapid middle miocene growth of the antarctic ice cap appendices 1 4 6 and 7 are available withentire article on microfiche order fromamerican geophysical union 2000 floridaavenue n w washington dc 20009 document 88p 002 5 00 payment mustaccompany order

world map of nd isotopes in sea floor ferromanganese deposits the nd isotopic compositions of hydrogenous ferromanganese encrustations and nodules largely reflect those of the overlying seawater integrated over the few millions of years over which they grow available nd isotope data on ocean floor hydrogenous deposits suggest control by nd from particulates blown from the continents and released by volcanic activity associated with subduction zones and midocean ridges the regional nd isotope variations display a marked similarity to the broad patterns of present day deep ocean circulation suggesting that the particulates that are the sources of oceanic nd travel along with water masses despite rapid 10 100 ka perturbations during glacial periods the long term patterns of water mass movements and oceanic fluxes of particulates have remained essentially stable over the last few million years

neodymium isotopic study of baffin bay water sources of ree from very old terranes water samples were collected from baffin bay and surrounding areas in order to evaluate this region as a potential source of nd from old continental material to atlantic water the isotopic data ranged from e nd 0 9 0 to 26 with most of the data around e nd 0 20 compared with values of north atlantic deep water nadw with e nd 0 13 5 the concentration of nd in baffin bay waters was as high as 6 10 12 g g compared with 2 5 10 12 g g for nadw the combination of low e nd and high nd concentration indicates that baffin bay may be a significant source of nd from very old crustal material a simple box model was used to evaluate the contribution to the nd budget of nadw and it was concluded that a substantial fraction of the nd from ancient crustal sources that is required to maintain the isotopic composition of nadw could be supplied by baffin bay outflow

an evaluation of strontium isotopic dating of ferromanganese oxides in a marine hydrogenous ferromanganese crust abstract in order to assess the utility of dating marine hydrogenous ferromanganese crusts with strontium isotopes we conducted a series of leaching experiments on samples selected from several stratigraphic layers of a well preserved 9 5 cm thick crust from the pacific ocean initially using a method similar to that of previous studies acetic acid leached residues of twelve subsamples were leached with hcl and the leachates analyzed for their isotopic composition the results did not make stratigraphic sense so we carried out two sequences of more detailed leaches on subsamples from four layers representing the chemical and textural spectrum observed in the crust in one sequence mineral specific leaches were selected for optimal extraction of strontium associated with the respective manganese and iron oxide phases the main compositional endmembers within ferromanganese crusts strontium isotopic ratios for these leaches were compared with those from the other sequence involving acetic acid plus hcl leaches on the same material residues remaining at the end of each leaching sequence were also analyzed it is evident from the results that hcl extracts strontium associated with both phosphatic material and aluminosilicate detritus present within the ferromanganese layers 87 sr 86 sr ratios obtained on the manganese oxides indicate that the strontium within the oxides exchanges with seawater strontium throughout the history of the deposit regardless of the porosity of the layer assuming that this crust is representative of other marine hydrogenous crusts strontium isotopes appear to be of little value for the direct dating of ferromanganese oxide deposits

40 ar 39 ar and k ar geochronology of magmatic and hydrothermal events in a classic low suphidation epithermal bonanza deposit el penon northern chile the epithermal el penon gold silver deposit consists of quartz adularia veins emplaced within a late upper paleocene rhyolitic dome complex located in the paleocene lower eocene au ag belt of northern chile detailed k ar and 40ar 39ar geochronology on volcano plutonic rocks and hydrothermal minerals were carried out to constrain magmatic and hydrothermal events the paleocene to lower eocene magmatism in the el penon area is confined to a rhomb shaped basin which was controlled by n s trending normal faults and both ne and nw trending transtensional fault systems the earliest products of the basin filling sequences comprise of middle to upper paleocene 59 55 ma welded rhyolitic ignimbrites and andesitic to dacitic lavas with occasional dacitic dome complexes later rhyolitic and dacitic dome complexes 55 52 ma represent the waning stages of volcanism during the latest upper paleocene and the earliest eocene lower eocene porphyry intrusives 48 43 ma mark the end of the magmatism in the basin and a change to a compressive tectonomagmatic regime 40ar 39ar geochronology of hydrothermal adularia from the el penon deposit yields ages between 51 0 0 6 and 53 1 0 5 ma these results suggest that mineralization occurred slightly after the emplacement of the el penon rhyolitic dome at 54 5 0 6 ma 40ar 39ar age and was closely tied to later dacitic rhyodacitic bodies of 52 to 53 ma k ar ages probably as short lived pulses related to single volcanic events

sm nd in marine carbonates and phosphates implications for nd isotopes in seawater and crustal ages this study explores the possibility of establishing nd isotopic variations in seawater over geologic time calcite aragonite and apatite are examined as possible phases recording seawater values of nd modern biogenic and inorganically precipitated calcite and aragonite from marine environments were found to have nd concentrations of from 0 2 to 70 ppb showing that primary marine caco 3 contains little ree and that nd ca is not greatly enhanced relative to seawater during carbonate precipitation very young marine limestone and dolomite containing no continental detritus have 200 ppb nd all the carbonates are lree enriched 0 16 sm nd 0 45 modern and very young atlantic and pacific carbonates have nd in the range of shallow atlantic and pacific seawater respectively implying that they derive their ree from local seawater the nd in well preserved carbonate fossils is 4 10 4 ppb much greater than in their modern counterparts but like the high values found for carbonates in other studies we believe the high ree contents at the 500 ppb level in some detritusfree carbonates are due to ree rich fe hydroxide in on the carbonate in favorable cases such material may record seawater nd values however introduction of extraneous ree may obscure the original isotopic composition of pure caco 3 because of its very low intrinsic primary ree abundance modern biogenic apatite is also shown to have very low ree content 150 ppb nd but appears to quickly scavenge ree from seawater inorganically precipitated apatite from phosphorites has high concentrations of seawater derived ree young phosphorite apatite from the atlantic and pacific oceans has nd in the range of the seawater from these oceans older apatite samples of similar age from different localities bordering common oceans record similar values of nd t sedimentary apatite has sr t values in good agreement with the curves for 87 sr 86 sr of seawater as a function of time individual conodonts from a single formation yield the same sr t and nd t other workers have shown that sedimentary apatite preserves seawater ree patterns these characteristics suggest that sedimentary apatite can be used to determine nd t in ancient seawater the seawater values so inferred range between 1 7 and 8 9 over the last 700 my and lie in the range of modern seawater showing no evidence for drastic changes high values of seawater nd t in the triassic and latest precambrian may correlate with the breakup of large continental landmasses the initial nd t 15 0 of a 2 ae old phosphorite implies the presence of 1 5 ae old continental crust at 2 ae ago the approach outlined here can be used to constrain the age of the exposed crust as a function of time

controls on skarn mineralization and alteration at the cadia deposits new south wales australia the cadia fe cu au skarns in eastern new south wales australia are associated with the late ordovician shoshonitic cadia intrusive complex which hosts the largest mineralized intrusion related system in eastern australia both the big cadia and little cadia skarns are developed within a 40 m thick volcanic derived calcareous sandstone and adjacent units which represent the most important control on skarn formation the ore zones have undergone postore displacement along steeply dipping predominantly reverse faults without significant remobilization of ore chalcopyrite and minor native gold formed in intimate association with epidote chlorite quartz calcite and in the interstices of bladed hematite and magnetite aggregates the age of the au cu mineralizing event at cadia is constrained by 40ar 39ar dating of accompanying muscovite at 438 2 4 2 ma 1 gold to copper ratios within the skarns are generally lower than for porphyry au cu deposits at cadia particularly the high tonnage low grade cadia hill deposit ore and calc silicate gangue mineralogy are similar to other oxidized fe cu au skarns associated with porphyry deposits iron skarn at big cadia occurs several hundred meters distant from the margin of a mineralized intrusion of quartz monzonite porphyry magmatic dominated fluids reacted with carbonate in the wall rocks and produced classic skarn zonation over an 800 m interval from the intrusive contact located at cadia quarry toward more distal environments at big cadia skarn zonation comprises 1 proximal garnet pyroxene 2 intermediate garnet pyroxene scapolite and 3 distal fe au cu skarn alteration of noncalcareous metavolcanic rock units fine grained pyroxene phyric volcanic rocks and volcaniclastic rocks adjacent to the mineralizing quartz monzonite porphyry includes hydrothermal biotite k feldspar quartz hornfels and magnetite quartz biotite hornfels gold copper mineralization formed adjacent to garnet bearing veins peripheral to the main garnet rich zone indicating that garnet forming fluids carried the ore metals at little cadia and cadia east mineralization and skarn zonation similar to big cadia developed above additional intrusions of quartz monzonite porphyry retrograde hydrous alteration replaced much of the prograde garnet dominant mineralogy at cadia with the strongest overprint at little cadia the styles and distribution of alteration and mineralization suggest that the big cadia skarn formed from fluids that migrated laterally within calcareous units from strongly altered quartz monzonite phases of the cadia intrusive complex at cadia quarry at little cadia skarn formation was probably related to fluids derived from the cadia intrusive complex that migrated vertically and laterally within permeable calcareous units structural controls were important in focusing fluids and localizing the emplacement of late mineralizing phases of the cadia intrusive complex

rare earth element transport in the western north atlantic inferred from nd isotopic observations the isotopic composition of nd in the water column from several western north atlantic sites and formational areas for north atlantic deep water shows extensive vertical structure at all locations in regions where a thermocline is well developed large isotopic shifts 2 to 3 units are observed across the base of the thermocline regions without a thermocline are characterized by much more gradual shifts in isotopic composition with depth in general the data reveal an excellent correlation between the nd isotopic distribution in the western north atlantic water column and the distribution of water masses identified from temperature and salinity characteristics nadw as identified from t s properties is also characterized by a well defined isotopic composition having nd 0 13 5 0 5 this signature is associated with waters identified as nadw from high latitudes near formational areas in the labrador sea down to the equatorial region the isotopic signature of nadw would appear to be formed by a blend of more negative waters originating in the labrador sea nd 0 18 and more positive waters originating in the overflows from the norwegian and greenland seas nd 0 8 to 10 and is consistent with classical theories on the formation of nadw the isotopic signature of nadw is propagated southward to the equator where it is gradually being thinned out by mixing from above and below with more radiogenic nd associated with northward spreading antarctic intermediate and bottom waters the preservation of the isotopic signature of nadw over these large distances indicate that the ree undergo extensive lateral transport the isotopic composition of nd is largely conservative over the time scales of mixing within the atlantic in spite of the intrinsic nonconservative behavior of neodymium nd concentration gradients generally show surface waters to be depleted in nd relative to deep waters which must require vertical transport processes however isotopic differences in the water column preclude the local downward transport of ree from the surface into underlying deep waters as a simple explanation of the concentration gradient the apparent decoupling of ree in nadw from overlying local surface waters and the increasing concentration with depth provide a conflict with simple vertical transport mechanisms that is not yet resolved

argon release systematics of hypogene and supergene alunite based on progressive heating experiments from 100 to 1000 c abstract few minerals have as many different geological applications for k ar or 39 ar 40 ar dating as alunite kal 3 so 4 2 oh 6 but a basic understanding of the systematics of argon release from this mineral is lacking progressive heating experiments from 100 1000 c at 100 c intervals were conducted in this study to obtain argon release patterns from hypogene and supergene alunite the samples analyzed were selected from well understood deposits and are representative of the principal natural environments of acid sulfate generation and related advanced argillic alteration and alunite formation for gas extraction and purification a low blank metallic extraction purification system used for dating silicates was complemented with a quartz pyrex glass line that included an external resistance furnace the resulting patterns of argon release vs temperature are well defined and similar for all the samples quantitative release of radiogenic 40 ar occurs during the dehydroxylation reaction of alunite at temperatures 400 and 800 c is mainly derived from the extraction system the results indicate that the retentivity of ar in fine grained supergene alunite is similar to that of coarsely crystalline hypogene alunite the data support the reliability of k ar dating of hypogene and supergene alunite and provide practical information to conduct routine high precision analyses