the relationship between silicon isotope fractionation in sponges and silicic acid concentration modern and core top studies of biogenic opal recent work has shown the silicon isotope composition denoted by 30si of deep sea sponges reflects the concentration of ambient silicic acid si oh 4 in seawater however existing calibrations are based predominantly on living sponges collected from the southern ocean these data cannot however be used to determine whether other parameters that correlate with silicic acid in the southern ocean such as temperature and salinity influence 30si of sponges furthermore the published data do not demonstrate whether disaggregated core top sedimentary spicules preserve the primary 30si signal recorded in living sponges here we address both of these issues we refine and widen the existing calibration by including a global distribution of modern sponges in addition we provide the first systematic calibration from spicules picked from core top sediments that covers sites from different ocean basins the relationship between si oh 4 and 30si in sponge spicules is the same in different ocean basins between specimens that grew in different temperature and salinity conditions our core top data agree well with the modern sponge calibration indicating there are no significant post depositional effects or early diagenetic overprints these two new datasets support the assertion that sponge 30si can be used as a proxy for silicic acid concentrations in the past

marine oxygenation lithistid sponges and the early history of paleozoic skeletal reefs abstract microbial carbonates were major components of early paleozoic reefs until coral stromatoporoid bryozoan reefs appeared in the mid ordovician microbial reefs were augmented by archaeocyath sponges for 15 myr in the early cambrian by lithistid sponges for the remaining 25 myr of the cambrian and then by lithistid calathiid and pulchrilaminid sponges for the first 25 myr of the ordovician the factors responsible for mid late cambrian microbial lithistid sponge reef dominance remain unclear although oxygen increase appears to have significantly contributed to the early cambrian explosion of marine animal life it was followed by a prolonged period dominated by greenhouse conditions as sea level rose and co2 increased the mid late cambrian was unusually warm and these elevated temperatures can be expected to have lowered oxygen solubility and to have promoted widespread thermal stratification resulting in marine dysoxia and hypoxia greenhouse conditions would also have stimulated carbonate platform development locally further limiting shallow water circulation low marine oxygenation has been linked to episodic extinctions of phytoplankton trilobites and other metazoans during the mid late cambrian we propose that this tendency to dysoxia hypoxia in shallow marine environments also limited many metazoan reef builders in contrast during the mid late cambrian the ability of lithistid sponges to withstand low oxygen levels allowed them to create a benthic association with microbial carbonates that dominated global reefs these conditions ameliorated during the ordovician as temperature decline promoted ocean ventilation the prolonged time gap occupied by low diversity reefs between the cambrian explosion and the great ordovician biodiversification event reflects elevated temperatures and reduced marine oxygenation that limited metazoan diversification in shallow marine environments

a combined in situ oxygen silicon isotopic and fluid inclusion study of a chert sample from onverwacht group 3 35ga south africa new constraints on fluid circulation abstract cherts have been extensively used as a proxy of precambrian environmental conditions of the earth cherts from the onverwacht group 3 5 3 3 ga south africa are considered as well preserved and have been used as a paleo oceanic thermometer especially by using the oxygen isotope composition of the chert nevertheless there are several lines of evidence size and number of the quartz veins that fluids circulated through these cherts after their deposition these cherts are composed mainly of microquartz with large quartz veins 100 m and iron carbonates as ankerites a combined study of oxygen silicon and fluid inclusions study have been performed on one chert sample from the onverwacht group in order to assess the effect of these fluid circulations on the chert oxygen and silicon isotopic compositions in situ oxygen and silicon isotopic compositions on microquartz and vein quartz were measured at high spatial resolution using a multicollector cameca 1270 ion probe crpg cnrs nancy moreover fluid inclusions present in the quartz veins are considered to determine the origin and conditions of the fluid circulation three types of primary fluid inclusions carbonic lc aqueous carbonic fluids lc w and aqueous fluid lw have been observed the temperatures of this fluid circulation were determined by microthermometry and using quartz carbonate pair thermometry at between 200 c and 300 c fluid pressures range from 100 mpa to 30 mpa which correspond to depths ranging from 4 to 1 km this evolution could be related to an exhumation associated with a dilution and cooling process this fluid circulation could be associated to the gold bearing fluid responsible of the sheba fairview ore system dated at 3074 6 ma the results of the isotopic study show that these fluid circulations homogenised the oxygen isotope composition but not the entire silicon isotope composition of the microquartz the 18 o value calculated for the fluid show that it equilibrated with carbonate rocks and is similar to the composition of the gold bearing fluid the silicon isotope composition of the microquartz allows determination of the initial 30 si of the microquartz precursor this precursor has been precipitated from a hydrothermal fluid hence a combined 18 o 30 si and fluid inclusion study allows the conditions of the fluid circulation to be constrained and the origins of chert to be determined

behavior of gold in the lateritic equatorial environment weathering and surface dispersion of residual gold particles at dondo mobi gabon abstract a supergene gold dispersion halo has developed at dondo mobi southern gabon from the weathering of an auriferous lisvenite under equatorial humid rain forest conditions morphological and chemical studies were performed on gold particles extracted from both fresh and weathered rocks from different parts of the dispersion halo the results demonstrate that the gold particles are residual and are subjected to increasing weathering upward through the profile from the protore to the surface in the central part of the halo and at the surface both laterally upslope and downslope weathering continues downward into the formerly barren saprolites which were derived from proterozoic schist and archean gneiss rim of the halo thus the au content progressively decreases as does the ag content of gold particles with distance from the center of the ore body gold particles subjected to increased weathering are smaller with more rounded shapes and surfaces pitted by dissolution the dispersion pattern is controlled by au dissolution and translocation of residual gold particles these processes involve an autochthonous evolution in the central part of the halo and a para autochthonous evolution at the rim of the halo at dondo mobi chloride and humic substances are the only ligands present in significant concentrations that are capable of complexing au thermodynamic calculations demonstrate that under such acidic conditions ph ranges from 4 to 6 gold stability increases with increasing silver content the dominant au complexes formed have been calculated to be an aurous hydroxichloride complex and an organometallic complex both of which are stable at surficial conditions thus both chemical and translocation processes control the mobility of au and gold particles in equatorial lateritic weathering profiles

crystal features of supergene gold at hannan south western australia deep weathering of basalt hosted sulphidic gold mineralisation has resulted in remobilisation of the gold and the formation of a supergene deposit the deposit occurs beneath an ephemeral lake system and is at least partially saturated by highly saline groundwater a general downward movement of an iron redox front associated with the weathering has controlled the distribution of the gold and its morphology it is unusual in that the high fineness gold crystals formed are coarse and well preserved small octahedra up to 50 m are the most abundant form of gold crystals but large up to 3 mm octahedral plates are common crystal morphology variations described here suggest that the gold is initially precipitated as euhedral octahedra and plates subsequent dissolution and recrystallisation associated with fluctuations in the chemical environment above the redox front has led to the development of the more common irregular dendritic or wire gold forms observed in other supergene deposits

ordovician k bentonites issues in interpreting and correlating ancient tephras abstract prominent upper ordovician k bentonites include the deicke and millbrig beds in the late ordovician of eastern north america and the kinnekulle k bentonite in the uppermost sandbian of baltoscandia these beds are thought to represent some of the largest ash fall deposits known in the phanerozoic record this report presents a comprehensive study of biotite compositions from the three beds covering a far more extensive geographic range than has previously been described these data show that the kinnekulle and millbrig are multiple event ash beds some parts of which are indistinguishable from one another tectonomagmatic discrimination diagrams combined with mg number data indicate that the deicke millbrig kinnekulle sequence represents the progressive transformation from calc alkaline to peraluminous magmatic sources consistent with a model of progressively evolving magmatism during the closure of the iapetus ocean it is concluded that the millbrig and kinnekulle beds are coeval and represent primarily simultaneous episodes of explosive volcanism although it cannot be excluded that some portions of both beds were produced by the same event

alteration characteristics of the archean golden grove formation at the gossan hill deposit western australia induration as a focusing mechanism for mineralizing hydrothermal fluids the archean golden grove formation is a 550 m thick rhyodacitic tuffaceous volcaniclastic succession that hosts the cu zn rich gossan hill volcanic hosted massive sulfide vhms deposit in the yilgarn craton western australia the golden grove formation consists of volcanic quartz and altered pumice and shards which were deposited during successive episodes of subaqueous mass flow coherent volcanics are absent from the golden grove formation at gossan hill but form the main rock type in the hanging wall scuddles formation a massive dacite dome that overlies its volcanic feeder dike is inferred to occupy a synvolcanic structure that focused mineralizing fluids during the formation of the gossan hill deposit the gossan hill deposit consists of two stratigraphically separate ore zones interconnected by stockwork the cu rich lower ore zone and the zn rich upper ore zone occur in the middle and upper parts of the golden grove formation respectively podiform zones of massive magnetite occur in the cu rich ore zone where the formation of magnetite predates massive sulfide the asymmetry of massive sulfide massive magnetite and alteration zones at the gossan hill deposit attest to synvolcanic structural control during mineralization the principal lithofacies of the golden grove formation are sandstone and pebble breccia which have a regionally extensive quartz fe rich chlorite and lesser muscovite alteration at gossan hill this alteration has resulted in near complete replacement of tuffaceous components causing substantial chemical modification of the primary lithologies quartz chlorite muscovite alteration is characterized by severe k2o na2o and cao depletion with rocks consisting principally of sio2 feo al2o3 and mgo along a quartz chlorite mixing trend widespread preservation of pumice and shard volcanic textures within the golden grove formation indicates that quartz chlorite muscovite alteration occurred soon after or possibly during sedimentation the absence of diagenetic compaction textures further suggests induration of the succession during this early alteration stage with the tuffaceous succession largely sealed from the texturally destructive effects of subsequent hydrothermal alteration except where mineralizing fluids were locally channeled along synvolcanic feeder conduits local intense hydrothermal alteration zones surround the gossan hill deposit and overprint earlier quartz chlorite muscovite alteration these local alteration zones have the same extent as the sulfide vein envelope and represent hydrothermal alteration formed during sulfide magnetite mineralization intense fe rich chlorite ankerite siderite alteration occurs as a strata bound envelope around massive magnetite cu rich veins and massive sulfide in the lower ore zone this chlorite rich alteration has strong feo and mgo enrichment with minor chloritoid and andalusite that reflect intense acid leaching during hydrothermal alteration iron chlorite ankerite siderite alteration grades upward into discordant to strata bound intense quartz alteration intense quartz alteration forms an envelope around zn rich veins and massive sulfide in the stockwork and upper ore zone the trend from fe chlorite ankerite siderite to quartz alteration toward the top of the deposit is consistent with the cooling of hydrothermal mineralizing fluids nearing the sea floor rhyodacite and dacite volcanics of the hanging wall scuddles formation have a pervasive muscovite calcite alteration muscovite calcite alteration led to na2o depletion and cao and k2o enrichment associated with burial of the gossan hill mineralizing system we propose that the gossan hill sulfide magnetite vhms deposit formed during an evolving archean hydrothermal system that began as part of a regional scale low temperature seawater convection alteration system initially this system caused extensive replacement of the golden grove formation by quartz and fe chlorite muscovite a process that sealed and indurated the volcaniclastic rocks by infilling of primary porosity and permeability structures due to subsequent impermeability of the host rock succession later and hotter mineralizing fluids that generated alteration and massive magnetite and sulfide at the gossan hill deposit were constrained to and focused upward along a synvolcanic feeder structure

paleoceanographic modeling of temperature salinity profiles from stable isotopic data temperature salinity t s profiles in ancient seawater can be estimated from oxygen isotopic compositions of well preserved calcitic fossils sampled from known paleodepth gradients the proposed model uses isotopic compositions of fossils to determine possible combinations of temperature and salinity during deposition given estimates of average salinity isotopic composition and net global hydrologic fractionation for ancient oceans increasing density with depth constrains those combinations to narrow ranges this graphic method superposes curves for density and isotopic composition in t s space to reveal possible ancient t s profiles application of this method to oxygen isotope data from ordovician brachiopods deposited on the eastern continental slope of north america demonstrates its usefulness and yields a stable seawater column in which brackish intermediate waters overlie warmer more saline deep waters

oxygen isotope analysis of the eyes of pelagic trilobites testing the application of sea temperature proxies for the ordovician the oxygen isotope composition of well preserved trilobite eye calcite retaining its original optical properties represents a possible source of information on paleozoic sea temperatures species of the epipelagic telephinid genera carolinites and opipeuterella from strata of early to middle ordovician age in spitsbergen and australia were analyzed and compared with benthic asaphid species scanning electron microscope sem cathodoluminescence cl electron microprobe and electron backscatter diffraction ebsd techniques were used to assess eye preservation prior to isotope analysis some apparently well preserved eyes are identified from the valhallfonna spitsbergen and emanuel australia formations the eyes show a wide variation in 18o values 6 2 to 9 8 for the valhallfonna formation 3 2 to 10 4 for the emanuel formation and 3 6 to 7 4 for the horn valley siltstone australia intra eye secondary ion mass spectrometry sims isotope results reveal an even larger range in 18o in some specimens 18o of 2 4 to 10 4 suggesting that the trilobite eyes have undergone cryptic recrystallization a sub set of trilobite cuticle from the three formations were analyzed for their carbonate clumped isotope compositions 47 and yielded crystallization temperatures above 50 c consistent with diagenetic alteration the sims and 47 results suggest that classic preservation assessment protocols for the stable isotope study of deep time carbonate samples may be insufficient especially for these techniques there is a need for extensive microstructural characterization of lower paleozoic biogenic carbonates by techniques including ebsd sims and 47 before their stable isotope signatures can be used with certainty in paleoclimate studies

micrometer scale chemical and isotopic criteria o and si on the origin and history of precambrian cherts implications for paleo temperature reconstructions abstract oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperature during the precambrian these reconstructions have been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation existing criteria used to assess the pristine sedimentary origin of a chert are based on petrography criterion 1 chert is composed mostly of microquartz on the bulk oxygen isotopic composition criterion 2 bulk 18 o has to be close enough to the maximum 18 o value previously measured in other cherts of the same age and on the presence of a large 18 o range at the micrometer scale criterion 3 18 o range of 10 at 2 m however these criteria remain incomplete in determining precisely the origin and degree of preservation of ancient cherts we report in situ si and o isotope compositions and trace element concentrations in seven chert samples ranging from 1 88 to 3 5 ga in age correlations between 30 si and al 2 o 3 and k 2 o tio 2 reveal that microquartz is of three different origins i e diagenetic hydrothermal or silicification moreover chert samples composed mostly of diagenetic microquartz show a large range of 30 si at the micrometer scale 1 7 4 5 consistent with the large range of 18 o previously found in the gunflint diagenetic cherts we propose two further quantitative criteria to assess the origin state of preservation and diagenetic history of cherts criterion 4 uses trace element concentrations coupled with 30 si to ascribe the origin of cherts among three possible end members diagenetic hydrothermal and silicified criterion 5 is the presence of a large range of 30 si in pure diagenetic microquartz in the seven samples analyzed in this study only one from the gunflint iron formation at 1 88 ga passes all the criteria assessed here and can be used for paleo temperature reconstructions the criteria outlined here will help guide to the identification of pristine diagenetic cherts and or to better constrain seawater paleo temperature reconstructions by taking into account the effect of diagenesis

the influence of co2 on the solubility of quartz in single phase hydrothermal fluids implications for the formation of stockwork veins in porphyry copper deposits

fibrous calcite from the ordovician of tennessee preservation of marine oxygen isotopic composition and its implications three categories of fibrous calcite from early to middle caradoc platform marginal buildups in east tennessee can be delineated using cathodoluminescent microscopy minor element chemistry and stable c o isotopic composition bright luminescent fibrous cement has elevated mn 1000 p p m negative 13c and intermediate 18o values relative to other types of fibrous calcite this cement reflects fibrous calcite that interacted with reducing mn rich fluids dully luminescent fibrous cement has elevated fe 400 p p m positive 13c and negative 18o values relative to other fibrous cements this cement was stabilized by burial fluids nonluminescent fibrous cement has low mn and fe generally below 400 p p m and positive 13c and 18o values relative to other types of fibrous calcite the latter cement is interpreted to be the best material for determining the isotopic composition of calcite precipitated in equilibrium with early to middle caradoc seawater which is 13c 1 pdb and 18o 4 to 5 pdb results from this study and ashgillian brachiopods indicate that the average 18o composition of the ordovician ocean during nonglacial periods was probably never more negative than 3 smow assuming an ordovician seawater 18o value of 1 smow holston formation fibrous cements would have precipitated at temperatures between 27 and 36 c which is near the upper temperature limit for metazoans a seawater 18o value of 2 smow yields temperatures ranging from 23 to 31 c while a 3 smow value yields temperatures of 18 26 c

supergene gold in silcrete as a vector to the scuddles volcanic massive sulfides western australia this study reports data on the first occurrence of economic supergene au in a near surface silcrete horizon over the scuddles volcanic massive sulfide vms deposit in the golden grove district western australia a deep weathering profile was developed on dacite rhyodacite siltstone and breccia that host primary cu zn and pb sulfides with au ag ore from the base the weathering profile at scuddles is subdivided into five main zones 1 supergene sulfide enrichment zone 2 supergene oxide enrichment zone 3 ferruginous saprolite 4 leached zone of kaolinitic saprolite and silcrete and 5 lateritic zone of mottled clays ferruginous duricrust and gravels silcrete at scuddles hosts supergene au deposit that formed in two generations the first is intimately associated with ag halides during supergene enrichment of the primary vms and the second is associated with kaolinite in dissolution cavities inside ag halides during lateritic weathering these two au generations imply more than one mechanism of au remobilization and formation multiple fluid pathways and superimposed episodes of weathering under variable timing and climatic conditions gold grains are pure nanocrystalline up to 10 nm and clustered together forming microcrystalline aggregates a few au grains are residual in silcrete with ag sb rich cores and ag poor rims possibly formed during dealloying of ag and sb chemically au in silcrete is associated with a multi element concentration of ag i br cl sb sn bi hg mo w te and ge gold and ag in the supergene weathering profile were mobilized to silcrete as a halide complex under acidic and saline conditions generated during the oxidation of massive sulfides at depth the precipitation of au ag halides in the silcrete may have taken place in response to a rise in ph gold was likely remobilized with kaolinite from the surface lateritic zone facilitated by decays of plant roots and bioturbation the clustered spongy cube octahedral platy six sided dendritic and reniform like morphologies of au in cavities inside ag halides may indicate biogenic related processes in its precipitation recognizing au ag rich silcrete over the buried vms at scuddles highlights the significance of the silcrete in finding buried vms particularly if the gossan is absent

geochemistry of late ordovician viola limestone oklahoma implications for marine carbonate mineralogy and isotopic compositions late ordovician caradocian age viola limestones from the arbuckle mountains oklahoma have sr concentrations from 216 ppm to 3720 ppm average 1600 ppm 18o values from 5 1 to 3 1 pdb and 87sr 86sr ratios from 0 70775 to 0 70793 although textural and chemical evidence indicates that these limestones suffered diagenetic alteration the elemental and isotopic compositions still reveal important information about the primary marine mineralogy and seawater composition during late ordovician time first the high sr concentrations suggest that original viola sediments were composed mainly of aragonite second the highest 18o value 3 1 of the limestones which is interpreted to be the least altered constrains the 18o values of seawater to l smow assuming that ocean temperatures were 27 5 c third the 87sr 86sr ratios of viola limestones probably better represent the 87sr 86sr ratios of coeval seawater because the ratios from these sr rich limestones show less scatter than those

biogenic chert and the ordovician silica cycle abstract analysis of 161 nodular and bedded cherts shows that shallow water biosiliceous facies which were common in the lower ordovician had mostly retreated from peritidal lagoonal facies to deeper settings by the middle ordovician for decades this retreat was thought to have spanned the cambrian and ordovician periods our data suggest that the basinward shift may have taken approximately 2 14 m y in the late lower to early middle ordovician a likely cause for the departure of these often siliceous sponge cherts from shallow waters was depletion of shallow water dissolved silica dsi increased use of dsi by radiolarians may have commenced in the middle ordovician when radiolarians increased just as a pulse of radiolarite deposition began siliceous sponges may have been forced to relocate to deeper waters because radiolarians depleted shallow water dsi apparent low relative sea level in the middle ordovician may have hampered the formation and or preservation of shallow water cherts by reducing available shelf area to host such deposits however the resurgence of shelf chert in the upper ordovician while peritidal cherts remained virtually absent suggests that biology was critical to driving the basinward shift of cherty facies formation and preservation of abundant latest darriwilian through sandbian cherts may have been favored in deep water settings during a possible warming interval marked by sea level rise and extensive anoxia and euxinia that coincided with an ongoing decline in 87sr 86sr ratios and increased sea floor volcanic activity that probably enriched the dsi content of deep ocean waters ocean circulation associated with the onset of cooling in the katian may have optimized chert accumulation by stimulating surface water biosiliceous productivity before dsi rich chert preserving anoxic basins were ventilated as late ordovician glaciation intensified deep ocean ventilation by thermohaline sinking of oxygen rich waters probably decreased preservation potential of biosiliceous deposits while increasing oceanic recycling of silica

pure siliceous ooze a diagenetic environment for early chert formation the formation of marine opal ct nodules or layers as early diagenetic deposits has been documented only in antarctic deep sea sediments in contrast porcellanites and cherts in land sections and deep sea drilling project and ocean drilling program drill sites are usually found in sediment sections of miocene age and older during r v polarstem cruises ant ix 3 and 4 young porcellanites were recovered for the first time in contact with their host sediment in two cores from the atlantic sector of the southern ocean chemical and mineralogical studies of these deposits and their surrounding sediments have increased knowledge about very early chert formation in both cores the porcellanites are embedded in sediments rich in opal a with extremely low levels of detrital minerals an environment that seems conducive to a rapid transformation of biogenic silica into porcellanites

the silicon and oxygen isotope compositions of precambrian cherts a record of oceanic paleo temperatures oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperatures during the precambrian during the past decade the advance of in situ analysis of si isotopes has enhanced the interest on cherts as a paleo environmental proxy the coupled o and si isotope composition variations show secular and correlated trends that have been interpreted as a progressive cooling of the ocean however this reconstruction has been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation in this case the secular oxygen and silicon isotope variation are considered as reflecting a mixing between seawater and hydrothermal sources a key point in this discussion deals with the origin of cherts sedimentary hydrothermal or chemically silicified therefore several petrographical and geochemical criteria are proposed to recognize the pristine sedimentary origin of a chert namely they are 1 the occurrence of microquartz 2 a 18o rich bulk oxygen isotopic composition 3 the occurrence of large 18o ranges at a micrometer scale 4 variation of trace element compositions coupled with 30si 5 the occurrence of large ranges of 30si in pure microquartz these criteria should be regarded as guides to the identification of pristine diagenetic cherts in order to better constrain seawater paleo temperature reconstructions by taking into account the effect of diagenesis this article will review the different interpretations about o and si isotope variation and propose a model of formation based on ancient and modern chert studies

a first look at the distribution of the stable isotopes of silicon in natural waters the first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported both marine and riverine 30si are more positive than 30si of igneous rocks suggesting isotopic fractionation during weathering and clay formation and or biomineralization the 30si value for dissolved silicic acid from several ocean basins is 1 1 0 3 n 69 more positive values are found in surface waters due to discrimination against 30si during biogenic opal formation by diatoms the deep pacific ocean is depleted in 30si relative to the deep atlantic by roughly 0 4 30si of riverine dissolved silicon averages 0 8 0 3 n 8 the biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea the average 30si value of inputs of silicon to the ocean most likely falls between 0 3 and 0 9 and that of the outputs falls between 0 9 to 1 9 probably in the neighborhood of 1 0

coordinated biotic and abiotic change during the great ordovician biodiversification event darriwilian assembly of early paleozoic building blocks abstract the ordovician period records an extraordinary biodiversity increase known as the great ordovician biodiversification event gobe which coincided with a series of environmental changes to the earth system notably a cooling global ocean increased oxygenation and increased nutrient supply from volcanism and continental weathering the co evolution of earth and its biota during this interval has been studied in various contexts on multiple paleocontinents emerging patterns depend on the lens of investigation here we summarize the current state of understanding by reviewing and synthesizing the fossil and sedimentary records recent paleontological studies mainly focused on rhynchonelliform articulated brachiopods bryozoa cephalopods trilobites graptolites echinoderms and reef organisms have documented details of diversification body size increase development of ecosystem complexity and intensification of inter continental dispersal from the late cambrian through late ordovician biomass increased markedly between the early and middle ordovician furthermore diversification rates increase statistically during the mid darriwilian age both globally and regionally coincident with these biotic changes geochemical proxies record significant changes to earth s physical system oceanic temperatures decreased and atmospheric oxygen levels increased to near modern levels in concert with the middle ordovician diversification of shelly fauna anoxic pulses ceased and evidence for deep ocean ventilation prevails in middle ordovician strata furthermore a major middle late ordovician change in oceanic strontium isotopic composition indicates increased weathering of juvenile volcanic rocks and delivery of nutrients to marine settings this multi proxy dataset records near simultaneous changes in fossil rich shallow marine environments during exactly the interval of greatest diversification by integrating biotic and geochemical datasets a clear picture of the co evolution of earth and its biota emerges indicating that the darriwilian was the critical interval facilitating increased capacity of ecosystems we therefore recommend restricting the term gobe to indicate this short interval of rapid diversification and ecosystem change and using ordovician radiation when referencing the sum of diversifications that occurred throughout the ordovician period

supergene features and evolution of gossans capping massive sulphide deposits in the iberian pyrite belt abstract twelve massive sulphide deposits from the iberian pyrite belt ipb show well preserved iron caps some of which were mined during the last century to recover precious metals e g tharsis rio tinto san miguel field observations and correlation assays between the distinct mineral sequences at different deposits suggest that all the gossans were developed under similar conditions and have undergone the same geological events all the gossans have a mushroom like morphology in sharp contact with the underlying massive sulphide orebodies in most cases these are located over an apparent supergene enrichment zone rich in secondary sulphides some gossans extend into tongues of alluvial heterolithic breccias consisting of eroded transported gossans displaced as far as several hundred meters away from their sources the distribution of major minerals throughout the gossan profiles goethite hematite quartz and jarosite and the statistical analysis of the geochemical data distinguish three separate zones with gradual contacts roughly parallel to the current topography 1 the lower zone dominated by goethite and subordinate jarosite with significant enrichment in s as p pb sn sb ag and au 2 the middle or principal zone dominated by goethite and lacking jarosite which is depleted in s and as as well as heavy and precious metals and 3 the upper zone near the surface mainly composed of hematite and quartz with only weak anomalies in p pb and sn the origin and variations occurred in the profiles are explained by a three stage process this involves an initial acidic stage of gossan development centred on the oxidation of sulphides that lead to the formation of the first fe rich oxyhydroxides and sulphates mainly goethite and jarosite respectively over time a progressive stage of maturity is reached progressively downwards through the gossan profile due to the intensification of the oxidation and leaching processes the ongoing gossan formation produced alteration and reprecipitation of pre existing oxyhydroxides the loss of the majority of the previously sorbed heavy metals and a major dilution of trace elements especially in the zones near the surface the main results of this stage of formation are the production of heavy metal depleted oxyhydroxides most commonly goethite and hematite and the disappearance of jarosite subsequently local uplift of the gossanous rocks by neotectonic movements facilitated the rejuvenation of the oxidation of the ores this final stage complicated the previously developed zonation with the formation of jarosite in mature areas possible major breaks in this gossan development ocurred in messinian times 7 8 ma and at the beginning of the early quaternary 1 2 ma

natural gold nanoparticles abstract the nanoparticulate gold component in ore deposits and exploration samples is yet to be fully realised but new methods of characterisation and analysis access to high grade gold samples and a strong focus on au nanoparticles in the manufacturing sector provide a new impetus to quantify its significance in geology nanoparticles are increasingly recognised as a fundamental step in geochemical reactions a critical component in weathering processes in biomineralisation and metal migration in ore systems the nanoparticulate population of au is a significant factor in the formation of economic deposits as a refractory component locked up in sulphides in secondary supergene enrichments the formation of surface geochemical anomalies and in the formation of high grade accumulations a better understanding and characterisation of this nanoparticulate gold could unlock hidden resources in known deposits through easier beneficiation and processing but also assist in the discovery of new deposits through advanced exploration methods

preservation of premetamorphic oxygen isotope ratios in granitic orthogneiss from the adirondack mountains new york usa the adirondack mountains new york expose a diverse group of proterozoic igneous rocks that were metamorphosed to granulite facies conditions during the ottawan phase of the grenville orogeny oxygen isotope data for seventy whole rock samples of gabbroic to granitic meta igneous rocks primarily from the charnockite suites from the tupper and saranac sheets in the central adirondacks demonstrate a correlation between 18 o wr and major element composition within continuous mappable meta plutonic units no such relationship is seen among nonconsanguineous granitoids variations in mineral 18 o values and large differences in 18 o between rocks with nonconsanguineous protoliths but similar bulk composition demonstrate that these rocks were not infiltrated by or isotopically equilibrated through a pervasive metamorphic fluid values of 18 o for mineral separates preserve generally high temperature fractionations indicative of dominantly closed system retrograde exchange values of 18 o wr in adirondack orthogneisses were not significantly shifted during granulite facies metamorphism and were dominantly controlled by processes active during premetamorphic magmatism values of 18 o wr may thus serve as a petrogenetic indicator allow discrimination of rock units and serve as a source constraint for meta igneous rocks fayalite meta granites with low 18 o values can be discriminated from surrounding granitoids having high 18 o values a record of assimilation in the evolution of differentiated granitic units is preserved the preservation of primary igneous 18 o values in granulite facies orthogneiss imposes constraints on the synmetamorphic and postmetamorphic fluid history of the adirondack highlands oxygen isotopic compositions at the hand sample scale have been preserved through granulite facies metamorphism fluid absence or low fluid rock ratios on a regional scale are indicated

the las cruces deposit iberian pyrite belt spain abstract the las cruces deposit is located at the eastern margin of the iberian pyrite belt ipb beneath the neogene quaternary deposits of the guadalquivir basin these sediments covered and preserved the supergene profile somewhat after its generation until its recent discovering for this reason the las cruces ore deposits together with lagoa salgada in portugal are the only known deposits in the ipb that preserve the entire weathering profile including both the gossan and the enrichment zone in that sense the present study provides new data on the ipb ore deposits and their post palaeozoic evolution the primary mineralization consists of massive and semi massive polymetallic sulfides overlaying a cupriferous and pyritic stockwork the supergene enrichment extends from the primary zone to gossan and consists of a thick cementation zone characterized by intense replacement of part of the primary sulfides by cu rich supergene sulfides at present the only economic resource of las cruces is the cementation zone with initial reserves of 17 6 mt 6 2 cu but the orebody also includes significant contents of cu and zn within the primary mineralization and au ag and pb in the gossan mineralogical and geochemical data confirm that the primary mineralization at las cruces resembles other ipb deposits in terms of distribution and evolution however the gossan and cementation zone show an uncommon evolution for this kind of supergene profile the major singularity that makes the las cruces supergene mineralisation noticeably different from others in the ipb or elsewhere resides in its unusual mineralogical composition mostly comprised of newly formed siderite calcite fe sulfides and galena the genetic model proposed for the las cruces deposit includes three main stages 1 ore genesis and evolution of primary mineralization which seems to be comparable to other ipb deposits 2 genesis of cu rich secondary mineralization and gossan by weathering after miocene exhumation of the upper part of the massive sulfide deposit under oxidizing and acidic conditions and 3 late evolution of supergene profile below the carbonated rich sediments of the guadalquivir basin with the subsequent change in the redox conditions due to water rock interaction between the supergene profile and basinal fluids

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

oxygen and carbon isotopic composition of ordovician brachiopods implications for coeval seawater one hundred and two articulate brachiopods and twenty one marine cements from ordovician strata were analyzed for oxygen and carbon isotopes these together with 200 previously published analyses of ordovician brachiopods mostly of upper ordovician age define a general trend of increasing 18o values with decreasing age from a minimum of 10 5 in the earliest ordovician to a maximum of 1 5 at the ordovician silurian boundary the 13c values show a general positive correlation with 18o and increase from 2 5 in the earliest ordovician to as high as 7 at the ordovician silurian boundary the magnitudes of ordovician excursions for both 18o and 13c are similar to those reported previously for the entire phanerozoic different genera of brachiopods from the same stratigraphie level did not yield any large differences in their isotopic compositions suggesting that vital effects did not influence decisively the isotope values of the studied skeletal parts petrographic examination and trace element analyses indicate that brachiopod shells from llandeilo to ashgill strata are well preserved and thus may have retained their primary isotopic signatures the same criteria for the tremadoc arenig and some llanvirn brachiopods suggest partial diagenetic alteration of 18o values in these samples but this does not preclude their utility as a recorder of a primary 13c signal the observed 18o trend for the well preserved portion of the record may reflect progressive cooling during ordovician perhaps complemented by a changing 18o values of seawater the latter could have been a consequence of a net increase in the rate of high temperature water rock interaction relative to its low temperature counterpart as indicated also by the large coeval decrease of marine 87sr86sr superimposed on the above long term 18o trend is a positive excursion of about 2 at the end of the ordovician probably reflecting a large expansion of polar ice caps the magnitude of this excursion indicates a degree of glaciation comparable to that at the height of the quaternary glacial episodes the secular 13c trend may reflect a progressive increase in marine organic productivity and or enhanced organic deposition in the ordovician oceans particularly noticeable at the time of the terminal ordovician glaciation

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

iodine rich waters involved in supergene enrichment of the mantos de la luna argentiferous copper deposit atacama desert chile recent studies have suggested the involvement of highly saline deep formation waters that modified preexisting cu assemblages to form atacamite during supergene oxidation of cu deposits in the atacama region in this report we document the occurrence of ag i inclusions hosted by supergene chalcocite from mantos de la luna an argentiferous upper jurassic stratabound cu deposit in the coastal range of northern chile the presence of this unusual mineral assemblage indicates that iodargyrite precipitated from reducing iodine rich waters suggesting that the fluids involved in supergene enrichment of cu deposits in the coastal range were more complex than previously thought this suggests the prevalence of hyperarid conditions during the latest stages of supergene enrichment of the mantos de la luna cu deposit in the atacama region supporting the notion that supergene enrichment processes in hyperarid areas are dynamic in nature and do not exclusively require the presence of meteoric water

paleozoic and mesozoic silica rich seawater evidence from hematitic chert jasper deposits laterally extensive beds of highly siliceous hematitic chert jasper are associated with many volcanogenic massive sulfide vms deposits of late cambrian to early cretaceous age yet are unknown in analogous younger including modern settings textural studies suggest that vms related jaspers in the ordovician lokken ophiolite of norway were originally deposited as si and fe rich gels that precipitated from hydrothermal plumes as colloidal silica and iron oxyhydroxide particles rare earth element patterns and ge si ratios of the jaspers reflect precipitation from plumes having seawater dilution factors of 10 3 to 10 4 similar to modern examples we propose that silica in the ancient jaspers is not derived from submarine hydrothermal fluids as suggested by previous workers but instead was deposited from silica rich seawater flocculation and precipitation of the silica were triggered inorganically by the bridging effect of positively charged iron oxyhydroxides in the hydrothermal plume a model involving amorphous silica opal a precursors to the jaspers suggests that silica contents of cambrian early cretaceous oceans were at least 110 mg l sio 2 compared to values of 40 60 mg l sio 2 estimated in other studies the evolution of ancient silica rich to modern fe rich precipitates in submarine hydrothermal plumes reflects a changeover from silica saturated to silica depleted seawater through phanerozoic time due mainly to ocean wide emergence of diatoms in the cretaceous

another continental pool in the terrestrial silicon cycle silicon is the second most abundant element on earth and is involved in important terrestrial and marine biogeochemical processes however its terrestrial cycle is not well quantified and there is a missing reservoir in the budget of silicon isotopes water in seas and rivers is enriched in silicon 30 compared with igneous rocks suggesting that there is an unknown continental reservoir that is depleted in silicon 30 now with the help of a new generation mass spectrometer basile doelsch et al have obtained in situ measurements of silicon isotopes in quartz from sandstones in southern france the results showing greater silicon 30 depletion than in any previously reported terrestrial samples suggest that silicified rocks formed from quartz precipitates in soils and groundwaters can account for the missing reservoir silicon is the second most abundant element on earth it is an important nutrient for phytoplankton1 and is readily absorbed by terrestrial vegetation2 it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates3 but the continental cycle of silicon is not well known and only a few studies have attempted to use silicon stable isotopes 28si 29si and 30si 4 5 6 7 8 9 10 11 12 13 to quantify the continental silicon reservoirs dissolved silicon in sea and river waters forms a reservoir of mean isotopic value 1 1 refs 7 10 it is enriched in 30si with respect to the igneous rocks reservoir which has a mean isotopic value of 0 3 refs 4 9 this enrichment can only be produced by a major fractionation during weathering and should result in the formation of a continental 30si depleted reservoir such a reservoir however has not been identified to date here we analyse silicon isotopes of in situ quartz from a sandstone series in france using a new generation secondary ion mass spectrometry apparatus we show that quartz that precipitates as siliceous cements forms a strongly 30si depleted reservoir with isotopic values down to 5 7 a more negative value than any previously published for terrestrial samples our findings suggest that quartz re precipitation plays an important role in the biogeochemical cycle of silicon

sims analyses of silicon and oxygen isotope ratios for quartz from archean and paleoproterozoic banded iron formations banded iron formations bifs are chemical marine sediments dominantly composed of alternating iron rich oxide carbonate sulfide and silicon rich chert jasper layers isotope ratios of iron carbon and sulfur in bif iron bearing minerals are biosignatures that reflect microbial cycling for these elements in bifs while much attention has focused on iron banded iron formations are equally banded silica formations thus silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in bifs bifs are banded by definition and microlaminae or sub mm banding are characteristic of many bifs in situ microanalysis including secondary ion mass spectrometry is well suited for analyzing such small features in this study we used a cameca ims 1280 ion microprobe to obtain highly accurate 0 3 and spatially resolved 10 lm spot size analyses of silicon and oxygen isotope ratios for quartz from several well known bifs isua southwest greenland 3 8 ga hamersley group western australia 2 5 ga transvaal group south africa 2 5 ga and biwabik iron formation minnesota usa 1 9 ga values of d 18 o range from 7 9 to 27 5 and include the highest reported d 18 o values for bif quartz values of d 30 si have a range of 5 from 3 7 to 1 2 and extend to the lowest d 30 si values for precambrian cherts isua bif samples are homogeneous in d 18 o to 0 3 at mm to cm scale but are heterogeneous in d 30 si up to 3 similar to the range in d 30 si found in bifs that have not experienced high temperature metamorphism up to 300 c values of d 30 si for quartz are homogeneous to 0 3 in individual sub mm laminae but vary by up to 3 between multiple laminae over mm to cm of vertical banding the scale of exchange for si in quartz in bifs is thus limited to the size of microlaminae or less than 1 mm we interpret differences in d 30 si between microlaminae as preserved from primary deposition silicon in bif quartz is mostly of marine hydrothermal origin d 30 si 0 5 but silicon from continental weathering d 30 si 1 was an important source as early as 3 8 ga

silicon fluxes and isotope composition of direct groundwater discharge into the bay of bengal and the effect on the global ocean silicon isotope budget abstract this study presents silicon isotope data for rivers and groundwater from the bengal basin variations of si isotope ratios were analyzed by means of high resolution mc icp ms using a nuplasma hr the rivers show typical positive 30 si values between 1 3 and 1 7 whereas the groundwater samples show decreasing 30 si values from 1 3 in shallow groundwater to 0 2 in the deeper groundwater beside a very distinctive isotope composition the concentration of dissolved si in these groundwater samples is 2 3 times higher than in river samples taken during dry season the resulting si flux by groundwater 9 3 10 10 mol yr 1 is on the order of the combined ganges brahmaputra si fluxes and equals 40 of the total river groundwater annual si flux into the bay of bengal given the significant large flux and distinctive isotope composition means that the overall isotopic input into the ocean is different from riverine values however a sound knowledge of all inputs into the ocean and of how these inputs might vary throughout time is mandatory in order to correctly interpret palaeo records of 30 si variations as recorded in sedimentary diatom opal from the last glacial maximum we extrapolate our results from the bengal basin onto a global scale and assess the ocean s sensitivity to changes in inputs as triggered by large scale events such as glaciation periods where the hydrological cycle might be out of steady state due to the build up of large continental ice shields in such a glaciation scenario riverine vs groundwater inputs can be shifted favoring isotopically lighter groundwater over heavier river inputs into the ocean the model proves impossible to change the biogenic output to a significant degree on time scales of a few thousand years by just changing the isotope inputs into the ocean

micro scale tracing of fe and si isotope signatures in banded iron formation using femtosecond laser ablation we have detected micrometre scale differences in fe and si stable isotope ratios between coexisting minerals and between layers of banded iron formation bif using an uv femtosecond laser ablation system connected to a mc icp ms in the magnetite carbonate chert bif from the archean old wanderer formation in the shurugwi greenstone belt zimbabwe magnetite shows neither intra nor inter layer trends giving overall uniform d 56 fe values of 0 9 but exhibits intra crystal zonation bulk iron carbonates are also relatively uniform at near zero values however their individual d 56 fe value is highly composition dependent both siderite and ankerite and mixtures between both are present and d 56 fe end member values are 0 4 for siderite and 0 7 for ankerite the data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by fe iii reducing bacteria or more likely an abiotic reaction of organic carbon and fe iii during low grade metamorphism si isotope composition of the old wanderer bif also shows significant variations with d 30 si values that range between 1 0 and 2 6 for bulk layers these isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal rich waters interestingly fe and si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures moreover the changes of fe and si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal rich water which govern the rates of fe and si precipitation and therefore also the development of layering during periods of low hydrothermal activity precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate chert layers during periods of intensive hydrothermal activity precipitation rates of ferric oxyhydroxide were high and subsequent diagenesis triggered only partial reduction forming magnetite carbonate chert layers we are confident that our micro analytical technique is able to detect both the solute flux history into the sedimentary bif precursor and the bif s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors

atacamite in the oxide zone of copper deposits in northern chile involvement of deep formation waters atacamite a copper hydroxychloride is an important constituent of supergene oxide zones of copper deposits in northern chile whereas in similar deposits elsewhere it is rare in chile it has generally been assumed to be a primary constituent of the supergene zones there are two difficulties with this supposition the first is that atacamite requires saline water for its formation whereas supergene oxidation was caused by percolating oxygenated meteoric water mainly rainwater the second is that atacamite dissolves rapidly or undergoes phase change when exposed to fresh water supergene enrichment of copper deposits in northern chile extended over a long period 44 to 9 ma being terminated by the onset of hyperaridity during this period there was at least intermittent rainfall exposing previously formed atacamite to dissolution or phase change furthermore atacamite bearing oxide zones in several deposits are directly overlain by thick miocene alluvial gravels the stream waters that transported these gravels would have permeated the oxide zones in some deposits atacamite bearing assemblages occur both in the oxide zones and in contiguous gravels we suggest that atacamite bearing oxide assemblages are more likely to have been a replacement of preexisting oxide phases after the onset of hyperaridity at about 9 ma a hyperarid climate made possible evaporation and concentration of chloride in meteoric waters in this paper we discuss another source of saline waters to modify oxide zones dewatering of the domeyko basin expelled brines along faults some of which had earlier guided the location of porphyry deposits at the spence porphyry copper deposit saline waters which d vs 18o isotope analyses identify as basinal brines are presently rising through the deposit then flowing away along the base of the covering gravels compositions of these waters lie within the stability fields of atacamite and brochantite the two minerals that comprise the oxide zone evidence is presented for other porphyry deposits radomiro tomic and gaby sur that basinal brines may have been involved in the late formation of atacamite

a petrographic and isotopic criterion of the state of preservation of precambrian cherts based on the characterization of the quartz veins the coupled o and si isotope variations of precambrian cherts show secular and correlated variations that have been interpreted as a progressive cooling of the oceans on earth however this reconstruction has been challenged because cherts can have various origins hydrothermal sedimentary volcanic silicification and their isotopic compositions might have been reset by metamorphic fluid circulation even pristine samples show evidence of fluid circulation by the presence of various quartz veins here we have determined the effect of fluid circulation on the isotopic composition of the microquartz chert matrix by studying the mineralogical relationships between quartz veins and microquartz in two distinctive chert samples these samples show different quartz habit and texture in their quartz veins which imply various fluid system conditions therefore the presence of quartz spherulites in the quartz veins indicates a more moderate fluid circulation than the presence of columnar quartz these observations were also confirmed by the oxygen isotope compositions of both microquartz matrix chert and quartz veins we have demonstrated that in some conditions fluid circulations do not modify the oxygen isotope composition of the chert matrix and thus the microquartz can still record paleo environmental conditions the petrography of quartz vein associated with in situ oxygen isotope composition provides one criterion to characterize the state of preservation of the microquartz

very high purity gold form lateritic weathering profiles of nilambur southern india laterite profiles in the nilambur gold prospect of southern india contain dust grains and nuggets of supergene gold whereas chemical rounding and corrosion cavities characterize the gold grains in upper zones of intense weathering the lower moderately to slightly weathered horizons contain grains with jagged contours and less common etch pits scanning electron microscopy reveals a variety of incipient growth patterns of secondary gold including spongy filamental petalitic or dendritic patterns spectacular examples of painted gold with a bright luster that precipitated over dull primary grains are documented electron microprobe analyses of secondary gold grains show up to 99 93 au and exceedingly high fineness values i e 1000 au au ag ranging from 991 to 999 compared to the normal fineness of 950 reported for supergene gold from other terrains particle induced x ray emission analyses indicate that fe is the commonly associated trace metal thus linking supergene gold genesis in nilambur to a model involving ferrolysis whereby mobility of gold in weathering profiles was controlled by pyrite oxidation the nature of occurrence grain morphology textural parameters and unusual fineness of gold grains in nilambur laterites testify to distinct low temperature processes attending the chemical dissolution migration and reprecipitation of gold during tropical weathering

silicon isotope compositions of dissolved silicon and suspended matter in the yangtze river china abstract silicon isotope compositions of main channel samples of the yangtze river were systematically investigated along with their chemical compositions the concentration of suspended matter in the yangtze river tends to decrease from the upper reaches to the lower reaches corresponding to settling of the sediments in the lakes and reservoirs due to reduction of the velocity of water flow the silica contents of suspended matter vary from 52 1 to 56 9 and their 30si values vary from 0 to 0 7 both similar to those of shales from the upper to lower reaches the silica contents of suspended matter tend to increase whilst their 30si values tend to decrease both trends reflect the increase of clay minerals and decrease of carbonates in suspended matter the concentrations of dissolved silicon vary from 97 to 121 mol l and their 30si values vary over a wide range from 0 7 to 3 4 from the upper to lower reaches dissolved silica concentrations tend to decrease and their 30si values tend to increase these trends mainly reflect the change of chemical and isotopic characteristics of the tributaries from the upper to lower reaches the major factors responsible for these changes may be the high meteoric precipitation and significant silicon absorption by grass in wetlands and rice in paddy fields in drainage areas of the middle and lower reaches there is no correlation between 30si of dissolved silicon and that of suspended matter the 30sidiss spm values vary over a wide range of 1 0 3 7 indicating that 1 they are out of isotopic equilibrium 2 dissolved silicon and the associated suspended matter do not belong to one physico chemical system and 3 isotopic exchange rate between them is very slow the 30si value of dissolved silicon output from the yangtze river to the east sea is estimated to be 3 0 much higher than the values reported for the amazon and congo rivers this increases the 30si range of dissolved silicon in the world s rivers from 0 4 1 2 to 0 4 3 4

sensitivity of silicon isotopes to whole ocean changes in the silica cycle a 2 box model has been used to assess the impact of both long and short term budgetary imbalance in the silica cycle on the average silicon isotopic composition 30si of the ocean and marine sediments over a 100 ky time span such as a quaternary glacial cycle a sustained change in the riverine flux of silicon to the oceans could alter the average 30si of seawater and the average 30si of opal outputs by a few hundredths to a few tenths of permil this would be largely tied to a change in the 30si of silicon entering the ocean due to a shift in the proportion of riverine and non riverine sources of silicon a doubling of the riverine flux of silicon would have little impact on average marine 30si but a sustained halving of river inputs could interfere with use of 30si as a tracer of nutrient utilization studies on the longer term focussed on the transition from a high silicic acid to low silicic acid ocean associated with the rise of the diatoms this transition is marked by drop in the average 30si of seawater from greater than 1 9 down to about 0 8 the isotopic composition of diatom opal however has an isotopic composition that sticks close to the 0 8 of the inputs and is thus unlikely to provide information about the transition to the low silicic acid ocean of the modern day however the 30si of opal produced in the deep sea for example by sponges should document this transition

the development of regolith exploration geochemistry in the tropics and sub tropics abstract this essay traces the development of geochemical exploration from its early beginnings in the modern era during the 1930s concentrating especially in its application to deeply weathered terrain in the tropics and sub tropics following promising results obtained in temperate regions in north america and europe test orientation surveys were conducted to see whether similar procedures were applicable in the tropics where conventional geological prospecting was largely precluded due to the extensive cover of a deep lateritic regolith and consequent lack of outcrop after initial work in sierra leone and nigeria the emphasis transferred to east africa in the 1950s and 1960s aimed principally at cu exploration many of the basic principles for exploration in dominantly residual free draining terrain were quickly established in this period exploration in terrains with more complex weathering histories however raised a number of difficulties due to leaching and secondary concentrations of elements problems in selecting and identifying appropriate sample media and extensive transported overburden these were encountered especially in more arid regions in australia and africa during exploration for ni and au during the 1970s and 1980s this led to a change in approach placing weathering and geochemical dispersion in the context of regolith and landscape evolution a return to the early concept of landscape geochemistry the 3d expression of mineralization in the landscape is depicted as empirical conceptual models that account for both relict features and active processes and portray element associations dispersion mechanisms and host materials they also indicate suitable sample media sampling intervals and procedures for analysis and interpretation

temperature evolution and the oxygen isotope composition of phanerozoic oceans from carbonate clumped isotope thermometry abstract surface temperature is among the most important parameters describing planetary climate and habitability and yet there remains considerable debate about the temperature evolution of the earth s oceans during the phanerozoic eon 541 million years ago to present the time during which complex metazoan life radiated on earth here we critically assess the emerging record of phanerozoic ocean temperatures based on carbonate clumped isotope thermometry of fossil brachiopod and mollusk shells and we present new data that fill important gaps in the late paleozoic record we evaluate and reject the hypothesis that solid state reordering of 13c 18o bonds has destroyed the primary clumped isotope temperature signal of most fossils during sedimentary burial at elevated temperatures the resulting phanerozoic record which shows a general coupling between tropical seawater temperatures and atmospheric carbon dioxide co2 levels since the paleozoic indicates that tropical temperatures during the icehouse climate of the carboniferous period were broadly similar to present 25 30 c and suggests that benthic metazoans were able to thrive at temperatures of 35 40 c during intervals of the early and possibly the latest paleozoic when co2 levels were likely 5 10 higher than present day values equally important there is no resolvable trend in seawater oxygen isotope ratios 18 o over the past 500 million years indicating that the average temperature of oxygen exchange between seawater and the oceanic crust has been high 270 c since at least the early paleozoic which points to mid ocean ridges as the dominant locus of water rock interaction over the past half billion years

cherty carbonate facies of the montoya group southern new mexico and western texas and its regional correlatives a record of late ordovician paleoceanography on southern laurentia abstract the upper ordovician montoya group in southern new mexico and westernmost texas records predominantly subtidal deposition on a gently dipping carbonate ramp that was subsequently nearly entirely dolomitized the medial unit of the montoya group the aleman formation is unique because it contains abundant chert 10 70 by volume the chert occurs as 1 thin continuous beds of sponge spicules within mudstone or calcisiltite 2 discontinuous lenses or nodules within skeletal wackestones and packstones or 3 as a replacement of skeletal grains and burrows coeval skeletal grainstones and muddy peritidal facies contain little chert phosphate up to 5 wt occurs within the underlying upham formation and the aleman formation as replacement of fossils and peloids the abundance of chert and phosphate in these subtidal facies indicates they formed within a region of strong upwelling regional correlation with upper ordovician cherty units along the periphery of southern laurentia and other low latitude continents suggests that upwelling was widespread and long lived during the late ordovician the upwelling is interpreted to record vigorous oceanic circulation produced by the onset of glaciation on gondwana during this period late ordovician relative sea level curves around the periphery of laurentia indicate correlative third order 1 3 my duration fluctuations that may provide a means for high resolution global correlations however the mechanism s that produced these long term fluctuations are unclear

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

silicon isotope compositions of euhedral authigenic quartz crystals implications for abiotic fractionation at surface temperatures abstract silicon si isotopes have been demonstrated as proxies for the paleoenvironmental conditions of various silica deposits in an effort to investigate the petrogenesis and paleoenvironments of phanerozoic silica deposits a suite of euhedral megaquartz crystals in the cretaceous edwards formation from central texas was petrographically analyzed and their si isotopic compositions analyzed by bulk and in situ techniques petrographic analysis shows a close relationship between megaquartz and evaporite minerals the lithologic association of silica with evaporite bearing dolomitized carbonate strata suggests that the silicification probably developed in a back reef tidal flat environment in which quartz crystals formed after the primary calcite cementation and partial dissolution of evaporites silicon isotopic mapping across the megaquartz crystals show that 30si nbs28 values range from 2 90 to 2 94 these values span the majority of published silicon isotopic values observed in nature and indicate complex growth histories the negative 30si values are attributed to the dissolution of sponge spicules which likely act as the primary source of the authigenic megaquartz the observed range of 30si values in megaquartz crystals is interpreted using a two stage model in which amorphous silica from sponge spicules is dissolved and re precipitated as megaquartz in a closed system during diagenesis this rayleigh type fractionation model also adds a new level of insight into the abiotic fractionation factor between dissolved and precipitated silica based on temperature estimates of 20 to 50 c for megaquartz precipitation the fractionation factor was determined to be between 1 8 and 2 1 the estimated average 30si value of early cretaceous seawater is 2 7 to 3 0 significantly higher than modern seawater

continental drift and phanerozoic carbonate accumulation in shallow shelf and deep marine settings knowledge of past rates of transfer of rock forming materials among the principal geologic reservoirs is central to understanding causes and magnitudes of change in earth surface processes over phanerozoic time to determine typical rates of global sediment cycling we compiled information on area volume and lithology of shallow water sediments by epoch for both terrigenous clastics and marine carbonates data on amounts of surviving continental terrigenous rock as opposed to deep oceanic and including terrestrial marginal marine and marine deposits exhibit positive age area trends wherein greatest areas and volumes of conglomerate sandstone and shale are represented by younger sequences global volumes of terrigenous clastic sediment yield a mean cycling rate of 0 00124 m yr similar to that determined for eurasian 0 00127 north american 0 00058 a b ronov 0 00352 t d cook and a w bally african 0 0017 and south american 0 0021 clastic sequences surficial erosion results in the mean destruction of 0 124 of terrigenous rock volume per million years of reservoir age in contrast surviving epicratonic and shelf margin carbonate sequences yield negative cycling rates of about 0 164 m yr surviving areas and volumes increase with sequence age that is the amount of limestone and dolostone preserved in shallow water settings increases back in time to maximal areal extents in the middle and upper cambrian mass age data on terrigenous clastic successions indicate generally constant rates of crustal erosion over phanerozoic time decrease in size of the shallow marine carbonate reservoir forward in time therefore suggests generally invariant rates of global limestone accumulation and a shift in sites of accumulation from shallow cratonic to deep oceanic settings over much of the past 540 m yr causes of this eon scale depositional translocation of carbonate sediment from shallowto deep marine settings cannot be satisfactorily linked to either changes in global sea level or the evolution of carbonate producing plankton because neither exhibits a pattern of unidirectional change in position or abundance since the early phanerozoic however tabulation of long term variation in areas of continental shelves from paleogeographic maps reveals a generally uniform decrease in low latitude 30 platform area with decreasing age over most of the phanerozoic moreover rate of change of low latitude shelf area km 2 m yr is almost exactly

structural event framework for the eastern yilgarn craton western australia and its implications for orogenic gold abstract the nnw trending tectonic grain of the eastern yilgarn craton eyc was established as a result of predominantly ene wsw directed extension d1 and d3 and e ene w wsw d2 d4 to ne sw directed d5 contraction the result has been a succession of nnw striking temporally discrete fabric elements which can be difficult to interpret reliably at any single location despite this many past workers interpreted the nnw striking fabric as the result of only one regional contractional event and used it as a marker for correlating structural events across the region in order to unravel the complexity this paper presents a new sixfold d1 d6 deformation nomenclature based on 10 000 new mesoscale structural observations including their kinematic analysis and cross cutting relationships these mesoscale data were referenced with regional 3d map patterns stratigraphic magmatic metallogenic considerations and deep seismic reflection images this integrated geodynamic architectural approach is applicable to solving structural event histories in other polydeformed terrains gold mineralisation occurred during the first five events but was particularly concentrated from d3 onwards the d3 event marked the most profound change in the tectonic evolution of the eyc with changes in greenstones granites and tectonic mode lithospheric extension and core complexes with the first significant gold deposited within extensional shear zones that dissect the crust later contraction d4 was imposed at a high angle to the previously established anisotropic architecture the outcome was the creation of a new dynamic permeability framework which resulted in gold mineralisation during nnw striking sinistral strike slip faulting and associated thrusting a further stress switch d5 further modified the architecture resulting in n to nne striking dextral strike slip faulting and the final period of gold mineralisation before late stage extension d6

supergene gold transformation secondary and nano particulate gold from southern new zealand biogeochemical processes drive the transformation of gold au in surface environments in this study we assess the link between surface morphologies of au grains and supergene transformation processes with a focus on the formation of nano particulate au in temperate settings gold grains were collected from six localities across the south island of new zealand deposit styles vary from eluvial alluvial and beach placer deposits in areas of moderate to very high levels of precipitation gold grains were assessed using optical microscopy om field emission scanning electron microscopy feg sem focused ion beam scanning electron microscopy fib sem coupled with x ray dispersive analysis edxa and electron microprobe analyses epma morphologies indicative of au and ag dissolution e g grain boundary dissolution as well as abundant au neoformation and aggregation morphologies were observed on all grains the latter include a variety of secondary au morphotypes in particular nano particulate and crystalline forms as well as bacteriomorphic au sheet au and porous branched au networks pervasive dissolution features on grains from an outcropping quartz vein system as well as extensive nano particle formation on weathered quartz vein and placer grains from the west coast of new zealand s south island which is subject to very heavy orographic precipitation suggest that these climatic conditions enhance the transformation of au grains at these sites au nano particles are most abundant in the polymorphic layer i e a coating of biofilms secondary au siliceous and carbonaceous materials on the surface of transforming au grains and in soil materials associated with the grains nano particulate au is also highly abundant in carbonaceous likely exopolymeric coatings on au grains from orepuki beach suggesting that au dissolution in seawater and microbial biomineralization are important contributors to au alteration in beach placer deposits in conclusion surface morphologies of au grains from new zealand are the result of supergene transformations occurring in current environments the formation of nano particulate au which was previously thought to be evaporation driven is in these high rainfall environments likely due to other mechanisms such as biomineralization

texture specific si isotope variations in barberton greenstone belt cherts record low temperature fractionations in early archean seawater sedimentary cherts are unusually abundant in early archean pre 3 0 ga sequences suggesting a silica cycle that was profoundly different than the modern system previously applied for the purpose of paleothermometry si isotopes in ancient cherts can offer broader insight into mass fluxes and mechanisms associated with silica concentration precipitation diagenesis and metamorphism early archean cherts contain a rich suite of sedimentological and petrographic textures that document a history of silica deposition cementation silicification and recrystallization to add a new layer of insight into the chemistry of early cherts we have used wavelength dispersive spectroscopy and then secondary ion mass spectrometry sims to produce elemental and si and o isotope ratio data from banded black and white cherts from the onverwacht group of the barberton greenstone belt south africa this geochemical data is then interpreted in the framework of depositional and diagenetic timing of silica precipitation provided by geological observations sims allows the comparison of si and o isotope ratios of distinct silica phases including black carbonaceous chert beds and bands many including well defined sedimentary grains white relatively pure chert bands including primary silica granules early cavity filling cements and later quartz filled veins including all chert types and textures analyzed the 30 si dataset spans a range from 4 78 to 3 74 with overall mean 0 20 median 0 51 and standard deviation 1 30 n 1087 most samples have broadly similar 30 si distributions but systematic texture specific 30 si differences are observed between white chert bands mean 0 60 n 750 which contain textures that represent primary and earliest diagenetic silica phases and later cavity filling cements mean 1 41 n 198 we observed variations at a 100 m scale indicating a lack of si isotope homogenization at this scale during diagenesis and metamorphism although fractionations during diagenetic phase transformations may have affected certain textures we interpret these systematic variations to reflect fractionation during silica precipitation as well as isotopically distinct fluids from which later phases originated sims 18 o values fall in a range from 16 39 to 23 39 n 381 similar to previously published data from bulk gas source mass spectrometry of onverwacht cherts we observed only limited examples of texture related variation in 18 o and did not observe correlation of 18 o with 30 si trends this is consistent with hypotheses that si isotope ratios are more resistant to alteration under conditions of rock buffered diagenesis marin carbonne et al 2011 our results indicate that low temperature processes fractionated silicon isotopes in early archean marine basins a behavior that probably precludes the application of chert 30 si as a robust paleothermometer the values we observe for facies that sedimentological and petrographic observations indicate formed as primary and earliest diagenetic silica precipitates from seawater are more 30 si rich than that expected for bulk silicate earth this is consistent with the hypothesis that the silicon isotope budget is balanced by the coeval deposition of 30 si enriched cherts and 30 si depleted iron formation lithologies precipitation of authigenic clay minerals in both terrestrial and marine settings may have also comprised a large 30 si depleted sink with the corollary of an important non carbonate alkalinity sink consuming cations released by silicate weathering

secular distribution of biogenic silica through the phanerozoic comparison of silica replaced fossils and bedded cherts at the series level abstract the modern marine silica cycle is dominated by silica secreting phytoplankton principally diatoms but this cycle has evolved considerably during the phanerozoic we analyzed the temporal distribution of silica replaced fossils and bedded chert to determine the influence of factors such as extinctions and climate change on siliceous facies trends in silica replacement of fossils match faunal radiations during the ordovician and siluro devonian with peaks in silica replacement in the late ordovician and middle devonian corresponding to peaks in the abundance and diversity of siliceous sponges sharp drops in the abundance of silica replaced fossils and or bedded cherts coincide with four of the five major mass extinctions no discernible decrease marks the extinction at the end of the triassic we expected peaks in bedded chert deposition during glacial episodes because enhanced ocean circulation should favor more and stronger upwelling orogenic episodes which may trigger continental glaciati

δ30si and δ18o of multiple silica phases in chert implications for δ30siseawater of darriwilian seawater and sea surface temperatures abstract a promising quantitative method is the use of fluctuations of seawater silicon isotope compositions 30siseawater to estimate the silicon cycle and silicon isotopic budgets of both the marine and terrestrial systems in a pre diatom ocean however a thorough global calibration of phanerozoic 30siseawater which is linked to continental tectonics and silicate weathering has not been established the silicon and oxygen isotope compositions of darriwilian cherts 30sichert and 18ochert respectively have considerable potential as proxies for 30siseawater and sea surface temperatures ssts which can expand our understanding of the influence of continental tectonics and temperature changes on the great ordovician biodiversification event gobe in this study primary depositional and early diagenetic cherts were sampled from bedded chert lump shaped chert nodular chert and banded chert from five darriwilian outcrops in the kalpin and yijianfang areas of the tarim basin in china the preservation conditions of these cherts were assessed based on petrographic criteria such as the absence of calcite veins and fractures under macroscopic observation and the absence of aggregate chalcedony cha microquartz mq and cryptocrystalline quartz mqc using optical microscopy and scanning electronic microscopy sem the ssts and 30siseawater of darriwilian seawater were reconstructed using the 18ochert and 30sichert of well preserved cherts our results show that the ssts were approximately 32 5 c in the early darriwilian and approximately 30 5 c in the late darriwilian the 30siseawater was approximately 3 1 in the kalpin and yijianfang areas during the darriwilian stage this is the first estimation of 30siseawater in the darriwilian stage which provides a parameter for the global calibration of the 30siseawater of the pre diatom ocean in the phanerozoic eon and validates the hypothesis that the pre diatom ocean waters had higher 30si values than modern seawater

naturally occurring gold nanoparticles and nanoplates during the weathering of gold deposits exceptionally pure 200 nm diameter nanoparticulate gold plates 6 nm thick are formed the particles display controlled growth of both size and shape and signs of assembly to form belts and sheets the gold is associated and intergrown with minerals formed by evaporation and is interpreted to have been deposited rapidly from saline groundwater during a drying event the size and morphology of the gold nanoparticles and nanoplates are identical to the products of experimentally manufactured gold colloids this represents the fi rst direct observation of colloidal nanoparticulate gold in nature confi rming this as an active mechanism of gold transport during the weathering of gold deposits

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

new isotopic temperature estimates for early silica diagenesis in bedded cherts when analyzed by a stepwise fluorination technique that reduces undesired oxygen contributions from hydroxyl and water opal ct samples from the siliceous member of the miocene monterey formation california yield 18 o values that contribute to mounting evidence that silica diagenesis in bedded chert can occur at much lower temperatures than previously supposed monterey opal ct 18 o values of 37 5 and 36 3 point to diagenetic temperatures at least as low as 17 21 c rather than the 35 50 c values normally inferred our results suggest that hydration waters can significantly reduce the bulk 18 o of opal ct and that special analytical techniques and consideration of water rock ratios are warranted for isotope thermometry off diagenetic silica transformations

weathering profile of the cerro de maimon vms deposit dominican republic textures mineralogy gossan evolution and mobility of gold and silver abstract cerro de maimon in the central dominican republic is currently the only vms deposit under production in the caribbean region it is hosted in the maimon formation of early cretaceous age which is part of the oldest and chemically most primitive island arc in the caribbean from bottom to top this deposit can be divided into i a primary sulfide zone ii a supergene enrichment zone and iii an oxidized zone this study reports new data on the textural and mineralogical characteristics of the oxidized zone gossan leached capping zone with emphasis in the au ag bearing phases the mineral paragenesis of the oxidized zone is essentially composed of goethite hematite quartz and barite botryoidal cellular and brecciated textures can be distinguished botryoidal and brecciated textures dominate in the upper parts of the oxidized zone whereas cellular textures are more common in the intermediate and lower parts however the weathering profile is very heterogeneous the leached capping profile shows evidence of both transported and indigenous gossans gold in the oxide paragenesis is extremely pure 99 au suggesting that chemical refining took place silver occurs mainly as iodargyrite agi and minor agbr agcl in botryoidal aggregates particles of au ag alloy in the primary mineralization exposed to a weathering environment can be leached and transported by various agents chemical and biochemical that may exist simultaneously in the presence of halides gold and silver can be leached and transported in a wide range of ph eh conditions especially if iodine is present silver is leached more rapidly and over a broader range of ph eh conditions and is preferentially transported as iodine complexes than other halides the presence of iodargyrite in the cerro de maimon gossan fully developed under tropical conditions suggests that this mineral cannot be considered an indicator of extremely arid environments as typically claimed by many authors iodargyrite occurrence would rather point to the presence of saline halide rich groundwater in oxide deposits

silica diagenesis origin of inorganic and replacement cherts abstract silicification of originally non siliceous sediments affects a wide variety of rock types and materials and ranges from minor to pervasive partial and minor chertification occur mostly in phanerozoic carbonates carbonate bearing sandstones evaporites and fossil wood the source of the silica is predominantly biogenic in petrified wood the silicification mechanism is a permeation or void filling process not a replacement in this example the sequence of silica phase transformation is the same as that in deep sea siliceous sediments in many silicified rocks particularly in certain carbonates the transformation sequence is different from that in radiolarites or diatomites the chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of knauth 1979 but remain unknown for most other types an exception are the flint nodules and bands of the english chalk a detailed geochemical study of the paramoudra flint structures by clayton 1986 provided remarkable insight into the replacement process seven different recurring silica fabrics have been recognized in chert replaced carbonates including equigranular microcrystalline quartz or microquartz and megaquartz and fibrous types chalcedony quartzine or length slow chalcedony lutecite zebraic chalcedony and microflamboyant quartz among the latter quartzine and microflamboyant quartz are common in but by no means restricted to chert replaced evaporites for which milliken 1979 recognized a sequence of seven quartz fabrics as a single criterion only anhydrite inclusions in megaquartz quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor the relative timing between silicification and well established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur chertification of carbonate host sediment thus may take place during early intermediate or late diagenesis and even during anchimetamorphism pervasive to complete silicification has been described in lacustrine pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds members or entire formations it may affect some of the aforementioned rock types too for example carbonates the source of the silica is predominantly inorganic chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage magadi type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in east african rift valley lakes such as lake magadi in late pleistocene time they are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes silcretes originate from weathering and soil forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites in more humid climates however non weathering profile silcretes occur which have been distinguished from weathering profile silcretes four to five different silcrete fabric types have been established including the 1 grain supported or quartzitic 2 floating terrazzo 3 matrix albertinia and opaline and 4 conglomeratic types in volcanic edifices silicification related to hydrothermal activity occurs 1 along the ascent routes of the fluids vents within the volcanic complexes 2 in isolated ponds and depressions on the sea floor where the fluids discharge mostly in the median rift valley of mid ocean ridges and 3 in geothermal areas on land associated with spreading lineaments volcanic islands arcs transform faults or continental hot spots attention has focussed on the associated base metal concentrations and less on the silicification and accompanying iron enrichment processes the precambrian banded iron formations may be indirectly or directly related to hydrothermal volcanic processes the origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it

oxygenation as a driver of the great ordovician biodiversification event the largest radiation of phanerozoic marine animal life quadrupled genus level diversity towards the end of the ordovician period about 450 million years ago a leading hypothesis for this great ordovician biodiversification event is that cooling of the ordovician climate lowered sea surface temperatures into the thermal tolerance window of many animal groups such as corals a complementary role for oxygenation of subsurface environments has been inferred based on the increasing abundance of skeletal carbonate but direct constraints on atmospheric o2 levels remain elusive here we use high resolution paired bulk carbonate and organic carbon isotope records to determine the changes in isotopic fractionation between these phases throughout the ordovician radiation these results can be used to reconstruct atmospheric o2 levels based on the o2 dependent fractionation of carbon isotopes by photosynthesis we find a strong temporal link between the great ordovician biodiversification event and rising o2 concentrations a pattern that is corroborated by o2 models that use traditional carbon sulfur mass balance we conclude that that oxygen levels probably played an important role in regulating early palaeozoic biodiversity levels even after the cambrian explosion an increase in biodiversity 450 million years ago coincided with a rise in atmospheric oxygen concentrations suggests a geochemical analysis oxygen availability may have thus helped spur the radiation alongside climatic cooling

iodargyrite from zalas cracow area poland as an indicator of oligocene miocene aridity in central europe abstract iodargyrite the hexagonal silver iodide was found in the permian rhyodacite quarry at zalas near cracow southern poland in a fault zone cutting middle jurassic sandy limestone it occurs within a tectonic breccia encrusted by chalcopyrite pyrite ag containing chalcocite covellite galena native bismuth malachite cuprite fe and mn dominant oxides cu sulfates and barite the hydrothermal encrustacion is most likely connected with rejuvenation of old paleozoic faults in the sava phase of the alpine orogeny on the oligocene miocene boundary the periods of aridity when iodargyrite precipitated determined by evaporation of salt lakes or the inland miocene sea that delivered halide ions to the groundwater oxidation system or leaching of weathered ag containing mineralization by saline i rich deep formation waters derived from paleogene sediments during folding and formation of the carpathians nappe structure are connected rather with the low to middle miocene period at this time steppe areas with a dry climate existed on the north foreland of the carpathians in southern poland as a result of a displacement to the north of a dry subtropical zone because of uplift of the carpathians and regression of the inland miocene sea

a silicon depleted north atlantic since the palaeogene evidence from sponge and radiolarian silicon isotopes despite being one of earth s major geochemical cycles the evolution of the silicon cycle has received little attention and changes in oceanic dissolved silica dsi concentration through geologic time remain poorly constrained silicon isotope ratios expressed as 30si in marine microfossils are becoming increasingly recognised for their ability to provide insight into silicon cycling in particular the 30si of siliceous sponge spicules has been demonstrated to be a useful proxy for past dsi concentrations we analysed 30si in radiolarian tests and sponge spicules from the blake nose palaeoceanographic transect odp leg 171b spanning the palaeocene eocene ca 60 30 ma our 30si results range from 0 32 to 1 67 and 0 48 to 0 63 for the radiolarian and sponge records respectively using an established relationship between ambient dissolved si dsi concentrations and the magnitude of silicon isotope fractionation in siliceous sponges we demonstrate that the western north atlantic was dsi deplete during the palaeocene eocene throughout the water column a conclusion that is robust to a range of assumptions and uncertainties these data can constitute constraints on reconstructions of past ocean circulation previous work has suggested ocean dsi concentrations were higher than modern ocean concentrations prior to the cenozoic and has posited a drawdown during the early palaeogene due to the evolutionary expansion of diatoms our results challenge such an interpretation we suggest here that if such a global decrease in oceanic dsi concentrations occurred it must predate 60 ma

a paired apatite and calcite clumped isotope thermometry approach to estimating cambro ordovician seawater temperatures and isotopic composition the secular increase in 18 o values of both calcitic and phosphatic marine fossils through early phanerozoic time suggests either that 1 early paleozoic surface temperatures were high in excess of 40 c tropical mat 2 the 18 o value of seawater has increased by 7 8 vsmow through paleozoic time or 3 diagenesis has altered secular trends in early paleozoic samples carbonate clumped isotope analysis in combination with petrographic and elemental analysis can deconvolve fluid composition from temperature effects and therefore determine which of these hypotheses best explain the secular 18 o increase clumped isotope measurements of a suite of calcitic and phosphatic marine fossils from late cambrian to middle late ordovician aged strata the first paired fossil study of its kind document tropical sea surface temperatures near modern temperatures 26 38 c and seawater oxygen isotope ratios similar to today s ratios

experimental constraints on kinetic and equilibrium silicon isotope fractionation during the formation of non biogenic chert deposits silicon isotopic compositions 30si of modern and ancient siliceous sedimentary rocks provide valuable information on conditions in depositional environments but interpretations are hampered by the lack of experimentally validated fractionation factors here we present new constraints on the magnitudes of kinetic and equilibrium isotope effects during chemical precipitation of amorphous silica in batch reactors at low temperature 10 35 c and near neutral ph 7 5 8 5 as analogue for non biogenic chert formation instantaneous fractionation factors derived from 30si values of the total dissolved sitd silica and mass balance computations with inst 30sippt 1000 30sitd 1000 decrease with progressive precipitation and reduced reaction rates this suggests that silica deposition in the batch reactors is kinetically dominated at the start of the experiments but approaches a metastable equilibrium after ca 400hours modelled kinetic fractionation factors range from 0 9965 at 10 c to 0 9976 at 20 c and 0 9993 at 35 c and ph8 5 whereas equilibrium isotope effects are smaller and range from 0 9995 at 10 c to 1 000 at 20 c and 1 0005 at 35 c our results suggest that large isotope effects are only expressed in natural systems where dissolved and precipitated silica are not equilibrated implying that the kinetic conditions of non biogenic silica precipitation provide important constraints on silicon isotope ratios of siliceous rocks with particular relevance for those preserved in the archean chert record

the association of gold with calcrete abstract the discovery that au accumulates in calcrete pedogenic carbonate or caliche was made in 1987 by csiro calcrete is a general term describing accumulation of alkaline earth metals in soils of arid and semi arid terrains around the world the principal constituent of calcrete is calcite while au is a noble metal calcrete has been a significant tool in a number of au deposit discoveries so understanding the mechanisms by which these diametrically different components come together is valuable for enhancing future discovery numerous laboratory experiments case histories and exploration models have been published most from australia yet we do not fully understand the mechanisms involved it is timely therefore twenty five years on since the first publication of this phenomenon to review this highly unusual but economically important association critical to any review on au in calcrete is to first consider calcretes themselves the nature of a particular calcrete where it has formed and mode of formation is relevant to how where and why au accumulates within it this review commences with a background nomenclature history classification and some examples of calcrete types found near au deposits how calcretes form their origins and the role of biota is considered their locations in the regolith and landscape as well as exploration models for au in calcrete are discussed a section on the chemistry of au in calcretes details what we know about possible mechanisms of formation and considers what laboratory experiments on microorganisms and abiotic experiments tell us following on is a summary of practical aspects of identifying collecting and analysing samples for exploration purposes selected mineral exploration case histories are described and how they fit into models of exploration and different regolith settings concluding sections include a summary and implications of this accumulated knowledge to discovering au deposits

ordovician oxygen isotopes and paleotemperatures abstract valid marine 18o proxies have been defined from three ordovician units and with previously estimated sea water 18o values we have calculated paleotemperatures for low latitude settings during a greenhouse climatic era valid marine proxies include marine equant bladed and translucent fibrous calcite these cements are interpreted to be valid marine proxies based on cathodoluminescence minor element content abundance of microdolomite and stable isotopic both altered marine and non marine calcite phases values valid marine proxies from the chickamauga alabama holston tennessee and kullsberg sweden formations have an overall range of 18o values in each unit that is confined to within 1 unlike altered marine cements which exhibit significantly wider ranges of 18o values we suggest a likely non glacial ordovician sea water value of 2 1 smow from which paleotemperatures can be calculated calculated paleotemperatures are broadly consistent with crowley and baum s 1995 late ordovician paleoclimatic model

micro scale silicon isotope heterogeneity observed in hydrothermal quartz precipitates from the 3 7 ga isua greenstone belt sw greenland pillow basalt and chert form integral lithologies comprising many archean greenstone belt packages to investigate details of these lithologies in the 3 7 ga isua greenstone belt sw greenland we measured silicon isotope compositions of quartz crystals by secondary ion mass spectrometry from a quartz cemented quartz amygdaloidal basaltic pillow breccia recrystallized chert and chert clasts thought to represent silica precipitation under hydrothermal conditions the recrystallized chert chert clasts and quartz cement have overlapping 30si values while the 30si values of the quartz amygdules span nearly the entire range of previously published values for quartz precipitates of any age despite amphibolite facies metamorphism we suggest that the heterogeneity is derived from kinetic isotope fractionation during quartz precipitation under disequilibrium conditions in a hydrothermal setting consistent with the pillow breccia origin on the basis of the present data we conclude that the geological context of each sample must be carefully evaluated when interpreting 30si values of quartz