Isotopic Compositions Of Fluids Research Articles

First-principles study of Ge isotope fractionation during adsorption onto Fe(III)-oxyhydroxide surfaces

Iron oxides and oxyhydroxides (or iron-rich marine sediments) are assumed to be one of the candidate ‘missing sinks’ of Ge that account for half of the total precipitated Ge from seawater. Information on equilibrium Ge isotope fractionation during Ge adsorption onto Fe(III)-oxyhydroxide surfaces plays an essential role in understanding the Ge isotope global budget. In this study, the possible adsorbed Ge surface complexes on Fe(III)-oxyhydroxide surfaces are carefully verified by comparing their energies and geometries with known experimental results. Moreover, the equilibrium Ge isotope fractionations between the adsorbed Ge complexes and the dominant Ge species in aqueous solution are investigated by using the Urey model or the Bigeleisen–Mayer equation. First-principles density functional theory is used for optimizing the structures and obtaining harmonic frequencies at the B3LYP/6-311+G(d,p) level. For accurate estimation of solvation effects, the “water-droplet” explicit solvent method is also used. Our results suggest that bidentate corner-sharing ( 2C) > Fe 2O 2Ge(OH) 2(aq) is the dominant adsorbed Ge surface complex during adsorption processes in the neutral or acidic condition. Under basic conditions, the dominant surface structure is changed to 2C > Fe 2O 2GeOOH − (aq) complex. These results are consistent with existing EXAFS experimental evidences (Pokrovsky et al., 2006). Our calculations show that Ge isotopes can be distinctly fractionated by such adsorption processes to about 1.7‰ (25 °C, in terms of 74Ge/ 70Ge). Light Ge isotopes are preferentially enriched on the surfaces of Fe-oxyhydroxides. The data generally agree with the observations of Galy et al. (2002) and Rouxel et al. (2008). Such Ge adsorption might affect the Ge isotope compositions of rivers, hydrothermal fluids and seawaters. Because of the broad distribution and large amount of iron-rich deposits everywhere, such adsorption processes will greatly influence the Ge isotope global budget.

The Darasun gold deposit, Eastern Transbaikal region: Chemical composition, REE patterns, and stable carbon and oxygen isotopes of carbonates from ore veins

The chemistry, REE patterns, and carbon and oxygen isotopic compositions of carbonates from ore veins of the Darasun deposit are discussed. In addition to the earlier described siderite, calcite, and carbonates of the dolomite-ankerite series, kutnahorite is identified. The total REE content in Fe-Mg carbonates of the dolomite-ankerite series (2.8–73 ppm) is much lower than in later calcite (18–390 ppm). δ13C of Fe-Mg carbonates and calcite varies from +1.1 to −6.7‰ and from −0.9 to −4.9‰, respectively. δ18O of Fe-Mg carbonates and calcite varies from +17.6 to 3.6‰ and from +15.7 to −0.5‰, respectively. The REE sum and carbon and oxygen isotopic compositions reveal zonal distribution relative to the central granodiorite porphyry stock. The correlation between the carbon and oxygen isotopic compositions and REE sum reflects variations in the physicochemical formation conditions and composition of ore-forming fluid. The isotopic composition of fluid is calculated, and possible sources of its components are considered. Earlier established evidence for a magmatic source of ore-forming fluid and participation of meteoric water in ore formation is confirmed. Geochemical evidence for interaction of ore-forming fluid with host rocks is furnished. The relationships between the REE sum, on the one hand, and carbon and oxygen isotopic compositions of hydrothermal ore-forming fluid, on the other, are established.

Oxygen Isotope Compositions of Iron Oxides from High-Grade BIF-Hosted Iron Ore Deposits of the Central Hamersley Province, Western Australia: Constraints on the Evolution of Hydrothermal Fluids

The Hamersley province of northwest Western Australia is one of the world’s premier iron ore regions with high-grade martite-microplaty hematite iron ore deposits mostly hosted within banded iron formation (BIF) sequences of the Brockman Iron Formations of the Hamersley Group. These high-grade iron ores contain between 60 and 68 wt percent Fe, and formed by the multistage interaction of hydrothermal fluids with the host BIF formation. The oxygen isotope compositions of magnetite and hematite from BIF, hydrothermal alteration assemblages, and high-grade iron ore were analyzed from the Mount Tom Price, Paraburdoo, and Channar iron ore deposits. The δ18O values of magnetite and hematite from hydrothermal alteration assemblages and high-grade iron ore range from −9.0 to −2.9 per mil, a depletion of 5 to 15 per mil relative to the host BIF. The δ18O values are spatially controlled by faults within the deposits, a response to higher fluid flux and larger influence on the isotopic compositions by the hydrothermal fluids. The oxygen isotope composition of hydrothermal fluids (δ18Ofluid) indicates that the decrease in the 18O content of iron oxides was due to the interaction of both basinal brines and meteoric fluids with the original BIF. Late-stage talc-bearing ore at the Mount Tom Price deposit formed in the presence of a pulse of 18O-enriched basinal brine, indicating that hydrothermal fluids may have repeatedly interacted with the BIFs during the Paleoproterozoic.

Oxygen Isotope Compositions of Iron Oxides from High-Grade BIF-Hosted Iron Ore Deposits of the Central Hamersley Province, Western Australia: Constraints on the Evolution of Hydrothermal Fluids

The Hamersley province of northwest Western Australia is one of the world’s premier iron ore regions with high-grade martite-microplaty hematite iron ore deposits mostly hosted within banded iron formation (BIF) sequences of the Brockman Iron Formations of the Hamersley Group. These high-grade iron ores contain between 60 and 68 wt percent Fe, and formed by the multistage interaction of hydrothermal fluids with the host BIF formation. The oxygen isotope compositions of magnetite and hematite from BIF, hydrothermal alteration assemblages, and high-grade iron ore were analyzed from the Mount Tom Price, Paraburdoo, and Channar iron ore deposits. The δ18O values of magnetite and hematite from hydrothermal alteration assemblages and high-grade iron ore range from −9.0 to −2.9 per mil, a depletion of 5 to 15 per mil relative to the host BIF. The δ18O values are spatially controlled by faults within the deposits, a response to higher fluid flux and larger influence on the isotopic compositions by the hydrothermal fluids. The oxygen isotope composition of hydrothermal fluids (δ18Ofluid) indicates that the decrease in the 18O content of iron oxides was due to the interaction of both basinal brines and meteoric fluids with the original BIF. Late-stage talc-bearing ore at the Mount Tom Price deposit formed in the presence of a pulse of 18O-enriched basinal brine, indicating that hydrothermal fluids may have repeatedly interacted with the BIFs during the Paleoproterozoic.

Mineralogical, fluid inclusion, and stable isotope constraints on mechanisms of ore deposition at the Samgwang mine (Republic of Korea)—a mesothermal, vein-hosted gold–silver deposit

The Samgwang mine is located in the Cheongyang gold district (Cheonan Metallogenic Province) of the Republic of Korea. It consists of eight massive, gold-bearing quartz veins that filled NE- and NW-striking fractures along fault zones in Precambrian granitic gneiss of the Gyeonggi massif. Their mineralogy and paragenesis allow two separate vein-forming episodes to be recognized, temporally separated by a major faulting event. The ore minerals occur in quartz and calcite of stage I, associated with fracturing and healing of veins. Hydrothermal wall-rock alteration minerals of stage I include Fe-rich chlorite (Fe/(Fe+Mg) ratios 0.74-0.81), muscovite, illite, K-feldspar, and minor arsenopyrite, pyrite, and carbonates. Sulfide minerals deposited along with electrum during this stage include arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena, argentite, pyrargyrite, and argentian tetrahedrite. Only calcite was deposited during stage II. Fluid inclusions in quartz contain three main types of C–O–H fluids: CO2-rich, CO2–H2O, and aqueous inclusions. Quartz veins related to early sulfides in stage I were deposited from H2O–NaCl–CO2 fluids (1,500–5,000 bar, average 3,200) with T htotal values of 200°C to 383°C and salinities less than about 7 wt.% NaCl equiv. Late sulfide deposition was related to H2O–NaCl fluids (140–1,300 bar, average 700) with T htotal values of 110°C to 385°C and salinities less than about 11 wt.% NaCl equiv. These fluids either evolved through immiscibility of H2O–NaCl–CO2 fluids as a result of a decrease in fluid pressure, or through mixing with deeply circulated meteoric waters as a result of uplift or unloading during mineralization, or both. Measured and calculated sulfur isotope compositions (δ34SH2S = 1.5 to 4.8‰) of hydrothermal fluids from the stage I quartz veins indicate that ore sulfur was derived mainly from a magmatic source. The calculated and measured oxygen and hydrogen isotope compositions (δ18OH2O = −5.9‰ to 10.9‰, δD = −102‰ to −87‰) of the ore-forming fluids indicate that the fluids were derived from magmatic sources and evolved by mixing with local meteoric water by limited water–rock exchange and by partly degassing in uplift zones during mineralization. While most features of the Samgwang mine are consistent with classification as an orogenic gold deposit, isotopic and fluid chemistry indicate that the veins were genetically related to intrusions emplaced during the Jurassic to Cretaceous Daebo orogeny.

Lithogeochemical and Stable Isotopic Insights into Submarine Genesis of Pyrophyllite-Altered Facies at the Boco Prospect, Western Tasmania

The Boco prospect is a large, fault dismembered, pipelike, hydrothermally altered zone in the Mount Read Volcanics of western Tasmania. It is a synvolcanic alteration zone hosted by felsic volcanic rocks formed in a subaqueous proximal intracaldera setting. Previous detailed geochemical and geophysical surveys and extensive drill testing have indicated it contains no economic metals. The strong to intense, pervasively quartz + phyllosilicate + pyrite-altered northern segment of the prospect is semiconcentrically zoned. Short wavelength infrared (SWIR) spectral analysis has revealed that phyllosilicate assemblages grade from phengitic white mica in the least altered peripheries, through normal potassic white mica, to central zones containing kaolinite, slightly sodic white mica, and pyrophyllite. Mass balance calculations indicate average net mass losses in the altered facies were about 10 to 30 g/100 g, mainly owing to loss of SiO2, which implies very high hydrothermal water-rock ratios. Whole-rock oxygen isotope compositions of the enclosing least altered felsic rocks (δ18O values 8.2–11.7‰) are indistinguishable from those of altered facies (9.6–11.8‰). We attribute the former to low-temperature diagenetic isotopic exchange with 0 per mil δ18O seawater in the peripheral least-altered zones, and the latter to exchange with 3 to 6 per mil δ18O hydrothermal fluids at high water/rock ratios and temperatures generally greater than 220°C, and locally greater than 270°C, in the intensely altered facies. Pyrite sulfur isotope compositions in the Boco altered facies (δ34S values 1.2–7.2‰) are distinctly lower than most Tasmanian massive sulfide deposits (6–15‰), compatible with a dominantly magmatic source of sulfur. The alteration mineral assemblages, estimated mass changes, and isotopic data show that the Boco alteration system was formed by a large volume of focused acidic hydrothermal fluid which had an oxygen isotope composition of 3 to 6 per mil δ18O at and temperature greater than 270°C. The slightly 18O-enriched fluid isotope composition suggests derivation from either mixed magmatic fluid and seawater or isotopically evolved sea water. Its advanced argillic altered facies place Boco among a newly recognized class of southeast Australian Cambrian volcanic-hosted prospects and deposits. These include Chester, Basin Lake, Western Tharsis, and North Lyell in Tasmania, and Rhyolite Creek, Hill 800, and Mike’s Bluff in eastern Victoria. SWIR spectral analyses with field-portable spectrometers allow early discrimination of this type of hydrothermally altered system, and can potentially assist subsequent exploration in mapping facies zonation.

Pore-fluid Fe isotopes reflect the extent of benthic Fe redox recycling: Evidence from continental shelf and deep-sea sediments

Pore-fluid Fe isotopes may be a unique tracer of sediment respiration by dissimilatory Fe-reducing bacteria, but to date, pore-fluid Fe isotope measurements have been restricted to continental shelf settings. Here, we present δ56Fe values of pore fluids from two distinct sedimentary settings: (1) a riverine-dominated site on the northern California margin (Eel River shelf; 120 m water depth) and (2) biogenic opal-rich volcaniclastic deep-sea sediments from the Southern Ocean (north and south of the Crozet Plateau; 3000–4000 m water depth). The Fe isotope compositions of Crozet region pore fluids are significantly less fractionated (δ56Fe = +0.12‰ to −0.01‰) than the Eel River shelf (δ56Fe = −0.65‰ to −3.40‰) and previous studies of pore-fluid Fe isotopes, relative to average igneous rocks. Our data represent the first measurements of Fe isotope compositions in pore fluids from deep-sea sediments. A comparison of pore-fluid δ56Fe with the relative abundance of highly labile Fe in the reactive sedimentary Fe pool demonstrates that the composition of Fe isotopes in the pore fluids reflects the different extent of sedimentary Fe redox recycling between these sites.

Isotopic constraints on ice age fluids in active geothermal systems: Reykjanes, Iceland

The Reykjanes geothermal system is located on the landward extension of the Mid-Atlantic Ridge in southwest Iceland, and provides an on-land proxy to high-temperature hydrothermal systems of oceanic spreading centers. Previous studies of elemental composition and salinity have shown that Reykjanes geothermal fluids are likely hydrothermally modified seawater. However, δD values of these fluids are as low as −23‰, which is indicative of a meteoric water component. Here we constrain the origin of Reykjanes hydrothermal solutions by analysis of hydrogen and oxygen isotope compositions of hydrothermal epidote from geothermal drillholes at depths between 1 and 3 km. δD EPIDOTE values from wells RN-8, -9, -10 and -17 collectively range from −60 to −78‰, and δ 18 O EPIDOTE in these wells are between −3.0 and 2.3‰. The δD values of epidote generally increase along a NE trend through the geothermal field, whereas δ 18 O values generally decrease, suggesting a southwest to northeast migration of the geothermal upflow zone with time that is consistent with present-day temperatures and observed hydrothermal mineral zones. For comparative analysis, the meteoric-water dominated Nesjavellir and Krafla geothermal systems, which have a δD FLUID of ∼ −79‰ and −89‰, respectively, show δD EPIDOTE values of −115‰ and −125‰. In contrast, δD EPIDOTE from the mixed meteoric-seawater Svartsengi geothermal system is −68‰; comparable to δD EPIDOTE from well RN-10 at Reykjanes. Stable isotope compositions of geothermal fluids in isotopic equilibrium with the epidotes at Reykjanes are computed using published temperature dependent hydrogen and oxygen isotope fractionation curves for epidote–water, measured isotope composition of the epidotes and temperatures approximated from the boiling point curve with depth. Calculated δD and δ 18 O of geothermal fluids are less than 0‰, suggesting that fluids of meteoric or glacial origin are a significant component of the geothermal solutions. Additionally, δD FLUID values in equilibrium with geothermal epidote are lower than those of modern-day fluids, whereas calculated δ 18 O FLUID values are within range of the observed fluid isotope composition. We propose that modern δD EPIDOTE and δD FLUID values are the result of diffusional exchange between hydrous alteration minerals that precipitated from glacially-derived fluids early in the evolution of the Reykjanes system and modern seawater-derived geothermal fluids. A simplified model of isotope exchange in the Reykjanes geothermal system, in which the average starting δD ROCK value is −125‰ and the water to rock mass ratio is 0.25, predicts a δD FLUID composition within 1‰ of average measured values. This model resolves the discrepancy between fluid salinity and isotope composition of Reykjanes geothermal fluids, explains the observed disequilibrium between modern fluids and hydrothermal epidote, and suggests that rock–fluid interaction is the dominant control over the evolution of fluid isotope composition in the hydrothermal system.

Sediment-Hosted Gold Deposits in Guizhou, China: Products of Wall-Rock Sulfidation by Deep Crustal Fluids

Sediment-hosted gold deposits in Guizhou, China, are hosted in late Paleozoic and early Mesozoic sedimentary rocks along the southwest margin of the Precambrian Yangtze craton. They have characteristics similar to Carlin-type gold deposits in Nevada and are notably enriched in As, Sb, Hg, and Tl. The Shuiyindong and Yata deposits consist of disseminated, strata-bound sulfides in Permian bioclastic limestone and fault-controlled mineralization in Middle Triassic calcareous clastic rocks, respectively. Mineralization in both deposits consists of barren milky quartz veins, disseminated gold-bearing arsenian pyrite and arsenopyrite, stibnite, realgar, and orpiment. The barren milky quartz veins occur in the ore-controlling structures with an envelope of gold mineralization in the host rock consisting of disseminated gold-bearing arsenian pyrite and arsenopyrite and replacement-style quartz veinlets. Later drusy quartz, stibnite, realgar, and orpiment fill fractures and vugs on the periphery of gold mineralization. Petrography, microthermometry, laser Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometric (LA-ICP-MS) analyses of fluid inclusions are used to characterize the chemical evolution of ore fluids at Shuiyindong and Yata. Early barren milky quartz veins trapped aqueous fluid with moderate salinity (6.0 wt % NaCl equiv) and minor CO2 (<2.4 mol %) at a temperature of 230° ± 30°C. This fluid contains measurable B, Na, K, Cs, Sr, As, Sb, and Au (3.8 ± 0.5–5.7 ± 2.3 μ g/g) determined by LA-ICP-MS and is interpreted to approximate the mineralizing fluid responsible for precipitation of early barren quartz veins. Low-salinity (0.9–2.3 wt % NaCl equiv), CO2-bearing (6.3–8.4 mol %), aqueous fluids are recorded in replacement-style quartz veinlets. They formed gold-bearing arsenian pyrite and arsenopyrite and quartz veinlets at a temperature of 210° ± 20°C and pressures of 450 to 1,150 bars, corresponding to depths between 1.7 and 4.3 km under lithostatic conditions. Fluid inclusions in late stibnite-orpiment-realgar-quartz veins contain high CO2 (58–64 mol %) and N2 (19.2–23.7 mol %) with trace CH4 (up to 1.6 mol %). The increasing content of CO2 and decrease in the concentrations of Au and As in aqueous-carbonic inclusions are interpreted to result from carbonate dissolution at the level of the deposits during gold mineralization. Covariance of Au, As, Sb, and Sr concentrations in the fluids is interpreted to reflect deposition of As and Sb sulfides and gold concurrent with carbonate crystallization. Iron is below detection limit (~400 μ g/g) in all fluid inclusion types, suggesting that the ore fluids were Fe poor but possibly sulfur rich, possibly explaining their exceptionally high gold contents. Iron in sulfide minerals was probably derived from dissolution of ferroan minerals in the host rocks, sulfidized by H2S-rich fluids to precipitate arsenian pyrite, arsenopyrite, and gold. Pressure fluctuation induced by faulting resulted in local fluid immiscibility and led to late deposition of realgar, orpiment, stibnite, and calcite. Hydrogen and oxygen isotope compositions of ore fluids ( δ D(H2O) = −35 to −68‰, δ 18O(H2O) = 4–16.5‰) indicate a metamorphic origin, possibly related to crustal thickening and prograde metamorphism during the late Yanshanian orogeny.

Fluid regime and origin of gold-bearing rodingites from the Karabash alpine-type ultrabasic massif, Southern Ural

The rodingites of the Karabash Massif are distinguished by the presence of native cupriferous gold. This zonal hydrothermal-metasomatic complex was formed in three stages. The inner zone of rodingite proper is made up of chlorite-andradite-diopside rocks of stage 1, which are cut by diopside veinlets of stage 2 and calcite veinlets of stage 3. The intermediate zone consists of chloritolites, which give way to the antigorite and chrysotile-lizardite serpentinites of the outer zone. Thermometric and cryometric studies and gas chromatography showed that the gold-bearing rodingites of stages 1 and 2 were formed at t = 420–470°C, P = 2–3 kbar, and \(X_{CO_2 } \) = 0.001–0.02, i.e., under conditions typical of rodingite formation. The final stage was accompanied by a decrease in P-T parameters (0.5–1.0 kbar and 230–310°C) and an increase in \(X_{CO_2 } \) up to 0.04. The rodingite-forming fluid was extremely rich in water (\(X_{H_2 O} \) = 0.942–0.981) and contained hydrogen as the major gas component (\(X_{H_2 } \) = 0.012–0.023); its composition was essentially chloride-magnesium with minor amounts of CaCl2 and FeCl2 and a low salinity of 2.6–8.0 wt % NaCl equiv. The rodingite minerals showed the following isotopic characteristics (‰): δ18O from 5.5 to 6.6 and δD from 42.8 to −44.3 for chlorite, δ180 from 2.0 to 3.8 for andradite, δ18O from 6.0 to 6.6 for diopside, and δ18O from 10.6 to 11.4 and δ13C from 0.1 to −1.8 for calcite. The chloritolite is characterized by δ18O from 5.9 to 6.6 and δD from −49.8 to −64.4; the antigorite serpentinite shows δ18O=6.5 and δD=−65.2; and the antigoritized chrysotile-lizardite serpentinite shows δ18O from 6.8 to 6.9 and δD from −127 to −128. The calculated isotopic composition of fluid in equilibrium with various rocks suggested its metamorphic origin. It was formed from the water released during dehydration of oceanic serpentinites, from the components of ultrabasic and basic magmatic rocks, and, at the final stage, from marine carbon.

Multi-isotopic tracing (δ 7Li, δ 11B, 87Sr/ 86Sr) and chemical geothermometry: evidence from hydro-geothermal systems in France

In the present work, we report the results of a multi-isotopic study (δ 7Li, δ 11B and 87Sr/ 86Sr) on waters collected from different hydro-and geothermal sites in France. The aim of this study was to characterize hydro-and geothermal reservoirs by coupling a multi-isotopic approach with in-depth temperature of the reservoir calculated using chemical geothermometers. A database constituted by 36 water samples (thermal, thermo-mineral and geothermal waters) was selected in order to cover a wide range of temperatures and to be representative of water/rock interactions with various types of lithologies. We clearly observed that the wide range of 87Sr/ 86Sr isotopic ratios in the waters (0.70755-0.72187) appear to be largely controlled by the dominant host lithologies with Sr isotopic signatures ranging from high values (resulting from interaction from granite and/or gneiss type > 0.714) to lower 87Sr/ 86Sr ratios (closer to Sr isotopic ratios observed in the sedimentary zones < 0.712). Boron isotopic ratios lay between - 8.3 and + 21.6‰, also reflecting a lithological control over δ 11B signatures. In addition, no simple or direct relationship was observed for either boron or strontium isotopic signatures with the temperature of the reservoir. In contrast, temperature plays an important role in controlling the lithium isotope composition of the hydro-and geothermal fluids. δ 7Li signatures are comprised between - 0.1 and + 10.0‰ for temperatures ranging from 60 to 230 °C. This result seems to prove that lithium isotopic systematics represents a useful tool suitable for the characterization of hydrothermal and geothermal waters and suggests that the δ 7Li composition of geothermal waters may be utilised as a geothermometer (δ 7Li = - 0.043 ± 0.003 T + 11.9 ± 0.5).

Multistage Au-As-Sb Mineralization and Crustal-Scale Fluid Evolution in the Kwekwe District, Midlands Greenstone Belt, Zimbabwe: A Combined Geochemical, Mineralogical, Stable Isotope, and Fluid Inclusion Study

The Kwekwe district comprises the central part of the Archean Midlands greenstone belt of Zimbabwe and represents one of few Archean gold provinces in the world hosting gold deposits associated with massive antimony mineralization. The deposits typically occur along late Archean transcrustal deformation zones. Mineralization is characterized by an early Au-As stage 1, followed by a pronounced Au-Sb stage 2, and a low-grade Sb-bearing stage 3. Four gold deposits within the Kwekwe district have been studied. The Primrose and Globe and Phoenix gold deposits display typical features of Archean orogenic lode gold systems such as fluid inclusions with low salinity, mixed aqueous-carbonic fluids, formation temperatures between 300° and 400°C, and a common stable isotope composition of fluid and mineral precipitates. Deposits of this type formed in the brittle-ductile crustal transition zone at 1.5 to 3.0 kbars. In contrast, gold mineralization at Jojo and especially the Indarama gold deposits probably formed at lower temperatures (<<300°C) and from dominantly aqueous, early moderate- to late high-salinity fluids. At Indarama, microthermometric studies indicate stibnite-gold mineralization at temperatures between 120° and 230°C at about 0.8 to 1.8 kbars. Low formation temperatures and extremely low δ34S values of pyrite (−52‰) and stibnite (−36‰) of the latest mineralization stage at Indarama reveal reactivation of the hydrothermal system within a near subsurface, organic-rich epithermal or hot spring-type environment during the late Archean or even early Proterozoic. Such low δ34S values have not been reported previously from gold deposits in Archean greenstone belts. We propose that hydrothermal activity and gold mineralization has not only taken place at different crustal levels but also at different times.

Stable isotope compositions of the Penninic ophiolites of the Kõszeg-Rechnitz series

The ophiolitic rocks of the easternmost Penninic unit, the Kõszeg-Rechnitz series, were analyzed for their H, C and O stable isotope compositions. Serpentinite, gabbro, blueschist, talc deposits, ophicarbonates, as well as calcite and inclusion fluids from quartz segregation veins were analyzed in order to determine the effects of different metamorphic events on the stable isotope compositions. The oxygen isotope compositions have a wide range depending on rock type and locality. Gabbro and serpentinite of Bienenhütte (Bernstein Window) have preserved mantle-like δ 18O values (5.9 to 6.3‰; all values are in ‰ relative to V-SMOW), whereas the serpentinite of Glashütten and Rumpersdorf (Kõszeg-Rechnitz Window) and the silicate minerals of the ophicarbonate rocks show a strong 18O-enrichment (up to 16.2‰). The 18O-enrichment may have been induced by low-temperature serpentinization or interaction with 18O-rich fluids that had been in equilibrium with sedimentary rocks. Contrary to the O isotope compositions, the H isotope compositions seem to be homogeneous in the entire series, with D values of −63 ± 7‰. Only some serpentinite rocks were depleted in D (down to −106‰), usually regarded as a result of interaction with meteoric water infiltrating during late-stage metamorphism. The meteoric water infiltration was rather limited, as even samples taken directly from slickensides within serpentinite bodies preserved isotopic compositions close to those of the bulk series. H and O isotope compositions of fluids mobilized in the metasedimentary rocks of the Penninic unit during the main metamorphic stage were determined by analyzing inclusion fluids and calcites in quartz-carbonate veins. The isotope compositions indicate interaction between these fluids and the ophiolite series, although relative deuterium enrichment has been preserved in the ophiolitic rocks. The strong D-enrichment characteristic for oceanic crust that has experienced high-temperature interaction with seawater was not detected. However, the H isotope compositions obtained for the Kõszeg-Rechnitz series indicate that subduction of the Penninic oceanic crust and the associated devolatilization may have been potentially responsible for mantle metasomatism, resulting in H isotope compositions of about −40‰, similar to the range determined from mantle-derived amphibole megacrysts (Demény et al. 2005). To conclude, the present dataset is discussed in the light of earlier studies on the formation of the Sopron leucophyllite.

Thermal and fluid flow history of the Lawn Hill Platform, northern Australia: implications for the formation of Zn-Pb deposits

Isa Superbasin strata in the Lawn Hill Platform host major base metal sulfide mineralization including the giant Century Zn-Pb deposit. Mineral paragenesis, organic maturation, K-Ar dating and stable isotope studies demonstrate that long-lived structures such as the Termite Range Fault acted as hot fluid conduits several times during the Paleoproterozoic and Mesoproterozoic. K-Ar dating of illites, in combination with other datasets, may identify three thermal events in the Lawn Hill Platform at 1500, 1440-1400 and 1250-1150 Ma. 1500 Ma is a Late Isan Orogeny age recorded only in the south that may reflect exhumation of a provenance region. The 1440-1300 Ma ages are related to fault reactivation and a thermal/?fluid pulse at ~1440-1400 Ma, with subsequent enhanced cooling. The youngest thermal/fluid flow event at 1250-1150 Ma is recorded mainly in the northeast and is probably related to the assembly of Rodinia. An extensive carbonate alteration halo in the footwall of the circa 1575 Ma Century Zn-Pb deposit extends some 15 km along strike. Calculations of fluid oxygen and carbon isotope composition based on model temperatures of 120°C for the ore zone siderites and 180°C for siderites and ankerites in the regional carbonate alteration halo indicate similar ore and alteration fluid compositions (d18O = 3 to 10 per mil; d13C = -7 to -3 per mil). This fluid isotope composition is consistent with highly evolved basinal brines and mixed inorganic and organic carbon sources. The good agreement between maximum temperature estimates from illite crystallinity and organic reflectance and inverse correlation with carbonate oxygen isotopes in northeast Lawn Hill platform boreholes indicate thermal maturation and carbonate formation resulted from interaction with the 1250-1150 Ma fluids. The calculated fluid isotope compositions are consistent with an evolved basinal brine (d18O = 5.1 to 9.4 per mil V- SMOW; d13C = -13.2 to -3.7 per mil V-PDB) that contained a significant organic carbon component. Differences in carbon and oxygen isotope systematics of alteration carbonate are significant to their use with lithogeochemical vectors in mineral exploration in the northern Australian basins.

Origin of fluids in iron oxide–copper–gold deposits: constraints from δ 37Cl, 87Sr/86Sri and Cl/Br

The origin of the hypersaline fluids (magmatic or basinal brine?), associated with iron oxide (Cu–U–Au–REE) deposits, is controversial. We report the first chlorine and strontium isotope data combined with Cl/Br ratios of fluid inclusions from selected iron oxide–copper–gold (IOCG) deposits (Candelaria, Raúl–Condestable, Sossego), a deposit considered to represent a magmatic end member of the IOCG class of deposit (Gameleira), and a magnetite–apatite deposit (El Romeral) from South America. Our data indicate mixing of a high δ 37Cl magmatic fluid with near 0‰ δ 37Cl basinal brines in the Candelaria, Raúl–Condestable, and Sossego IOCG deposits and leaching of a few weight percent of evaporites by magmatic-hydrothermal (?) fluids at Gameleira and El Romeral. The Sr isotopic composition of the inclusion fluids of Candelaria, Raúl–Condestable, and El Romeral confirms the presence of a non-magmatic fluid component in these deposits. The heavy chlorine isotope signatures of fluids from the IOCG deposits (Candelaria, Raúl–Condestable, Sossego), reflecting the magmatic-hydrothermal component of these fluids, contrast with the near 0‰ δ 37Cl values of porphyry copper fluids known from the literature. The heavy chlorine isotope compositions of fluids of the investigated IOCG deposits may indicate a prevailing mantle Cl component in contrast to porphyry copper fluids, an argument also supported by Os isotopes, or could result from differential Cl isotope fractionation processes (e.g. phase separation) in fluids of IOCG and porphyry Cu deposits.

Germanium isotope measurements of high-temperature geothermal fluids using double-spike hydride generation MC-ICP-MS

We present a double-spike isotope dilution MC-ICP-MS technique for the determination of germanium (Ge) isotope fractionation. Using this technique we determined Ge isotope compositions of geothermal spring fluids, a Columbia River Basalt sample, and an in-house diatom standard. Our technique uses a 73Ge/ 70Ge double spike in combination with hydride generation for Ge extraction from the sample matrix. Fractionation is determined on the 74Ge/ 72Ge mass ratio. The double spike allows us to effectively correct analytical isotope fractionation. Our external standard reproducibility is 0.4‰ (2 SD) over the course of several months. The minimum quantity of Ge needed for isotope analysis is approximately 2 ng. Consistent with previous work on geothermal fluids, Ge in the geothermal spring samples presented here is enriched over Si as compared to low temperature weathering signatures. This observation is typically interpreted as Ge exclusion during silicate mineral precipitation (e.g., quartz). Our isotope results indicate that the analyzed high temperature fluids fractionate Ge isotopes with a range in δ 74Ge between −0.4‰ and −1.4‰ relative to a Columbia River basalt. We cautiously interpret the observed fractionation as preferential removal of heavy Ge isotopes out of solution during cooling of the hydrothermal fluid and subsequent precipitation of quartz.

Gold-Bearing Mesothermal Veins from the Gubong Mine, Cheongyang Gold District, Republic of Korea: Fluid Inclusion and Stable Isotope Studies

The Gubong deposit consists of five massive, gold-bearing mesothermal quartz veins that fill fractures oriented northeast and northwest along fault shear zones over an area of 14 km2 in Precambrian metasedimentary rocks of the Gyeonggi massif. The veins are divided into three groups, based on their orientation and location. They have a ribbon texture that is interpreted to result from repeated hydraulic fracturing events. Mineral deposition was associated with hydrothermal fluid overpressuring within and below the active fault shear zones. The vein mineralogy and paragenesis of the veins allow two separate discrete mineralization episodes to be recognized, separated by a major faulting event. The ore minerals are contained within quartz and calcite associated with fracturing and healing of veins that occurred during both mineralization episodes. Wall-rock alteration minerals during stage I, the main ore stage, include sericite, chlorite, and minor pyrite and carbonates. Sulfide minerals deposited along with electrum during this stage include arsenopyrite, pyrite, pyrrhotite, sphalerite, marcasite, chalcopyrite, galena, and argentite. Electrum also was deposited during stage II mineralization in the one group of veins, along with pyrite, sphalerite, chalcopyrite, and galena, but the second stage of deposition in other veins was barren. Petrographic examination of textural relationships among sulfides, fluid inclusions, microfracturing, and quartz shows chronological and genetic relationships between gold deposition and fluid entrapment. Systematic studies of fluid inclusions in stage I vein indicate two contrasting events: a relatively high temperature (203°–432°C) and pressure (943–2,098 bars) event related to early sulfide deposition and associated with H2O-CO2-CH4-NaCl ± N2 fluids (less than about 13.4 wt % NaCl), and a lower temperature (202°–399°C) and pressure (670–850 bars) late sulfide depositional event involving H2O-NaCl fluids (3.9–17.3 wt % NaCl). The H2O-NaCl fluid involved in ore-related stage II mineralization had a salinity of 0.4 to 4.2 wt percent NaCl and a homogenization temperature of 201° to 378°C. The calculated sulfur isotope compositions of hydrothermal fluids from the stage I veins (δ34SH2S = 3.5–10.5‰) indicate that ore sulfur was derived mainly from a magmatic source but also in part from sulfur contained in the host rocks. The calculated and measured oxygen and hydrogen isotope compositions of the ore-forming fluids (stage I: δ18OH2O = 1.1–9.0‰, δD = −92 to −21‰; stage II: δ18OH2O = −0.1 to +0.3‰, δD = −95 to −93‰) indicate that the fluids were derived from magmatic and/or deep-seated metasedimentary rocks (stage I) and evolved by mixing with local meteoric water (stage II), by limited water-rock exchange and by degassing during mineralization in uplift zones. The H2O-NaCl fluids involved in the stage I development of these veins represent fluids that evolved either through unmixing of H2O-CO2-CH4-NaCl ± N2 fluids following a decrease in fluid pressure or through mixing with deeply circulating meteoric waters, possibly as a result of uplift and/or unloading during mineralization. The H2O-NaCl fluid involved in stage II was derived from meteoric water. Early deposition of gold in stage I was caused by a decrease in sulfur fugacity (H2S loss) that accompanied the immiscible separation of carbonic vapor from H2O-CO2-CH4-NaCl ± N2 fluids. Gold in late stage I and stage II veins was precipitated from H2O-NaCl fluids by cooling and by dilution caused by mixing with meteoric water.

Genetic Environment of the Intrusion‐related Yuryang Au‐Te Deposit in the Cheonan Metallogenic Province, Korea

Abstract. The Yuryang gold deposit, comprising a Te‐bearing Au‐Ag vein mineralization, is located in the Cheonan area of the Republic of Korea. The deposit is hosted in Precambrian gneiss and closely related to pegmatite. The mineralized veins display massive quartz textures, with weak alteration adjacent to the veins. The ore mineralization is simple, with a low Ag/Au ratio of 1.5:1, due to the paucity of Ag‐phases. Ore mineralization took place in two different mineral assemblages with paragenetic time; early Fe‐sulfide mineralization and late Fe‐sulfide and Au‐Te mineralization. The early Fe‐sulfide mineralization (pyrite + sphalerite) occurred typically along the vein margins, and the subsequent Au‐Te mineralization is characterized by fracture fillings of galena, sphalerite, pyrrhotite, Te‐bearing minerals (petzite, altaite, hessite and Bi‐Te mineral) and electrum.Fluid inclusions characteristically contain CO2 and can be classified into four types (Ia, Ib, IIa and IIb) according to the phase behavior. The pressure corrected temperatures (≥500d̀C) indicate that the deposit was formed at a distinctively high temperature from fluids with moderate to low salinity (&lt;12 wt% equiv. NaCl) and CH4 (1˜22 mole %). The sphalerite geo‐barometry yield an estimated pressure about 3.5 ˜2.1 kbar. The dominant ore‐deposition mechanisms were CO2 effervescence and concomitant H2S volatilization, which triggered sulfidation and gold mineralization. The measured and calculated isotopic compositions of fluids (δ18OH2O = 10.3 to 12.4 %o; δDH2O = ‐52 to ‐77 %o) may indicate that the gold deposition originated from S‐type magmatic waters.The physicochemical conditions observed in the Yuryang gold deposit indicate that the Jurassic gold deposits in the Cheonan area, including the Yuryang gold deposit are compatible with deposition of the intrusion‐related Au‐Te veins from deeply sourced fluids generated by the late Jurassic Daebo magmatism.

Clay mineralogical and isotopic (K–Ar, δ18O, δD) constraints on the evolution of the North Anatolian Fault Zone, Turkey

This study presents the first attempt to constrain the evolution of the North Anatolian Fault Zone (NAFZ) by age dating and isotope tracing of clay minerals formed during near-surface faulting. Extensive illitic clay mineralisation occurred along the NAFZ related to hydrothermal alteration of the fault gouges and pseudotachylytes. Samples representing the pre-fault protoliths outside the fault zone do not contain authigenic illitic clay minerals indicating that hydrothermal processes were confined to the areas within the fault zone. K–Ar age data indicate that the hydrothermal system and the associated illite authigenesis initiated at ∼57 Ma. This process is interpreted to reflect the onset of significant strike-slip or transtensional faulting immediately after the continental collision related to the closure of the Neotethys Ocean. Following the initiation of the fault movements in the latest Paleocene–Early Eocene, displacements along the NAFZ have continued, with probably intensified fault activities at ∼26 Ma and later than ∼8 Ma. Oxygen isotope compositions of the illitic clays from different locations along the NAFZ are similar, with narrow ranges in δ 18O values indicating clay precipitation from fluids with similar oxygen isotope compositions and crystallisation temperatures. The δ 18O and δD values of the calculated fluid isotopic composition ( δ 18O = 5.9‰ to 11.2‰, δD = − 59‰ to − 73‰) are consistent with metamorphic and magmatic origin of fluids mobilised during active tectonism. The interpretation of the fluid flow history of the NAFZ is in agreement with that reported previously for some well-known large-scale high-angle fault zones, which similarly developed along collisional-type orogenic belts and are commonly associated with significant mesothermal ore mineralisation.

Character of fluids associated with hydrothermal alteration and metamorphism of Palaeoproterozoic submarine volcanic rocks, Baffin Island, Nunavut, Canada

The Bravo Lake Formation on central Baffin Island, Canada formed in a Palaeoproterozoic (∼1.9 Ga) intrabasinal submarine volcanic rift, the rocks of which preserve mineral assemblages formed during submarine hydrothermal alteration, amphibolite facies regional metamorphism, and retrograde metamorphism. Both stable isotope and cation exchange geothermometry of co-existing mineral assemblages indicate preservation of hydrothermal alteration at ∼300 °C, amphibolite facies metamorphism at ∼590 °C, and retrograde metamorphism at ∼300 °C. Fluid isotopic compositions calculated from δ 18O and δD values measured from minerals suggest that three isotopically distinct fluids interacted with the Bravo Lake Formation during its evolution: (1) Palaeoproterozoic seawater infiltration during hydrothermal alteration; (2) regional metamorphic fluids at low water–rock ratios; and (3) retrograde fluids indicating increasing water–rock ratios. Hydrothermal fluids have δ 18O and δD values near 0‰, similar to modern seawater values. The similarity in the isotopic composition of Palaeoproterozoic and modern seawater is consistent with the buffering of the isotopic composition of ocean water by hydrothermal circulation at mid-ocean ridges by similar processes as operate today. The results of this study show that the isotopic composition of prior fluid events can be preserved in amphibolite facies submarine mafic volcanic rocks if there is minimal penetrative structural deformation and dry prograde metamorphism.