copper isotope fractionation in the meiduk porphyry copper deposit northwest of kerman cenozoic magmatic arc iran the meiduk deposit possesses three different cu reservoirs each with a unique cu isotope signature 65cu for the leached cap minerals ranges from 2 5 to 0 49 to 0 45 to 0 3 for hypogene minerals and from 1 3 to 4 4 for supergene enrichment minerals oxidation of hypogene sulphides and effective trapping of copper from solutions derived from the leached cap in the supergene enrichment zone caused this relationship a systematic pattern of low cu isotope values close to the surface and higher isotope values with depth reveals a palaeo fluid pathway in the northwest southeast direction over the deposit thus the copper isotope data from leached cap and enrichment minerals can be used to monitor copper migration during supergene weathering at the meiduk deposit

simulation of preferential flow in three dimensional heterogeneous conductivity fields with realistic internal architecture subsurface flow is primarily controlled by the distribution of hydrogeological properties spatially correlated random fields currently the primary means of describing heterogeneity in these properties underutilizes the existing knowledge of geological systems a new computer code the braided channel simulator bcs 3d is used to geometrically simulate the spatial heterogeneity of hydrogeological properties using representations of surface topography to estimate the three dimensional arrangement of subsurface units four methods of assigning conductivity were used to generate conductivity fields as a basis for saturated flow simulation variograms measured for each of the conductivity fields give no indication of underlying discrete structure however particle tracking simulations showed that flow occurred along paths of preferentially high hydraulic conductivity results indicate that in systems where the contrast in conductivity is sufficiently great the location and magnitude of flow is constrained by discrete internal structure additionally geometrical simulation techniques such as bcs 3d can be used to produce property fields that embody these discrete geological structures

hyporheic transport and biogeochemical reactions in pool riffle systems under varying ambient groundwater flow conditions at the interface between stream water groundwater and the hyporheic zone hz important biogeochemical processes that play a crucial role in fluvial ecology occur solutes that infiltrate into the hz can react with each other and possibly also with upwelling solutes from the groundwater in this study we systematically evaluate how variations of gaining and losing conditions stream discharge and pool riffle morphology affect aerobic respiration ar and denitrification dn in the hz for this purpose a computational fluid dynamics model of stream water flow is coupled to a reactive transport model scenarios of variations of the solute concentration in the upwelling groundwater were conducted our results show that solute influx residence time and the size of reactive zones strongly depend on presence magnitude and direction of ambient groundwater flow high magnitudes of ambient groundwater flow lower ar efficiency by up to 4 times and dn by up to 3 orders of magnitude compared to neutral conditions the influence of stream discharge and morphology on the efficiency of ar and dn are minor in comparison to that of ambient groundwater flow different scenarios of o2 and no3 concentrations in the upwelling groundwater reveal that dn efficiency of the hz is highest under low upwelling magnitudes accompanied with low concentrations of o2 and no3 our results demonstrate how ambient groundwater flow influences solute transport ar and dn in the hz neglecting groundwater flow in stream groundwater interactions would lead to a significant overestimation of the efficiency of biogeochemical reactions in fluvial systems

benthic nitrogen fluxes and fractionation of nitrate in the mauritanian oxygen minimum zone eastern tropical north atlantic we present sedimentary geochemical data and in situ benthic flux measurements of dissolved inorganic nitrogen din no3 no2 nh4 and oxygen o2 from 7 sites with variable sand content along 18 n offshore mauritania nw africa bottom water o2 concentrations at the shallowest station were hypoxic 42 m and increased to 125 m at the deepest site 1113 m total oxygen uptake rates were highest on the shelf 10 3 mmol o2 m 2 d 1 and decreased quasi exponentially with water depth to 3 2 mmol o2 m 2 d 1 average denitrification rates estimated from a flux balance decreased with water depth from 2 2 to 0 2 mmol n m 2 d 1 overall the sediments acted as net sink for din observed increases in 15nno3 and 18ono3 in the benthic chamber deployed on the shelf characterized by muddy sand were used to calculate apparent benthic nitrate fractionation factors of 8 0 15eapp and 14 1 18eapp measurements of 15nno2 further demonstrated that the sediments acted as a source of 15n depleted no2 these observations were analyzed using an isotope box model that considered denitrification and nitrification of nh4 and no2 the principal findings were that i net benthic 14n 15n fractionation eden was 12 9 1 7 ii inverse fractionation during nitrite oxidation leads to an efflux of isotopically light no2 22 1 9 and iii direct coupling between nitrification and denitrification in the sediment is negligible previously reported eden for fine grained sediments are much lower 4 8 we speculate that high benthic nitrate fractionation is driven by a combination of enhanced porewater seawater exchange in permeable sediments and the hypoxic high productivity environment although not without uncertainties the results presented could have important implications for understanding the current state of the marine n cycle

the effect of redox conditions and bioirrigation on nitrogen isotope fractionation in marine sediments abstract nitrogen isotopic signatures of sources and sinks of fixed nitrogen n can be used to constrain marine nitrogen budgets however the reported fractionation during benthic n 2 production varies substantially to assess the range and mechanisms responsible for such observations we conducted a model study to evaluate the extent to which nitrification denitrification and anaerobic ammonium oxidation contribute to the isotopic composition of in situ n 2 production different hydrodynamic regimes were taken into account ranging from bioirrigation to diffusion dominated transport the benthic redox conditions were found to control the n isotope effect which under reducing conditions is driven by fractionation during nitrification and anaerobic ammonium oxidation and under oxidizing conditions by fractionation during denitrification environmental parameters such as the mineralization rate the bioirrigation intensity and chemical composition of the overlying water affect the benthic redox zonation and therefore also the benthic n isotope effect the n isotope effect of benthic n 2 production was computed for a wide range of bioirrigation intensities and mineralization rates and found to be approximately 3 for commonly encountered conditions this value is similar to previous estimates of the global n isotope effect of benthic n 2 production and further constrains the relative importance of water column vs benthic n 2 production

benthic mineralization and solute exchange on a celtic sea sand bank jones bank benthic carbon mineralization and solute exchange was investigated on a celtic sea sandbank during july 2008 the sediment on the top of the bank consisted of consolidated sand characterized by advective porewater transport and was difficult to sample this to some extent compromised the investigations at this site however intact sediment cores were sampled at 4 stations at the slopes base of the bank and off the bank reference site this sediment was used to assess rates and pathways for benthic diagenesis total sediment o2 uptake tou ranged from 5 8 to 9 0 mmol m 2 d 1 and total sediment release rates of dissolved inorganic carbon dic ranged between 8 60 and 13 8 mmol m 2 d 1 microbial denitrification and sulfate reduction accounted for 2 and 12 28 of the total benthic carbon mineralization respectively the remaining mineralization is ascribed to o2 and fe mn respiration respectively activity profiles of unsupported 210pb from all stations indicated deep mixing presumably caused by intense trawling activity in the area calculation based on satellite tracking of fishing vessels suggest that on average 33 of the sediment is affected by trawling activity every year this is presumed to facilitate high metal respiration rates by continuously oxidizing reduced fe that would otherwise accumulate in the sediment extracted porewater profiles reflected elevated no3 levels as compared to microsensor determined no3 profiles that were measured in parallel we suggest that this reflects no3 leakage from meiofauna and that the intracellular no3 pool in the top 5 cm of the sediment exceeds the porewater pool up to a factor of 45 however this intracellular pool is presumably turned over at very slow rates compared to the porewater pool subject to microbial denitrification

copper isotopic zonation in the northparkes porphyry cu au deposit se australia abstract significant systematic cu isotopic variations have been found in the northparkes porphyry cu au deposit nsw australia which is an orthomagmatic porphyry cu deposit copper isotope ratios have been measured in sulfide minerals chalcopyrite and bornite by both solution and laser ablation multi collector inductively coupled plasma mass spectrometry mc icp ms the results from both methods show a variation in 65cu of hypogene sulfide minerals of greater than 1 relative to nist976 significantly the results from four drill holes through two separate ore bodies show strikingly similar patterns of cu isotope variation the patterns are characterized by a sharp down hole decrease from up to 0 8 0 29 0 56 1 n 20 in the low grade peripheral alteration zones phyllic propylitic alteration zone to a low of 0 4 0 25 0 36 1 n 30 at the margins of the most mineralized zones cu grade 1 wt in the high grade cores of the systems the compositions are more consistent at around 0 2 0 19 0 14 1 n 40 the cu isotopic zonation may be explained by isotope fractionation of cu between vapor solution and sulfides at high temperature during boiling and sulfide precipitation processes sulfur isotopes also show an isotopically light shell at the margins of the high grade ore zones but these are displaced from the low 65cu shells such that there is no correlation between the cu and s isotope signatures fe isotope data do not show any discernable variation along the drill core this work demonstrates that cu isotopes show a large response to high temperature porphyry mineralizing processes and that they may act as a vector to buried mineralization

geologic setting and genesis of the mule canyon low sulfidation epithermal gold silver deposit north central nevada the mule canyon mine exploited shallow low sulfidation epithermal au ag deposits that lie near the west side of the northern nevada rift in northern lander county nevada mule canyon consists of six small deposits that contained premining reserves of about 8 2 mt at an average grade of 3 81 g au tonne it is an uncommon mafic end member of low sulfidation au ag deposits associated with tholeiitic bimodal basalt rhyolite magmatism the ore is hosted by a basalt andesite eruptive center that formed between about 16 4 to 15 8 ma during early mafic eruptions related to regionally extensive bimodal magmatism hydrothermal alteration and au ag ores formed at about 15 6 ma and were tightly controlled by north northwest to north striking high angle fault and breccia zones developed during rifting emplacement of mafic dikes and eruption of mafic lava flows hydrothermal alteration assemblages are zoned outward from fluid conduits in the sequence silica adularia adularia smectite smectite intermediate argillic and smectite carbonate propylitic all alteration types contain abundant pyrite and or marcasite arsenopyrite field relations indicate that silica adularia alteration is superimposed on argillic and propylitic alteration little or no steam heated acid sulfate alteration is present probably the result of a near surface water table during hydrothermal alteration and ore deposition two distinct ore types are present at mule canyon early replacement and later open space filling replacement ores consist of disseminated and vesicle filling pyrite marcasite and arsenopyrite in argillically altered or weakly silicified rocks ore minerals consist of au bearing arsenopyrite and arsenian pyrite overgrowths on earlier formed pyrite and marcasite open space filling ores include narrow stockwork quartz adularia veins banded and crustiform opaline and chalcedonic silica adularia veins silica adularia cemented breccias and sparse carbonate pyrite and or marcasite veins ore minerals consist mostly of electrum and ag sulfide and selenide minerals with minor to major amounts of pyrite marcasite and arsenopyrite and local stibnite both types of ores have similar geochemical signatures characterized by high au ag as sb and se contents locally high hg mo tl and w contents and low cu pb and zn contents stable isotope data indicate that ore fluids consisted dominantly of meteoric water that evolved by deep circulation through paleozoic sedimentary rocks at low water rock ratios about 1 and high temperatures 200 c calculated isotopic compositions of ore fluids are 18oh2o 3 to 7 per mil dh2o 107 to 124 per mil 13cco2 0 to 6 per mil and 34sh2s 3 to 8 per mil the ore fluids obtained much of their h2s and co2 and probably scavenged ore metals and trace elements from the paleozoic sedimentary rocks some h2s and co2 may have been derived from degassing miocene magmas mule canyon formed at shallow depths probably about 100 m below the paleosurface ore fluids were dilute nearly neutral in ph reduced h2s rich and co2 bearing peak temperatures in ore zones reached 230 to 265 c at nearly lithostatic pressures when some crystalline quartz adularia precipitated but most ore formed at temperatures 200 c at near hydrostatic pressures and was accompanied by precipitation of opaline and chalcedonic silica adularia calcite and dolomite deposition of gold in as rich overgrowths on pyrite and or marcasite in disseminated ores occurred owing to decreasing h2s in the ore fluids resulting from sulfidation reactions later electrum and ag selenide precipitation in open spaces occurred owing to boiling loss of h2s to the vapor phase and cooling mule canyon is similar to most other low sulfidation au ag deposits associated with miocene tholeiitic bimodal basalt rhyolite magmatism in the great basin such as sleeper midas and buckhorn major differences at mule canyon are the high fe sulfide contents of some ores and altered wall rocks the paucity of silicification and the presence of only narrow and discontinuous gold bearing siliceous veins these differences primarily are due to the mafic composition of the host rocks mule canyon differs from low sulfidation deposits associated with subduction related calc alkaline porphyry intrusions such as the comstock lode in that a much smaller proportion of water salts and metals was derived from crystallizing magmas

advective transport affecting metal and nutrient distributions and interfacial fluxes in permeable sediments abstract our laboratory flume experiments demonstrate that advective porewater flows produce biogeochemical reaction zones in permeable sediments leading to specific and reproducible complex patterns of fe mn and nutrients oxygenated water forced into the sediment when boundary flows were deflected by protruding sediment structures generated distinct zones of nitrification and ferric iron precipitation this inflow was balanced by ammonium rich porewater ascending from deeper sediment layers thereby creating an anoxic channel where dissolved fe2 and mn2 could reach the surface between the zones of ferric iron precipitation and fe2 upwelling a layer with increased manganese oxide and solid phase fe ii concentrations formed indicating redox reaction between these components the establishment of topography on the previously smooth sediment surface reversed the net interfacial flux of solutes while the smooth control core was found to be a sink for metals and nutrients the sediment with mounds acted as a source for these substances our experiments show that in sandy sediment with an oxidised surface layer reduced metal species can be released to the water column by flow topography interactions we conclude that advective transport processes constitute an important process controlling biogeochemical zonations and fluxes in permeable sea beds

the negligible effect of bed form migration on denitrification in hyporheic zones of permeable sediments bed form celerity the migration rate of ripples along a sediment bed has previously been shown to have dramatic effects on oxygen distribution and transport within the hyporheic zone of permeable sediments this has the potential to influence denitrification rates in particular by increasing the coupling of nitrification and denitrification to further understand this we numerically modeled nitrogen cycling under migrating ripples while the simulated oxygen profiles match with expected behavior almost no effect on denitrification or coupled nitrification denitrification was observed with increasing celerity instead denitrification rates were dominantly controlled by the flow velocity of water overlying the sediment

new isotopic evidence bearing on bonanza au ag epithermal ore forming processes new cu s and pb isotope data provide evidence for a magmatic source of metal loid s and sulfur in epithermal au ag deposits even though their ore forming solutions are composed primarily of heated meteoric ground waters the apparent isotopic discrepancy between ore metals and ore forming solutions and even between the ore and associated gangue minerals indicates two different sources of epithermal ore forming constituents 1 a shallow geothermal system that not only provides the bulk of water for the ore forming solutions but also major chemical constituents leached from host rocks silica aluminum potassium sodium calcium to make gangue minerals and 2 metals and metalloids as te sb etc and sulfur se derived from deeper magma bodies isotopic data are consistent with either vapor phase transport of metal loids and sulfur and their subsequent absorption by shallow geothermal waters or formation of metallic au ag cu phases nanoparticles at depth from magmatic fluids prior to encountering the geothermal system the latter is most consistent with ore textures that indicate physical transport and aggregation of nanoparticles were significant ore forming processes the recognition that epithermal au ag ores form in tectonic settings that produce magmas capable of releasing metal rich fluids necessary to form these deposits can refine exploration strategies that previously often have focused on locating fossil geothermal systems

denitrification in shallow sublittoral gulf of mexico permeable sediments we examined nitrogen cycling over a 1 yr period in shallow sandy sediments at two contrasting sites near a barrier island in the northeastern gulf of mexico and we provide here the direct determination of dinitrogen gas n2 production at ambient nitrate concentrations in permeable marine sediments nitrogen stable isotope tracer techniques were used to quantify n2 production rates and pathways in sediment cores and slurries to simulate pore water advection the dominant transport process in the upper layer of the permeable sand beds intact sediment cores were perfused with aerated seawater this perfusion increased denitrification rates up to 2 fold in apalachicola bay sands and up to 17 fold in gulf of mexico sublittoral sands respectively relative to static cores seasonal n2 production rates were highest in spring and fall denitrified nitrate originated almost entirely from benthic nitrification at the exposed gulf site whereas water column nitrate dominated sedimentary denitrification at the sheltered bay site sediment incubations in stirred chambers were used to determine net fluxes of oxygen o2 n2 nitrate and ammonium across the sediment water interface during varied degrees of continuous pore water exchange rates of n2 efflux correlated with rates of pore water flow increasing from 0 12 mmol n m22 d21 under diffusion limited transport conditions up to 0 87 mmol n m22 d21 with pore water advection mineralized nitrogen was completely converted to n2 gas in gulf of mexico sediments our results demonstrate that advective pore water circulation will accelerate benthic n2 production by coupled nitrification denitrification and that substantial nitrogen loss occurs from coastal permeable sediments

regulation of benthic oxygen fluxes in permeable sediments of the coastal ocean large areas of the oceanic shelf are composed of sandy sediments through which reactive solutes are transported via porewater advection fueling active microbial communities the advective oxygen transport in permeable sands of the north sea was investigated under in situ conditions using a new benthic observatory to assess the dynamic interaction of hydrodynamics sediment morphodynamics and oxygen penetration depth during 16 deployments concurrent measurement of current velocity sediment topography and porewater oxygen concentration were carried out in all cases the oxyclines were found at depths of 1 6 cm correlating with the topography of stationary and migrating bedforms ripples different conditions in terms of bottom water currents and bedform migration led to fluctuating oxygen penetration depths and hence highly variable redox conditions in up to 2 5 cm thick layers beneath the surface volumetric oxygen consumption rates of surface sediments were measured on board in flow through reactors bedform migration was found to reduce consumption rates by up to 50 presumably caused by the washout of organic carbon that is otherwise trapped in the pore space of the sediment based on the observations we found oxygen penetration depths to be largely controlled by oxygen consumption rates grain size and current velocity these controlling variables are summarized by an adapted damkohler number which allows for prediction of oxygen penetretion depths based on a simple scaling law by integrating the oxygen consumption rates over the oxygen penetration depth oxygen fluxes of 8 34 mmol m 2 d 1 were estimated

enhancement of denitrification in permeable carbonate sediment due to intra granular porosity a multi scale modelling analysis abstract it has recently been hypothesized that bulk denitrification rates in carbonate sands may be enhanced by reactions occurring in the intra granular pores cracks and crevices we tested this hypothesis using a series of flow and reactive transport models spanning from the pore scale mm to the continuum scales 10 cm bedforms pore scale simulations solved the coupled navier stokes and brinkman equations and represented flow through reactor experiments previously performed on coral reef sands the results revealed that intra granular transport and reactions can explain over all denitrification enhancement a sensitivity study with a single grain diffusive transport model showed that in the majority of cases the resultant increase in denitrification was not coupled to nitrification within a single grain only for large grain diameters of 2 and 4 mm was coupled nitrification denitrification important in most cases coupled nitrification denitrification instead arose as conditions became more reducing along a flow path as is the case in quartz sands without intragranular pores an intra granular reaction rate based on a single grain model was incorporated into a continuum scale darcy flow and reactive transport model for a rippled sand bed where porewater flow is driven by the turbulent current over the ripple the results of the darcy scale model suggest that intra granular pores increase the amount of slow flowing areas within which coupled nitrification denitrification can occur we conclude that the complex advective flow field does not strongly inhibit denitrification enhancement by carbonate sand grains as it does in silica sands thus intra granular reactions may enhance bulk denitrification in carbonate sediment with porous grains under natural advective conditions

single discharge events increase reactive efficiency of the hyporheic zone in this study we investigate the impact of single stream discharge events on water exchange solute transport and reactions in the hyporheic zone below a natural in stream gravel bar we set up a reactive transport groundwater model with streamflow scenarios that vary by event duration and peak discharge a steady ambient groundwater flow field is assumed that results in losing neutral or gaining stream conditions depending on the stream stage across the streambed dissolved oxygen organic carbon and nitrate are transported into the subsurface additional nitrate is received from upwelling groundwater aerobic respiration and denitrification are simulated for scenarios with different stream solute concentrations results show that hyporheic exchange flux solute transport and consumption increase during events however their intensities depend highly on the interplay between event characteristics and ambient groundwater conditions during events where reversals in the hydraulic gradient occur stream water and solutes infiltrate deeper into the aquifer where they have more time to react for those events the reactive efficiency of the hyporheic zone solute consumption as fraction of influx for aerobic respiration and denitrification is up to 2 7 and 10 times higher compared to base flow conditions the fraction of stream nitrate load consumed in the hyporheic zone increases with stream discharge up to 150 mg m2 h but remains below the value under base flow conditions for weak events events also increase denitrification of groundwater borne nitrate but groundwater nitrate flux to the stream decreases by up to 33 due to temporary gradient reversals

quantifying denitrification in rippled permeable sands through combined flume experiments and modeling we measured denitrification in permeable sediments in a sealed flume tank with environmentally representative fluid flow and solute transport behavior using novel measurements numerical flow and reactive transport models representing the flume experiments were implemented to provide mechanistic insight into the coupled hydrodynamic and biogeochemical processes there was broad agreement between the model results and experimental data the model showed that the coupling between nitrification and denitrification was relatively weak in comparison to that in cohesive sediments this was due to the direct advective transport between anoxic pore water and the overlying water column and little interaction between the mostly oxic advective region and the underlying anoxic region denitrification was therefore mainly fueled by nitrate supplied from the water column this suggests that the capacity of permeable sediments with nonmigratory ripples to remove bioavailable nitrogen from coastal ecosystems is lower than that of cohesive sediments we conclude that while experimental measurements provide a good starting point for constraining key parameters reactive transport models with realistic kinetic and transport parameters provide critical insight into biogeochemical processes in permeable sediment that are difficult to experimentally evaluate

postsubduction porphyry cu au and epithermal au deposits products of remelting of subduction modified lithosphere porphyry cu mo au and some epithermal au deposits are formed from hydrothermal fluids exsolved from cooling water rich calc alkaline magmas emplaced in volcanoplutonic arcs above subduction zones these magmas originate by partial melting of the metasomatized asthenospheric mantle wedge however there is increasing evidence for the existence of a suite of porphyry cu au and epithermal au deposits related to magmas generated after subduction beneath the arc has ceased associated magmas tend to be mildly alkaline relatively sulfur poor and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs they are likely formed by remelting of previously subduction modified arc lithosphere triggered by postsubduction lithospheric thickening lithospheric extension or mantle lithosphere delamination metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first stage arc magmatism contain small amounts of chalcophile and siderophile element rich sulfides and constitute a fertile source for hydrous au rich but relatively sulfur poor magmas during later remelting the recognition that porphyry cu au and related epithermal au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits

hyporheic zone hydrologic science a historical account of its emergence and a prospectus the hyporheic zone defined by shallow subsurface pathways through river beds and banks beginning and ending at the river is an integral and unique component of fluvial systems it hosts myriad hydrologically controlled processes that are potentially coupled in complex ways understanding these processes and the connections between them is critical since these processes are not only important locally but integrate to impact increasingly larger scale biogeochemical functioning of the river corridor up to the river network scale thus the hyporheic zone continues to be a growing research focus for many hydrologists for more than half the history of water resources research this manuscript partly summarizes the historical development of hyporheic zone hydrologic science as gleaned from papers published in water resources research from the birth of the concept of the hyporheic zone as a hydrologic black box sometimes referred to as transient storage zone to its adolescent years of being torn between occasionally competing research perspectives of interrogating the hyporheic zone from a surface or subsurface view to its mature emergence as an interdisciplinary research field that employs the wide array of state of the art tools available to the modern hydrologist the field is vibrant and moving in the right direction of addressing critical fundamental and applied questions with no clear end in sight in its growth there are exciting opportunities for scientists that are able to tightly link the allied fields of geology geomorphology hydrology geochemistry and ecology to tackle the many open problems in hyporheic zone science

small scale permeability heterogeneity has negligible effects on nutrient cycling in streambeds 1 aquatic sediment hosts coupled pore water flow and biogeochemical reactions that mediate water quality across the fluvial corridor including adjacent alluvial aquifers however the effect of small scale variations in sediment hydraulic properties on nutrient transformation occurring within the sediment is poorly understood we show through numerical flow and transport simulations for two realistic heterogeneous permeability cases typical of lowland rivers and two idealized homogeneous ones that there is little difference in the reactive transport fields the bulk reaction rates and nutrient sink source function despite stark differences in flow fields this is because the reactions are ultimately controlled by characteristic or bulk residence times that are similar for the heterogeneous and homogeneous cases this is a promising result for predictive models based solely on relative ratios of residence times and reactive time scales

colloidal transport of gold and silica in epithermal precious metal systems evidence from the sleeper deposit nevada bonanza vein gold ore from the sleeper deposit nevada was deposited at a depth of less than 500 m and exhibits a characteristic texture of alternating colloform bands of fine grained quartz and bands of opaline silica containing abundant electrum textural evidence indicates that the quartz bands recrystallized from a gel like amorphous precursor whereas the gold rich bands were deposited originally as amorphous precipitate formed by the coagulation of colloidal silica and gold particles gold and silica precipitated initially as colloidal particles at deeper levels in the hydrothermal system and were mechanically transported upward by the hydraulic action of the ore forming fluids coagulation of the colloidal particles may have been caused by solution cooling or boiling and catalyzed by hydrated polycationic al 3 aqueous species transport of gold as colloidal particles in epithermal systems indicates that at least in some instances higher gold content is possible than in true solution a gold enriched ore fluid appears necessary to explain the origin of the bonanza grade ore at the sleeper mine and the colloidal gold transport process locally might have played a role in the genesis of the more common disseminated epithermal precious metal deposits

extensive nitrogen loss from permeable sediments off north west africa the upwelling area off north west africa is characterized by high export production high nitrate and low oxygen concentration in bottom waters the underlying sediment consists of sands that cover most of the continental shelf due to their permeability sands allow for fast advective pore water transport and can exhibit high rates of nitrogen n loss via denitrification as reported for anthropogenically eutrophied regions however n loss from sands underlying naturally eutrophied waters is not well studied and in particular n loss from the north west african shelf is poorly constrained during two research cruises in april may 2010 2011 sediment was sampled along the north west african shelf and volumetric denitrification rates were measured in sediment layers down to 8 cm depth using slurry incubations with 15n labeled nitrate areal n loss was calculated by integrating volumetric rates down to the nitrate penetration depth derived from pore water profiles areal n loss was neither correlated with water depth nor with bottom water concentrations of nitrate and oxygen but was strongly dependent on sediment grain size and permeability the derived empirical relation between benthic n loss and grains size suggests that pore water advection is an important regulating parameter for benthic denitrification in sands and further allowed extrapolating rates to an area of 53 000 km2 using detailed sediment maps denitrification from this region amounts to 995 kt yr 1 average 3 6 mmol m 2 d 1 which is 4 times higher than previous estimates based on diffusive pore water transport sandy sediments cover 50 60 of the continental shelf and thus may contribute significantly to the global benthic n loss

hyporheic flow and dissolved oxygen distribution in fish nests the effects of open channel velocity permeability patterns and groundwater upwelling many fish lay their eggs in nests or redds which they construct in sediment the viability of eggs depends on many factors particularly their oxygenation because dissolved oxygen is typically saturated within the stream channel the dissolved oxygen distribution within the redd depends on whether or not hyporheic flow and transport occur within the sediment we conducted a series of flume and numerical flow and age transport modeling experiments with the aim of understanding the effects of salmonid redds on the hyporheic transport of young oxygenated water hyporheic flow was visualized directly through dye injections dissolved oxygen throughout the fish nest was measured using a planar optode experiments were conducted at various open channel flow velocities in order to understand their effect on dissolved oxygen and computational simulations considered various sediment textures and ambient groundwater upwelling rates to add process level insight we found that as also shown by previous studies the redd topography induces multiscale hyporheic flow that effectively flushes the egg pocket location with younger presumably oxygenated water older water upwells and forms anoxic zones this pattern persists even at the lowest channel flow rates and at small upwelling velocities of older ambient groundwater which splits the multiscale hyporheic flow cells into isolated pockets large groundwater upwelling rates can shut down all the hyporheic flushing the relatively coarse texture of the redd further promotes hyporheic flushing of the redd sediment with oxygenated water thus redd morphology and sediment texture optimally combine to induce hyporheic exchange flow that delivers young oxygenated water to the egg pocket

the importance and challenge of hyporheic mixing the hyporheic zone is the interface beneath and adjacent to streams and rivers where surface water and groundwater interact the hyporheic zone presents unique conditions for reaction of solutes from both surface water and groundwater including reactions which depend upon mixing of source waters some models assume that hyporheic zones are well mixed and conceptualize the hyporheic zone as a surface water groundwater mixing zone but what are the controls on and effects of hyporheic mixing what specific mechanisms cause the relatively large 1m mixing zones suggested by subsurface solute measurements in this commentary we explore the various processes that might enhance mixing in the hyporheic zone relative to deeper groundwater and pose the question whether the substantial mixing suggested by field studies may be due to the combination of fluctuating boundary conditions and multi scale physical and chemical spatial heterogeneity we encourage investigation of hyporheic mixing using numerical modeling and laboratory experiments to ultimately inform field investigations