Hydrothermal Source Research Articles

Silicon isotopes and trace elements in chert record early Archean basin evolution

Silicon isotopes of chemical sediments have received growing attention, given their applicability in the search for properties of ancient seawater. An important target is the reconstruction of secular changes in surface temperature of the Precambrian Earth, but interpretations are problematic since controls of the isotopic signals are potentially manifold. Here we report the existence of significant silicon isotope variability in chemically precipitated chert layers covering a continuous stratigraphic section across the ~3.42Ga Buck Reef Chert (BRC) in the Barberton Greenstone Belt (South Africa). Black chert bands are variably but consistently higher in δ30Si (up to ca. 0.9‰) than co-existing translucent counterparts at the same stratigraphic level, which is interpreted as a primary feature acquired during deposition upon interaction between submarine discharging hydrothermal water and a stratified water body. Averaged δ30Si values range from +0.3‰ (maximum +0.5‰) for shallow-marine sediments in the lower part of the section to −0.5‰ (minimum −1.3‰) for deeper water deposits in the upper part when results from translucent and black cherts are taken together. An average shift of ca. 0.8‰ (maximum 1.8‰) thus accompanied the formation of chemical cherts in a single Archean basin. Cherts at the base of the section represent silicified felsic volcanics and volcaniclastics and have exclusively positive δ30Si values between +0.1‰ and +0.7‰.Rare-earth-elements-and-yttrium patterns confirm the marine origin of the cherts and support the presumed basin evolution. From the combined evidence we infer that the δ30Si variations in the BRC reflect changes in the predominant origin of the silica, with terrigenous input supplying positive δ30Si to shallow waters and seafloor hydrothermal sources negative δ30Si to deeper levels. Our findings demonstrate the viability of silicon isotopes in cherts for reconstructing the evolution of ancient marine basins.

Isotopic geochemistry of the Jinwozi gold deposit in the eastern Tianshan orogen, NW China: implications for the ore genesis

The Jinwozi gold deposit in the eastern Tianshan orogenic belt in northwest China has been mined for more than 100 years and previous studies have mostly classified it as an intrusion‐related gold deposit. In this paper, the age of the Jinwozi intrusion and the isotopic geochemistry characteristics of the deposit were studied to investigate the ore genesis and ore genetic type. Results showed that: (1) The Jinwozi pluton, which is the host rock of the orebodies, was emplaced during 395–390 Ma, thus substantially predating gold mineralization (243–228 Ma); (2) δ18O values of fluid in the main mineralized stage (stage II) vary from −0.9 to 5.0‰, with an average of 2.0‰. Those of late quartz–calcite stage (stage III) fluids are between −13.3 and 0.4‰, with an average of −7.6‰. δD values of fluids in stage II vary from −97 to −84‰, with an average of −89‰, slightly higher than those of stage III fluids (−85 to −120‰, average −108‰). δ13C values of the ore‐forming fluid in stage II vary from −0.3 to 3.2‰, with an average of 1.3‰. δ34S values of pyrite from both stage II and III are generally high, ranging from 3.9 to 7.7‰, with an average of 6.0‰. Lead isotopic compositions of sulphides from stage II and III are similar, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios have ranges from 17.7780 to 18.4750, 15.4881 to 15.6965, 38.1560 to 38.7370, respectively. Two Pb isotope model ages of 450 and 268 Ma were yielded from these data. The oxygen, hydrogen, carbon, sulphur and lead isotope compositions all indicate that the ore‐forming fluids at Jinwozi may dominantly derive from a mixture of metamorphic and meteoric water, but is unlikely from a magmatic–hydrothermal source. Based on the geological and geochemical evidence, we infer that the Jinwozi gold mineralization may have occurred during the collision between the Kazakhstan–Junggar Plate and the Tarim Plate, accompanied by metamorphism which generated the ore‐forming fluids through the dehydration and decarbonation of country rocks. Such processes, combining with other geological and geochemical characteristics, indicate that the Jinwozi gold deposit can be classified as orogenic type. Copyright © 2014 John Wiley & Sons, Ltd.

Geology and fluid characteristics of the Ulu Sokor gold deposit, Kelantan, Malaysia: Implications for ore genesis and classification of the deposit

The Ulu Sokor gold deposit is one of the most famous and largest gold deposits in Malaysia and is located in the Central Gold Belt. This deposit consists of three major orebodies that are related to NS- and NE-striking fractures within fault zones in Permian-Triassic meta-sedimentary and volcanic rocks of the East Malaya Block. The faulting events represent different episodes that are related to each orebody and are correlated well with the mineralogy and paragenesis. The gold mineralization consists of quartz-dominant vein systems with sulfides and carbonates. The hydrothermal alteration and mineralization occurred during three stages that were characterized by (I) silicification and brecciation; (II) carbonatization, sericitization, and chloritization; and (III) quartz–carbonate veins.Fluid inclusions in the hydrothermal quartz and calcite of the three stages were studied. The primary CO2–CH4–H2O–NaCl fluid inclusions in stage I are mostly related to gold mineralization and display homogenization temperatures of 269–389°C, salinities of 2.77–11.89wt.% NaCl equivalent, variable CO2 contents (typically 5–29mol%), and up to 15mol% CH4. In stage II, gold was deposited at 235–398°C from a CO2±CH4–H2O–NaCl fluid with a salinity of 0.83–9.28wt.% NaCl equivalent, variable CO2 contents (typically 5–63mol%), and up to 4mol% CH4. The δ18OH2O and δD values of the ore-forming fluids from the stage II quartz veins are 4.5 to 4.8‰ and −44 to −42‰, respectively, and indicate a metamorphic–hydrothermal origin. Oxygen fugacities calculated for the entire range of T-P-XCO2 conditions yielded log fO2 values between −28.95 and −36.73 for stage I and between −28.32 and −39.18 for stage II. These values indicate reduced conditions for these fluids, which are consistent with the mineral paragenesis, fluid inclusion compositions, and isotope values.The presence of daughter mineral-bearing aqueous inclusions is interpreted to be a magmatic signature of stage IIIa. Combined with the oxygen and hydrogen isotopic compositions (δ18OH2O=6.8 to 11.9‰, δD=−77 to −62‰), these inclusions indicate that the initial fluid was likely derived from a magmatic source. In stage IIIb, the gold was deposited at 263° to 347°C from a CO2–CH4–H2O–NaCl fluid with a salinity of 5.33 to 11.05wt.% NaCl equivalent, variable CO2 contents (typically 9–15mol%), and little CH4. The oxygen and hydrogen isotopic compositions of this fluid (δ18OH2O=8.1 to 8.8‰, δD=−44 to −32‰) indicate that it was mainly derived from a metamorphic–hydrothermal source. The CO2–H2O±CH4–NaCl fluids that were responsible for gold deposition in the stage IIIc veins had a wide range of temperatures (214–483°C), salinities of 1.02 to 21.34wt.% NaCl equivalent, variable CO2 contents (typically 4–53mol%), and up to 7mol% CH4. The oxygen and hydrogen isotopic compositions (δ18OH2O=8.5 to 9.8‰, δD=−70 to −58‰) were probably acquired at the site of deposition by mixing of the metamorphic–hydrothermal fluid with deep-seated magmatic water and then evolved by degassing at the site of deposition during mineralization. The log fO2 values from −28.26 to −35.51 also indicate reduced conditions for this fluid in stage IIIc. Moreover, this fluid had a near-neutral pH and δ34S values of H2S of −2.32 to 0.83‰, which may reflect the derivation of sulfur from the subducted oceanic lithospheric materials.The three orebodies represent different gold transportation and precipitation models, and the conditions of ore formation are related to distinct events of hydrothermal alteration and gold mineralization. The gold mineralization of the Ulu Sokor deposit occurred in response to complex and concurrent processes involving fluid immiscibility, fluid–rock reactions, and fluid mixing. However, fluid immiscibility was the most important mechanism for gold deposition and occurred in these orebodies, which have corresponding fluid properties, structural controls, geologic characteristics, tectonic settings, and origins of the ore-forming matter. These characteristics of the Ulu Sokor deposit are consistent with its classification as an orogenic gold deposit, while some of the veins are genetically related to intrusions.

Dissolved Al in the zonal N Atlantic section of the US GEOTRACES 2010/2011 cruises and the importance of hydrothermal inputs

The distribution of dissolved aluminium determined during GA03, the US GEOTRACES North Atlantic Transects (US GT NAZT) shows large inputs to the basin from three main sources, atmospheric deposition, outflow from the Mediterranean, and inputs from hydrothermal sources along the Mid Atlantic Ridge (MAR).The partial dissolution of atmospheric aerosols emanating from the Sahara yield high concentrations of dissolved Al in the surface waters of the basin and are used to estimate the geographical pattern of dust deposition. The Mediterranean outflow delivers a large source of dissolved Al to the intermediate waters of the eastern basin and its subsequent distribution within the basin can be explained by simple isopycnal mixing with surrounding water masses.Hydrothermal venting at the Trans-Atlantic Geotraverse (TAG) hydrothermal field in the MAR produces a neutrally buoyant plume that introduces copious quantities of dissolved Al (with concentrations of up to 40nM) to the deeper waters of the North Atlantic that can be seen advecting to the west of the MAR. The concentration of dissolved Al in the deep waters of the eastern basin of the Atlantic can be accounted for by admixing the MAR Al enriched plume water and Antarctic Bottom Water (AABW) as they pass through the Vema Fracture Zone. The data sets show no evidence for biological remineralisation of dissolved Al from Si carrier phases in deep waters.

Exceptional silica speleothems in a volcanic cave: A unique example of silicification and sub‐aquatic opaline stromatolite formation (Terceira, Azores)

AbstractSilica stromatolites occur in a number of modern hydrothermal environments, but their formation in caves is very rare. The silica stromatolitic speleothems of the Branca Opala cave (Terceira Island, Azores), however, provide an excellent opportunity for their study. These formations may be analogous to ancient silica stromatolites seen around the world. Petrographic, mineralogical and geochemical analyses were undertaken on the silica speleothems of the above cave, and on the silica‐tufa deposits outside it, with the aim of understanding their genesis. The possible hydrothermal origin of their silica is discussed. X‐ray diffraction analyses showed opal‐A to be the sole silica phase. Negligible ordering of this opal‐A showed ageing to be insignificant, as expected for recent silica deposits. Most of the silica speleothems examined were definable as sub‐aquatic opaline stromatolites that are not currently growing. Optical microscopy clearly revealed a lower microlaminated, an intermediate and an upper microlaminated zone within the stromatolites. Stromatolite types (I, II and III) were classified with respect to their internal structure and distribution throughout the cave. Scanning electron microscopy showed silicified bacterial filaments within the stromatolites, the silicified plant remains and the silica‐tufa deposits. Bacteria therefore played a major role in the precipitation of the opal‐A. Plasma emission/mass spectrometry showed major, minor and rare earth elements to be present in only small quantities. The rare earth elements were mainly hosted within volcanic grains. Rapid silica precipitation from highly super‐saturated water would explain the intense silicification of the plant remains found inside and outside the cave. The opaline stromatolites, the silica‐tufa deposits and the above‐mentioned intense general silicification suggest a local hydrothermal source for the silica. Indeed, these deposits strongly resemble plant‐rich silica sinter associated with low‐temperature hot spring deposits that include bacterial filaments. However, no geochemical signals that might indicate a hydrothermal origin could be found.

The gravimetric picture of magmatic and hydrothermal sources driving hybrid unrest on Tenerife in 2004/5

We present results from the inversion of gravity changes observed at the central volcanic complex (CVC) of Tenerife, Canary Islands, between May 2004 and July 2005. Marking a period of elevated activity and a reawakening of the volcanic system, the data depict spatial and temporal variations in the sub-surface processes that defined this period of unrest at the Pico Viejo (PV)–Pico Teide (PT) complex, after the last volcanic eruption on Tenerife in 1909. An initial non-linear inversion, based on 3D line segments approximation, yielded three line segments at depths between 1kma.s.l. and 2km b.s.l. Our interpretation of the initial inversion results is that the line segments represent apparent composite sources, a superposition of deep and shallow seated sources. We therefore decomposed the gravity changes into shallow and deep parts (fields) using a procedure based on triple harmonic continuation. The shallow and deep fields could then be inverted separately, using the same inversion methodology. The deep field constrains two connected line segments at the depth of about 6km b.s.l., in the center of the NW seismogenic zone of VT event swarm of the seismic unrest, that we interpret as magma input. The inversion of the shallow field images three weak line segments that are all situated at very shallow, near-surface depths. We interpret the weak segments as hydrothermal sources potentially excited by the deeper magma injection. Our results indicate no significant input into the shallow phonolitic plumbing system of the PV–PT complex, but rather a deeper-seated rejuvenation of the mafic feeder reservoir. The emerging picture from our analysis is that the 2004/5 unrest on Tenerife was of a hybrid nature due to the combination of a deep magma injection (failed eruption?) coupled with fluid migration to shallow depths. The identified causative link between deep and shallow unrest sources indicates the presence of permeable pathways for shallow fluid migration at the CVC.

Anomalously high concentrations of metals in fluid of the Semenov modern hydrothermal system (Mid-Atlantic Ridge, 13°31′ N): LA-ICP-MS study of fluid inclusions in minerals

How and how much do hydrothermal fluids trans� port metals from the area of their involvement in the oreforming process to the zone of their precipitation? What should the composition of fluids, which may transport metals in the proportions necessary for the formation of mineral deposits, be? These problems have been studied by more than one generation of researchers in the field of ore deposits. An important role in the solution of this problem was played by the experimental and thermodynamic studies of mineral solubility in aqueous fluids at high temperatures. In spite of significant achievements in these areas, we cannot assume that the obtained results reflect the chemical composition of the natural oreforming fluid adequately, since modeling of metal behavior in it is limited by our knowledge of the thermodynamic con� stants of minerals and components of fluid. Significant achievements in improving our knowledge on the chemistry of mineralforming fluids were gained dur� ing the study of modern hydrothermal systems and fluid inclusions in minerals of hydrothermal deposits (1). A huge volume of data was obtained after the dis� covery of modern hydrothermal sources precipitating sulfide ores on the floor of the World Ocean (21° N) on the East Pacific Rise in 1978. During the past 35 years, more than 300 hydrothermal sites have been discov� ered and studied by submersibles. The major data on the chemistry of fluid and concentrations of metals in it were mostly based on the results of the analyses of its samples collected on the hydrothermal fields by sub� mersibles. However, study of fluid inclusions in miner� als from modern sulfide edifices has shown that the conditions of deposition, the evolution of salinity, and the temperature of fluids discharged on the sea floor were much more variable than followed from direct measurements (2-4). Development of new highpre� cision apparatus and analytical methods has provided completely different possibilities for the study of a wide spectrum of chemical elements in fluids trapped by fluid inclusions during mineral crystallization. Study of fluid inclusions in minerals from a number of ore deposits and analysis of the concentrations of met� als in fluids precipitating ores have been performed (5-7). Using these methods, it is very interesting to analyze the fluid captured by minerals during the for� mation of modern sulfide ores. For this purpose, we carried out investigations of fluid inclusions in miner� als from the Semenov modern ore cluster.

Clues to the cause of the 2011-2013 Campi Flegrei caldera unrest, Italy, from continuous GPS data

From 2006 to spring 2013, Campi Flegrei (CF) caldera, Italy, was mostly uplifting at an increasing rate, particularly high from 2011. We show that the 2011–2013 accelerated uplift and 1980–2010 inflation and deflation phases can be explained by a two-source conceptual model similar to that proposed by Amoruso et al. (2014) (reference model). However, pressurization of the sole thin quasi-horizontal ∼4000 m deep source, responsible for large-scale 1980–2010 deformation, can explain the whole 2011–2013 deformation, while activity of the shallower Solfatara hydrothermal source, responsible for residual 1980–2010 deformation, appears constant. These results suggest a predominantly magmatic unrest in 2011–2013. Near-real-time comparison of observations and reference model predictions can provide additional information for short-term eruption forecasting at CF; a similar approach could be followed also in other volcanic environments.

The Syabru‐Bensi hydrothermal system in central Nepal: 2. Modeling and significance of the radon signature

AbstractThe Syabru‐Bensi hydrothermal system (SBHS), located in the Nepal Himalayas, is characterized by numerous hot (>30°C) springs and the release of dry, cold (<35°C) CO2 associated with radon‐222, detailed in the companion paper. In the SBHS, CO2 and radon fluxes on the ground vary over 5–6 orders of magnitude, reaching exceptional mean values of 100 kg m−2 d−1 and 12 Bq m−2 s−1, respectively. This paper extends the companion paper by developing three quantitative models for the radon signature of CO2 based on measurements of radon and radium concentrations in the spring waters and effective radium concentration of rocks and soils. The first model considers near‐surface radon and CO2 degassing from water, considered unlikely unless there exist currently unidentified large discharges of hydrothermal water. The second model considers CO2, arising from deeper hydrothermal sources, incorporating radon from shallow radium sources as it percolates upward toward the surface, considered more likely as a percolation depth of 100 m is sufficient to account for the observed radon discharge. The third model considers the observed peak radon concentrations in the gas zones and assumes that gaseous CO2 can be transported from kilometer‐scale depths through a fault network connected to the zones. This latter model affords the possibility that variations of physical parameters at depths associated with earthquake nucleation might be detectable at the surface. Gas‐dominated transport might operate in other locations in Himalayas and elsewhere and may be an important aspect of the coupled mechanisms associated with seismically active orogens.

Metacolloid pyrite-pitchblende veinlets of high-grade hydrothermal ores at the Dalmatovskoe uranium deposit, Transural Region, Russia: New data on the mineralogy, geochemistry, age, and uranium sources

Micro to nano-scale structures of pockets of solidified uranium oxide gel were found for the first time and deciphered ontogenetically during electron-microscopic mineralogical study of four samples of high-grade uranium ores from the Dalmatovskoe deposit. Mineralogical-geochemical features of paragenesis and structural features are reported for unique hydrothermal veinlets consisting of variably crystallized gelpyrite and subveinlets of gel-pitchblende of several generations. Mineralogical features indicate repeated opening of veinlets during hydrothermal ore formation and provide insight into physical and chemical features of non-solidified gel, its mobility, and aggressive behavior during metasomatism. It was proposed that the veinlets are genetically related to the main hydrothermal sources and possibly to the endogenous uranium occurrence, which was formed owing to opening of main fault marked by paleoriver bed. The find of such veinlets is considered as evidence for repeated influx of hydrothermal uranium in the uranium ores of the sedimentary-epigenetic deposit simultaneously with accumulation and transformation of sediments, which made it possible to ascribe this deposit to the hydrothermal-sedimentary type. The depth of core sampling of high-grade ores corresponds to the “ore zone” of the Dalmatovskoe Deposit, near the Paleozoic basement. For this reason, the entire “zone” was ascribed to the specific endogenous metasomatic epihydrothermal occurrences, possibly, of elision nature, with approximate temperature of ore formation of 150–250°C.

Composition and origin of the dolostones of Um Bogma Formation, Lower Carboniferous, West Central Sinai, Egypt

Dolostones of Um Bogma Formation, Lower Carboniferous, have been possibly originated due to two different fluids. The main fluid source is related to meteoric water. The second fluid originated from deeper hydrothermal sources ascended through faults. The diagenetic features of Um Bogma Formation show many processes with conspicuous effects, including dolomitization, neomorphism, mechanical and chemical compaction, as well as it characteristics with stylolites, sutured and concave–convex contacts between grains. Based on mineralogical composition and petrography, the diagenetic environments are distinguished into shallow marine, near surface meteoric and burial environments. Dolostones in the studied area are possibly subjected to chemical and biochemical weathering. Um Bogma dolostones are nearly stoichiometric and ordered. Mineralogical and geochemical studies indicated that dolomite crystals were deposited in mixing zone of marine and meteoric water. In addition, it is formed in the near-surface oxidizing environment. The Sr distribution coefficient and Y content of the studied dolostones indicated that they are formed in temperature >110 and <150 °C. The CF (Contamination factor) and Igeo (Geo-accumulation index) have doubtlessly indicated that the Um Bogma dolomite rocks are polluted especially for Fe, Rb, Co, Cu, W, S, Ba, Zn, Zr and As. The studied dolomites of Area 45 and Um Bogma Mine sections are characterized by enrichment of Co and Cu, which indicated their deposition in marine environment. The studied dolomite of Um Bogma Mine is characterized by the enrichment of Zn, W, Ba, Mn, Ti and As which derived from hydrothermal solution. Consequently, the dolostones of the Um Bogma Formation are similar to those in Cayman Formation, Indies.

Cobalt and marine redox evolution

Cobalt (Co) is a bio-essential trace element and limiting nutrient in some regions of the modern oceans. It has been proposed that Co was more abundant in poorly ventilated Precambrian oceans based on the greater utilization of Co by anaerobic microbes relative to plants and animals. However, there are few empirical or theoretical constraints on the history of seawater Co concentrations. Herein, we present a survey of authigenic Co in marine sediments (iron formations, authigenic pyrite and bulk euxinic shales) with the goal of tracking changes in the marine Co reservoir throughout Earth's history. We further provide an overview of the modern marine Co cycle, which we use as a platform to evaluate how changes in the redox state of Earth's surface were likely to have affected marine Co concentrations. Based on sedimentary Co contents and our understanding of marine Co sources and sinks, we propose that from ca. 2.8 to 1.8 Ga the large volume of hydrothermal fluids circulating through abundant submarine ultramafic rocks along with a predominantly anoxic ocean with a low capacity for Co burial resulted in a large dissolved marine Co reservoir. We tentatively propose that there was a decrease in marine Co concentrations after ca. 1.8 Ga resulting from waning hydrothermal Co sources and the expansion of sulfide Co burial flux. Changes in the Co reservoir due to deep-water ventilation in the Neoproterozoic, if they occurred, are not resolvable with the current dataset. Rather, Co enrichments in Phanerozoic euxinic shales deposited during ocean anoxic events (OAE) indicate Co mobilization from expanded anoxic sediments and enhanced hydrothermal sources. A new record of marine Co concentrations provides a platform from which we can reevaluate the role that environmental Co concentrations played in shaping biological Co utilization throughout Earth's history.

Unusual signals recorded by ocean bottom seismometers in the flooded caldera of Deception Island volcano: volcanic gases or biological activity?

AbstractAn ocean bottom seismometer (OBS) network was deployed for 1 month at Deception Island volcano, Antarctica, in early 2005. Although only two volcano-tectonic and three long-period events were observed, the three OBSs located &gt; 2 km apart inside the caldera detected over 3900 events that could not be attributed to known volcanic or hydrothermal sources. These events are found on one instrument at a time and occur in three types. Type 1 events resemble impulsive signals from biological organisms while type 2 and type 3 events resemble long-period seismicity. The largest number of events was observed in a region of volcanic resurgence and hydrothermal venting. All three types occur together suggesting a common cause and they show evidence for a diurnal distribution. The events are most likely to be due to aquatic animals striking the sensors, but a geological source is also possible. In the first case, these signals indicate the presence of a biological community confined to the caldera. In the second case, they imply widespread hydrothermal activity in Port Foster. Future OBS experiments should bury the seismometers, include a hydrophone, deploy instruments side-by-side, or include a video camera to distinguish between biological and geological events.

RAMAN SPECTROSCOPY AND X-RAY FLUORESCENCE IN MOLECULAR ANALYSIS OF YELLOW BLOCKS FROM THE ARCHEOLOGICAL SITE PLAYA MILLER 7 (NORTHERN CHILE)

Yellow blocks from the archaeological site Playa Miller 7 (PLM7), on the coast of Atacama Desert in northern Chile, were analyzed by Raman spectroscopy and X-ray fluorescence (XRF) portable. Our results identify for the first time the use of K-jarosite and natrojarosite in prehispanic times (approx. 2500 year BP). In search of a possible source of supply for this mineral hydrothermal origin, our surveys were focused on Andean geothermal areas with identification, so far, from a single source in the region of Arica and Parinacota: Jurasi (JU), located at 4000 mamsl. Comparison of the Raman spectra between samples archaeological and Jurasi, allow us to infer that this hydrothermal source could be used as obtaining source of yellow pigment by prehispanic inhabitant of Formative period (3700-1500 years B.P.).

∼750 Ma banded iron formation from the Arabian-Nubian Shield—Implications for understanding neoproterozoic tectonics, volcanism, and climate change

Neoproterozoic Banded Iron Formation (BIF) from Sawawin, NW Saudi Arabia and the Central Eastern Desert of Egypt define the 200×100km Arabian-Nubian Shield (ANS) BIF basin. ANS BIF formed ∼750Ma, prior to the Sturtian glacial episode (which began ∼716Ma). BIF deposition occurred in a marine basin associated with arc/backarc basin volcanism and immature clastic sedimentation. Beds are composed of alternating iron- and silica-rich laminae, which may reflect seasonal changes in deposition of Fe vs. Si. Fe-rich layers are dominantly composed of primary fine-grained hematite “dust” and minor apatite, with abundant secondary magnetite. Rapid deposition is revealed by: (1) major and trace element data indicating that ANS-BIF are very pure (<20% detrital input) chemical sediments in spite of being deposited in a basin with high sedimentation rates, and (2) pervasive evidence for soft-sediment deformation, suggesting that rapid sedimentation of dense, weak materials resulted in slumping. Nd and Pb isotopic compositions are predominantly mantle-like, indicating the dominance of hydrothermal sources or weathering of juvenile ANS crust for these elements. REE data show HREE-enriched patterns typical of modern seawater, with small positive Eu and small negative Ce anomalies. Low abundances of transition elements that are commonly abundant in proximal hydrothermal deposits of modern oceans may indicate that ANS-BIF formed at some distance from hydrothermal vents, or may reflect prior sulfide scavenging by euxinic and sulfidic deep ocean waters. REE data and Zn/Co share characteristics of both modern seawater and hydrothermal vent fluids suggesting derivation from a mixture of shallow suboxic seawater with a dilute, low-T hydrothermal vent fluid. Considered in conjunction with BIF of similar age on other paleocontinents, these observations support that rapid and widespread re-oxygenation of Fe+2 in previously anoxic or suboxic seawater led to rapid precipitation of hematite “dust” and BIF deposition ∼750Ma. Sulfate limitation or diminished bacterial sulfate reduction, required to form BIF instead of pyrite, may reflect large-scale glaciation but evidence for deep ferruginous conditions prior to Cryogenian glaciations suggests than any scenario that substantially reduced continental weathering (i.e., hard snowball or slushball) could have primed the oceans for BIF deposition. The likely short duration of ANS BIF deposition (a few to 10s of kyr) and apparent timing well before the “Sturtian” glaciation suggest that conditions favoring Neoproterozoic BIF formation could have existed over an extended period (10s of myrs) and that Neoproterozoic Oxidation Event began during the “Kaigas-Sturtian” time frame.

Natural attenuation process via microbial oxidation of arsenic in a high Andean watershed

Rivers in northern Chile have arsenic (As) concentrations at levels that are toxic for humans and other organisms. Microorganism-mediated redox reactions have a crucial role in the As cycle; the microbial oxidation of As (As(III) to As(V)) is a critical transformation because it favors the immobilization of As in the solid phase. We studied the role of microbial As oxidation for controlling the mobility of As in the extreme environment found in the Chilean Altiplano (i.e., >4000meters above sea level (masl) and <310mm annual rainfall), which are conditions that have rarely been studied. Our model system was the upper Azufre River sub-basin, where the natural attenuation of As from hydrothermal discharge (pH4–6) was observed. As(III) was actively oxidized by a microbial consortium, leading to a significant decrease in the dissolved As concentrations and a corresponding increase in the sediment's As concentration downstream of the hydrothermal source. In-situ oxidation experiments demonstrated that the As oxidation required biological activity, and microbiological molecular analysis confirmed the presence of As(III)-oxidizing groups (aroA-like genes) in the system. In addition, the pH measurements and solid phase analysis strongly suggested that the As removal mechanism involved adsorption or coprecipitation with Fe-oxyhydroxides. Taken together, these results indicate that the microorganism-mediated As oxidation contributed to the attenuation of As concentrations and the stabilization of As in the solid phase, therefore controlling the amount of As transported downstream. This study is the first to demonstrate the microbial oxidation of As in Altiplano basins and its relevance in the immobilization of As.

Time series analysis of high temperature fumaroles monitored on the island of Vulcano (Aeolian Archipelago, Italy)

The exhalation activity at the La Fossa cone (Vulcano Island, Aeolian Archipelago, Italy) has been ongoing for more than 1 century. Many of the monitored geochemical and geophysical parameters have showed transient variations of energy release. The time-series analyses of fumarole temperatures presented in this paper enabled the sequence of observations to be defined and information from different monitoring stations to be integrated. The motion of fluids feeding the fumaroles of the La Fossa cone is driven by the thermal and kinetic energies that balance the seismic and volcanic forces active in the region, and the temperatures of the fumaroles reflect the local response of the hydrothermal system to these forces. During a 14-year period of observation, from 1998 to 2012, fumarole temperatures showed various trends but also cyclic variations characterized by sharp increases. The repetition of these variations during periods with different trends indicates that no physical variation occurred from the hydrothermal source to the surface during the analyzed period, and after each periodic geochemical crisis the previous thermal conditions were restored. Although the continuous monitoring of high-temperature fumaroles was limited to only a few sites, the observed trends characterized the most important fumaroles in the area of Vulcano Island. An evaluation of thermal-energy release based on these spatially discrete measurements would be a speculative exercise in thermodynamics, but the analyses of the recorded data represent a step forward in interpreting the signals from ongoing volcanic activity and in assessing the seismic risk.

Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic

The global variability in nickel (Ni) isotope compositions in ferromanganese crusts is investigated by analysing surface samples of 24 crusts from various ocean basins by MC–ICPMS, using a double-spike for mass bias correction. Ferromanganese crusts have δNi60 isotopic compositions that are significantly heavier than any other samples thus far reported (−0.1‰ to 0.3‰), with surface scrapings ranging between 0.9‰ and 2.5‰ (relative to NIST SRM986). There is no well resolved difference between ocean basins, although the data indicate somewhat lighter values in the Atlantic than in the Pacific, nor is there any evidence that the variations are related to biological fractionation, presence of different water masses, or bottom water redox conditions. Preliminary data for laterite samples demonstrate that weathering is accompanied by isotopic fractionation of Ni, which should lead to rivers and seawater being isotopically heavy. This is consistent with the slightly heavier than average isotopic compositions recorded in crusts that are sampled close to continental regions. Furthermore, the isotopic compositions of crusts growing close to a hydrothermal source are clustered around ∼1.5‰, suggesting that hydrothermal fluids entering the ocean may have a Ni isotopic composition similar to this value. Based on these data, the heavy Ni isotopic compositions of ferromanganese crusts are likely due to input of isotopically heavy Ni to the ocean from continental weathering and possibly also from hydrothermal fluids. A depth profile through one crust, CD29-2, from the north central Pacific Ocean displays large variations in Ni isotope composition (1.1–2.3‰) through the last 76Myr. Although there may have been some redistribution of Ni associated with phosphatisation, there is no systematic difference in Ni isotopic composition between deeper, older parts and shallower, younger parts of the crust, which may suggest that oceanic sources and sinks of Ni have largely remained in steady state over the Cenozoic. Additionally, the isotope profile is in agreement with a profile of Mn concentration through the same crust. This implies a link between the Ni isotopic composition recorded in ferromanganese crusts and the release of Ni into the ocean through hydrothermal activity. This supports the conclusions drawn from surface data, that Ni isotope ratios in ferromanganese crusts are largely controlled by the isotopic compositions of the Ni oceanic input sources.

Genesis of the Nasirabad manganese occurrence, Fars province, Iran: Geochemical evidences

The Nasirabad manganese occurrence is located to the south of the Neyriz in the Fars province. Structurally and lithologically, this occurrence lies in the southwest part of the Zagros Thrust Belt and was deposited as Mn-nodules and interconnected ore-bearing interlayer's with radiolarite cherts in the vicinity of the Neyriz ophiolites. The present work deals with the geology and geochemistry of the Nasirabad manganese occurrence with a discussion of its genesis. High Mn/Fe (average 18.85) and high Ba (average 28,830ppm) with low Pb (2.0ppm) and LREE>HREE, Lan/Ndn (average 4.5), Dyn/Ybn (average 1.2) and negative Eu anomaly suggest distal hydrothermal source. The Co/Zn (average 2.2), Ce/La ratio (average 0.67) and trace element discrimination diagrams indicate hydrothermal–hydrogenous processes. Y/Ho ratio (average 24.85) and strong positive correlation coefficient between major oxides and some high field strength elements (HFSE) like; TiO2 vs Fe2O3 (r=0.98), Al2O3 vs Zr (r=0.97), Al2O3 vs Fe2O3 (r=0.98), Zr vs K2O (r=0.98), Nb vs TiO2 (r=0.92), Th vs Fe2O3 (r=0.76), Th vs MgO (r=0.86) reveal the presence of volcaniclastics and (or) terrigenous detritals of mafic composition being deposited into the depositional basin. It seems that intermittent interlayering of mafic detrital material derived from volcanic eruption of the Neyriz island arc directly affected the physicochemical conditions of hydrothermal ore precipitation in this basin and consequently the Nasirabad manganese ores represent hydrothermal–hydrogenous geochemical characteristics. The Nasirabad is an example of non-sulphidic, oxic Mn-mineralization. Similar trend between the enrichment and depletion of some bioessential elements (e.g., Mn, As, Ba, Sr, Co, Ce) might have been resulted from selective sequestering of metal ions by microbial processes and hence hydrogenous characteristics may also be the result of biogenetic processes. Moreover the high dilution of distal hydrothermal exhalations by sea water cannot be ruled out.

Geochemical characteristics of rare earth elements and their implications for the Huachanggou gold deposit in Shaanxi Province, China

The Huachanggou gold deposit is located in the south part of the Mian-Lue suture zone in the Qinling orogenic belt. Rare earth element (REE) concentrations determined by ICP-MS are shown to characterize the ore samples and their wall rocks in three ore zones in order to reveal the origin of ore-forming materials and fluid. In AuI, REE chondrite normalized patterns of ore are similar to those of ore-controlling spilite; the ore-forming materials originated from deep magma, and magmatic activity offered main hydrothermal source for gold mineralization. The REE characteristics of AuII and AuIII are similar, and most of the ore samples are similar with the wall rocks. The ore-forming fluids of AuII and AuIII were metamorphic hydrothermal fluids which had extracted ore-forming materials part from the wall rocks, and part from the spilite in AuI.