Uptake and fractionation of rare earth elements on hydrothermal plume particles at 9°45′N,East Pacific Rise

Particulate samples (>0.45 μm) from a neutrally buoyant hydrothermal plume at 9°45′N on the northern East Pacific Rise were collected using large volume in situ filtration and analyzed for Fe, Al, Mn, Ni, and fourteen rare earth elements (REE). The Sm/Fe ratio (a proxy for overall REE/Fe) and Nd/Er (light/heavy REE fractionation) increased moderately with decreasing particulate Fe. Chemically, the sense of these relationships matched that documented previously in the TAG plume on the Mid-Atlantic Ridge (German et al., 1990), although particulate Fe was about 10 fold lower at 9°45′N. Spatial trends relative to the vent source, however, were opposite of expectation because slow Fe(II) oxidation and Fe(III) colloid aggregation over this interval led to increased particulate Fe (10–26 nM) with distance from source (Field and Sherrell, submitted). After subtraction of non-plume background particle composition, plume particles at 9°45′N and TAG had indistinguishable ranges of light REE-enriched fractionation relative to ambient seawater and had very similar Sm/Fe (therefore Kd for Fe oxyhydroxides), demonstrating that plume particles in both oceans reflect to a first degree the local seawater REE composition. Within-plume REE variations at 9°45′N were investigated using a simple mixing model which accounts for the bulk Fe-Al-Mn variations in the plume using two endmembers: fresh hydrothermal oxyhydroxide precipitates and ridge-crest background particles (composed largely of locally resuspended sediment). Sm/Fe and Nd/Er plot linearly with mixing ratio (R > 0.96), implying that the observed REE trends result from mixing of these two endmembers. Extrapolation to the composition of pure hydrothermal precipitates suggests that Nd/Er is fractionated relative to seawater by a factor of 1.8 during adsorption onto fresh Fe oxyhydroxide particles. The ridge-crest background particles are 5 fold higher in Sm/Fe and Nd/Er is 2.49 relative to seawater, partly a result of enriched terrigenous component in the resuspended matter. A reinterpretation of REE at TAG reveals that positive curvature in REE vs. Fe plots, argued previously to reflect continuous REE uptake (i.e., increasing Kd; German et al., 1990), may result from local depletion of the dissolved REE pool by partitioning onto Fe particles at Fe > 100 nM. Similar drawdown effects could contribute to the variable degrees of curvature observed for all seawater-source particle-reactive species in plumes that are sampled at high particulate Fe concentration. In sum, REE behavior in hydrothermal plumes is more consistent with equilibrium adsorption and mixing of distinct particle types, than with kinetic uptake control. Precise measurements of REEs in modern ridge-crest metalliferous sediments could be compared to the endmember composition calculated from the plume data to evaluate long-term changes in REE of the hydrothermal component.     

Fluxes to sediments underlying the Rainbow hydrothermal plume at 36°14′N on the Mid-Atlantic Ridge

A geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the Rainbow hydrothermal field. As well as a large biogenic component (>80% CaCO₃) these sediments record clear enrichments of the elements Fe, Cu, Mn, V, P, and As from hydrothermal plume fallout but only minor detrital background material. Systematic variations in the abundances of “hydrothermal” elements are observed at increasing distance from the vent site, consistent with chemical evolution of the dispersing plume. Further, pronounced Ni and Cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near-field locations. Radiocarbon dating reveals mean Holocene accumulation rates for all four cores of 2.7 to 3.7 cm.kyr⁻¹, with surface mixed layers 7 to 10+ cm thick, from which a history of deposition from the Rainbow hydrothermal plume can be deduced. Deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced (e.g., Fe, Mn, Cu) or scavenged from seawater via the hydrothermal plume (e.g., V, P, As). Holocene fluxes into to the cores’ surface mixed layers are presented which, typically, are an order of magnitude greater than “background” authigenic fluxes from the open North Atlantic. One core, collected closest to the vent site, indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 ¹⁴C kyr ago; the preferred explanation is that this variation reflects the initiation/intensification of hydrothermal venting at the Rainbow hydrothermal field at this time—perhaps linked to some specific tectonic event in this fault-controlled hydrothermal setting.     

Mineralogy, geochemistry, and Nd isotope composition of the Rainbow hydrothermal field, Mid-Atlantic Ridge

Petrological, geochemical, and Nd isotopic analyses have been carried out on rock samples from the Rainbow vent field to assess the evolution of the hydrothermal system. The Rainbow vent field is an ultramafic-hosted hydrothermal system located on the Mid-Atlantic Ridge characterized by vigorous high-temperature venting (∼365°C) and unique chemical composition of fluids: high chlorinity, low pH and very high Fe, and rare earth element (REE) contents (Douville et al., Chemical Geology 184:37–48, 2002). Serpentinization has occurred under a low-temperature (<270°C) retrograde regime, later overprinted by a higher temperature sulfide mineralization event. Retrograde serpentinization reactions alone cannot reproduce the reported heat and specific chemical features of Rainbow hydrothermal fluids. The following units were identified within the deposit: (1) nonmineralized serpentinite, (2) mineralized serpentinite—stockwork, (3) steatite, (4) semimassive sulfides, and (5) massive sulfides, which include Cu-rich massive sulfides (up to 28wt% Cu) and Zn-rich massive sulfide chimneys (up to 5wt% Zn). Sulfide mineralization has produced significant changes in the sulfide-bearing rocks including enrichment in transition metals (Cu, Zn, Fe, and Co) and light REE, increase in the Co/Ni ratios comparable to those of mafic Cu-rich volcanic-hosted massive sulfide deposits and different ¹⁴³Nd/¹⁴⁴Nd isotope ratios. Vent fluid chemistry data are indicative of acidic, reducing, and high temperature conditions at the subseafloor reaction zone where fluids undergo phase separation most likely under subcritical conditions (boiling). An explanation for the high chlorinity is not straightforward unless mixing with high salinity brine or direct contribution from a magmatic Cl-rich aqueous fluid is considered. This study adds new data, which, combined with the current knowledge of the Rainbow vent field, brings compelling evidence for the presence, at depth, of a magmatic body, most likely gabbroic, which provides heat and metals to the system. Co/Ni ratios proved to be good tools used to discriminate between rock units, degree of sulfide mineralization, and positioning within the hydrothermal system. Deeper units have Co/Ni <1 and subsurface and surface units have Co/Ni >1.     

Iron isotope fractionation in a buoyant hydrothermal plume, 5°S Mid-Atlantic Ridge

Fe isotopes are a potential tool for tracing the biogeochemical redox cycle of Fe in the ocean. Specifically, it is hypothesized that Fe isotopes could enable estimation of the contributions from multiple Fe sources to the dissolved Fe budget, an issue that has received much attention in recent years. The first priority however, is to understand any Fe isotope fractionation processes that may occur as Fe enters the ocean, resulting in modification of original source compositions. In this study, we have investigated the Fe inputs from a basalt-hosted, deep-sea hydrothermal system and the fractionation processes that occur as the hot, chemically reduced and acidic vent fluids mix with cold, oxygen-rich seawater.The samples collected were both end-member vent fluids taken from hydrothermal chimneys, and rising buoyant plume samples collected directly above the same vents at 5°S, Mid-Atlantic Ridge. Our analyzes of these samples reveal that, for the particulate Fe species within the buoyant plume, 25% of the Fe is precipitated as Fe-sulfides. The isotope fractionation caused by the formation of these Fe-sulfides is δ_Fe(II)–FeS = +0.60 ± 0.12‰.The source isotope composition for the buoyant plume samples collected above the Red Lion vents is calculated to be −0.29 ± 0.05‰. This is identical to the value measured in end-member vent fluids collected from the underlying “Tannenbaum” chimney. The resulting isotope compositions of the Fe-sulfide and Fe-oxyhydroxide species in this buoyant plume are −0.89 ± 0.11‰ and −0.19 ± 0.09‰, respectively. From mass balance calculations, we have been able to calculate the isotope composition of the dissolved Fe fraction, and hypothesize that the isotope composition of any stabilised dissolved Fe species exported to the surrounding ocean may be heavier than the original vent fluid. Such species would be expected to travel some distance from areas of hydrothermal venting and, hence, contribute to not only the dissolved Fe budget of the deep-ocean but also it’s dissolved Fe isotope signature.     

Geochemistry of a sediment push-core from the Lucky Strike hydrothermal field,Mid-Atlantic Ridge

Hydrothermal sediment mineralogy and geochemistry can provide insights into seafloor mineralization processes and changes through time. We report a geochemical investigation of a short (22 cm) near-vent hydrothermal metalliferous sediment core from the Lucky Strike site (LS), on the Mid-Atlantic Ridge (MAR). The sediment was collected from the base of an active white smoker vent and comprises pure hydrothermal precipitates, mainly chalcopyrite, sphalerite, pyrite and barite, with negligible detrital and biogenic inputs. Geochemically, the core is enriched in elements derived from high-temperature hydrothermalism (Fe, Cu, Zn and Ba) and depleted in elements derived from low-temperature hydrothermalism (Mn), and metasomatism (Mg). The U/Fe content ratio is elevated, particularly in the deeper parts of the core, consistent with uptake from seawater associated with sulphide alteration. Rare earth elements (REE) concentrations are low and chondrite-normalized patterns are characteristic of high-temperature vent fluids with an enrichment in light REE and a pronounced positive Eu anomaly. A stronger positive Eu anomaly associated with higher La_n/Sm_n at the core top is controlled by barite precipitation. The hydrothermal influence on the REE decreases downcore with some evidence for a stronger seawater influence at depth. Nd isotopes also exhibit an increased detrital/seawater influence downcore. Pb isotope ratios are uniform and plot on the Northern Hemisphere Reference Line in a small domain defined by LS basalts and exhibit no detrital or seawater influence. Lucky Strike sediments are derived from high-temperature mineralization and are overprinted by a weak seawater–sediment interaction when compared with other Atlantic hydrothermal sites such as TAG. The larger seawater input and/or a larger detrital contribution in deeper layers can be explained by variable hydrothermal activity during sediment formation, suggesting different pulses in activity of the LS hydrothermal system.     

Tracing iron-fueled microbial carbon production within the hydrothermal plume at the Loihi seamount

The Loihi hydrothermal plume provides an opportunity to investigate iron (Fe) oxidation and microbial processes in a system that is truly Fe dominated and distinct from mid-ocean ridge spreading centers. The lack of hydrogen sulfide within the Loihi hydrothermal fluids and the presence of an oxygen minimum zone at this submarine volcano’s summit, results in a prolonged presence of reduced Fe within the dispersing non-buoyant plume. In this study, we have investigated the potential for microbial carbon fixation within the Loihi plume. We sampled for both particulate and dissolved organic carbon in hydrothermal fluids, microbial mats growing around vents, and the dispersing plume, and carried out stable carbon isotope analysis on the particulate fraction. The δ¹³C values of the microbial mats ranged from −23‰ to −28‰, and are distinct from those of deep-ocean particulate organic carbon (POC). The mats and hydrothermal fluids were also elevated in dissolved organic carbon (DOC) compared to background seawater. Within the hydrothermal plume, DOC and POC concentrations were elevated and the isotopic composition of POC within the plume suggests mixing between background seawater POC and a ¹³C-depleted hydrothermal component. The combination of both DOC and POC increasing in the dispersing plume that cannot solely be the result of entrainment and DOC adsorption, provides strong evidence for in-situ microbial productivity by chemolithoautotrophs, including a likelihood for iron-oxidizing microorganisms.     

Rare Earth, Major and Trace Elements in the Kunimiyama Ferromanganese Deposit in the Northern Chichibu Belt, Central Shikoku, Japan

Abstract. Rare earth, major and trace element geochemistry is reported for the Kunimiyama stratiform ferromanganese deposit in the Northern Chichibu Belt, central Shikoku, Japan. The deposit immediately overlies greenstones of mid-ocean ridge basalt (MORB) origin and underlies red chert. The ferromanganese ores exhibit remarkable enrichments in Fe, Mn, P, V, Co, Ni, Zn, Y and rare earth elements (excepting Ce) relative to continental crustal abundance. These enriched elements/ Fe ratios and Post-Archean Average Australian Shale-normalized REE patterns of the ferromanganese ores are generally analogous to those of modern hydrothermal ferromanganese plume fall-out precipitates deposited on MOR flanks. However in more detail, Mn and Ti enrichments in the ferromanganese ores are more striking than the modern counterpart, suggesting a significant contribution of hydrogenetic component in the Kunimiyama ores. Our results are consistent with the interpretation that the Kunimiyama ores were umber deposits that primarily formed by hydrothermal plume fall-out precipitation in the Panthalassa Ocean during the Early Permian and then accreted onto the proto-Japanese island arc during the Middle Jurassic. The presence of strong negative Ce anomaly in the Kunimiyama ores may indicate that the Early Permian Panthalassa seawater had a more striking negative Ce anomaly due to a more oxidizing oceanic condition than today.     

安徽霍邱BIF铁矿地球化学特征及其成矿意义：以班台子和周油坊矿床为例

安徽霍邱铁矿田位于华北克拉通南缘,是一个大型BIF铁矿田.本文对霍邱矿田班台子矿区和周油坊矿区的铁矿石及其赋存的岩石共28件样品进行了详细的主微量元素地球化学分析.分析结果表明,班台子矿区的片麻岩和角闪岩的原岩属于一套亚碱性系列的岩石,具有大离子亲石元素(LILE)富集,高场强元素(HFSE)明显亏损的火山弧岩石的特征.班台子角闪岩具有低的K2O含量和Ti/V值,Ti/V=22.7 ～ 25.9,平均24.5,与岛弧拉斑玄武岩一致.弧后盆地玄武岩化学组成具有类似岛孤拉斑玄武岩的特征.BIFs的形成往往需要构造稳定的半深水-深水盆地,孤后盆地能够为BIFs韵律条带的产生提供稳定的沉积环境,因此霍邱BIFs铁矿的大量出现说明班台子矿区角闪岩形成于弧后盆地,代表了霍邱铁矿形成的构造环境.班台子矿区铁矿石的(Eu/Eu*)SN=1.57 ～1.82,与Superior型(简称S型)BIFs特征一致;而周油坊矿区假象镜铁矿的(Eu/Eu*)SN=1.93 ～3.41,与Algoma型(简称A型)BIFs特征比较吻合.正Eu异常的强弱反应了成矿位置距离海底火山热液喷气口的远近.因此,我们推断霍邱地区BIFs型铁矿形成位置与海底火山热液喷气口的距离比较特别,处于A型向S型过渡的位置.角闪岩和片麻岩及其赋存的铁矿石的Al2O3和TiO2良好的线性相关性说明铁矿石铁质部分来源于侵蚀的弧后盆地玄武岩.Y/Ho比值=31.05 ～56.67,平均为46.65,说明霍邱铁矿继承了海水与热液的混合特征,其中,海水的贡献更大一些.周油坊矿区的大理岩主要化学组成CaO为28.49％ ～29.10％,MgO为20.25％ ～ 21.22％以及少量的SiO2(2.45％～6.10％).与平均显生宙石灰岩相比,周油坊大理岩亏损LILE和HFSE;与后太古代平均澳大利亚页岩(PAAS)相比,周油坊假象镜铁矿稀土元素总量低,明显正Eu异常,Ce无明显异常,Y/Ho比值介于35.00～56.67,平均48.81.这些特征显示大理岩及其赋存的假象镜铁矿形成于缺氧的海洋环境,海水中的氧能使亚铁离子氧化成三价铁离子沉淀出Fe(OH)3,但不足以使Ce3+氧化成Ce4+.     

渔塘坝硒矿床多元素富集与沉积环境

对采自湖北西部渔塘坝二叠系独立硒矿区的硅质岩和页岩样品进行了元素测试（微量元素、总铁、总有机碳、总硫）,分析了该区域的多元素富集及沉积环境。该区域V、Cu、As、Cd、Tl、Se、Mo、V、 Sb、 U、Ag、 Cr、Ni相对富集, Ba、Hf、Ta、W、Pb、Th相对贫化或亏损。全硫—全铁（TS—TFe）的关系表明页岩及部分硅质岩的S/Fe比值均大于1.15,说明有过量的硫存在,可能为有机硫或为其他亲硫元素沉淀提供条件。TOC—TS—TFe三角图及V/(V + Ni)表明该区域的岩石形成于缺氧或静海相环境。U/Th、Zr—Cr、P—Y等微量元素地球化学参数表明该区域有热水沉积的存在。通过对微量元素可能的来源研究分析,该区域的微量元素可能存在多种来源:热水来源、海水沉积、峨眉山玄武岩。     

Multi-element and rare earth element composition of lichens,mosses, and vascular plants from the Central Barrenlands,Nunavut, Canada

Lichen (n=12) and moss (n=6) species from a remote region of northern Canada have remarkably similar multi-element patterns suggesting they are non-specific accumulators of metals under existing conditions. Within individual species the concentration of many metals analyzed range over an order of magnitude. Many elements have a positive correlation with multi-element (n=48) and REE (rare earth element) totals. Others, such as Cd, K, and Zn have relatively consistent concentrations across all lichen and moss species, and across all sampling sites, indicating different accumulation and/or retention processes. Lichens and mosses have REE concentrations 1–3 orders of magnitude less than those of the average upper continental crust (UCC) but yield identical patterns. The correlation of other poorly soluble elements and key elemental ratios in lichen and moss are also similar to UCC and modern river sediment values. Metals including Sc, V, Cr, Fe, Co, Ga, Y, Hf, W, Pb, Th, and U show strong positive correlations with REE in lichen and moss. Rare earth elements may be useful as reference elements in environmental studies because of transport in the particulate phase, lack of significant anthropogenic sources, coherent group geochemistry, generally robust concentrations, and upper crustal signatures. Further, the REE may be helpful in identifying particulate deposition related to anthropogenic activities and enrichment of other elements by biogenic processes. The multi-element compositions of vascular plants (leaves and twigs) are fundamentally different from those of lichen and moss, lack correlation with REE, and are extremely enriched for many elements (100–1000× average upper continental crust) relative to the REE; perhaps because of limited REE solubility and transport via root systems. Enrichment factors for most metals of environmental concern are low; Pb is elevated but may be an artifact of low concentrations in local bedrock. Trace metal concentrations in lichen and moss at Otter Lake are similar to those measured across the Northwest Territories over 25 a ago.     

Vent fluid chemistry of the Rainbow hydrothermal system (36°N, MAR): Phase equilibria and in situ pH controls on subseafloor alteration processes

The Rainbow hydrothermal field is located at 36°13.8′N-33°54.15′W at 2300 m depth on the western flank of a non-volcanic ridge between the South AMAR and AMAR segments of the Mid-Atlantic Ridge. The hydrothermal field consists of 10-15 active chimneys that emit high-temperature (∼365 °C) fluid. In July 2008, vent fluids were sampled during cruise KNOX18RR, providing a rich dataset that extends in time information on subseafloor chemical and physical processes controlling vent fluid chemistry at Rainbow. Data suggest that the Mg concentration of the hydrothermal end-member is not zero, but rather 1.5-2 mmol/kg. This surprising result may be caused by a combination of factors including moderately low dissolved silica, low pH, and elevated chloride of the hydrothermal fluid. Combining end-member Mg data with analogous data for dissolved Fe, Si, Al, Ca, and H₂, permits calculation of mineral saturation states for minerals thought appropriate for ultramafic-hosted hydrothermal systems at temperatures and pressures in keeping with constraints imposed by field observations. These data indicate that chlorite solid solution, talc, and magnetite achieve saturation in Rainbow vent fluid at a similar pH_(T,P) (400 °C, 500 bar) of approximately 4.95, while higher pH values are indicated for serpentine, suggesting that serpentine may not coexist with the former assemblage at depth at Rainbow. The high Fe/Mg ratio of the Rainbow vent fluid notwithstanding, the mole fraction of clinochlore and chamosite components of chlorite solid solution at depth are predicted to be 0.78 and 0.22, respectively. In situ pH measurements made at Rainbow vents are in good agreement with pH_(T,P) values estimated from mineral solubility calculations, when the in situ pH data are adjusted for temperature and pressure. Calculations further indicate that pH_(T,P) and dissolved H₂ are extremely sensitive to changes in dissolved silica owing to constraints imposed by chlorite solid solution-fluid equilibria. Indeed, the predicted correlation between dissolved silica and H₂ defines a trend that is in good agreement with vent fluid data from Rainbow and other high-temperature ultramafic-hosted hydrothermal systems. We speculate that the moderate concentrations of dissolved silica in vent fluids from these systems result from hydrothermal alteration of plagioclase and olivine in the form of subsurface gabbroic intrusions, which, in turn are variably replaced by chlorite + magnetite + talc ± tremolite, with important implications for pH lowering, dissolved sulfide concentrations, and metal mobility.     

辽宁歪头山铁矿床两类矿石地球化学特征及其对成矿作用的制约

歪头山铁矿床是鞍山-本溪地区条带状铁建造（BIFs）的典型代表,并且有一定规模的富铁矿分布。本文主要报道了矿区内较贫铁矿石和较富铁矿石的地球化学特征,结果表明两类矿石既有一致性又存在差异性。一致性表现在：所有矿石主要由TFe2O3和SiO2组成,其他氧化物含量很少,结合微量元素和稀土元素特征,指示其为一种化学沉积岩,但受到了火山热液作用的影响;矿石的稀土元素总量很低,经太古界后平均澳大利亚页岩（PAAS）标准化后,呈现重稀土相对富集,轻稀土相对亏损的配分模式,都具有明显的Eu正异常,和比较高的Y/Ho比值,与热液相关的Cr、Co、Ni含量也相对较高,暗示其成矿物质来源于海底热液,同时具有海水的特征。差异性表现在：较富铁矿石具有明显的热液特征,并且K2O含量大于Na2O含量,某些微量元素组成也与混合花岗岩具有相似性,结合野外地质特征,暗示较富铁矿石可能是在较贫铁矿石的基础上受混合岩化热液作用形成的。     

Geochemistry of rare earth elements in micro-and macronodules from the pacific bioproductive zone

Concentrations and compositions of rare earth elements (REE) in three micronodule fractions (50–250, 250–500, and >500 μm), coexisting macronodules, and host sediments are examined. The samples were collected from three sites (Guatemala Basin, Peru Basin, and northern equatorial Pacific) located in elevated bioproductivity zones of the surficial water. The influence of micronodule size is dominant for REE compositions and subordinate for REE concentrations. For example, the Ce concentration inversely correlates with the micronodule fraction dimension and drops to the lowest value in macronodules and host sediments. The Ce decrease is generally accompanied by the Mn/Fe increase in micro- and macronodules. Hence, the role of diagenetic source of material directly correlates with the micronodule dimension. The contribution of diagenetic source is maximal for macronodules. The REE signature distinctions of micronodules and macronodules can be attributed to variations of hydrogenic iron oxyhydroxides and diagenetic (hydrothermal) iron hydroxophosphates that are the major REE carriers in ferromanganese ore deposits. The relationship and general trend in the chemistry of coexisting macronodules suggest that they can represent products of the initial stage of nodule formation.     

The chemistry of lava-seawater interactions II: the elemental signature

The flow of lava into the ocean at the shoreline of Kilauea Volcano during the ongoing Pu’u O’o eruption has allowed a detailed study of the geochemical interaction between lava and seawater. This paper focuses on the chemistry of the major and minor elements in the fluids that resulted from this interaction. The elemental enrichments in these fluids are dominated by three processes: (1) evaporation of water from seawater, which creates solutions enriched in the major elements found in seawater, (2) congruent dissolution of the basalt glass matrix, which is limited by the solubility of some of the elements in seawater, and (3) removal of volatile phases from the lava on contact with seawater.Using a simple model of volatile emanation (using published emanation coefficients) and congruent dissolution, we are able to explain the concentrations observed for the majority of elements in precipitation from the steam plume at the shoreline lava entry and in water allowed to interact with molten lava in controlled experiments. Fe, Al, Ti, and some of the rare earth elements (REEs) in precipitation samples from the steam plume at the lava entry were > 10,000-times enriched over their ambient seawater concentrations, suggesting that these elements may be useful for identifying submarine eruptions. The flux of elements from the Kilauea ocean lava entry is greater than that from a typical midocean ridge hydrothermal vent field for Al, Cd, Co, and the REEs, whereas the opposite is true for the remainder of the elements studied.     

Geochemical Constraints on the Origin of Banded Iron Formation‐Hosted Iron Ore from the Archaean Ntem Complex (Congo Craton) in the Meyomessi Area,Southern Cameroon

Banded iron formations (BIFs) are the most significant source of iron in the world. In this study, we report petrographic and geochemical data of the BIF from the Meyomessi area in the Ntem Complex, southern Cameroon, and discuss their genesis and the iron enrichment process. Field investigations and petrography have revealed that the studied BIF samples are hard; compact; weakly weathered; and composed of magnetite, subordinate quartz, and geothite. The geochemical composition of the whole rock reveals that iron and silica represent more than 98 wt% of the average composition, whereas Al₂O₃, TiO₂, and high‐field strength elements (HFSE) contents are very low, similar to detritus‐free marine chemical precipitates. The total iron (TFe) contents range from 48.71 to 65.32 wt % (average of 53.29 wt %) and, together with the low concentrations of deleterious elements (0.19 wt % P on average), are consistent with medium‐grade iron ores by global standards. This interpretation is confirmed by the SiO₂/Fe₂O_3total versus (MgO + CaO + MnO)/Fe₂O_3total discrimination plot in which most of the Meyomessi BIF samples fall in the field of medium‐grade siliceous ore. Only one sample (MGT94) plots in the high‐grade magnetite–geothite ore domain. The high Fe/Ti (376.36), Fe/Al (99.90), and Si/Al (29.26) ratios of the sample are consistent with significant hydrothermal components. The rare earth elements (REE) contents of the studied BIF samples are very low (∑REE: 0.81–1.47 ppm), and the Post‐Archaean Australian Shale (PAAS)‐normalized patterns display weak positive Eu anomalies (Eu/Eu*: 1.15–1.33), suggesting a syngenetic low‐temperature hydrothermal solutions, similar to other BIF worldwide. However, the Meyomessi BIFs show high Fe contents when compared to the other BIFs. This indicates an epigenetic mineralization process affected the Meyomessi BIF. From the above results and based on the field and analytical data, we propose that the genetic model of iron ores at the Meyomessi area involves two stages of the enrichment process, hypogene enrichment of BIF protore by metamorphic and magmatic fluids followed by supergene alteration as indicated by the presence of goethite in the rocks.     

In-situ LA-ICP-MS trace elemental analyses of magnetite: The Bayan Obo Fe-REE-Nb deposit,North China

The Bayan Obo Fe-REE-Nb deposit in northern China is the world's largest light REE deposit, and also contains considerable amounts of iron and niobium metals. Although there are numerous studies on the REE mineralization, the origin of the Fe mineralization is not well known. Laser ablation (LA) ICP-MS is used to obtain trace elements of Fe oxides in order to better understand the process involved in the formation of magnetite and hematite associated with the formation of the giant REE deposit. There are banded, disseminated and massive Fe ores with variable amounts of magnetite and hematite at Bayan Obo. Magnetite and hematite from the same ores show similar REE patterns and have similar Mg, Ti, V, Mn, Co, Ni, Zn, Ga, Sn, and Ba contents, indicating a similar origin. Magnetite grains from the banded ores have Al + Mn and Ti + V contents similar to those of banded iron formations (BIF), whereas those from the disseminated and massive ores have Al + Mn and Ti + V contents similar to those of skarn deposits and other types of magmatic-hydrothermal deposits. Magnetite grains from the banded ores with a major gangue mineral of barite have the highest REE contents and show slight moderate REE enrichment, whereas those from other types of ores show light REE enrichment, indicating two stages of REE mineralization associated with Fe mineralization. The Bayan Obo deposit had multiple sources for Fe and REEs. It is likely that sedimentary carbonates provided original REEs and were metasomatized by REE-rich hydrothermal fluids to form the giant REE deposit.     

Insights to magmatic-hydrothermal processes in the Manus back-arc basin as recorded by anhydrite

Microchemical analyses of rare earth element (REE) concentrations and Sr and S isotope ratios of anhydrite are used to identify sub-seafloor processes governing the formation of hydrothermal fluids in the convergent margin Manus Basin, Papua New Guinea. Samples comprise drill-core vein anhydrite and seafloor massive anhydrite from the PACMANUS (Roman Ruins, Snowcap and Fenway) and SuSu Knolls (North Su) active hydrothermal fields. Chondrite-normalized REE patterns in anhydrite show remarkable heterogeneity on the scale of individual grains, different from the near uniform REE_N patterns measured in anhydrite from mid-ocean ridge deposits. The REE_N patterns in anhydrite are correlated with REE distributions measured in hydrothermal fluids venting at the seafloor at these vent fields and are interpreted to record episodes of hydrothermal fluid formation affected by magmatic volatile degassing. ⁸⁷Sr/⁸⁶Sr ratios vary dramatically within individual grains between that of contemporary seawater and that of endmember hydrothermal fluid. Anhydrite was precipitated from a highly variable mixture of the two. The intra-grain heterogeneity implies that anhydrite preserves periods of contrasting hydrothermal versus seawater dominant near-seafloor fluid circulation. Most sulfate δ³⁴S values of anhydrite cluster around that of contemporary seawater, consistent with anhydrite precipitating from hydrothermal fluid mixed with locally entrained seawater. Sulfate δ³⁴S isotope ratios in some anhydrites are, however, lighter than that of seawater, which are interpreted as recording a source of sulfate derived from magmatic SO₂ degassed from underlying felsic magmas in the Manus Basin. The range of elemental and isotopic signatures observed in anhydrite records a range of sub-seafloor processes including high-temperature hydrothermal fluid circulation, varying extents of magmatic volatile degassing, seawater entrainment and fluid mixing. The chemical and isotopic heterogeneity recorded in anhydrite at the inter- and intra-grain scale captures the dynamics of hydrothermal fluid formation and sub-seafloor circulation that is highly variable both spatially and temporally on timescales over which hydrothermal deposits are formed. Microchemical analysis of hydrothermal minerals can provide information about the temporal history of submarine hydrothermal systems that are variable over time and cannot necessarily be inferred only from the study of vent fluids.     

The effects of the San Francisco Bay plume on trace metal and nutrient distributions in the Gulf of the Farallones

The distributions of particulate elements (Al, P, Mn, Fe, Co, Cu, Zn, Cd, and Pb), dissolved trace metals (Mn, Fe, Co, Cu, Zn, and Cd), and dissolved nutrients (nitrate, phosphate, and silicic acid) were investigated in the Gulf of the Farallones, a region of high productivity that is driven by the dynamic mixing of the San Francisco Bay plume, upwelled waters, and California coastal surface waters. Particulate metals were separated into >10 and 0.4-10 μm size-fractions and further fractionated into leachable (operationally defined with a 25% acetic acid leach) and refractory particulate concentrations. Dissolved metals (< 0.4 μm pore-size filtrate) were separated into colloidal (0.03-0.4 μm) and soluble (<0.03 μm) fractions. The percent leachable particulate fractions ranged from 2% to 99% of the total particulate concentration for these metals with Mn and Cd being predominantly leachable and Fe and Al being predominantly refractory. The leachable particulate Pb concentration was associated primarily with suspended sediments from San Francisco Bay and was a tracer of the plume in coastal waters. The particulate trace metal data suggest that the leachable fraction was an available source of trace metal micronutrients to the primary productivity in coastal waters. The dissolved trace metals in the San Francisco Bay plume and freshly upwelled surface waters were similar in concentration, with the exception of Cu and Co, which exhibited relatively high concentrations in plume waters and served as tracers of this water mass. The dissolved data and estimates of the plume dynamics suggest that the impact of anthropogenic inputs of nutrients and trace metals in the San Francisco Bay plume contributes substantially to the concentrations found in the Gulf of the Farallones (10-50% of estimated upwelled flux values), but does not greatly disrupt the natural stoichiometric balance of trace metal and nutrient elements within coastal waters given the similarity in concentrations to sources in upwelled water. In all, the data from this study demonstrate that the flux of dissolved nutrients and bioactive trace metals from the San Francisco Bay plume contribute to the high and relatively constant phytoplankton biomass observed in the Gulf of the Farallones.     

Metavolcanic host rocks,mineralization, and gossans of the Shaib al Tair and Rabathan volcanogenic massive sulphide deposits of the Wadi Bidah Mineral District,Saudi Arabia

The Wadi Bidah Mineral District of Saudi Arabia contains more than 16 small outcropping stratabound volcanogenic Cu–Zn–(Pb) ± Au-bearing massive sulphide deposits and associated zones of hydrothermal alteration. Here, we use major and trace element analyses of massive sulphides, gossans, and hydrothermally altered and least altered metamorphosed host rock (schist) from two of the deposits (Shaib al Tair and Rabathan) to interpret the geochemical and petrological evolution of the host rocks and gossanization of the mineralization. Tectonic interpretations utilize high-field-strength elements, including the rare earth elements (REE), because they are relatively immobile during hydrothermal alteration, low-grade metamorphism, and supergene weathering and therefore are useful in constraining the source, composition, and physicochemical parameters of the primary igneous rocks, the mineralizing hydrothermal fluid and subsequent supergene weathering processes. Positive Eu anomalies in some of the massive sulphide samples are consistent with a high temperature (>250°C) hydrothermal origin, consistent with the Cu contents (up to 2 wt.%) of the massive sulphides. The REE profiles of the gossans are topologically similar to nearby hydrothermally altered felsic schists (light REE (LREE)-enriched to concave-up REE profiles, with or without positive Eu anomalies) suggesting that the REE experienced little fractionation during metamorphism or supergene weathering. Hydrothermally altered rocks (now schists) close to the massive sulphide deposits have high base metals and Ba contents and have concave-up REE patterns, in contrast to the least altered host rocks, consistent with greater mobility of the middle REE compared to the light and heavy REE during hydrothermal alteration. The gossans are interpreted to represent relict massive sulphides that have undergone supergene weathering; ‘chert’ beds within these massive sulphide deposits may be leached wall-rock gossans that experienced silicification and Pb–Ba–Fe enrichment from acidic groundwaters generated during gossan formation.     

Dissolved iron anomaly in the deep tropical-subtropical Pacific: Evidence for long-range transport of hydrothermal iron

Dissolved iron profiles along a north-south transect along 158°W in the tropical Pacific show evidence of two deepwater anomalies. The first extends from Station ALOHA (22.78°N) to the equator at ∼1000-1500 m and lies below the maximum apparent oxygen utilization and nutrient (N, P) concentrations. The feature is not supported by vertical export processes, but instead corresponds with the lateral dilution field of δ³He derived from the Loihi seamount, Hawaii, though a sediment source associated with the Hawaiian Island Chain cannot be entirely ruled out. The second, deeper (2000-3000 m) anomaly occurs in tropical South Pacific waters (7°S) and also does not correlate with the depths of maximum nutrient concentrations or apparent oxygen utilization, but it does coincide closely with δ³He emanating from the East Pacific Rise, more than 5000 km to the east. We hypothesize that these anomalies represent the long-range (>2000 km) transport of hydrothermal iron residuals, stabilized against scavenging by complexation with excess organic ligands in the plume source regions. Such trace leakage of hydrothermal iron to distal plume regions would have been difficult to identify in most hydrothermal vent mapping studies because low analytical detection limits were not needed for the proximal plume regions. These findings suggest that hydrothermal activity may represent a major source of dissolved iron throughout the South Pacific deep basin today, as well as other regions having high mid-ocean spreading rates in the geologic past. In particular, we hypothesize that high spreading rates along the South Atlantic and Southern Ocean mid-oceanic ridges, combined with the upwelling ventilation of these distal hydrothermal plumes, may have increased ocean productivity and carbon export in the Southern Ocean. Assessing the magnitude and persistence of dissolved hydrothermal iron in basin scale deep waters will be important for understanding the marine biogeochemistry of iron and, potentially, on ocean productivity and climate change during the geologic past.