Variations In Fluid Composition Research Articles

The Capture of Cadmium from Solution during the Replacement of Calcite by Apatite.

The capture of cadmium (Cd) from phosphate-containing solutions during the replacement of CaCO3 by phosphate phases such as hydroxylapatite (HAP) and tricalcium phosphate (TCP) has been studied under high and low temperature and pressure conditions using atomic force microscopy, scanning electron microscopy equipped with an X-ray spectrometer and a backscattered electron detector, Raman spectroscopy, and microprobe analysis. Starting with cubes of Carrara Marble (polycrystalline calcite) and single crystals of calcite, a new solid phosphate phase was observed to incorporate Cd from solution, formed under different pressure and temperature conditions tested. Results showed that Cd precipitated in a new phase on the surface of all samples tested. In Carrara Marble, pseudomorphic replacement of CaCO3 is restricted possibly due to kinetic limitation caused by the adsorption of Cd complexes formed in solution at reactive surface sites and the variation of fluid composition inside the sample. However, on the sample surface, this kinetic limitation is less influential, so the new phase could incorporate higher amounts of Cd faster. Furthermore, this reaction at room temperature was found to have similar and/or better Cd-uptake efficiency as HAP and CaCO3 in pure Cd solution through the precipitation of Cd-containing phosphate crystals on the sample surface. Both reactions were able to capture Cd in the precipitating phase structure and could provide a mechanism for simultaneous Cd and phosphate removal from solutions contaminated with both, in industrial or natural settings.

Spatial variations in fluid composition along structures hosting unconformity-related uranium deposits in the Athabasca Basin, Canada: implications for ore-controlling factors

Spatial variations in fluid composition along structures hosting unconformity-related uranium deposits in the Athabasca Basin, Canada: implications for ore-controlling factors

In Situ LA-ICP-MS of Zoned Garnets from the Huanggang Skarn Iron–Tin Polymetallic Deposit, Southeastern Mongolia, Northern China

The Huanggang deposit is the most important and largest skarn Fe–Sn polymetallic deposit in the Southern Great Xing’an Range of Northeast China. Cassiterite, magnetite, and other metal minerals are related to the garnets within skarn systems. The zoned garnets from various skarn stages are able to record numerous geological and mineralizing processes including variations in physicochemical conditions and hydrothermal fluid evolution. In this contribution, we present the mineralogy, systematic major, trace, and rare earth element (REE) concentrations of zoned garnets from the Huanggang Fe–Sn polymetallic skarn deposit. The in situ analytical results of garnets in the prograde skarn stage from andradite core (Grt I) to grossular rim (Grt II) reveal that core sections were formed from a fluid that was generally LREE-rich, with relatively high ∑REE, high LREE/HREE ratios, and weak negative Eu anomalies, whereas rim sections were crystallized from a fluid that was typically HREE-rich, with relatively low ∑REE, low LREE/HREE ratios, and obviously negative Eu anomalies. The garnets of the retrograde skarn stage from Fe3+-rich andradite core (Grt III) to andradite rim (Grt IV) demonstrate that the core sections have a flat trend with high ∑REE and obvious negative Eu anomalies, whereas rim sections were formed from a fluid with relatively low ∑REE, HREE-rich and obviously negative Eu anomalies. The garnets from the prograde skarn stage principally display relatively lower U and higher Y and F concentrations than those from the retrograde skarn stage. Based on optical and textural characteristics, REE patterns, Eu anomalies, and trace element variations in zoned garnets, it can be shown that, during skarn formation, Huanggang hydrothermal fluids shifted from near-neutral pH, oxidizing conditions, and high W/R ratios with relatively low LREE/HREE ratios characteristics to acidic, reducing conditions, and low W/R ratios with relatively high LREE/HREE ratios characteristics. We infer that variations in fluid compositions and physicochemical conditions may exert major control on formation and evolution of garnets and skarn hydrothermal fluids.

Interface-resolved direct numerical simulations of interphase momentum, heat, and mass transfer in supercritical water gasification of coal

Interactions between a reacting particle and the surrounding fluid are complex due to the interplay between flow dynamics, heat and mass transfer, and chemical reactions. In the present work, particle–fluid transport processes in supercritical water gasification of coal are studied using high-fidelity interface-resolved direct numerical simulations. The impact of different factors on the particle–fluid interactions are evaluated by performing simulations of the flow around two-dimensional particles considering different numerical configurations. The outgoing Stefan flow from the particle surface is found to cause expanded boundary layers for velocity, heat, and species. The temperature-induced changes in transport properties around a heated particle lead to a higher drag force and decreased heat/mass transport; those differences are further enlarged when taking into account the volumetric expansion of the fluid. Transport limitation for coal gasification in a realistic configuration is finally investigated. Temperature-induced fluid dilatation is then the major factor affecting drag force and heat transfer around the reacting particle, and mass transport is significantly impacted by species production or consumption in the boundary layer. Reaction heat release and variations in fluid composition within the thermal boundary layer lead to a slight enhancement of heat transfer. This work reveals and quantifies the main mechanisms affecting the exchanges between a reacting coal particle and surrounding supercritical water regarding both thermal and chemical aspects. It also provides high-fidelity data to later fit the reduced models needed for simulations of large-scale supercritical water gasification installations.

Fluid inclusions in vein quartz as a reflection of the deformation stages of the junction area of Baikal-Muisk and Barguzin-Vitim structural-formation zones (Western Transbaikalia)

Research subject. Fluid inclusions in quartz of three vein systems associated with tectonic cracks occurred during the development of thrust and shear deformations of the Western Transbaikalia. Vein systems localized in volcanic, terrigenous and granite rocks. Methods. The composition of fluid inclusions was studied by microthermometry, Raman spectroscopy and gas chromatography-mass spectrometry. Results. The quartz under study was formed by high-density medium-temperature NaCl-KCl and a low- and medium-salinity water-carbon dioxide fluid (2-5 wt % NaCl eq.). A distinctive feature of quartz of different vein systems was found to be the gas phase composition of inclusions associated with the composition of host rocks. The minimum temperatures of vein quartz formation were determined to range from 180 to 450°C under the minimum fluid pressure values of 0.7-2.9 kbar. Conclusions. Complex vein systems of the junction zone of the Baikal-Muiskaya and Barguzino-Vitimskay SFZ, confined to different host rocks and having different structural positions, are characterized by a similar fluid composition and a common thermodynamic history reflecting the change of deformation aggregates. The metamorphogenic-hydrothermal nature of the fluid involved in the formation of quartz vein systems was established. The development of the hydrothermal system was accompanied by its influence on the host rocks, which was manifested in the variations of fluid composition. The evolution of a single hydrothermal system is associated with tectonic deformation processes, which can be described by three dynamic stages corresponding to three quartz generations.

A HR-TEM study on two generations of magnetite from the Alemao IOCG deposit, Carajás, Brazil: Implication for Fe-Cu mineralization

Multiple generations of magnetite are common in many magmatic-hydrothermal Fe deposits, which can be formed by different mechanisms. This study investigates magnetite from the Alemao IOCG deposit (Brazil) characterized by inclusion-rich cores (Mag-1) and inclusion-poor rims (Mag-2), using electron probe microanalyzer and high-resolution transmission electron microscopy (HRTEM), to understand its formation mechanism and implications for Fe and Cu mineralization. Electron probe microanalysis shows that Mag-1 has Si and Mg contents of 0.28–1.15 wt% and 0.11–0.49 wt%, respectively, higher than Mag-2. Mineral nanoinclusions of minnesotaite, chlorite, quartz, and minor Ti-rich magnetite in the Mag-1 account for the relatively high Si and Mg in the Mag-1, as measured by electron probe microanalysis. The crystal orientation of nanoinclusions relative to the host Mag1 indicates formation by growth entrapment, where they crystallized from boundary layer supersaturated fluids under non-equilibrium conditions.Identification of a reaction front, spatial coupling of parent and product phases at micro- and nano-meter scales support the formation of Alemao Mag2 by coupled dissolution and reprecipitation (CDR). Variations in fluid composition rather than fluctuations in conditions such as temperature and oxygen fugacity are the factors inducing CDR. A conceptual model is proposed for the formation of Alemao magnetite where Fe-rich fluids formed inclusion-rich primary magnetite and CDR reactions modified the texture and chemical composition of primary magnetite to form a core rich, and a rim poor, in nanoinclusions. During CDR, impurities of Si, Ca, Al, Mn, and Mg and nanoinclusions are removed from the primary magnetite and formed product magnetite with higher Fe. The progressive enrichment of Cu from Mag-1 to Mag-2 during CDR indicates that a Cu-rich fluid related to the Cu-Au mineralization is responsible the formation of Cu-rich Mag-2 and chalcopyrite. Therefore, micron- to nano-scale characterization of magnetite with multiple generations can provide important constraints on fluid evolution in ore system.

Fluid evolution in the Beidabate porphyry Cu-Mo deposit, Xinjiang, northwest China: Evidence from fluid inclusions and H-O-C-S isotopes

• A fluid evolution model for Beidabate deposit is presented. • The model relies on fluid phase separation. • The precipitation mechanism for Mo and Cu is proposed. The Beidabate deposit is a porphyry Cu-Mo deposit located in the West Tianshan Orogenic Belt , Xinjiang Uygur Autonomous Region. The fluid evolution path and Cu-Mo mineralization mechanism remain obscured by multiple hydrothermal pulses. Systematic fluid inclusion and H-O-C-S isotope analyses were performed on five identifiable mineralization stages. Variations in fluid composition, temperature, and pressure record the fluid evolution path: (1) During the formation of barren quartz veins (stage I), fluid inclusions were trapped under single- to two-phase transitional conditions, as evidenced by the presence of intermediate-density inclusions as well as coexisting daughter mineral-bearing inclusions and vapor-rich inclusions (boiling FIAs). Fluid phase separation occurred at temperatures of 387–412 °C and pressures of 180–300 bar. (2) Fluid inclusions in K-feldspar-quartz veins (stage II) and molybdenite-quartz veins (stage III) were trapped under two-phase conditions. Boiling FIAs were also recognized, with homogenization temperatures of 306–368 °C and entrapment pressures of 80–200 bar. Fluid boiling is the main precipitation mechanism of molybdenite . (3) Fluid inclusions in pyrite-chalcopyrite-quartz veins (stage IV) and fluorite-chalcopyrite veins (stage V) are predominantly liquid-rich and CO 2 -bearing inclusions, which represent extraneous fluids. These inclusions homogenized at temperatures of 192° to 285 °C, and minimum entrapment pressures of 40–50 bar. Fluid mixing is the main precipitation mechanism of chalcopyrite . H-O-C-S isotope data (δ 18 O = 3.2 to 15.2‰, δD = −103.6 to −75.1‰, δ 13 C = −24.2 to −9.7‰ and δ 34 S = 4.6 to 7.0‰) indicate that the ore-forming fluids have a predominantly magmatic signature with subsequent dilution by late meteoric water and intense water–rock interaction. We established a genetic model of the Beidabate porphyry Cu-Mo deposit with the aim of providing insight into prospecting at a regional scale.

Fluid inclusion and stable isotope constraints on the source and evolution of ore-forming fluids in the Bailongshan pegmatitic Li-Rb deposit, Xinjiang, western China

The newly discovered Bailongshan pegmatitic lithium–rubidium (Li–Rb) deposit in West Kunlun–Karakorum (western China) is a world-class Li deposit. In this paper, we present detailed field observations and fluid inclusion (FI) data for four major mineral zones at Bailongshan; i.e., the fine albite (FAZ), blocky feldspar (BFZ), quartz–muscovite (QMZ), and spodumene–quartz (SQZ) zones. FIs are abundant in quartz and spodumene, and include four types: (1) two-phase L-type, (2) two-phase V-type, (3) three-phase S-type, and (4) three-phase (CO2–H2O–NaCl) or two-phase (CO2-rich) C-type. Quartz contains all four FI types, whereas spodumene contains mainly C-type FIs. Microthermometric measurements show that the L-type FIs in the FAZ, BFZ, and SQZ homogenised at 283–338, 156–224, and 223–421 °C, respectively, with corresponding salinities of 15.4–26.5, 4.4–10.4, and 10.5–30.6 wt% NaCl equivalent. The S-type FIs in the FAZ and SQZ homogenised at 183–285 and 214–298 °C, respectively. The C-type FIs in spodumene and quartz homogenised at 256–321 and 234–286 °C, with corresponding salinities of 4.4–18.1 and 4.4–12.6 wt% NaCl equivalent, respectively. The ore-forming fluids were of medium–low-temperature and medium–low-salinity, and no systematic fluid compositional variations were identified in each zone. The average CO2 densities and trapping pressures were estimated at 0.72–0.91 g/cm3 and 3.00–3.75 kb, respectively, corresponding to mineralisation depths of 9–11 km. Supercritical fluids likely contributed to the concentration of ore-forming elements in the fluids, and fluid boiling may have led to CO2 degassing and Li ore deposition. H–O isotope data show that quartz grains from all four zones have δ18OV-SMOW values of 8.6‰–10.2‰ and δDV-SMOW values of −38‰ to 107‰. The ore-forming fluids were likely sourced from pegmatitic magmas.

Geochemical characteristics of magnetite in Longqiao skarn iron deposit in the Middle-Lower Yangtze Metallogenic Belt, Eastern China

The Longqiao Fe deposit is the only large stratabound Fe skarn in Eastern China. This extensive ore body is suitable for investigating the spatial variations in trace elements in magnetite from the intrusion to the wall rocks and tracing the evolution of the mineralizing fluid in a skarn deposit. Magnetite samples were collected every 200 m from the ore-forming gabbro-diorite to the edge of the ore body, including the diopside-magnetite and epidote-chlorite-magnetite stages. Our results show that magnetite proximal to the gabbro-diorite displays lower Mg, Al, and Si and higher contents of Ti and V than those found further from the gabbro-diorite. The hydrothermal fluids in both diopside-magnetite and epidote-chlorite-magnetite stages were influenced by reaction with marine sedimentary rocks. Magnetites from different parts of the ore body display a wide range of trace element contents and fall into the porphyry and skarn fields on a (Mn + Al) vs. (Ti + V) discrimination diagram. Based on our results and published data, we propose that the early-stage magnetite from skarn and iron oxide-apatite (IOA) Fe deposits is compositionally similar, and fluid compositional variations are largely dependent on differences in the composition of the wall rock and the intensity of fluid-rock interactions. This results in a large variation in (Ti + V) in magnetite compositions from skarn deposits in the (Mn + Al) vs. (Ti + V) discrimination diagram, which is different from IOA deposits. This study shows that magnetite trace element evolution trends can provide a reliable indication to recognize skarn and IOA deposit types.

Assessment of Plasma Resistivity as a Surrogate for Extracellular Fluid Resistivity: Analytical Performance and Impact of Fluid Composition.

Bio-electrical impedance analysis (BIA) is frequently used to assess body composition in man. Its accuracy in patients is limited, possibly because the employed algorithms are based on the assumption that total body electrical resistance (TBER) is exclusively related to body water volume, and that variation in fluid composition and its effect on fluid resistivity can be ignored. This may introduce substantial calculation errors. The aim of this study was to develop an objective method to assess plasma resistivity (ρplasma) based on measurements by a conductivity probe, as a surrogate for extracellular fluid resistivity (ρe). Sample measurements were standardized at body temperature. Analytical variation was 0.6% within runs and 0.9% between runs. The critical difference, i.e. the smallest difference needed to consider changes within individuals significant, was 1.8% for measurements within runs and 4.3% for measurements between runs. The normal range was defined by a mean ± SD of 66.9 ± 1.8Ωcm. Multiple regression demonstrated that ρplasma was inversely related to plasma sodium and chloride concentrations, and positively related to total protein (overall R2 = 0.92, p < 0.001). In conclusion, ρplasma measurements were sufficiently robust to be useful as a tool to examine and improve the validity of BIA in clinical settings.

Mineral geochemistry of the Sangan skarn deposit, NE Iran: Implication for the evolution of hydrothermal fluid

The Sangan iron skarn deposit is located in the Sabzevar-Dorouneh Magmatic Belt of northeastern Iran. The skarn contains zoned garnet, clinopyroxene and magnetite. Cores and rims of zoned garnets are generally homogeneous, having a relatively high ΣREE, low ΣLREE/ΣHREE ratios, and positive Eu anomalies. The cores of the zoned clinopyroxenes are exceptionally HREE-rich, with relatively high ΣREE and HREE/LREE ratios, as well as positive Eu anomalies. Clinopyroxene rims are LREE-rich, with relatively low ΣREE contents and HREE/LREE ratios, and do not have Eu anomalies. Magnetite grains are enriched in LREEs in comparison with the HREEs and lack Eu anomalies. Variations of fluid composition and physicochemical conditions rather than YAG-type substitution mechanism are considered to have major control on incorporating trace elements, including REE, into the skarn mineral assemblage. Based on baro-acoustic decrepitation analysis, the calc-silicate and magnetite dominant stages were formed at similar temperatures, around 350–400°C. In the Sangan skarns, hydrothermal fluids shifted from near-neutral pH, reduced conditions with relatively high ΣREE, low LREE/HREE ratios, and U-rich characteristics towards acidic, oxidized conditions with relatively low ΣREE, high LREE/HREE ratios, and U-poor characteristics.

Equation of State for Relative Permeability, Including Hysteresis and Wettability Alteration

SummaryCommercial compositional simulators commonly apply correlations or empirical relations that are based on fitting experimental data to calculate phase relative permeabilities. These relations cannot adequately capture the effects of hysteresis, fluid compositional variations, and rock-wettability alteration. Furthermore, these relations require phases to be labeled, which is not accurate for complex miscible or near-miscible displacements with multiple hydrocarbon phases. Therefore, these relations can be discontinuous for compositional processes, causing inaccuracies and numerical problems in simulation.This paper develops for the first time an equation-of-state (EOS) to model robustly and continuously the relative permeability as a function of phase saturations and distributions, fluid compositions, rock-surface properties, and rock structure. Phases are not labeled; instead, the phases in each gridblock are ordered on the basis of their compositional similarity. Phase compositions and rock-surface properties are used to calculate wettability and contact angles. The model is tuned to measured two-phase relative permeability curves with very few tuning parameters and then is used to predict relative permeability away from the measured experimental data. The model is applicable to all flow in porous-media processes, but is especially important for low-salinity polymer, surfactant, miscible gas, and water-alternating-gas (WAG) flooding. The results show excellent ability to match measured data, and to predict observed trends in hysteresis and oil-saturation trapping, including those from Land's model and for a wide range in wettability. The results also show that relative permeabilities are continuous at critical points and yield a physically correct numerical solution when incorporated within a compositional simulator (PennSim 2013). The model has very few tuning parameters, and the parameters are directly related to physical properties of rock and fluid, which can be measured. The new model also offers the potential for incorporating results from computed-tomography (CT) scans and pore-network models to determine some input parameters for the new EOS.

Impact of natural convection and diffusion on variation of oil composition through a fractured model

The first step to study oil reservoirs is determination of reservoir fluid composition in both horizontal and vertical directions. In this research, fluid composition distribution is investigated as a result of diffusion and natural convection in a naturally fractured model. A two-component single phase fluid and a rectangular two-dimensional fractured oil reservoir with a known temperature profile are used in this study. MATLAB software is exploited to solve governing equations, numerically. According to results, thermal diffusion has the greatest impact on the variation of fluid composition, followed by pressure and molecular diffusions, respectively. Moreover, it is found out that even a small convection would change the fluid composition distribution, considerably. Fluids tend to flow through the fracture due to its high permeability; therefore, a counterclockwise convective cell would flow in the reservoir.

In-situ Isotopic and Chemical Study of Pyrite from Chu-Sarysu (Kazakhstan) Roll-front Uranium Deposit

Pyrite is common in roll-front type uranium deposit in Chu-sarysu basin, Kazakhstan. Combined in-situ microstructural, isotopic and chemical analysis of pyrite indicates variation in precipitation conditions and in fluid composition. Broad-scale δ34S heterogeneity indicates a complex multi-facet evolution. First generation authigenic framboïdal aggregates are biogenic as demonstrated by the lowest δ34S values of -48‰ to -28‰. The latest generation pyrites are probably hydrothermal with greater δ34S variation (-30‰ to +12‰). This hydrothermal pyrite commonly displays variable enrichment of several trace elements especially As, Co and Ni. Strong variation in δ34S values and variable trace element enrichment is interpreted in terms of continuous variations in fluid composition.

LA-ICP-MS analyses of minor and trace elements and bulk Ge isotopes in zoned Ge-rich sphalerites from the Noailhac – Saint-Salvy deposit (France): Insights into incorporation mechanisms and ore deposition processes

The increasing worldwide demand in germanium (Ge) is driving renewed research for understanding its geological cycle and the factors controlling its concentration in minerals. The advent of accurate, high-resolution trace element analysis by LA-ICP-MS, as well as the advances in MC-ICP-MS technique for Ge isotopes in sulphides, has enhanced studies in this field. Ge isobaric interferences, standard calibration and data interpretation remain outstanding issues needing to be addressed for more precise and comprehensive LA-ICP-MS analyses.An integrated mineralogical and geochemical study was carried out on typical sphalerite (ZnS) samples from the main Ge deposit in western Europe: the vein-type Zn–Ge–Ag–(Pb–Cd) deposit of Noailhac – Saint-Salvy (Tarn, France). In situ coupled measurements of trace elements and S isotopes were performed using LA-ICP-MS and SIMS, respectively, together with bulk Ge isotopes by MC-ICP-MS. Principal component analyses revealed element clusters antithetically distributed within distinct zoning types in sphalerite: sector zonings are enriched in Cu, Ge, Ga, Sb and As, whereas rhythmic bandings (dark brown bands primarily) are enriched in Fe, Cd, In and Sn. This typical distribution points to crystallographic controls on trace element uptake during sphalerite growth, occurring with concomitant microscale variations in fluid compositions at the fluid–crystal interface. Regardless of the zoning type, in all spots, Cu contents approach the sum of tri- and tetravalent cations (Ge, Ga, In, etc.) so that Cu could provide charge-balance for the entire set of coupled substitution mechanisms responsible for the incorporation of the whole range of trace elements in this sphalerite. Strong binary correlations suggest direct substitutions as Zn2+↔(Fe2+, Cd2+) and coupled substitutions as 2Zn2+↔Cu++Sb3+, 3Zn2+↔Ge4++2Ag+, and 3Zn2+↔In3++Sn3++□ (vacancy) despite no clear evidence for the presence of Sn4+.δ74GeNIST3120a in bulk sphalerite varies from −2.07±0.37‰ to +0.91±0.16‰ (2σ SD) and positively correlates with bulk Ge content. This indicates considerable Ge isotopic fractionation within sphalerite during low-T hydrothermal deposition and zoning processes, associated with possible microscale open system fluid mixing. The trace element features in sphalerite from Saint-Salvy compared with those of other deposits confirm their use as discriminators among genetic types of ores (e.g., high In contents for magmatic-related deposits, and Ge for low-temperature deposits).

A combined chemical, isotopic and microstructural study of pyrite from roll-front uranium deposits, Lake Eyre Basin, South Australia

The common sulfide mineral pyrite is abundant throughout sedimentary uranium systems at Pepegoona, Pepegoona West and Pannikan, Lake Eyre Basin, South Australia. Combined chemical, isotopic and microstructural analysis of pyrite indicates variation in fluid composition, sulfur source and precipitation conditions during a protracted mineralization event. The results show the significant role played by pyrite as a metal scavenger and monitor of fluid changes in low-temperature hydrothermal systems.In-situ micrometer-scale sulfur isotope analyses of pyrite demonstrated broad-scale isotopic heterogeneity (δ34S=−43.9 to +32.4‰VCDT), indicative of complex, multi-faceted pyrite evolution, and sulfur derived from more than a single source. Preserved textures support this assertion and indicate a genetic model involving more than one phase of pyrite formation. Authigenic pyrite underwent prolonged evolution and recrystallization, evidenced by a genetic relationship between archetypal framboidal aggregates and pyrite euhedra. Secondary hydrothermal pyrite commonly displays hyper-enrichment of several trace elements (Mn, Co, Ni, As, Se, Mo, Sb, W and Tl) in ore-bearing horizons. Hydrothermal fluids of magmatic and meteoric origins supplied metals to the system but the geochemical signature of pyrite suggests a dominantly granitic source and also the influence of mafic rock types. Irregular variation in δ34S, coupled with oscillatory trace element zonation in secondary pyrite, is interpreted in terms of continuous variations in fluid composition and cycles of diagenetic recrystallization. A late-stage oxidizing fluid may have mobilized selenium from pre-existing pyrite. Subsequent restoration of reduced conditions within the aquifer caused ongoing pyrite re-crystallization and precipitation of selenium as native selenium.These results provide the first qualitative constraints on the formation mechanisms of the uranium deposits at Beverley North. Insights into depositional conditions and sources of both sulfide and uranium mineralization and an improved understanding of pyrite geochemistry can also underpin an effective vector for uranium exploration at Beverley North and other sedimentary systems of the Lake Eyre Basin, as well as in comparable geological environments elsewhere.

Trace element zoning of sulfides and quartz at Sheba and Fairview gold mines: Clues to Mesoarchean mineralisation in the Barberton Greenstone Belt, South Africa

The Fairview and Sheba mines are two of the major gold mines in the Paleoarchean Barberton Greenstone Belt of Southern Africa. At these mines, gold is associated with quartz–carbonate±rutile veins and occurs both as “invisible” gold finely dispersed in sulfides (primarily pyrite and arsenopyrite), and as visible electrum grains hosted in pyrite. Up to approximately 1000ppm Au are contained in pyrite, and up to approximately 1700ppm in arsenopyrite. Mapping of trace element distribution in sulfide minerals using electron microprobe and proton probe techniques revealed multiple events of ore formation and Au mineralisation. At Fairview mine, three stages of pyrite formation were identified, the last of which is associated with arsenopyrite, electrum and other sulfide minerals (sphalerite, chalcopyrite, galena, gersdorffite, and Sb-sulfides). At Sheba mine, pyrite was deposited in two stages, and electrum is associated with the second stage. At both mines, the last stage of sulfide formation is the main stage of Au deposition, and is associated with mobilisation of Au, As, Sb, Cu, Zn, and Ni. The host rock composition seems to have affected the composition of pyrite, since higher Ni and Co concentrations (up to 1.4 and 1.6wt.%, respectively) have been measured in meta-(ultra)mafic host rocks in comparison with chert and metagreywacke. Arsenopyrite is chemically zoned, and has Sb- and S-rich cores and As- and Ni-rich rims. This zoning indicates variations in fluid compositions (decreasing Sb and increasing Ni), and crystallisation conditions (increasing As content for increasing temperature). Geothermometric estimates based on the As content of arsenopyrite (As ≤32at.%) indicate temperatures up to ~420°C for the crystal rims. Petrographic and cathodoluminescence observations of quartz associated with gold mineralisation show only local brittle deformation, and no plastic deformation. This supports the notion that the ore-transporting veins were emplaced late in the deformation history. Variations of cathodoluminescence of quartz are correlated with changing Al contents (Al ≤0.16wt.%), and can be related to fluctuations in the pH of the mineralising fluids.

A mechanistic model development to overcome the challenges of subsea fluid sampling

Extracting fluid samples from actual flow stream being measured subsea is one of the operational requirements for obtaining sustained accurate measurement for calibration of a subsea multiphase flow meter (MPFM). Samples collected from topside facilities do not represent the fluid being measured due to chemical injection downstream the meter and possible liquid separation/hold-up. However, the issue of subsea intervention and transportation of fluid samples present another challenge with significant cost impact and risk to the subsea environment. To overcome these challenges, a virtual compositional fluid tracking model has been developed as an optimal solution in bridging the gaps in subsea fluid sampling. The virtual model is developed with a compositional fluid tracking module, capturing the essential building blocks of the Subsea Production System (SPS). Results from the mechanistic model demonstrates the capability in improving understanding of well flow stream, taking into considerations the variations in fluid compositions in real time, and calculating the physical properties in view of matching the hydrocarbons compositions to enable a proactive and cost effective fluid sampling operations. This has also enabled the development of advanced operations monitoring, as operational conditions – we cannot control – changes over the field life.

Appraisal of reservoir compartmentalization using fluid mixing time-scales: Horn Mountain Field, Gulf of Mexico

Reservoir compartmentalization can seriously compromise a project’s economics if left undetected during appraisal. Its early identification is made more likely if maximum use is made of available fluid appraisal data. This involves making a critical comparison of time-scales for various fluid properties to equilibrate compared with the actual time since those properties were initially disturbed. Spatially varying fluid properties indicate compartmentalization if they have existed for longer than the time needed for them to equilibrate. Here we use data from appraisal wells and reservoir mixing time-scales to investigate vertical and horizontal compartmentalization in the Horn Mountain Field (Gulf of Mexico) and to quantify the properties of the baffles/barriers identified. We compare our results with earlier work using time-lapse geochemistry and mud gas isotope logging. Present fluid compositional variations in the field are shown not to be diagnostic of horizontal compartmentalization as the mixing time-scales by molecular diffusion are longer than the time since the reservoir filled. In contrast, pressure shifts and density differences are diagnostic. They indicate that faults within the Horn Mountain Field are relatively impermeable and would act as barriers during oil production. They also confirm that a shale-filled channel acts as a barrier to vertical flow.

Garnet-forming reactions in calc-silicate rocks from the Polička Unit, Svratka Unit and SE part of the Moldanubian Zone

*Corresponding author Three main Variscan tectono-metamorphic units with distinct metamorphic evolution are present at the eastern boundary of the Bohemian Massif: Moldanubian Zone (MZ), Svratka Unit (SU) and Policka Unit (PU). All these metamorphic complexes contain calc-silicate rocks, which occur as bodies, layers or nodules within gneisses, mica schists, marbles, amphibolites or skarns. Calc-silicate rocks consist of quartz, plagioclase, diopside, titanite and sometimes amphibole, scapolite, garnet, minerals of the epidote group, K-feldspar and calcite. Two groups of calc-silicate rocks are distinguished according to the presence/absence of garnet. The appearance of garnet is the result of continuous reactions among Fe-Ca-Al minerals. The dominant garnet-producing reaction calcite + quartz + clinozoisite = garnet + H 2 O + 5CO 2 depends on the P–T conditions, X CO2 in the coexisting metamorphic fluids and bulk rock chemistry. Phase relations and mineral compositions document a polymetamorphic history of the calc-silicate rocks. Calc-silicate rocks from the middle subunit of the PU have been affected by LP–MT metamorphism M 1 and strongly overprinted by a temperature peak of the Variscan regional metamorphism under amphibolite-facies conditions (M 2 ; 560–650 °C and 5–7 kbar). Calc-silicate rocks of the MZ, SU, and lower subunit of the PU reflect post-peak metamorphic decompression (M 2 ) after MP–HT metamorphism (M 1 ). Metamorphic P–T conditions of 640–680 °C and 6–7 kbar obtained for the calc-silicate rocks from the MZ and SU correspond to data from adjacent metapelites (M 2 ) and are similar to the P–T conditions for metapelites from the lower subunit of the PU. The Variscan regional metamorphism in all the studied units was followed by younger, greenschist-facies metamorphism (M 3 ). The temperature, pressure and metamorphic fluids composition were estimated from the mineral assemblages of the calc-silicate rocks and surrounding metapelites. The observed mineral reaction sequences require very low X CO2 values for calc-silicate nodules from the middle subunit of the PU and variable X CO2 values for the rest of the studied calc-silicate rocks. These variations in fluid composition were controlled by the size of the bodies, their bulk rock chemistry and/or the presence of carbonates in the surrounding rock complexes. The metamorphic assemblages with coexisting garnet and epidote group minerals from calc-silicate rocks are widespread in all the tectono-metamorphic units at the eastern margin of the Bohemian Massif. Appearance of comparable mineral assemblages in the rocks with variable geological evolution can be explained by the higher activity of hydrous fluids during metamorphic reactions in the surrounding metapelites at 550–680 °C and 5–7 kbar. The bulk rock chemistry and mineralogy of the calc-silicate rocks indicate that they were derived from silica-rich calcareous protoliths, such as marls or altered tuffs.