Open Hydrothermal System Research Articles

Growth mechanism of double cone-shaped nano zinc oxide prepared from zinc oxidation flue dusts by acid leaching

Cone-shaped nano zinc oxide was prepared by acid leaching-alkali precipitation using zinc oxidation flue dusts as raw materials. The dynamic conditions of acid leaching and the growth mechanism of ZnO crystal were also discussed. It was shown that the optimum acid leaching conditions were H2SO4 concentration 2 mol/L, liquid–solid ratio 20:1, reaction time 4 h, reaction temperature 90 °C and Zn leaching rate was 94.98%. Then, the nano ZnO was prepared by acid leaching solution in an open hydrothermal system at low temperature. The formation process of nano ZnO crystals undergoes crystallization-dissolution-recrystallization under low temperature and alkaline conditions. Double cone-shaped ZnO nanoparticles could be obtained at 62 °C with n(Zn2+)/n(OH−) ratio of 1:5 and reaction time of 1 h. The average size of ZnO nanoparticles was about 100 nm.

Tunnelgeothermie – Ein Überblick

AbstractTunnel geothermal systems hold the potential to promote decarbonization of the building heating and cooling sector. They can be integrated into existing infrastructure, resulting in low additional costs. In addition, these systems have large contact areas with the ground leading to larger heat fluxes. However, tunnel geothermics is relatively unknown and rarely used. Thus, the objective of this study is to provide an overview of the two primary tunnel geothermal system types as well as their application and potential. Open hydrothermal systems use the tunnel drainage water as a heat source, whereas closed absorber systems harness the heat flux from the subsoil and the warm tunnel interior via heat exchangers. The evaluation of the global application of existing and planned tunnel geothermal systems shows that all open systems are currently located in mountainous regions with a thick rock overburden. In contrast, closed absorber systems are mostly installed in urban tunnel infrastructures. The spatial distribution of geothermal tunnel systems has a focus in central Europe with Switzerland, Germany and Austria being the countries with the highest number of installed systems. Finally, this study also presents a brief summary of existing methods to determine the geothermal potential of tunnels.

A Rayleigh model of cesium fractionation in granite-pegmatite systems

Abstract The K/Cs ratios of K-feldspars from granitic pegmatites are compared to models derived from the Rayleigh equation. The K/Cs and K/Rb ratios of K-feldspars and micas exhibit decreasing values when plotted against their Cs or Rb contents across cogenetic suites of granites and pegmatites and from margin to core of individual bodies. The trends in elemental ratios conform to Rayleigh fractionation for the crystallization of feldspars and micas from a silicate melt. Within two individual pegmatite bodies, the K/Cs ratio of K-feldspar initially falls more rapidly than the Rayleigh model predicts. That might reflect a local increase in the concentration of Cs relative to K due to the pile-up of incompatible elements in a boundary layer of melt adjacent to the crystal growth front. The addition of an aqueous solution to the Rayleigh model (i.e., the simultaneous crystallization of K-feldspars from melt and from aqueous solution) predicts high and increasing K/Cs ratios of K-feldspars that are not observed in natural rock suites, except when K-feldspars crystallize in miarolitic cavities or when primary K-feldspar recrystallizes to microcline perthite in an open hydrothermal system. In those cases, the Cs content of K-feldspars falls to nil because of the high solubility of Cs in aqueous solution and low compatibility of Cs in K-feldspar. Otherwise, the observed patterns of K/Rb or K/Cs in K-feldspar and micas in pegmatites conform to crystal-melt fractionation in which an aqueous solution plays no part. From the viewpoint of the geochemistry of Cs in pegmatites, these observations give support to the model proposed by Cameron et al. (1949) and endorsed by Jahns (1953a, 1953b).

NanoSIMS element mapping and sulfur isotope analysis of Au-bearing pyrite from Lannigou Carlin-type Au deposit in SW China: New insights into the origin and evolution of Au-bearing fluids

Sulfur isotope signatures of Au-bearing pyrite from Lannigou Au deposit, a typical Carlin-type Au deposit in SW China, provide valuable information about the origin of the ore-forming minerals. Analysis by NanoSIMS was used to determine S isotope compositions of Au-bearing pyrite and to map the grain-scale distributions of Au, Cu, As and S in pyrite from the deposit. Based on different textural pattern of pyrites revealed by back-scattered electron (BSE) images, they are divided into three types: Py-1 diagenetic pyrite without core-rim structure, Py-2 pyrite with an Au-free core and a rhythmically-zoned Au-bearing rim, Py-3 Au-bearing pyrite with rhythmic zoning across the entire grain. The element distributions and S isotope compositions of four paragenetic stages are recognized on the basis of textural observation. Py-1 grains and the Au-free homogeneous cores of zoned crystals were formed in Stage 1 while the Au-bearing rims of the zoned crystals with rhythmic zonation of As and Cu, and to a lesser degree Au, were formed in two superimposed stages: stage 2 formed the inner zone that is enriched in As alone; and stage 3 formed the outer zone that is enriched in both Au and As. Other sulfides such as realgar, cinnabar and stibnite are formed in the last stage. The relationship between Au and As distributions in pyrite rim is complicated, changing from coupled to decoupled at the nanoscale. Such complexity is interpreted to reflect fluctuation of fluid composition and temperature with time, which in turn affect the modes of occurrence of As and Au. It is inferred that As mainly occurs in the crystal lattice replacing S whereas Au is mainly present as nanoparticles that were trapped in pyrite during crystal growth. The Au-bearing rims of the zoned pyrite crystals are characterized by highly variable δ34S values from 1.1 to 18.1‰, which exceed the values of the Triassic calcareous host rocks (10–14‰). In contrast, the δ34S values of the Au-free cores of zoned pyrite crystals vary over a narrower interval and are mainly between 6 and 12‰, close to the values of pyrite crystals in the sedimentary country rocks. Our new analyses also reveals that the δ34S values of the Au-bearing fluids generally increase during the formation of the deposit. The observed S isotope variations are consistent with mixing between a magmatic-related fluid with mantle-like δ34S value (∼0‰) and a sedimentary or deep basin brine fluid with elevated δ34S value (>18‰), with an increasing contribution from the latter with time. The notably varied values of δ34S and the disseminations of Au and other trace elements such as As and Cu in pyrite crystals indicate that the process responsible for Au precipitation in this deposit occurred in an open hydrothermal system.

THE ISOTOPIC SIGNATURE OF THE MINERALIZING FLUID OF THE LAVRION CARBONATE-REPLACEMENT PB-ZN-AG DISTRICT

The Pb-Zn-Ag carbonate-replacement deposits in the Lavrion district are genetically related to a 7- 10 Ma-old granodiorite, felsic dikes and sills. These deposits are hosted in the Upper and Lower marble and schists of the Cyclades Blueschist unit and occur along the major Legraina detachment fault. Carbonate-replacement orebodies occur as “mantos” and veins, dominated by base metal sulfides and Ag, Bi, Sn, Sb, As, and Pb sulfosalts. Calculated carbon and oxygen isotope compositions of the hydrothermal fluid range from δ13CCO2 of -13.7 to 0.8 per mil and δ18OH2O of 4.2 to 27.4 per mil, at 400º, 350º, 320º, 300º, 250º and 200ºC. These isotopic compositions reveal water-torock ratios ranging from 4.8 to 52.6%, which reflect intense interaction of the ore fluid with the host rock in a water-dominated, transitional closed to open hydrothermal system. The range of δ34SH2S for sulfides in the deposits were from -8.5 to 6.8 per mil, for similar temperatures, whereas for barite-fluorite veins from δ34SH2S of -43.6 to -16.4 per mil, at 200º, 150º and 100ºC. This range implies that there was contribution from a magmatic sulfur component exsolved from the Plaka pluton, as well as contribution from a metasedimentary component. Based on the isotopic signature of sulfur for barite, the ranges from -6.7 to -7.6, comprising an increase in the fluid influx. Isotopic temperatures based on pyrite-galena and sphalerite-pyrite pairs revealed at least three major events of carbonate-replacement ore deposition, (i) at ~ 360º, (ii) 320º-280ºand (iii) 260º-200ºC.

Dual sources of water overprinting on the low zircon \u03b418O metamorphic country rocks: Disequilibrium constrained through inverse modelling of partial reequilibration

Since water is only composed of oxygen and hydrogen, δ18O and δ2H values are thus utilized to trace the origin of water(s) and quantify the water-rock interactions. While Triassic high pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks across the Dabie-Sulu orogen in central-eastern China have been well documented, postcollisional magmatism driven hydrothermal systems are little known. Here we show that two sources of externally derived water interactions were revealed by oxygen isotopes for the gneissic country rocks intruded by the early Cretaceous postcollisional granitoids. Inverse modellings indicate that the degree of disequilibrium (doD) of meteoric water interactions was more evident than that of magmatic one (−65 ± 1o vs. −20 ± 2°); the partial reequilibration between quartz and alkali feldspar oxygen isotopes with magmatic water was achieved at 340 °C with a water/rock (W/R) ratio of about 1.2 for an open-hydrothermal system; two-stage meteoric water interactions were unraveled with reequilibration temperatures less than 300 °C and W/R ratios around 0.4. The lifetime of fossil magmatic hydrothermal system overprinted on the low zircon δ18O orthogneissic country rocks was estimated to maintain up to 50 thousand years (Kyr) through oxygen exchange modellings. Four-stage isotopic evolutions were proposed for the magmatic water interacted gneiss.

Synthesis and Characterization of Ca and Na Zeolites (Non-Pozzolanic Materials) Obtained From Fly Ash–Ca(OH)2 Interaction

Abstract The conventional method for alkali activation of fly ash utilizes Ca(OH)2 and NaOH for the formation of pozzolanic material and fly ash zeolites, respectively. Sodium-based fly ash zeolites (say, Na-zeolite) mostly employ NaOH (the high-grade mineralizer and alkali) for activation of fly ash before its application as an absorbent. However, the Na-zeolites as absorbents (in agro- and aqua-culture) result in sodicity (i.e., excess of Na, present as impurity in the zeolite), which in turn reduces their holding capacity of moisture, nutrient [i.e., nitrogen, phosphorous, and potassium (NPK fertilizers)], microorganisms (viz., microbial spores), or heavy metals and negatively affect the growth of plant and aquatic life. To resolve such problems, the present study is focused on synthesis of agro-grade blend (dominated by Ca-zeolite) of zeolites by using Ca(OH)2 as major alkali and two well-established mineralizers, NaOH and NaCl, used in the trace quantity. To monitor activation of the fly ash in two different conditions, the synthesis of zeolites could be carried out by employing (1) conventional (the open hydrothermal system), and (2) autoclaving (the closed hydrothermal system) methods. The main attributes that control the entire study include temperature and reaction times for both methods. In addition, the present study demonstrates (1) effectiveness of Ca(OH)2 in creation of blend of zeolites with considerable cation exchange capacity when an optimum chemical composition (comparable to a pure agro- and aqua-grade zeolite 4A), and (2) suitability of the conventional hydrothermal method over autoclave method in synthesizing the blend of Ca- and Na-zeolites possessing a cation exchange capacity up to 394 meq/100 g. The formation of needle/star/spherule/small cube-shaped crystals (i.e., Na-P1, the Na-zeolite) and prismatic/cuboidal shaped crystals (i.e., heulandite, the Ca-zeolite), confirms suitability of the end product as a good sorbent and manure.

Analyze the Growth Habit of Mullite Nanocomposites by First-Principles Calculations

The plane surface energy of the mullite crystals (001), (010), (100) and (-100) is comparative calculated by using first-principles plane-wave pseudopotential method and the open hydrothermal system preparing mullite phase crystal test conditions. Also the plane growth habits can be predicted according to the results. The result shows that: the descending order of the mullite crystal surface energy is: E(001)> E(010)> E(-100)> E(100). On the basis of Cune-Woolf principle, the surface energy of mullite crystals (001) surface is the maximum and growth rate is the fastest. Thus the (001) crystal face is growth surface habits of mullite phase crystal. Crystal growth morphology mostly extends in one direction.

Preparation and mechanism of nano mullite powders from kaolin via open hydrothermal process.

Nano mullite powders have been synthesized from calcined kaolin via open hydrothermal process. The powder product was characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM) with selected area electron diffraction (SAED) analysis. The results indicated that single phase puncheon-shaped nano mullite with about 140-360 nm in length and 50-80 nm in diameter is produced when synthesized in 4 mol/L NaOH solution at 100 degrees C for 4 h. The grain size of mullite decrease slightly with increasing reaction time from 0 h to 4 h. In addition, nuclear magnetic resonance (NMR) analysis confirms presence of Al3+ during the whole reaction process, which indicates that there is an absence of growth units of mullite in the open hydrothermal system and the mullite precursor mainly underwent a solution process during open hydrothermal treatment.

Fluid inclusion and stable C-O isotope constraints on the origin of metasomatic magnesite deposits of the Western Carpathians, Slovakia

Primary fluid inclusions in metasomatic magnesite deposits hosted in Palaeozoic basement of the Western Carpathians are filled with of low-to-moderately saline aqueous solutions, with locally increased CO 2 concentrations (up to 34 mol.%). Brine inclusions with up to 42 wt.% of dissolved salts are less frequent. K/Na ratios in the fluid inclusion leachates indicate formation temperatures between 180–310 °C in the Gemeric unit and 230–300 °C in the Veporic tectonic unit. Carbon isotopes in metasomatic magnesite and dolomite show larger spread than those of oxygen. In some deposits, the δ 18O values are almost fixed in various generations of the metasomatic Mg-carbonates, while δ 13C values vary within several ‰. The C, O-isotope covariation reflects low concentrations of CO 2 (less than several mol.%) in the aqueous fluid precipitating the Mg-carbonates in an open hydrothermal system and high fluid/rock ratios ( w/r >5). Calculated δ 18O fluid values between 2 and 10‰ (V-SMOW) indicate isotopic exchange of the carbonate-precipitating fluid with crustal silicate rocks and/or marine carbonates at increased temperatures. Calculated δ 13C fluid values between –5 and 3‰ are thought to reflect dissolution of the metasomatised carbonate as well as the escape of lighter carbon isotope during the CO 2 degassing. Mg-carbonate-precipitating fluids typically contain increased Br-concentrations resembling the halite-fractionated residual brines originated by seawater evaporation. However, extent of the Br-enrichment substantially exceeds the buffering capacity of the seawater evaporation and it is even greater than that in the spatially associated siderite vein- and replacement-type deposits. Apart from the seawater evaporation, superimposed leaching of the organic matter from marine sediments probably played important role. This mechanism has, however, little effect in open hydrothermal systems. Hence, mechanism of the aditional Br-enrichment of the magnesite-forming fluids remains unknown. The observed stable isotope record is a result of Alpine hydrothermal processes as evidenced by coarse-grained dolomite with alpine-type mineral assemblage (rutile, apatite, zircon, muscovite-phengite) identified also in the spatially associated siderite vein- and replacement-type deposits. Another evidence for the Alpine origin are frequently observed primary CO 2-rich aqueous inclusions, up to 50 μm in diameter, which could not survive the Early Cretaceous Alpine metamorphic overprint, as well as different covariation of stable isotopes in siderite deposits, where larger oxygen isotope fluctuations are accompanied by less extensive carbon isotope fractionation. This indicates different precipitation mechanisms, i.e., CO 2-devolatilization in open system during Mg-metasomatism and devolatilization-absent precipitation of the siderite in a closed system triggered by rising temperature. The associated magnesite and siderite deposits may be co-sanguineous with respect to the presence of the evaporated seawater component in the ore-forming fluid, but they cannot be coeval owing to different composition of primary fluid inclusions, hydrologic regimes (open versus closed hydrothermal system) and precipitation mechanisms. The fluid inclusion and stable isotope evidence does not definitely discard genetic models, linking the Mg-metasomatism with infiltration of bitter brines along faults during Permo-Triassic rifting, but this process must have been entirely obliterated by the late Alpine (Cretaceous) hydrothermal activity along shear zones formed during middle-to-late Cretaceous transtension-extension of the orogenic wedge. The Permo-Triassic rift-related origin of the magnesite must cope with the problem of complete loss of pristine isotopic signature of the metasomatic Mg-carbonates during the superimposed Alpine hydrothermal activity, contrasting with none or negligible Alpine metamorphic/hydrothermal overprint of the spatially associated Fe-carbonate vein- and replacement-type deposits.

Fluid evolution in zoned Cordilleran polymetallic veins — Insights from microthermometry and LA-ICP-MS of fluid inclusions

Fluid inclusion analysis through the paragenetic sequence of one symmetrically zoned vein sample is used to reconstruct the P–T–X fluid evolution of a porphyry intrusion-related Cordilleran polymetallic vein from Morococha, central Peru. Results record an evolution from initial deep-seated precipitation of quartz–pyrite and base metal sulphides to final near-surface deposition of carbonates, demonstrating progressive mineralisation during uplift and erosion. This is the first detailed study addressing meso- to epithermal Zn–Pb–Ag–Cu-rich ore in a magmatic–hydrothermal system by combination of fluid inclusion microthermometry with laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) quantifying metal, as well as sulphur concentrations in the evolving hydrothermal fluid. Scanning electron microscopy cathodoluminescence (SEM-CL) imaging of quartz and detailed transmitted- and reflected-light petrography provide textural evidence that early, moderately saline (4–5 wt.% NaCl eq.) and CO 2-bearing fluids with homogenisation temperatures of 340°–380 °C precipitate Cu-bearing minerals. In this open hydrothermal system the fluids record decreasing salinities, CO 2-contents and temperatures, while Zn-, Pb-, and Ag-sulphides precipitate. Fluids related to early precipitation in the vein have metal contents of several 1000 μg/g S and Fe, over 1000 μg/g Cu, 100 μg/g Pb, 10 μg/g Ag, and several 100 μg/g Zn. Sulphur concentrations in the fluid are sufficiently high to precipitate all metals in solution as sulphides. The latest generation of fluid inclusions associated with abundant carbonate precipitation in the centre of the vein has homogenisation temperatures ranging from 260° to 220 °C, low metal concentrations, and no measurable CO 2. During vein formation, cooling and several kilometres of erosion resulted in “telescoping” of consecutively precipitated mineral assemblages. The deep input fluid dominating in the early vein stage is interpreted to be of magmatic origin, most likely a single phase magmatic fluid of intermediate salinity and density. It cooled to an aqueous liquid, separated minor CO 2-rich vapour, and was eventually diluted by meteoric water in the late stages of vein formation when the progressively eroded land surface was only several hundred meters above the vein location.

The fluid regime of ore formation in the Balei gold-bearing ore-magmatic system ( eastern Transbaikalia, Russia)

Based on comprehensive studies of fluid inclusions in quartz formed at different stages of hydrothermal process, we consider the physicochemical conditions of formation of epithermal ores (K 1) in the Balei ore field. The limiting parameters of hydrothermal process have been established: 353–131 °C, 150–30 bars, and salt concentrations of 7.6–0.5 wt.%-NaCl equiv. A specific feature of the ore-forming process at the Balei deposits is a rapid drop in temperature and pressure, which is typical of open hydrothermal systems. The temperature increase at the beginning of each stage evidences pulse-like ore formation. The productive stage coincides with the initiation of a drastic decrease in temperature (<225 °C) and salt concentration in the solution. The deposits resulted from the functioning of the common Balei ore-magmatic system at shallow depths with a high permeability of the host rocks. High-K calc-alkalic magmas might have been sources of gold mineralization. The ore formation zone is localized above intrusive bodies near their roof. It is not ruled out that the Balei gold was partly borrowed from the products of the early cycles (J 2-3) of gold mineralization and from the host rocks.

Mobilization of ore fluids during Alpine metamorphism: evidence from hydrothermal veins in the Variscan basement of Western Carpathians, Slovakia

AbstractHydrothermal polymetallic veins of the Gemeric unit of the Western Carpathians are oriented coherently with the foliation of their low‐grade Variscan basement host. Early siderite precipitated from homogeneous NaCl‐KCl‐CaCl2‐H2O brines with minor CO2, while immiscible gas–brine mixtures are indicative of the superimposed barite, quartz–tourmaline and quartz–sulphide stages. The high‐salinity aqueous fluid (18–35 wt%) found in all mineralization stages corresponds to formation water modified by interaction with crystalline basement rocks at temperatures between 140 and 300°C. High brominity (around 1000 ppm in average) resulted from evaporation and anhydrite precipitation in a Permo‐Triassic marine basin, and from secondary enrichment by dissolution of organic matter in the marine sediments at diagenetic temperatures. Sulphate depletion reflects thermogenic reduction during infiltration of the formation waters into the Variscan crystalline basement. Crystallization temperatures of the siderite fill (140–300°C) and oxygen isotope ratios of the parental fluids (4–10‰) increase towards the centre of the Gemeric cleavage fan, probably as a consequence of decreasing water/rock ratios in rock‐buffered hydrothermal systems operating during the initial stages of vein evolution. In contrast, buoyant gas–water mixtures, variable salinities and strongly fluctuating P–T parameters in the successive mineralization stages reflect transition from a closed to an open hydrothermal system and mixing of fluids from various sources. Depths of burial were 6–14 km (1.7–4.4 kbar, in a predominantly lithostatic fluid regime) during the siderite and barite sub‐stages of the north‐Gemeric veins, and up to 16 km (1.6–4.5 kbar, in a hydrostatic to lithostatic fluid regime) in the quartz–tourmaline stage of the south‐Gemeric veins. The fluid pressure decreased down to approximately 0.6 kbar during crystallization of sulphides. U‐Pb‐Th, 40Ar/39Ar and K/Ar geochronology applied to hydrothermal muscovite–phengite and monazite, as well as cleavage phyllosilicates in the adjacent basement rocks and deformed Permian conglomerates corroborated the opening of hydrothermal veins during Lower Cretaceous thrusting and their rejuvenation during Late Cretaceous sinistral transpressive shearing and extension.

High‐density nitrogen inclusions in barite from a giant siderite vein: implications for Alpine evolution of the Variscan basement of Western Carpathians, Slovakia

AbstractContrasting compositions and densities of fluid inclusions were revealed in siderite–barite intergrowths of the Droždiak polymetallic vein hosted in Variscan basement of the Gemeric unit (Central European Carpathians). Primary two‐phase aqueous inclusions in siderite homogenized between 101 and 165 °C, total salinity ranged between 18 and 27 wt%, and CaCl2/(NaCl + CaCl2) weight ratios were fixed at 0.1–0.3. By contrast, mono‐ and two‐phase aqueous inclusions in barite exhibited total salinities between 2 and 22 wt%, and the CaCl2/NaCl ratios ranged from NaCl‐ to CaCl2‐dominated compositions. The aqueous inclusions in barite were closely associated with very high‐density (0.55–0.745 g cm−3) nitrogen inclusions, in some cases containing up to 16 mol.% CO2. Crystallization P–T conditions of siderite (175–210 °C, 1.2–1.7 kbar) constrained by the vertical oxygen isotope gradient along the studied vein, isochores of fluid inclusions and the K/Na exchange thermometer corresponded to minimal palaeodepths between 4.3 and 6.3 km, assuming lithostatic load and average crust density of 2.75 g cm−3. Maximum fluid pressure during barite crystallization attained 3.6–4.4 kbar at 200–300 °C, and the most dense nitrogen inclusions maintained without decrepitation the residual internal pressure of 2.2 kbar at 25 °C. Contrasting fluid compositions, increasing depths of burial (∼4–14 km) and decreasing thermal gradients (∼40–15 °C km−1) during initial mineralization stages of the Droždiak vein reflect Alpine orogenic processes, rather than an incipient Permian rifting suggested in previous metallogenetic models. Siderite crystallized at rising P–T in a closed, rock‐buffered hydrothermal system developed in the Variscan basement during the north‐vergent Cretaceous thrusting and thickening of the Gemeric crustal wedge. Variable salinities of the barite‐hosted inclusions reflect a fluid mixing in open hydrothermal system, and re‐equilibration textures (lengths of decrepitation cracks proportional to fluid inclusion sizes) correspond to retrograde crystallization trajectory coincidental with transpression or unroofing. Maximum recorded fluid pressures indicate ∼12‐km‐thick pile of imbricated nappe units accumulated over the Gemeric basement during the Cretaceous collision.

Dynamic control on serpentine crystallization in veins: Constraints on hydration processes in oceanic peridotites

Deformation and hydration processes are intimately linked in the oceanic lithosphere, but the feedbacks between them are still poorly understood, especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and/or mass transfer. Serpentinization is accompanied by abundant veining marked by different generations of vein‐filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation. We use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow‐spreading ridges. We have selected a representative set of veins from ocean floor serpentinites of the Mid‐Atlantic Ridge near Kane transform fault (23°N) and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy (SEM, TEM) with electron microprobe analyses. Four main veining episodes (V1 to V4) accompany the serpentinization. The first episode, identified as vein generation V1, is interpreted as the tectonically controlled penetration of early seawater‐dominated fluid within peridotites, enhancing thermal cracking and mesh texture initiation at 3–4 km up to 8 km depth and at T &lt;300–350°C. The two following vein stages (V2 and V3) formed in a closed, diffusive system and accommodate volume expansion required to reach almost 50% serpentinization of the protolith. The cracks exploited by these veins were caused by the progressive unroofing at depths of ∼4 to ∼2 km along a detachment fault. Degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization, and this capacity is in turn linked to the exhumation rate and local tectonics. During this period, water consumed by hydration may prevent the establishment of convective hydrothermal cells. The onset of an open hydrothermal system in the shallow lithosphere (&lt;2 km), where brittle fracturing and advective transfer dominate and enable the completion of serpentinization, is marked by the last vein generation (V4). These results show a complete history of alteration, with the crystallization of different types of serpentine recording different tectonic events, chemical conditions, and modes of hydrothermal alteration of the lithosphere.

Crystal-Size Distributions of Clays During Episodic Diagenesis: The Salton Sea Geothermal System

AbstractCrystal-size distributions (CSDs) for clay minerals with depth were measured from the Salton Sea Geothermal Field (SSGF) as a test for the presence and meaning of theoretical crystal-size distributions in a natural system. The SSGF is a classic open hydrothermal system, and crystals are forming directly without apparent modification of early-formed crystals, over a wide range of temperature. Thus, the measured CSDs are the actual distributions for a single episode in which all crystals grew at the same time from solution at different temperatures and depths, rather than through modifications of shallower samples.Some TEM images of ion-milled samples from a range of depths were used to measure the crystal thicknesses of illite, chlorite and biotite. Grain-size histograms flatten, broaden and shift to larger sizes with increasing depth. Values of α and β were calculated and used to verify that the measured distributions are log normal. Reduced grain-size distributions for illite in SSGF samples obey steady-state constraints.The observations appear to be consistent with evolution of illite with increasing depth in the SSGF system by growth in an open system giving rise to log-normal distributions, followed by supply-controlled growth in an open system. Because crystals at different depths grew simultaneously under different temperature and fluid conditions as a function of depth, they do not represent different stages of a single evolving system. The relations imply that isochemical and isothermal systems which permit an evolving system to be sampled are rare or non-existent. The data for distributions for a given depth in the SSGF are consistent with growth in an open system. The collective relations therefore imply that caution should be used in interpreting conditions of crystal growth in natural systems even where CSDs give results which are necessary for, but not sufficient to prove, a given modeled mechanism.

Mesothermal gold lodes in the north-western Alps: A review of genetic constraints from radiogenic isotopes.

Identifying the source of the hydrothermal fluid responsible for mesothermal gold lodes in orogenic belts has proven to be a formidable hurdle. As a consequence, several key aspects of the genesis of this worldwide class of deposits remain poorly understood. This article reviews a wide spectrum of published data on the Monte Rosa Gold District, a belt of mesothermal gold lodes in the Alpine orogen. The data include Sr-, Pb- and He-Ar isotopes, 40 Ar/ 39 Ar chronology, fluid-inclusion compositions, mineralogy, and the geological framework of the deposits. It is demonstrated that simultaneous consideration of several radiogenic isotope systems is highly valuable in reconstructing the source characteristics and processes of ore deposition in open hydrothermal systems. The genetic model suggested by these data involves prograde metamorphic devolatilization of Mesozoic calcschists during mid-Tertiary continental collision and orogenic uplift. The liberated fluids scavenged gold from metabasites interlayered with the calcschists, then ascended to form auriferous quartz-carbonate-sulphide veins in rocks undergoing retrograde metamorphism. Metamorphic hydrothermal systems of this type recurred along the district over a period of at least 20 Ma, their location and timing being controlled by the progress of differential uplift of the north-western Alps. The nature of this “temporal continuum” of mineralisation contrasts with that reported for Archean gold-lode deposits. As well as clarifying aspects of gold-lode genesis, the isotopic approaches discussed have great potential to constrain scenarios of large-scale fluid flow – with or without mineralising potential – in orogenic belts of all ages.

The nature and dimensions of regional and local gold-related hydrothermal alteration in tholeiitic metabasalts in the Norseman goldfields: the missing link in a crustal continuum of gold deposits?

The Archaean lode-gold deposits at Norseman, Western Australia, consist of auriferous quartz veins in dextral-reverse ductile-brittle shear zones within tholeiitic metabasalts of upper-greenschist to amphibolite facies metamorphic grade. Three types of deposits (Northern, Central, Southern) are delineated on the basis of their spatial distribution, veining style, alteration mineraloty and metamorphic grade of host rocks. Northern deposits, hosted in upper-greenschist to lower-amphibolite facies rocks, comprise massive to laminated quartz veins with selvedges of quartz-chlorite-calcite-biotite-plagioclase assemblages. Central deposits, hosted in lower-amphibolite facies rocks, consist of laminated to massive quartz veins with selvedges of quartz-actinolite-biotite-plagioclasecalcite assemblages. Southern deposits, hosted in middleamphibolite facies metabasalts, consist of banded quartz-diopside-calcite-microcline-zoisite veins. All deposits exhibit variable ductile deformation of veins and contiguous alteration haloes, consistent with a syn-deformational genesis at high temperatures. From Northern to Southern deposits, the alteration assemblages are indicative of higher temperatures of formation, and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals. These observations imply that a thermal variation of gold-related hydrothermal alteration exists within the Norseman Terrane over a distance of 40 km, with TNorthern<TCentral<TSouthern This thermal zonation is corroborated by T−XCO 2 phase relations between vein selvedge assemblages, which signify formation temperatures of approximately 420°–475°C, 470°–495°C and >500°C for Northern, Central and Southern deposits, respectively. The sum of structural, petrographic and mineral chemistry data indicates that the alteration assemblages formed in high-temperature, open hydrothermal systems and have not been subsequently metamorphosed. The thermal differences between the deposit groups may reflect (1) a temperature gradient, at relatively constant P, corresponding to the proximity of the deposits to regional granitoid complexes, or (2) formation of the deposits at progressively deeper crustal levels from north to south. In either case the deposits represent a continuum of gold deposition from upper-greenschist to amphibolite facies, now exposed in an oblique section through the Archaean crust at Norseman.

Types of epithermal silver deposits, northeastern Russia

Three main epithermal silver provinces occur in northeastern Russia: (1) the Cretaceous Okhotsk-Chukotka volcanic plutonic belt (Russia's part of the Circum-Pacific belt), (2) adjacent Mesozoic orogenic belts, and (3) Paleozoic complexes formed of intermediate massifs. Within the most studied Okhotsk-Chukotka province, three types of deposits are distinguished by structural position and geologic environments: (1) those in volcanic intrusive domes, (2) those in siliceous subvolcanic bodies, and (3) those in contact halos around granodioritic intrusions. Gold-silver, silver-lead-zinc, and tin-silver ore deposits are distinguished, according to the ore composition. The silver deposits of the Ducat ore district are examples of these three types.Genetically, ore mineralization of the three types differs. Gold-silver mineralization was formed in near-surface, open hydrothermal systems during mixing of metalliferous solutions with meteoric waters, as indicated by low salinities (2.5-3.5% NaCl equiv) of the fluid inclusions. Subvolcanic and intrusive bodies were the local heat sources, which caused a field with high thermal gradients (avg 200 degrees -370 degrees C). Boiling of the fluids was the most effective mechanism of ore deposition with which ore shoot formation was connected as evidenced by texture and fluid inclusion data. In contrast, silver-lead-zinc and tin-silver mineralizations formed in closed hydrothermal systems in which boiling did not occur. Their relatively low uniform temperatures (avg 240 degrees -320 degrees C) and low thermal gradients were associated with a more steady regime of Ag-Pb-Zn-Sn ore deposition compared to the gold-silver deposits. The relatively high salinity of fluids (up to 7.5% NaCl equiv) resulted from the absence of mixing of metalliferous solutions and meteoric waters.

Kinetic theory of oxygen isotopic exchange between minerals and water

Kinetic and mass conservation equations are used to describe oxygen isotopic exchange between minerals and water in “closed” and open hydrothermal systems. In cases where n coexisting mineral phases having different reaction rates are present, the exchange process is described by a system of n + 1 simultaneous differential equations consisting of n pseudo first-order rate equations and a conservation of mass equation. The simultaneous solutions to these equations generate curved exchange trajectories on δ-δ plots. Families of such trajectories generated under conditions allowing for different fluid mole fractions, different fluid isotopic compositions, or different fluid flow rates are connected by positive-sloped isochronous lines. These isochrons reproduce the effects observed in hydrothermally exchanged mineral pairs including 1) steep positive slopes, 2) common reversals in the measured fractionation factors (δ), and 3) measured fractionations that are highly variable over short distances where no thermal gradient can be geologically demonstrated.