Hydrothermal Fluid Chemistry Research Articles

Gold and Arsenic in Pyrite and Marcasite: Hydrothermal Experiment and Implications to Natural Ore-Stage Sulfides

Hydrothermal synthesis experiments were performed in order to quantify the states of Au and As in pyrite and marcasite. The experiments were performed at 350 °C/500 bar and 490 °C/1000 bar (pyrite–pyrrhotite buffer, C(NaCl) = 15 and 35 wt.%). The synthesis products were studied by EPMA, LA-ICP-MS, and EBSD. The EPMA was applied for simultaneous determinations of Au, As, Fe, and S, with a Au detection limit of 45–48 ppm (3σ). The analyses were performed along profiles across zonal grains. The concentrations of As and Au up to 5 wt.% and 8000 ppm, respectively, were determined in pyrite and up to 6 wt.% and 1300 ppm in marcasite. In pyrite, the Au concentration decreases with fluid salinity and temperature increases. Strong positive Au–As correlation and strong negative Au–Fe and As–S correlation were identified in pyrite. Comparison of the correlations with theoretical lines implies Au–As clustering. The cluster stoichiometry is inferred to be [AuAs10]. Most probably, As in pyrite presents in the form of clusters and in the As→S solid solution. Incorporation of Au in As-rich pyrite can be controlled by the reductive deposition mechanism. In marcasite, the concentrations of Au are not correlated with the As content. The [AuAs10] clusters enrich the {210}, {113}, and {111} pyrite faces, where the former exhibits the highest affinity to Au and As. The affinity of {110} and {100} forms to Au and As is lower. Implication of the experimental results to data for natural auriferous pyrite shows that the increase of Au content at C(As) > 0.5–1 wt.% is caused by the incorporation of the Au-As clusters, but not because of the formation of Au→Fe solid solution. Therefore, the concentration of “invisible” gold in pyrite is dictated solely by the hydrothermal fluid chemistry and subsequent ore transformations.

Biotite Geochemistry and Its Implication for the Difference in Mineralization in the Xiongcun Porphyry Cu–Au Ore District, Tibet

The Xiongcun Cu–Au ore district is in the southern middle Gangdese Metallogenic Belt, Tibet, and formed during Neo-Tethyan oceanic subduction. The Xiongcun ore district mainly comprises two deposits, the No. I and No. II deposits, which were formed by two individual mineralization events according to deposit geology and Re–Os isotopic dating of molybdenite. The No. I deposit is similar to a reduced porphyry copper–gold deposit, given the widespread occurrence of primary and/or hydrothermal pyrrhotite and common CH4-rich and rare N2-rich fluid inclusions. The No. II deposit, similar to classic oxidized porphyry copper–gold deposits, contains highly oxidized minerals, including magnetite, anhydrite, and hematite. The halogen chemistry of the ore-forming fluid from the No. I and No. II deposits is still unclear. Biotite geochemistry with halogen contents was used to investigate the differences in ore-forming fluid between the No. I and No. II deposits. Hydrothermal biotite from the No. I deposit, usually intergrown with sphalerite, is Mg-rich and classified as phlogopite and Mg-biotite, and hydrothermal biotite from the No. II deposit is classified as Mg-biotite. Hydrothermal biotite from the No. I deposit has significantly higher SiO2, MnO, MgO, F, Li, Sc, Zn, Rb, Tl, and Pb contents and lower Al2O3, FeOtot, Cl, Ba, Cr, V, Co, Ni, Y, Sr, Zr, Th, and Cu contents than the biotite from the No. II deposit. Hydrothermal biotites from the No. I and No. II deposits yield temperatures ranging from 230 °C to 593 °C and 212 °C to 306 °C, respectively. The calculated oxygen fugacity and fugacity ratios indicate that the hydrothermal fluid of the No. I deposit has a higher F content, oxygen fugacity, and log(fHF/fHCl) value and a lower log(fH2O/fHF) value than the hydrothermal fluid from the No. II deposit. The biotite geochemistry shows that the No. I and No. II deposits formed from different hydrothermal fluids. The hydrothermal fluid of the No. I deposit was mixed with meteoric waters containing organic matter, resulting in a decrease in oxygen fugacity and more efficient precipitation of gold. The No. I and No. II deposits were formed by a Cl-rich hydrothermal system conducive to transporting Cu and Au. The decreasing Cl, oxygen fugacity, and temperature may be the key factors in Cu and Au precipitation. Biotite geochemistry allows a more detailed evaluation of the halogen chemistry of hydrothermal fluids and their evolution within porphyry Cu systems.

Apatite Texture, Composition, and O-Sr-Nd Isotope Signatures Record Magmatic and Hydrothermal Fluid Characteristics at the Black Mountain Porphyry Deposit, Philippines

AbstractThe trace elemental and isotopic signatures in apatite can be modified during hydrothermal alteration. This study investigates the suitability of apatite as an indicator of the source, chemistry, and evolution of magma and hydrothermal fluids. In situ textural, elemental, and O-Sr-Nd isotope analyses were performed on apatite in thin sections, from fresh and propylitically altered pre- and synmineralized dioritic porphyries from the Black Mountain porphyry Cu deposit in the Philippines.All studied apatite crystals have similar subhedral to euhedral shapes and are homogeneous in the grayscale in backscattered electron images. In cathodoluminescence images, the apatite in fresh and altered rocks displays yellow to yellow-green and green to brown luminescence, respectively. Apatite in fresh rocks has a higher Cl and Mn content, and lower Fe, Mg, Sr, Pb, and calculated XOH-apatite, compared to apatite in altered rocks. The content of F, rare earth elements (REEs), Y, U, Th, and Zr, and the Sr-Nd isotope signatures of apatite from fresh and altered rocks are similar in all apatite grains (87Sr/86Sr = 0.7034–0.7042 vs. 0.7032–0.7043, εNd(t) = 5.3–8.0 vs. 5.1–8.4). The X-ray maps and elemental and oxygen isotope signatures across individual apatite crystals are typically homogeneous in apatite from both fresh and altered rocks. The distinct luminescence colors, coupled with distinct mobile element compositions (Cl, OH, Mn, Mg, Fe, Sr, Pb), indicate modification of primary magmatic apatite during interaction with hydrothermal fluids. The similarities in Sr isotope ratios (87Sr/86Sr = 0.7032–0.7043) but slight differences in O isotope signatures (δ18O = 6.0 ± 0.3‰ vs. 6.6 ± 0.3‰) in apatite from fresh and altered rocks are consistent with the magma and hydrothermal fluids having the same source and suggest significant phase separation in the hydrothermal fluids given that 18O preferentially fractionates into the residual liquid relative to 16O during phase separation. The similarity of immobile element (REE, Y, U, Th, and Zr) contents in both populations of apatite, consistency of textures and Nd isotope compositions, and absence of obvious dissolution-reprecipitation features all suggest that altered apatite retains some magmatic characteristics. The apatite in fresh rocks has oxygen isotope compositions similar to that of zircons from the same sample (δ18O = 5.9 ± 0.3‰), indicating little to no oxygen isotope fractionation between zircon and apatite and that apatite can be a good proxy for the oxygen isotope composition of the magma. Based on the Cl contents of the magmatic and replacement apatite, and assuming their equilibrium with high-temperature magma fluid and replacement hydrothermal fluid, respectively, the calculated Cl content of the early magmatic fluid and the later replacement fluid can be estimated to be 6.4 to 15.1 wt % and ~0.25 ± 0.03 wt %, respectively. This indicates a depletion of Cl from the early high-temperature fluid to the replacement fluid, consistent with phase separation.This study demonstrates that cathodoluminescence, elemental compositions (such as Cl, Mn, Mg, Fe, Sr, Pb) and Sr-O isotope signatures in apatite can be modified during hydrothermal alteration, whereas other components (REE, Y, U, Th, and Zr) and the Nd isotope composition are preserved. These features can be used to constrain the origin, chemistry, and evolution of the primary magma and ore-forming hydrothermal fluids.

Chemical Gardens as Electrochemical Systems: In Situ Characterization of Simulated Prebiotic Hydrothermal Vents by Impedance Spectroscopy.

In an early earth or planetary chimney systems, hydrothermal fluid chemistry and flow durations play a large role in the chimney's ability to drive electrochemical reactions for the origin of life. We performed continuous electrochemical impedance spectroscopy (EIS) characterization on inorganic membranes representing prebiotic hydrothermal chimney vents in natural seafloor systems, by incorporating an electrode array into a chimney growth experiment. Localized potential and capacitances profiles in the chimney reveal a dynamic system where redox processes are driven by transport phenomena, increasing rapidly due to disequilibrium until achieving equilibrium at about 100 mV and 1000 μF/cm2 . The impedance in the chimney interior is three orders of magnitude lower (100 Ohms/cm2 vs 100 KOhms/cm2 ) than at the ocean or the ocean/chimney interface. The calculated peak dissipation factor (DF) values are more than ten times higher (40.0 vs 3.0) and also confirm the elevated chemical reactivity in the chimney interior.

Chlorite as an exploration indicator for concealed skarn mineralization: Perspective from the Tonglushan Cu–Au–Fe skarn deposit, Eastern China

Compositional variations of chlorite have been increasingly used to trace the magmatic–hydrothermal fluid evolution, as well as exploration vector toward concealed orebodies. This method has been widely applied to porphyry Cu (–Au) systems, but related research on skarn systems remain scarce. The Tonglushan Cu–Au–Fe deposit (86.3 Mt @ 1.66% Cu, 0.94 g/t Au and 39.4% Fe) is one of the largest Cu–Au polymetallic skarn deposits in the Middle–Lower Yangtze River metallogenic belt (MLYRB, Eastern China). Based on detailed petrographic observation, we present new EPMA and LA–ICP–MS elemental data for the Tonglushan chlorite, and discuss their implications on hydrothermal fluid evolution and exploration of concealed skarn orebodies there. At Tonglushan, chlorite alteration is well developed in the main Cu–Au sulfide ore stage. Three types of chlorite have been identified, i.e., disseminated chlorite replacing biotite and/or hornblende in the ore-causative quartz monzodiorite (porphyry), vein-type chlorite in quartz–chlorite–sulfides veins along/near the internal and external intrusive contact, and replacement-type chlorite that replaced garnet, diopside, epidote and Fe oxides in the skarn mineralization center.All the Tonglushan chlorite samples belong to trioctahedral chlorite and have similar crystallization temperatures (163–351 °C). The substitution mechanisms of Fe2+ ⇔ Mg2+, Tschermark (AlivAlvi ⇔ Si (Mg2+, Fe2+)) and di-trioctahedral (3(Mg2+, Fe2+) ⇔ □ + 2Alvi) are likely important for the Tonglushan chlorite formation. Replacement-type chlorite shows higher contents of FeOT, mobile elements (e.g., Ca, Na, B, Sr, Pb, Zn and Sn) and ore-forming elements (e.g., Cu and Co), whereas the disseminated chlorite has higher contents of Ti, Li, LILEs (e.g., K, Rb, Ba and Cs) and incompatible elements (e.g., V, Sc, Co and Ni). Meanwhile, the vein-type chlorite has similarly high contents of Ti, Li, V, Sc and Ni to the disseminated chlorite, but has higher FeOT and Cu contents but lower MgO content than the latter. Such compositional differences may have mainly attributed to the protolith (or precursor minerals) and hydrothermal fluid chemistry. Integrating the chlorite petrographic features and chemical compositions, we suggest that elevated FeOT and B contents and Fe/(Fe + Mg) ratios, and lower MgO, Al2O3, Ti, V and Sc contents can serve as proximal indicator for exploring concealed skarn orebodies at Tonglushan, as well as other similar Cu–Au–Fe skarn deposits in the MLYRB.

Chemical archeoceanography

Elemental fluxes to the ocean are expected to increase with the surface area of continental exposure to weathering and atmospheric PCO2. The record of phosphorus in sediments, which has no notable source within the ocean, and the radiogenic strontium isotopes in Archean carbonates indicate that, prior to the Great Oxidation Event (GOE), subaerial expanses represented only about 20% of the modern continental surface area, i.e. 7% of the surface of the Earth. Because these simple first-order observations, in contrast to the low oxygen content of the pre-GOE atmosphere, have so far received only little attention in the appraisal of the marine chemistry of the early Earth, a reassessment of the chemistry of the pre-GOE ocean is warranted. Here we discuss some of the geochemical cycles of the Archean world, including protons, alkalinity, electrons, and other electrolytes, and attempt to build a first conceptual framework for Chemical Archeoceanography. The smaller subaerial exposures characterizing the Archean and the low abundance of Archean carbonates and mudstones imply that the flux of alkalinity to the ocean was much weaker than today and therefore that the capacity of the runoff to neutralize the high-temperature hydrothermal fluids was less important. Such a reduced flux in turn entails lower seawater pH and higher chlorinity. The lack of atmospheric O2 allowed iron to be in its soluble Fe(II) form and to reduce HCO3− to CH4 while liberating large amounts of protons. The low pH of the pre-GOE ocean and the reduced alkalinity input, reinforced by scant P supply, account for reduced carbonate precipitation. Ocean chemistry evolved under the control of two changing factors: (i) the balance between chemical weathering and hydrothermal fluxes and (ii) the oxygen pressure in the ocean and the atmosphere. Seawater iron concentration was controlled by high [Fe2+]/[H+]2 ratios. With the temperature of hydrothermal fluids at mid-ocean ridges and other active volcanic edifices being determined by the expansion properties of seawater, the chemistry of hydrothermal fluids is controlled by the chlorinity of the ocean. Of all the major element cycles in seawater, those of Na, Mg, and P seem to be unbalanced when runoff falls below the modern value. The very low P content of banded iron formations is inconsistent with a major role of biological activity in the oxidation of Fe2+ dissolved in the Archean ocean. Prior to the GOE, banded iron formations were the major sedimentary sink of seawater cations, a role now played by carbonates. The controls of seawater alkalinity, which today are exerted by the Ca2+–CaCO3 couple, was exerted by dissolved Fe2+-magnetite or Fe2+-ferrihydrite. Plain language summaryWe here evaluate the constraints on seawater chemistry before free oxygen was introduced into the atmosphere some 2.4 billion years ago. We suggest a simple conceptual framework which we validated on modern seawater and which is based on two so-far largely overlooked observations, namely that the phosphorus concentration and strontium isotopic records in ancient sediments require that emerged continental land masses were covering only ~7% of Earth's surface at the time of the Great Oxidation Event instead of 30% today. Atmospheric and seawater chemistry critically depend on this proportion as well as on the chlorinity of the ocean. The oxygen-free atmosphere allowed iron to remain in solution and to reduce atmospheric carbon dioxide to methane. The pH of the ancient oceans must have been lower because less river water was available to neutralize the acidic submarine high-temperature hydrothermal fluids emitted at mid-ocean ridges. Limited continental surface starved the sedimentary column for clays. The low pH and scant phosphorus drainage led to rare carbonate deposition. We conclude by proposing a new scheme for Archean marine chemistry and geochemical cycles that can explain the chemical and geological observations from the Archean and early Proterozoic rock record.

Apatite as a tracer of the source, chemistry and evolution of ore-forming fluids: The case of the Olserum-Djupedal REE-phosphate mineralisation, SE Sweden

Abstract This study explores the suitability of apatite as a tracer of the source(s), chemistry, and evolution of ore-forming hydrothermal fluids. This is tested by analysing the halogen (F, Cl, Br, and I), stable Cl isotopic, and trace element compositions of fluorapatite from the regional-scale Olserum-Djupedal rare earth element (REE) phosphate mineralisation in SE Sweden, which is dominated by monazite-(Ce), xenotime-(Y), and fluorapatite. The primary hydrothermal fluid flow system is recorded in a sequence from proximal granite-hosted to distal metasediment-hosted fluorapatite. Along this sequence, primary fluorapatite shows a gradual increase of Cl and Br concentrations and in (Gd/Yb)N, a decrease of F and I concentrations, a decrease in δ37Cl values, in (La/Sm)N, and partly in (La/Yb)N and (Y/Ho)N. Local compositional differences of halogen and trace element concentrations have developed along rims and in domains adjacent to fractures of fluorapatite due to late-stage partial reaction with fracture fluids. These differences are insignificant compared to the larger deposit-scale zoning. This suggests that apatite can retain the primary record of the original ore-forming fluid despite later overprinting fluid events. The agreement between Br/Cl and I/Cl ratios of apatite and those of co-existing fluid inclusions at lower temperatures indicates that only a minor fractionation of Br from I occurs during apatite precipitation. The halogen ratios of apatite can thus be used as a first-order estimate for the composition of the ore-forming fluid. Taking the small fractionation factors for Cl isotopes between apatite and co-existing fluid at high temperatures into account, we propose that the Cl isotopic composition of apatite and the halogen ratios derived from the apatite composition can be used jointly to trace the source(s) of ore-forming fluids. By contrast, most trace elements incorporated in apatite are affected by the host rock environment and by fluid-mineral partitioning due to growth competition between co-crystallising minerals. Collectively, apatite is sensitive to changing fluid compositions, yet it is also able to record the character of primary ore-forming fluids. Thus, apatite is suitable for tracing the origin, chemistry, and evolution of fluids in hydrothermal ore-forming settings.

Application of SWIR spectrometry to the determination of biotite compositions in hydrothermally altered units of the Yangyang iron-oxide-apatite (IOA) deposit, South Korea

The Yangyang iron oxide-apatite deposit in South Korea has gone through multiple events for igneous activity, structural evolution, hydrothermal alteration, and mineralization. Trioctahedral mica is indicative of potassic alteration in the Yangyang IOA deposit and its mineral chemistry is quite variable. The variability can be described in terms of biotite colors and short wavelength infrared (SWIR) reflectance spectra. Brown to greenish brown biotite displays lower values of Mg# (0.48–0.58) than green biotite (0.62–0.78), whereas colorless phlogopite related to Fe–P mineralization has the highest values of Mg# (0.85–0.95). To compare Mg# values with the positions and intensities of absorption in the trioctahedral micas, we used three types of spectrum: reflectance, hull quotient, and Gaussian. Brown biotite produces a short Mg–OH peak near 2340 nm, green biotite has a peak near 2330 nm, and colorless phlogopite displays an absorption peak near 2310 nm. These positional variations are consistent with compositional changes. A mica alteration index (MAI) represents the ratio of Fe–OH and Mg–OH peak absorption intensities for trioctahedral micas. From brown to green biotite and colorless phlogopite, the MAI values gradually increase with decreasing Fe–OH intensity. Correlation coefficients for plots of Mg–OH absorption values vs. Mg# vary from 0.89 for the reflectance method, to 0.93 for the hull quotient method, and 0.93 for the Gaussian method, and for plots of MAI vs. Mg# they vary from 0.91 for the reflectance method, to 0.92 for the hull quotient method, and 0.92 for the Gaussian method. Analyses using our new SWIR spectroscopic methods confirm that the abundance of indicator minerals and the Fe:Mg ratios in trioctahedral micas vary systematically as a function of hydrothermal fluid chemistry and mineral composition within the altered lithologies. SWIR spectroscopy of trioctahedral micas could provide a useful tool for mapping vectors of alteration in the Yangyang IOA system.

Hydrothermal Venting at Hinepuia Submarine Volcano, Kermadec Arc: Understanding Magmatic‐Hydrothermal Fluid Chemistry

AbstractThe Hinepuia volcanic center is made up of two distinct edifices aligned northwest to southeast, with an active cone complex in the SE. Hinepuia is one of several active volcanoes in the northern segment of the Kermadec arc. Regional magnetic data show no evidence for large‐scale hydrothermal alteration at Hinepuia, yet plume data confirm present‐day hydrothermal discharge, suggesting that the hydrothermal system may be too young to have altered the host rocks with respect to measurable changes in magnetic signal. Gravity data are consistent with crustal thinning and shallow mantle under the volcanic center. Following the discovery of hydrothermal plumes over Hinepuia, the submersible Shinkai 6500 was used to explore the SE cone and sample hydrothermal fluids. The chemistry of hydrothermal fluids from submarine arc and backarc volcanoes is typically dominated by water‐rock interactions and/or magmatic degassing. Chemical analyses of vent fluids show that Hinepuia does not quite fit either traditional model. Moreover, the Hinepuia samples fall between those typically ascribed to both end‐member fluid types when plotted on a K‐Mg‐SO4 ternary diagram. Due to evidence of strong degassing, abundant native sulfur deposition, and H2S presence, the vent sampled at Hinepuia is ultimately classified as a magmatic‐hydrothermal system with a water‐rock influence. This vent is releasing water vapor and magmatic volatiles with a notable lack of salinity due to subcritical boiling and phase separation. Magmatic‐hydrothermal fluid chemistry appears to be controlled by a combination of gas flux, phase separation processes, and volcano evolution and/or distance from the magma source.

Geochemistry of hydrothermal vent fluids and its implications for subsurface processes at the active Longqi hydrothermal field, Southwest Indian Ridge

The Longqi hydrothermal field at 49.6°E on the Southwest Indian Ridge was the first active hydrothermal field found at a bare-rock ultra-slow spreading mid-ocean ridge. Here we report the chemistry of the hydrothermal fluids, for the first time, that were collected from the S zone and the M zone of the Longqi field by gas-tight isobaric samplers by the HOV “Jiaolong” diving cruise in January 2015. According to H2, CH4 and other chemical data of the vent fluid, we suggest that the basement rock at the Longqi field is dominantly mafic. This is consistent with the observation that the host rock of the active Longqi Hydrothermal field is dominated by extensively distributed basaltic rock. It was very interesting to detect simultaneously discharging brine and vapor caused by phase separation at vents DFF6, DFF20, and DFF5 respectively, in a distance of about 400m. Based on the end-member fluid chemistry and distance between the vents, we propose that there is a single fluid source at the Longqi field. The fluid branches while rising to the seafloor, and two of the branches reach S zone and M zone and phase separate at similar conditions of about 28–30.2MPa and 400.6–408.3°C before they discharge from the vents. The end-member fluid compositions of these vents are comparable with or within the range of variation of known global seafloor hydrothermal fluid chemical data from fast, intermediate and slow spreading ridges, which confirms that the spreading rate is not the key factor that directly controls hydrothermal fluid chemistry. The composition of basement rock, water-rock interaction and phase separation are the major factors that control the composition of the vent fluids in the Longqi field.

Texture and chemistry of pyrite at Chah Zard epithermal gold–silver deposit, Iran

Gold mineralization at Chah Zard, Iran, is mostly concentrated in breccia and veins, and is closely associated with pyrite. Optical and scanning electron microscopy-backscattered electron observations indicate four different pyrite types, each characterized by different textures: porous and fractured py1, simple-zoned, oscillatory-rimmed, framboidal and fibrous py2, colloform py3, and inclusion-rich py4. Laser ablation ICP–MS analysis and elemental mapping reveal the presence of invisible gold in all pyrite types. The highest concentrations (161–166ppm Au) are found in py2 and py4, which correlate with the highest As concentrations (73,000–76,000ppm). In As-poor grains, Au concentrations decrease by about two orders of magnitude. Copper, Pb, Zn, Te, Sb, and Ag occur with invisible gold, suggesting that at least part of the gold occurs in nanoparticles of sulfosalts of these metals and metalloids. Gold distribution patterns suggest that only negligible Au was originally trapped in py1 from the initial ore fluids. However, most, if not all, Au was transported and deposited during subsequent overprinting hydrothermal fluid flow in overgrowth rims around the margins of the py2 and within microfractures of py4 grains. Oscillatory zonation patterns for Co, Ni, Sb, Cu, Pb, and Ag in pyrite reflect fluctuations in the hydrothermal fluid chemistry. The LA-ICP–MS data reveal that Cu, Pb and Ag show systematic variations between different pyrite types. Thus, Cu/Pb and Pb/Ag ratios in pyrite may provide a potentially powerful exploration vector to epithermal gold mineralization at Chah Zard district and elsewhere.

Thermophilic Archaeal Diversity and Methanogenesis from El Tatio Geyser Field, Chile

ABSTRACTThe hydrothermal fluid chemistry at El Tatio Geyser Field (ETGF) in northern Chile suggests that biogenic CO2–CH4 cycling may play an important role in water chemistry, and relatively low sulfate (0.6–1 mM) and high molecular hydrogen (H2) concentrations (67–363 nM) suggest that methanogenic Archaea are present in ETGF microbial mats. In this study, δ13C analysis of dissolved inorganic carbon and methane was not indicative of biogenic methane production (δ13CCH4 values ranging from −15‰ to −5.3‰); however, methanogenic Archaea were successfully cultured from each of the hydrothermal sites sampled. Sanger sequencing using universal Archaea primers identified putative methanogenic orders with varying metabolic capabilities, including Methanobacteriales, Methanomicrobiales and Methanosarcinales.

Vapor–liquid partitioning of alkaline earth and transition metals in NaCl-dominated hydrothermal fluids: An experimental study from 360 to 465 °C, near-critical to halite saturated conditions

Multi-phase fluid flow is a common occurrence in magmatic hydrothermal systems; and extensive modeling efforts using currently established P–V–T–x properties of the NaCl–H2O system are impending. We have therefore performed hydrothermal flow experiments (360–465°C) to observe vapor–liquid partitioning of alkaline earth and first row transition metals in NaCl-dominated source solutions. The data allow extraction of partition coefficients related to the intrinsic changes in both chlorinity and density along the two-phase solvus. The coefficients yield an overall decrease in vapor affinity in the order Cu(I)>Na>Fe(II)>Zn>Ni(II)⩾Mg⩾Mn(II)>Co(II)>Ca>Sr>Ba, distinguished with 95% confidence for vapor densities greater than ∼0.2g/cm3. The alkaline earth metals are limited to purely electrostatic interactions with Cl ligands, resulting in an excellent linear correlation (R2>0.99) between their partition coefficients and respective ionic radii. Though broadly consistent with this relationship, relative behavior of the transition metals is not well resolved, being likely obscured by complex bonding processes and the potential participation of Na in the formation of tetra-chloro species. At lower densities (at/near halite saturation) partitioning behavior of all metals becomes highly non-linear, where M/Cl ratios in the vapor begin to increase despite continued decreases in chlorinity and density. We refer to this phenomenon as “volatility”, which is broadly associated with substantial increases in the HCl/NaCl ratio (eventually to >1) due to hydrolysis of NaCl. Some transition metals (e.g., Fe, Zn) exhibit volatility prior to halite stability, suggesting a potential shift in vapor speciation relative to nearer critical regions of the vapor–liquid solvus. The chemistry of deep-sea hydrothermal fluids appears affected by this process during magmatic events, however, our results do not support suggestions of subseafloor halite precipitation recorded in currently available field data. Ca–Cl systematics in vent fluids are specifically explored, revealing behavior consistent with partitioning due to phase separation. Interestingly, the effect of variable chloride on dissolved Na/Ca ratios associated with plagioclase solubility (in single-phase solutions) appears fundamentally similar to that of phase separation on vapor compositions such that vapors evolved in hydrothermal systems may naturally remain near equilibrium with the host lithology. Conversely, residual liquids/brines left behind in the crust may be undersaturated with metals, enhancing the rate and extent of hydrothermal alteration.

The Lost City hydrothermal system: Constraints imposed by vent fluid chemistry and reaction path models on subseafloor heat and mass transfer processes

Since the first reported discovery of the Lost City hydrothermal system in 2001, it was recognized that seawater alteration of ultramafic rocks plays a key role in the composition of the coexisting vent fluids. The unusually high pH and high concentrations of H2 and CH4 provide compelling evidence for this. Here we report the chemistry of hydrothermal fluids sampled from two vent structures (Beehive: ∼90–116°C, and M6: ∼75°C) at Lost City in 2008 during cruise KNOX18RR using ROV Jason 2 and R/V Revelle assets. The vent fluid chemistry at both sites reveals considerable overlap in concentrations of dissolved gases (H2, CH4), trace elements (Cs, Rb, Li, B and Sr), and major elements (SO4, Ca, K, Na, Cl), including a surprising decrease in dissolved Cl, suggesting a common source fluid is feeding both sites. The absence of Mg and relatively high concentrations of Ca and sulfate suggest solubility control by serpentine–diopside–anhydrite, while trace alkali concentrations, especially Rb and Cs, are high, assuming a depleted mantle protolith. In both cases, but especially for Beehive vent fluid, the silica concentrations are well in excess of those expected for peridotite alteration and the coexistence of serpentine–brucite at all reasonable temperatures. However, both the measured pH and silica values are in better agreement with serpentine–diopside–tremolite-equilibria. Geochemical modeling demonstrates that reaction of plagioclase with serpentinized peridotite can shift the chemical system away from brucite and into the tremolite stability field. This is consistent with the complex intermingling of peridotite and gabbroic bodies commonly observed within the Atlantis Massif. We speculate the existence of such plagioclase bearing peridotite may also account for the highly enriched trace alkali (Cs, Rb) concentrations in the Lost City vent fluids. Additionally, reactive transport modeling taking explicit account of temperature dependent rates of mineral dissolution and precipitation clarifies the feedback between permeability, heat loss, and changes in the dissolved Si of the vent fluids. Assuming both the Beehive and M6 vent fluids were sourced at similar subseafloor conditions (tremolite buffered at 200°C), model results indicate loss of approximately 30% Si upon cooling to ∼150°C during upflow. However, Si concentrations remained largely conservative with continued cooling to lower temperatures owing to unfavorable reaction kinetics. While consistent with the Beehive endmember composition, these results fail to explain the relative Si depletion in the lower temperature M6 fluids. Thus, it may be that more robust kinetic models for silicates are needed to accurately account for the mechanism and rate of silica removal in the unusually high pH of the Lost City vent fluids.

The Trans-Atlantic Geotraverse hydrothermal field: A hydrothermal system on an active detachment fault

Over the last ten years, geophysical studies have revealed that the Trans-Atlantic Geotraverse (TAG) hydrothermal field (26°08′N on the Mid-Atlantic Ridge) is located on the hanging wall of an active detachment fault. This is particularly important in light of the recognition that detachment faulting accounts for crustal accretion/extension along a significant portion of the Mid-Atlantic Ridge, and that the majority of confirmed vent sites on this slow-spreading ridge are hosted on detachment faults. The TAG hydrothermal field is one of the largest sites of high-temperature hydrothermal activity and mineralization found to date on the seafloor, and is comprised of active and relict deposits in different stages of evolution. The episodic nature of hydrothermal activity over the last 140ka provides strong evidence that the complex shape and geological structure of the active detachment fault system exerts first order, but poorly understood, influences on the hydrothermal circulation patterns, fluid chemistry, and mineral deposition. While hydrothermal circulation extracts heat from a deep source region, the location of the source region at TAG is unknown. Hydrothermal upflow is likely focused along the relatively permeable detachment fault interface at depth, and then the high temperature fluids leave the low-angle portion of the detachment fault and rise vertically through the highly fissured hanging wall to the seafloor. The presence of abundant anhydrite in the cone on the summit of the TAG active mound and in veins in the crust beneath provides evidence for a fluid circulation system that entrains significant amounts of seawater into the shallow parts of the mound and stockwork. Given the importance of detachment faulting for crustal extension at slow spreading ridges, the fundamental question that still needs to be addressed is: How do detachment fault systems, and the structure at depth associated with these systems (e.g., presence of plutons and/or high permeability zones) influence the pattern of hydrothermal circulation, mineral deposition, and fluid chemistry, both in space and time, within slowly accreted ocean crust?

Episodic Subseafloor Hydrothermal Activity Within the Eastern Manus Back-Arc Basin Determined by Uranium-Series Disequilibrium in Barite

The geochronology of seafloor hydrothermal activity from several mineralized, active and inactive vent fields at Suzette and Pual Ridge, eastern Manus back-arc basin, Bismarck Sea, Papua New Guinea, has been established using the 228Th/228Ra, 228Ra/226Ra, and 226Ra/Ba uranium-series disequilibrium methods for barite-rich samples. These hydrothermal systems underwent episodes of hydrothermal fluid circulation that produced barite over the last ~6,000 years. Precipitation of hydrothermal barite, disseminated within the host rock and lining fractures, vugs, and conduits, was controlled by decreasing temperatures and mixing of hydrothermal fluids with oxidized seawater. Repeated fluctuations in hydrothermal fluid temperature and chemistry resulted in complex sequences of barite and sulfide minerals lining vugs and conduits, whereas rapid cooling following fracture events led to precipitation of only barite and some Fe oxide coatings on barite crystals. A wide range in ages of barite within drill cores (i.e., 380–5,990 yrs before present (BP)) reflect repeated pulses of hydrothermal fluids through the altered volcanic and mineralized rocks that make up the hydrothermal mound structures, with the younger ages (i.e., 34–220 yrs BP), representing more recent barite-precipitating fluid circulation accessing fractures, veins, and vugs. Barite from chimney samples is mostly young (i.e., 23–1,680 yrs BP), with the majority precipitated between 23 to 41 yrs BP; reflecting the structural stability (standing life) of the chimneys and possible influences from varying hydrothermal fluid chemistry or reactivation of vent fields. Three generalized age groupings, representing major episodes of barite-precipitating hydrothermal fluid circulation, are resolved from the Suzette area (19–45, 810–1,900, and 3,740–4,710 yrs BP, including 1σ errors); with the most recent ages recorded from both the Suzette and Pual Ridge areas. Fast-spreading rates within the Manus basin, an abundance of faults and variable magmatic inputs are major factors controlling the frequency and longevity of these episodes of hydrothermal activity. Mineral resource models and comparisons indicate that the Cu-Zn-Ag-Au–rich Suzette deposit is small; however the measured age range is comparable to ages given for modeled (typical) seafloor massive sulfide deposits from back-arc and arc environments. We suggest that the scale of the Suzette deposit may have been limited during formation by a low hydrothermal fluid flux and/or inappropriate physical and chemical conditions for sufficient metal precipitation from hydrothermal fluids.

Magmatic eruptions and iron volatility in deep-sea hydrothermal fluids

Author(s): Pester, NJ; Ding, K; Seyfried, WE | Abstract: During periods of volcanic activity, hydrothermal fluid chemistry changes drastically, becoming unusually dilute due to enhanced degrees of phase separation. Despite decreases in nearly all other metals, these dilute fluids maintain surprisingly high dissolved Fe concentrations. This is demonstrated by a 17 yr time series from 9°50'N on the East Pacific Rise, where two eruption cycles are separated by a decade of steady-state chemical and physical conditions. We report experimental data confi rming a sharp increase in Fe solubility in low-salinity and low-density vapors that constitutes a reversal in behavior exhibited in near-critical vapors characteristic of the steady-state condition. In accordance with field observations during the eruptions, a fundamental divergence between the otherwise similar behaviors of Fe and Mn also results. This helps explain how Fe fluxes are maintained during magmatic events, which may have important implications for the succession and temporal evolution of vent-related fauna. Calibrated geochemical proxies for subseafl oor reaction conditions (pressure-temperature) now allow us to elucidate hydrothermal processes from steady state through eruptive and recovery stages at the 9°50'N system. © 2014 Geological Society of America.

Theoretical constraints of physical and chemical properties of hydrothermal fluids on variations in chemolithotrophic microbial communities in seafloor hydrothermal systems

In the past few decades, chemosynthetic ecosystems at deep-sea hydrothermal vents have received attention as plausible analogues to the early ecosystems of Earth, as well as to extraterrestrial ecosystems. These ecosystems are sustained by chemical energy obtained from inorganic redox substances (e.g., H2S, CO2, H2, CH4, and O2) in hydrothermal fluids and ambient seawater. The chemical and isotope compositions of the hydrothermal fluid are, in turn, controlled by subseafloor physical and chemical processes, including fluid–rock interactions, phase separation and partitioning of fluids, and precipitation of minerals. We hypothesized that specific physicochemical principles describe the linkages among the living ecosystems, hydrothermal fluids, and geological background in deep-sea hydrothermal systems. We estimated the metabolic energy potentially available for productivity by chemolithotrophic microorganisms at various hydrothermal vent fields. We used a geochemical model based on hydrothermal fluid chemistry data compiled from 89 globally distributed hydrothermal vent sites. The model estimates were compared to the observed variability in extant microbial communities in seafloor hydrothermal environments. Our calculations clearly show that representative chemolithotrophic metabolisms (e.g., thiotrophic, hydrogenotrophic, and methanotrophic) respond differently to geological and geochemical variations in the hydrothermal systems. Nearly all of the deep-sea hydrothermal systems provide abundant energy for organisms with aerobic thiotrophic metabolisms; observed variations in the H2S concentrations among the hydrothermal fluids had little effect on the energetics of thiotrophic metabolism. Thus, these organisms form the base of the chemosynthetic microbial community in global deep-sea hydrothermal environments. In contrast, variations in H2 concentrations in hydrothermal fluids significantly impact organisms with aerobic and anaerobic hydrogenotrophic metabolisms. Particularly in H2-rich ultramafic rock-hosted hydrothermal systems, anaerobic and aerobic hydrogenotrophy is more energetically significant than thiotrophy. The CH4 concentration also has a considerable impact on organisms with aerobic and anaerobic methanotrophic metabolisms, particularly in sediment-associated hydrothermal systems. Recently clarified patterns and functions of existing microbial communities and their metabolisms are generally consistent with the results of our thermodynamic modeling of the hydrothermal mixing zones. These relationships provide important directions for future research addressing the origin and early evolution of life on Earth as well as for the search for extraterrestrial life.

Chemical composition of rock-forming minerals in granitoids associated with Au–Bi–Cu, Cu–Mo, and Au–Ag mineralization at the Freegold Mountain, Yukon, Canada: magmatic and hydrothermal fluid chemistry and petrogenetic implications

The chemical compositions of rock-forming minerals have been determined for both altered and least-altered igneous rocks spatially associated with numerous mineralized zones (Nucleus Au–Bi–Cu–As deposit, Revenue Au ± Cu and Stoddart Cu–Mo ± W mineral occurrences, and Laforma Au–Ag deposit) across the Freegold Mountain area, Yukon, Canada. Within the study area, K-feldspar has a narrow compositional range (89.4–91% Or), whereas plagioclase spans a wide range (4.4–70.07% An). In all of the investigated samples, T Ab = T An = T Or, suggesting that magmatic equilibrium between the coexisting plagioclase and K-feldspar was maintained. Igneous amphibole phenocrysts from hypabyssal dikes are typically calcic, whereas the Stoddart Cu–Mo ± W, Laforma Au–Ag, and Goldy Au mineralization are associated with Mg-enriched primary amphibole of edenite composition, and Au–Bi–Cu–As mineralization from Nucleus is related to Al-enriched primary amphibole of ferropargasite composition. Primary biotite phenocrysts across the Freegold Mountain area re-equilibrated with oxidized magma (f(O2) values between 10–13 and 10–11.5 bars, lying between the Ni/NiO and the magnetite/haematite buffers). However, biotite and amphibole phenocrysts from Stoddart, Goldy, Laforma, and the Highway zones crystallized from a more oxidized magma, as indicated by their elevated X Mg up to 0.65, relative to biotite and hornblende from Nucleus and Revenue characterized by a lower X Mg (typically < 0.50). This suggests that various sources and (or) rapid emplacement were involved in magma genesis, as further supported by the considerable variation of pressure (1.8–7.3 kb) of amphibole crystallization and of the total Al content in least-altered biotite (2.6–2.9 afu) within the Freegold Mountain area. Biotite and apatite equilibrated within the T range of 520–780°C, consistent with temperatures of equilibration between ilmenite and magnetite, and their compositions indicate that they formed from an oxidized I-type magma. Magma differentiated by fractional crystallization (indicated by the presence of normally zoned plagioclase with Ca-rich cores and Na-enriched outer rims) and multiple magma mixing (supported by the presence of reversed zoned plagioclase and coexistence of normally and reversely zoned plagioclase). Lower X Mg biotite associated with the mineralized (Cu–Mo ± W) potassic alteration incorporated more F and Cl relative to least-altered biotite with higher X Mg. In both Nucleus and Revenue Au–Cu mineralizations, secondary biotite composition varies with respect to the associated alteration mineral assemblages. Although secondary biotite in the skarn re-equilibrated with F-poor fluids, secondary biotite from the pervasive biotitization is related to F- and Cl-enriched fluids, and secondary biotite from the phyllitic zone is related to F-, Cl-, and Mg-depleted fluids, thus consistent with a change in mineralizing fluid composition during mineralization.

Geochemical origin of hydrothermal fluid methane in sediment-associated fields and its relevance to the geographical distribution of whole hydrothermal circulation

We investigated the chemical composition of hydrothermal fluids and deduced the origin of hydrothermal fluid methane in a sediment-associated Minami-Ensei (ME) hydrothermal field, the Okinawa Trough. The endmember hydrothermal fluid of the ME field contained isotopically 13C-enriched methane (−26.0 to −24.7‰), which suggested a thermogenic origin. However, the observed methane-dominating hydrocarbon compositions (C1/C2+>103) suggested that biogenic methane, not thermogenic hydrocarbons, is predominant. The conflict between these geochemical indications can be resolved by the hypothesis that the thermal reaction of sedimentary organic matter initially generates the primary source of hydrocarbons and the secondary thermal decomposition of hydrocarbons increases the C1/C2+ ratio as high as that of biogenic methane. The low molecular hydrogen concentration (0.04mmol/kg) in the ME fluid suggests that the thermal fluid–sediment interaction occurred in the recharge stage, not the discharge stage, of hydrothermal circulation. Less biogenic methane in the ME fluid implies a laterally shrunken hydrothermal fluid circulation that reduced biogenic methane incorporation at the recharge stage. A comparison of hydrothermal fluid chemistry and geological/geographical settings among the global sediment-associated hydrothermal systems also suggests representative patterns in the geochemical origins of hydrothermal fluid methane and their relevance to hydrothermal fluid circulation as a whole.