CO2-rich Fluids Research Articles

Deep CO2 emissions facilitated by localized shear deformation: A case study of the Karakoram fault system, western Tibet

Strike-slip faults play a significant role in creating deeply penetrating fractures with high permeability, thus promoting rapid migration of CO2-rich fluids to the surface. However, there are rare observations regarding how strike-slip movement could affect deep CO2 emissions. Here, we focus on the Karakoram fault system (KKFS), western Tibet, to estimate diffuse soil CO2 fluxes and to unravel potential controlling factors for CO2 emissions. Average CO2 fluxes of geothermal fields range in 22–2475 g m−2 d−1, significantly higher than the across-fault profiles (6–116 g m−2 d−1). A mass balance model based on δ13C-CO2 and CO2 concentration of soil gases reveals that deep carbon constitutes 49.1–91.5 % (average = 73.9 %) and 0.2–40.5 % (average = 25.5 %) of soil-gas carbon released from geothermal fields and across-fault profiles, respectively. Deep carbon could be produced by thermal decomposition of crustal rocks considering CO2-rich fluids with radiogenic helium isotopes. Strikingly, higher CO2 fluxes preferentially occur in geothermal fields along a bending segment of the KKFS, where localized shear deformation is prominent as documented by high slip rates over geological timescales, dense splay faults, clustering of earthquake events, and elevated strain rates. We suggest that high stress acting on the KKFS bend could enhance the deformation and fracturing of fault zone rocks, leading to production of metamorphic CO2 and efficient release of CO2-rich fluids through the highly permeable fault system. Our results could shed new light on CO2 origins and fluxes of strike-slip faults that are characterized by spatially heterogeneous strain partitioning and thus localized enhanced shear deformation.

The use of combined C[sbnd]Mg isotope compositions of carbonates from orogenic Sb[sbnd]Au deposits as a tracer of fluid interaction with sea-floor altered crust

The Mesoarchaean Murchison Greenstone Belt is composed of a strongly deformed volcano-sedimentary succession metamorphosed up to amphibolite facies and surrounded by metaluminous and peraluminous granitoids in the north of the Kaapvaal Craton of southern Africa. A circa 40 km-long shear-zone of strongly carbonatised rocks (the Antimony Line, or Sb-Line) oriented along the main trend of the greenstone belt (WSW–ENE) hosts important Sb deposits with accessory Au. These commodities mostly occur as stibnite and native gold hosted by quartz‑carbonate rocks at the centre of the Sb-Line, underlining the importance of CO2-rich fluid flow for mineralisation during deformation and metamorphism.In this study, we compare the elemental and stable isotope (C-O-Mg) composition of carbonates from the Sb-Line with regionally distributed carbonates hosted by the volcano-sedimentary succession distal to mineralisation. The carbonates of the Sb-Line and regional rocks have overlapping major element compositions. Both magnesite and dolomite in most Sb-Line samples have marked light-REE-depleted patterns with variable positive Eu anomalies. Carbon isotope ratios define two clusters, with marked δ13C peaks at ca. -5 ‰ for Sb-Line rocks and ca. -2 ‰ for regional rocks, implying separate C-sources. The peak at ca. -2 ‰ likely represents early carbonatisation through sea-floor alteration, whereas the first peak at ca. -5 ‰ is indicative of deep CO2 (mantle, or magma-derived) introduced during tectonic activity of the Sb-Line. Magnesium isotope ratios of regional rocks reveal limited fractionation (bulk δ26Mg = −0.27 ± 0.10 ‰) that overlap with mantle values, but some carbonate-bearing veins present 26Mg-depleted compositions (bulk δ26Mg = −1.91 to −0.40 ‰). Sb-Line carbonated rocks have more fractionated Mg isotope compositions (bulk δ26Mg = −0.8 to 0.0 ‰) and carbonates display marked negative 26Mg values (δ26Mg from −1.46 to −0.31 ‰). We interpret these results in terms of preferential remobilisation of isotopically light Mg during fluid-rock interaction and dissolution of carbonate in the host-rocks. The lack of correlation between δ13C and δ26Mg indicates decoupling of these isotopic systems, implying contribution from isotopically distinct sources of C and Mg to the mineralised zone.Similar to other structurally controlled AuSb mineralisation in Archaean greenstone belts, metal transport and ore deposition in the Murchison Greenstone Belt was closely related to the deep cycle of carbon, linking C-draw-down during sea-floor alteration, carbonate re-mobilisation during metamorphism and CO2 degassing from deep sources along major crustal discontinuities. In contrast to models for orogenic AuSb deposits invoking a purely intra-crustal, metamorphic origin of mineralising fluids, our results underline the importance of deep-sourced fluids originating from an external source from the volcano-sedimentary succession.

The precipitation mechanisms of scheelite from CO2-rich hydrothermal fluids: Insight from thermodynamic modeling

Scheelite and wolframite are two main tungsten-bearing ore minerals in tungsten deposits. Compared to wolframite formed mostly by NaCl–H2O fluids, scheelite in many but not all tungsten deposits is associated with CO2–bearing hydrothermal fluids, but its precipitation mechanisms from these fluids are poorly understood. The replacement of wolframite by scheelite is common among tungsten deposits where these two tungstates coexist, but how CO2, as the pH-buffering agent, affects their replacement remains unclear. To answer these questions, we first tested the pH-buffering effect of CO2 by comparing available experimental pH values of H2O–CO2±NaCl solutions at 200–280 °C and the corresponding thermodynamic modeling results. The mean absolute errors between the calculated and experimental pH values are 0.14 log units in H2O–CO2 solutions and 0.13 log units in H2O–CO2–NaCl solutions, indicating that the calculated acitivities of H+ in CO2-bearing solutions are reliable. Then, the solubilities of scheelite, scheelite-ferberite, and scheelite-hübnerite in NaCl–H2O–CO2 systems were modeled in this study. Our modeling results suggest that scheelite solubility in CO2-rich fluids is highly dependent on fluid pressure but is insensitive to fluid temperature. A decrease in fluid pressure from lithostatic to hydrostatic levels at a depth of 3–8 km causes CO2 escape and pH rise and precipitates over 70 % of tungsten contents in fluids on average. Therefore, CO2 escape is an efficient mechanism for precipitating scheelite from CO2–rich hydrothermal fluids. The independence of scheelite solubility on temperature at high pressures and the slightly retrograde solubility at low pressures allows CO2–rich hydrothermal fluids to carry a great amount of tungsten far away from its sources. This may be one of reasons why some scheelite deposits occur at greater distances (>500 m) from the granite or extend along its strike for several kilometers. Similar to the cases in NaCl–H2O systems, two mechanisms for scheelite replacing wolframite in CO2–rich fluids are identified, a single decrease in fluid pressure or simple cooling plus an increase in the ratios of total Ca contents to total Fe (or Mn) contents in fluids.

Quantifying element mass transfer in the Jiling Na-metasomatic hydrothermal uranium deposit, Northwest China

The Na-metasomatic hydrothermal uranium deposits are relatively widespread, low in grade (less than 1 % U3O8) but high in tonnage. Although it has been considered that this type of deposit was formed due to hydrothermal alteration unrelated to magmatic activity, the detailed evolution of fluids and ore-forming process are still not well understood. Through element-mass-balance calculation and geochemical mapping of regional rocks, we investigated the Jiling uranium deposit in northwestern China and evaluated the composition and source of fluids and element-transfer behavior through Na-metasomatism and uranium mineralization. The findings show that, in the early Na-metasomatism stage, the Na-, HFSE- and REE-rich late-magmatic hydrothermal fluids caused Na-metasomatism of wall rocks, enriching Na2O (>53 %) while removing K2O (<-78 %), depleting SiO2 (30 % in granite and 3 % in diorite), and massively mass-transferring Fe, Ti, P and some incompatible elements. With increased rock permeability and the formation of partial Fe2+-bearing minerals, the Na-metasomatic alteration produced reducing agents and migration channels for ore-forming fluids, as well as the creation of ∼ 15 vol% porosity in the altered granite for metallogenic space. In the late uranium mineralization stage, CO2-rich fluids extracted uranium and HREEs, converted Fe2+ to Fe3+, and subsequently precipitated uranium to form pitchblende with apatite, calcite and chlorite. Thus, the Na-metasomatic alteration caused by late-magmatic hydrothermal fluids is critical for the production of large Na-metasomatic hydrothermal uranium deposits. Our new geochemical mapping reveals that the mass-concentration changes of Na, K and Si are more credible to defining Na-metasomatic alteration, while Fe, Ti, P, ∑(Zr-Hf-Nb-Ta) and ∑LREE/∑HREE vary strikingly during the uranium mineralization process.

Genesis and fluid evolution of the Hatu orogenic gold deposit in the West Junggar, Western China

The West Junggar in Xinjiang, western China, represents a significant gold mineralization belt hosting over 200 gold deposits, with the Hatu gold deposit being the largest among them. In this study, ore geology and fluid inclusion assemblages of quartz samples from the Hatu gold deposit were investigated in an effort to clarify the mineralization process and its relationship with the geodynamic settings. The mineralization process is delineated into early (pyrite-albite-quartz veins), middle-a (quartz-ankerite veins), middle-b (quartz-ankerite-sulfide-native gold veins), and late (quartz-calcite veins) stages. The early stage veins suggest a compressional setting while the middle-a and middle-b stage veins suggest a tensional shear setting. The late stage veins are typically filled fissures cutting through earlier veins. Scanning electron microscope-cathodoluminescence (SEM-CL) reveals that early stage quartz (Q1) exhibits concentric CL-oscillatory growth zoning, while middle-a stage quartz (Q2a) displays unidirectional zoning and ranges from CL-bright to CL-dark, middle-b stage quartz (Q2b) is CL-bright and weakly zoned, and late-stage euhedral quartz (Q3) shows sector zoning transitioning from CL-gray to CL-dark. Quartz that formed in these stages developed three types of fluid inclusions: pure CO2 (PC-type), CO2–H2O (C-type), and H2O–NaCl (W-type). The early stage quartz contains C- and W-type fluid inclusions homogenizing at 309–345 °C, while the late stage quartz contains only W-type fluid inclusions with homogenization temperatures of 164–229 °C. Microthermometry of fluid inclusion indicated the evolutionary of the metallogenic fluid from a high-temperature, CO2-rich, and minor CH4 metamorphic fluid to a low-temperature, CO2-poor meteoric fluid. From the early to middle-a stages, fluid boiling caused the unloading of ore-forming elements whereas fluid mixing led to the precipitation of polymetallic sulfides and gold in the middle-b stage. Trapping pressures in the middle-b stage were estimated using C-type inclusions, illustrating that gold mineralization took placed at depth of 8.0 km. We propose classifying the Hatu gold deposit as an orogenic deposit, originating from the collisional orogenesis between the Yili-Kazakhstan and Siberian continents in the Late Carboniferous.

CO2 retention in high-pressure/high-temperature reservoirs of the Yinggehai Basin, northwestern South China Sea

Global industry drillings targeted at deep-burial hydrocarbons have renewed the record of maximum sustainable overpressure in sedimentary basins. However, the influence of extremely high overpressure on natural fluid accumulation and artificial waste sequestration is not yet completely understood. To better understand the motion characteristics of the highly overpressured CO2-rich fluid, the CO2 retention capacity was quantified, and the CO2-rich fluid motion trails were evaluated in an ideal natural laboratory in the Yinggehai Basin. The hydraulic sealing capacity was higher than the capillary sealing capacity in the highly overpressured stratum. Relative to the situations of no breach or solely breached by capillary failure, the superposition of capillary and hydraulic failures resulted in the caprock integrity breakage by faults (or fractures), diapirs, and pipes. Meanwhile, the high expulsion flux of CO2-rich fluid caused the consumption of chlorite to generate illite in the caprock of dual-breached fields. The CO2-rich fluid flux of capillary invasion was limited by the inherently low permeability of caprock, which may be insufficient for a dramatic change of hydrogen ions or electron activities to induce remarkable chlorite dissolution in the caprock of the sole-breached field.

Petrology, phase equilibria modelling, and fluid inclusion study of mafic granulites from Bhavani Suture Zone, Southern India

We report new petrology, mineral chemistry, P–T conditions, and fluid inclusion data on mafic granulites from the Mettupalayam region along the Bhavani Suture Zone, Southern Granulite Terrane, India. Phase equilibria modelling of mafic granulites yielded peak P–T conditions of 780–860 °C and 7.6–10.1 kbar followed by a near isothermal decompression along a clockwise P–T path. The trapped fluid inclusions in the peak metamorphic minerals display a melting temperature range from −57.4 °C to −56.6 °C, close to the triple-point temperature of pure CO2. The primary inclusions homogenized at −18.9 °C to +0.2 °C, corresponding to density values of 0.93–1.03 g/cm3. Homogenization of the secondary inclusions occurred within the range from −6.3 to +18.1 °C, corresponding to low CO2 densities of 0.79–0.96 g/cm3. From the textural characteristics of the high-density primary carbonic fluid inclusions, we interpret these inclusions as the CO2-rich syn-metamorphic fluid present during the high-grade metamorphism. The secondary fluids characterised by lower densities have undergone re-equilibration during the exhumation stage (decompression) from the peak granulite-facies metamorphism along a clockwise P–T trajectory. This interpretation is consistent with the occurrence of hornblende + plagioclase symplectite around the porphyroblastic garnet, suggesting decompression. We infer that the high-density CO2 was the dominant syn-metamorphic fluid components present during the granulite-facies metamorphism in the Mettupalayam region. Such carbonic fluids, possibly derived by degassing from carbonates or mantle sources, probably played a significant role in stabilizing high-grade mineral assemblages along this collisional suture zone.

Magmatic water content in HIMU basalts from the Cook-Austral Islands: constraints on degassing processes and source composition from clinopyroxene phenocrysts

The amount of water recycled during subduction is unclear. Water/Ce estimates in HIMU (high-238U/204Pb) basalts are variable, ranging from < 100 to > 250 in glasses and melt inclusions. Because clinopyroxene (cpx) is a common early liquidus phase and the cpx/melt partitioning of water is well constrained, cpx phenocrysts provide an additional constraint on magmatic water contents. We present water and trace element concentrations in HIMU-basalt-hosted cpx phenocrysts from the Austral Islands. Calculated melt [H2O] (up to 4.3 wt.%) and H2O/Ce ratios (22–825) are higher than in olivine-hosted inclusions, as are estimated equilibration pressures. Correlation between estimated melt [H2O] and crystallization or entrapment pressure suggests significant water loss during magma ascent. Most ocean island basalts (OIBs) span a limited range in H2O (~ 1–1.5 wt.%), and low (< 100) H2O/Ce ratios are primarily observed in melts with unusually high [Ce] (up to 350 ppm). Additionally, [H2O] and H2O/Ce in some suites correlate with entrapment pressure despite having quench pressures high enough to prevent significant water loss from open- or closed-system degassing (< 100 MPa). Polybaric “sparging”, whereby low-P melts re-equilibrate with CO2-rich fluids exsolved at higher pressure, may result in water loss at pressures less than CO2 saturation. This may more accurately describe OIB degassing processes than open or closed system degassing. After correcting for degassing, primary Australs melts likely have H2O/Ce of ~ 300–600. If applicable to OIB sources in general, this limits the total water budget of the mantle, including the mantle transition zone, to < 2.4 ocean masses.

Magnetotelluric imaging of an iron-oxide copper gold (IOCG) deposit under thick cover

The presence of thick (>100 m) and electrically conductive (< 10 Ω.m) ground cover is a major impediment in mineral exploration of deep resources. Iron-oxide copper gold (IOCG) systems in Australia are often associated with a pronounced potential field anomaly, depending on the oxidation state of iron, but the gravity and magnetic field signatures have little vertical resolution unless constrained by drill hole petrophysics. Additionally, IOCG systems have deep magmatic sources, with their conductivity anomalies extending at least to the lithospheric mantle. The Vulcan IOCG prospect lies about 30 km northeast of the Olympic Dam IOCG mine and is defined by a significant gravity anomaly associated with brecciated haematite beneath 850 m of sedimentary cover sequences. To image the physical properties and structural geometry of the Vulcan IOCG prospect, a 100-site broadband MT and passive seismic array was deployed in a 1 km grid over a 9 by 9 km area. Three-dimensional inversion of MT responses resolve structure in three distinct domains. Firstly, broad limestone-quartzite-shale stratigraphy (1–30 Ω.m) in the 850 m cover is delineated in resistivity and corresponds well with changes in shear-wave velocity. Secondly, the region of brecciated haematite below the cover sequences is shown to have lower resistivity (< 60 Ω.m) than surrounding country rock (> 100 Ω.m). Thirdly, a more electrically conductive (< 30 Ω.m) vertical zone that extends > 5 km is imaged a few kilometers to the northeast of the Vulcan haematite breccia, and appears to be linked by a region of low shear-wave velocity in the depth range 1–2 km. Two-dimensional inversion of more regional MT responses along a 200-km line passing through the Vulcan prospect suggest this vertical region of low resistivity links to the lower-crust with anomalous resistivity of < 60 Ω.m in a similar way as imaged beneath the Olympic Dam mine. It is suggested that this conductive region is associated with graphite precipitated from magmatically derived CO2-rich fluids cooling in a reducing environment.

An unusual occurrence of carbonatites derived from the crust in the UHT granulite facies metamorphic terrain of Sri Lanka

The special calcite bodies (CBs) exposed within the ultrahigh temperature (UHT) granulite facies terrain in the Balangoda area of Sri Lanka preserve shreds of evidence for the generation of crust-derived carbonatite in the crust. The CBs are exclusively made up of massive calcites and appear as concordant bands extending tens of meters or as meso-scale isolated pockets hosting the massive dolomitic marble band. Various sizes of mafic and calc-silicate enclaves occur as rotated or tilted structures within the CBs. The contact between the CBs and the host marble is texturally and mineralogically gradational, while the contact between the enclaves and the CBs is sharp. The large-ion lithophile elements and rare earth element contents of the CBs show enrichment compared to the host marble, while depletion compared to typical carbonatites. Furthermore, the Sr content and C-O isotope values in CBs differ from those found in known carbonatites, hydrothermal carbonates, or metasomatic carbonates. We suggested that the crustal anatexis of marble should be hypothesized as the possible mechanism for the origin of the CBs. Microtexural evidence of the calcite grains shows indications of the melting of the host marble. The release of CO2-rich fluids from the collision and thrusting of HC over VC, or related metamorphic events, likely lowered the solidus of carbonates, triggering crustal anatexis of marble during UHT granulite facies metamorphism. The generated low viscous carbonate melt may have moved rapidly, resulting in a low degree of mixing of silicates and fragmentation and dislocation of enclaves. The results of the present study reflect the existence of anatexis of carbonates under extreme crustal conditions and provide a better understanding of the sources, migration paths and reservoirs of the carbon recycling processes.

The role of CO2 in the genesis of Dabie-type porphyry molybdenum deposits

Porphyry-type molybdenum deposits, many of which are in China, supply most of the World’s molybdenum. Of particular importance are the molybdenum deposits located in the Qinling-Dabie region that are responsible for more than half of China’s molybdenum production. A feature that distinguishes this suite of deposits from the better-known Climax and Endako sub-types of porphyry molybdenum deposits is their formation from CO2-rich magmatic-hydrothermal fluids. The role of CO2, if any, in the transport of molybdenum by these fluids, however, is poorly understood. We conducted experiments on the partitioning of molybdenum between H2O-CO2, H2O-NaCl, and H2O-NaCl-CO2 fluids and a felsic melt at 850 °C and 100 and 200 MPa. Here we show that the exsolution of separate (immiscible) brine and vapor leads to the very high brine DMo values needed for efficient extraction of Mo from the magmas forming Dabie-type porphyry molybdenum deposits.

Petrogenesis and Geodynamic Evolution of the Archean Shawmere Anorthosite Complex and Associated Gneisses, Kapuskasing Uplift, Superior Province, Canada

Abstract In this study, we integrated extensive field, petrographic, whole-rock major and trace element, and Nd–Pb–Sr–O isotope, and zircon U–Pb ages, trace element and Lu–Hf isotope data from the Neoarchean Shawmere Anorthosite Complex and surrounding gneisses to unravel their petrogenetic origin and tectonic history. The ~2765 Ma Shawmere Anorthosite Complex is interpreted to have been emplaced into a sequence of interlayered greywacke and basalt deposited in an intra-continental arc rift system above a north-dipping subduction zone. The complex consists mainly of anorthosite, leucogabbro, gabbro, and hornblendite that were emplaced as several batches of magmas and crystal mushes originating from sub-arc mantle sources. In contrast to the previous studies, our field and petrographic data suggest an igneous origin for the most hornblende in the complex, implying hydrous parental magmas. A hydrous magma origin is also consistent with the high-anorthite content (mostly 70–90%) of the plagioclase in the complex. Percolation of hydrous basaltic melts through gabbroic cumulates in crustal magma chambers led to extensive (&amp;gt;50%) replacement of igneous clinopyroxene by igneous hornblende. Continued subduction resulted in the closure of the intra-arc rift system and the intrusion of the complex by tonalite, granodiorite and diorite between 2765 and 2680 Ma in an Andean-type margin. The complex and surrounding gneisses underwent hornblende granulite-facies metamorphism mainly between 2680 and 2620 Ma, overlapping with mid-crustal east-west extension between 2660 and 2640 Ma. The granulite-facies metamorphism is recorded by the replacement of hornblende, plagioclase and clinopyroxene by garnet and the development of a garnet-orthopyroxene-plagioclase metamorphic assemblage with a granoblastic texture. Tectonic rebounding of mid-crustal rocks to upper crustal levels after 2620 Ma led to the formation of an extensive network of extensional fractures and retrograde metamorphism. Migration of CO2-rich hydrous fluids along the extensional fractures and grain boundaries resulted in the precipitation of many metasomatic minerals mainly at the expense of hornblende and plagioclase, including epidote, clinozoisite, tremolite, actinolite, paragonite, margarite, titanite, quartz, calcite, sillimanite, dolomite, and chlorite. Prevalent replacement of hornblende by garnet during prograde metamorphism and metasomatic replacement of hornblende and plagioclase by retrograde mineral assemblages disturbed the Sm–Nd, U–Th–Pb, and Rb–Sr isotope systems.

Low-temperature crystallization of kumdykolite, a polymorph of albite, during mineral carbonation within fluid inclusions in hornblendite from the Dabie orogen, central China

Abstract Kumdykolite is a polymorph of albite that has been predominantly identified within crystallized melt inclusions in high-temperature metamorphic rocks. This study reports a new occurrence of kumdykolite that formed during internal mineral carbonation within amphibole-hosted fluid inclusions in postcollisional hornblendite from the Dabie orogen, central China. Amphibole in the hornblendite trapped CO2-rich fluid inclusions at magmatic stage, and mineral carbonation, referring to the reaction of mineral rich in divalent cations and CO2 into carbonate, occurred in situ within the fluid inclusions due to interaction between trapped CO2-rich fluids and host amphibole during cooling of the hornblendite. Kumdykolite was produced along with calcite, dolomite, chlorite, talc, a SiO2 phase (quartz or cristobalite), a TiO2 phase (rutile or anatase), and mica during internal mineral carbonation within the fluid inclusions. It is estimated that kumdykolite in the fluid inclusions crystallized under near-surface conditions, which are significantly different from the conditions of crystallization proposed in previous studies. It is further inferred that kumdykolite may crystallize metastably across the stability field of albite, and presence of kumdykolite is not indicative of extreme thermobaric and fluid-absent conditions.

The role of hydrothermal processes and the formation of the J-M reef and associated rocks of olivine-bearing zone I of the Stillwater Complex, Montana

AbstractSeveral lines of evidence, including hydrous melt inclusions and unusually Cl-rich apatite, have been used to suggest that the reappearance of olivine and PGE-sulfide of the J-M Reef in the Stillwater Complex, Montana, is due to fluid infiltration and hydration melting. This study builds upon the hydration melting model using the programs MELTS and PELE with Stillwater bulk rock compositions for the original protolith. Cl-bearing phases are not modeled by MELTS and thus simple oxide mixtures of either a pure H2O or a H2O + Na2O “faux brine” are added to norite, gabbronorite, and melanorite protoliths at 1050 °C at 2 kbar pressure, conditions for which the nominally “dry” protolith is &gt; 95% solid. Incongruent hydration melting results in up to 37% olivine produced in the melanorite. The olivine Fo content is a function of the partial melt retained on cooling, and ranges between 76 and 86, overlapping the natural range of olivine compositions observed in the rocks. Modeling with the PELE program, which includes a silicate liquid Cl component, sulfur species, and a more complex C-O–H-S fluid, suggests that, for CO2-rich fluids, fluid metal concentrations on the order of 25 ppm Pt, 75 ppm Pd, 0.03 wt.% Cu, and 0.20 wt.% Ni at a fluid/rock mass ratio of ~ 0.25 are needed to account for the observed ore grades. Sulfide and ore metals are readily remobilized for more H2O-rich fluids, consistent with heterogeneous distribution of sulfide and regionally variable ore grades.

Genesis of a sediment-hosted vein-type Cu deposit in the Lanping Basin, SW China: New insights from geology and LA–ICP–MS analysis of fluid inclusions

Sediment-hosted Cu deposits, including stratiform and epigenetic vein-type Cu deposits, are the second largest source of Cu worldwide. However, many facets of these vein-type Cu deposits, such as the nature and origin of ore fluids and geodynamic mechanisms that produced these fluids, remain controversial, limiting our interpretations of the ore genesis and hampering the selection of exploration strategies. Numerous sediment-hosted vein-type Cu deposits are present in the Lanping Basin, SW China, of which the Jinman deposit is the largest. Copper orebodies at Jinman consist of Cu-sulfide–quartz–carbonate veins and altered wall-rocks hosted mainly in faults and fracture zones in metamorphosed and deformed clastic rocks. The deposit was formed in three stages, pre-, syn- and post-ore, with the first two stages producing hydrothermal veins. The veins in pre-ore stage consist of quartz + ankerite ± sericite ± pyrite, whereas those in syn-ore stage consist of Cu-sulfides + quartz + carbonate ± sericite. During the syn-ore stage, tennantite, quartz, and ankerite crystallized early, then bornite was precipitated, followed by chalcopyrite and chalcocite and finally calcite. Only minor sericite was formed during the post-ore stage. Two major types of fluid inclusions—CO2-rich and aqueous inclusions—are present in both pre- and syn-ore quartz, whereas only aqueous inclusions are present in syn-ore calcite. In pre- and syn-ore quartz and calcite, CO2-rich inclusions have higher homogenization temperatures (275–323 °C) and pressures (307–3495 bar) and lower salinities (0.22–2.7 wt% NaCl eqv.) than aqueous inclusions (164–299 °C, 6.5–80.4 bar and 3.7–20.9 wt% NaCl eqv.). However, the small variations in Cs/Na and Cs/(Na + K) ratios [log Cs/Na = –3.3 ∼ –1.9, log Cs/(Na + K) = –3.3 ∼ –2.1] of the fluid inclusions suggest a same source for the ore fluids. We propose that during both stages, CO2-rich inclusions resulted from unmixing of initial deep-seated, single-phase CO2-rich fluids, whereas aqueous inclusions likely represent the fluids that evolved from these unmixed fluids by extensive CO2 degassing. This interpretation, combined with microthermometric results of CO2-rich inclusions, suggests a mesothermal, low-salinity H2O–NaCl–CO2 ore fluid system. The fluid inclusions from Jinman have similar compositions to those of metamorphic fluids, suggesting a metamorphic origin for the ore-forming fluids. The evolution of fluid compositions suggests that fluid degassing likely acted as a major mechanism for Cu precipitation. We propose that the ore fluids were generated through metamorphic devolatilization of basement of the Lanping Basin in the Biluoxueshan–Chongshan metamorphic zone under a compressional or transpressional regime caused by Cenozoic India–Asia collision. The Jinman deposit has many similarities to orogenic gold deposits worldwide and thus can be thought of as an orogenic deposit. Based on a compilation of the geological characteristics of global sediment-hosted epigenetic vein-type Cu deposits, we suggest that other sediment-hosted epigenetic vein-type Cu deposits with similar genesis to the Jinman deposit are likely present in other regions worldwide. This highlights the exploration potential of this type of deposit in these regions and the need for further research.

Diagenetic Genesis and Evolution of Coal-Bearing Tight Sandstone Reservoir in the Yangxia Formation, Northern Kuqa Depression, Tarim Basin.

Coal seams of the Yangxia Formation are widespread in the northern part of the Kuqa Depression in the Tarim Basin. During the thermal evolution of the coal seams, the generated fluids of different periods and natures have a significant impact on tight sandstone reservoirs. To further investigate the diagenetic characteristics and reservoir genesis of the tight sandstones due to the influence of coal seams, an in-depth exploration of the causes of dissolution and cementation in the reservoirs was conducted through thin-section casting, cathode luminescence, scanning electron microscopy, carbon-oxygen isotopic analyses, and X-ray diffraction of whole rock and authigenic clay minerals, along with burial evolution history and fluid evolution history. It is suggested that two phases of acidic fluids are mainly produced during the thermal evolution process of coal seams, including an early humic acid and a late organic carboxylic acid. The early phase humic acid plays a purifying role in reservoirs with coarse particles, rigidity-rich particles, and good permeability conditions. It selectively dissolves sedimentary calcareous mud and calcite, and the dissolution products are completely migrated. At the same time, it inhibits early carbonate cementation. The late organic carboxylic acid will dissolve potassium feldspar and some volcanic rock debris, and the dissolution products are difficult to migrate under the sealing conditions caused by lithological differences, which often take the cementation form of siliceous overgrowth and kaolinite or illite. In addition to the cementation resulting from the dissolution products of acidic fluids produced by the coal seams, the CO2-rich fluids generated by the coal seam thermal evolution will combine with ions such as Ca2+ from different sources, resulting in two phases of carbonate cementation. Based on the above research, this study summarizes a set of diagenetic evolution models for coal-bearing reservoirs.

Fluids in the Shallow Mantle of Southeastern Australia: Insights from Phase Equilibria

Abstract Small amounts of water (10s to 100s of ppm) can have a profound effect on the properties of mantle peridotites, including viscosities, conductivities, and melting temperatures. Measuring the water content of nominally anhydrous minerals (NAMs) has provided insight into the amounts of water contained within mantle rocks. However, converting from NAM water contents to the activity of H2O is non-trivial. Equilibria involving amphibole can be used to determine values of the activity of H2O (aH2O) at the time of mineral equilibration. This approach yields low values of the activity of H2O (&amp;lt; 0.3) for four peridotite xenoliths from Southeastern Australia. These four xenoliths also record values of oxygen fugacity (fO2) that range from -0.2 to -1.2 (Log units relative to FMQ). All these values of fO2 are inconsistent with the presence of a CH4-rich fluid (too oxidizing), and the lowest value of oxygen fugacity, as recorded by one sample, is inconsistent with the presence of a CO2-rich fluid.

Direct measurement of hydrate equilibrium temperature in CO2 and CO2 rich fluids with low water content

A high-pressure experimental setup was used to obtain experimental data on the hydrate equilibrium (melting) temperature (HET) of hydrates formed from low water content CO2 and CO2 rich mixtures. A novel method was used to measure hydrate formation temperature in addition to melting temperature directly at realistic temperature/pressure conditions encountered in transport by pipelines or ship. The method can be used to evaluate the likelihood of hydrate formation in pipelines/ships transporting pure CO2 and CO2 coming from capture processes as well as from high CO2 content produced gas. The data generated from the measurement can also be used to investigate the kinetics of hydrate formation and dissociation in low water content fluids at pipeline/ship conditions. This paper details the experimental equipment, methods, test fluids and results. Experimental results are compared against predictions of the simplified cubic plus association equation of state (sCPA-EoS) coupled with the van der Waals and Platteeuw solid solution theory.

Mineral carbonation of peridotite fueled by magmatic degassing and melt impregnation in an oceanic transform fault

Most of the geologic CO2 entering Earth's atmosphere and oceans is emitted along plate margins. While C-cycling at mid-ocean ridges and subduction zones has been studied for decades, little attention has been paid to degassing of magmatic CO2 and mineral carbonation of mantle rocks in oceanic transform faults. We studied the formation of soapstone (magnesite-talc rock) and other magnesite-bearing assemblages during mineral carbonation of mantle peridotite in the St. Paul's transform fault, equatorial Atlantic. Clumped carbonate thermometry of soapstone yields a formation (or equilibration) temperature of 147 ± 13 °C which, based on thermodynamic constraints, suggests that CO2(aq) concentrations of the hydrothermal fluid were at least an order of magnitude higher than in seawater. The association of magnesite with apatite in veins, magnesite with a δ13C of -3.40 ± 0.04‰, and the enrichment of CO2 in hydrothermal fluids point to magmatic degassing and melt-impregnation as the main source of CO2. Melt-rock interaction related to gas-rich alkali olivine basalt volcanism near the St. Paul's Rocks archipelago is manifested in systematic changes in peridotite compositions, notably a strong enrichment in incompatible elements with decreasing MgO/SiO2. These findings reveal a previously undocumented aspect of the geologic carbon cycle in one of the largest oceanic transform faults: Fueled by magmatism in or below the root zone of the transform fault and subsequent degassing, the fault constitutes a conduit for CO2-rich hydrothermal fluids, while carbonation of peridotite represents a vast sink for the emitted CO2.

Seismic faulting and CO2-rich fluid interactions: Evidence from carbonate spherulitic grains in ultramafic fault damage zones

Faults and shear zones in the metamorphic ultramafic rocks of the Voltri Massif (Ligurian Alps, NW Italy) are often accompanied by intense or complete carbonation of the host rocks. One of these fault zones, hosted in peridotite and serpentinite, is characterized by distinctive structural features that can be related to a fluid-assisted multistage brittle deformation, indicative of paleoseismic activity. The fault rocks contain cataclasite and breccias with layers of coseismic spherulitic grains of dolomitic composition, silica/calchedony veins and cement, and saponite bearing gouge. The fault is part of a system of regional structures associated with gold mineralization in quartz veins and various stages of hydrothermal alterations.Here we present a multiscale study of this fault zone combining field observations, microstructural analysis, SEM-EDS, and electron backscattered diffraction (EBSD). Focus is on the microstructures of the fault core and the significance of the characteristic carbonate spherulite layers in association with silica/chalcedony cement and veins. Our results show that these structures are indicative of the interaction between CO2-rich fluids released during coseismic and interseismic phases of faulting, i.e., during cycles of fluid pressure build-ups, faulting, fluid flushing, and mineral precipitation and sealing during seismic failure of the fault.