High-salinity Fluids Research Articles

Fluid flow in the Katanga Supergroup: From Lufilian brittle tectonic stages to the post-Lufilian period (Democratic Republic of Congo)

The metasedimentary rock succession of the Neoproterozoic-Cambrian Katanga Supergroup in the Central Africa Copperbelt shows evidence of several complex tectonic events. The deformation of this supergroup started from the tectonic inversion at about 570 Ma and lasted up to today, but reached paroxysm at ∼550 Ma. This long period was characterized by folding and faulting throughout multiple compressive and extensional events, which controlled the regional fluid flow on the one hand, and played an important role during formation of the stratiform to stratabound Cu-Co (Ni, U) deposits and the polymetallic Cu-Zn-Pb (Ag, Ge, Mo, Cd) vein type deposits on the other hand. Based on the structural analysis and paleostress reconstruction, coupled with fluid inclusion characterization from mineralized structures in rocks from the Nguba, Kundelungu and Biano Groups, this study demonstrates that the composition of hydrothermal fluids changed during brittle tectonic deformation during the Lufilian orogeny and subsequent uplift and post-Lufilian faulting.During early brittle tectonic deformation along strike slip faults with sinistral and dextral movement related to a NE-SW transpression, the Cu-mineralizing fluid was hypersaline (27.9–31.1 eq. wt% NaCl) with moderate temperatures (Th = 128–216 °C). The subsequent Cu or Cu (Zn, Pb) mineralization formed within an E-W extensional stress regime, related to the late Lufilian orogenic collapse. The fluid inclusions present in the gangue minerals associated with this latter mineralization show a large range in Th (50–264 °C) and salinity (26.7–36.0 eq. wt% NaCl). The decrease in temperature is interpreted to be due to migration of the fluids at shallower depth in the subsurface after uplift and erosion of the orogen. The increased salinity of the fluid is related to the dissolution of evaporites, mainly NaCl. A second H2O-NaCl-CaCl2 fluid with a homogenization temperature below 55 °C has also been found associated with this brittle stage and mineralization phase, but only in rocks belonging to the Kundelungu Group. A third mineralization phase, also characterized by Cu or Cu (Zn, Pb), formed during the post-Lufilian period within a NW-SE transpressional inversion regime. The fluid inclusion in the gangue minerals of this mineralization phase have a smaller range in homogenization temperature (Th = 37–172 °C) and the largest range in salinity (0.71–30 eq. wt% NaCl), compared to the earlier fluid inclusions generations. This large range in salinity may be explained by the mixing of a high salinity fluid, already present during the earlier tectonic stages in the sedimentary basin, with meteoric water. During the more recent rift-related extension, a fluid with again a large and higher range in homogenization temperatures (Th = 47–257 °C) and with a typical low salinity (<10 eq. wt% NaCl) has been recognized in minerals filling NNE-SSW to NE-SW oriented faults and fractures. The upward migration of a relatively low-salinity fluid from deeper parts in the subsurface explains the variation in the temperatures observed with this tectonic event.

Albitization related U and Th mobilization under reducing conditions

Hydrothermally altered quartzofeldspathic gneisses, granites and pegmatites of the West Garo Hills, Shillong Plateau (eastern India) preserve alteration assemblages of monazite, xenotime and zircon comprising thorite and coffinite in the altered domains. Albitization of muscovite and K-feldspars, sericitization of K-feldspar and albite, alteration of biotite to hydrobiotite, and crystallization of tourmaline is suggestive of the involvement of acidic fluids. The microtextural relations and the mineral assemblages indicate prevalence of low-temperature (T < 300 °C) conditions during hydrothermal alteration. The light δ11B values of tourmaline (–15.1 to –13.5 ‰) suggests derivation/interaction of the fluids from/with metapelitic rock and/or S-type granite. Low ‘inferred’ Fe3+/Fe2+ ratio in tourmaline (based on high Al content in Y-site, 0.16–0.56 apfu, avg. 0.36 apfu), estimated excess charge (0.22–0.47, avg. 0.35 apfu) and lack of any Fe–Al relation support a reduced nature of the hydrothermal fluid. Mass balance calculations reveal that Th and U required for the formation of thorite and coffinite, respectively were likely derived from monazite and xenotime, and zircon. Significantly lower Cl─ contents in hydrobiotite (avg. 0.03 wt%) compared to the unaltered biotite (avg. 0.13 wt%) suggests release of Cl─ to the hydrothermal fluid during alteration. Textural evidences of albitization together with Cl─ release from biotite suggest increased Cl─ concentrations in the fluid during hydrothermal alteration. Based on solubility calculations for various U and Th species, we propose that high Cl─ content in the fluid aided mobilization of Th and U as ThCl40 and UCl40/UO2Cl20 complexes from accessory radioactive phases under reducing conditions. These results suggest that significant mobility of U and Th can be achieved in acidic high salinity fluid even under reducing conditions. Thermodynamic calculations for the solubility of Th- and U-chloride complexes and stability of monazite/xenotime in a range of pH and temperature suggest that thorite and coffinite precipitation was the result of an increase in pH.

A numerical model for the quantification of fluid inclusion property variations caused by heterogeneous entrapment and post-entrapment modifications in the H2O-NaCl system

The primary geological conditions for hydrothermal systems can be reconstructed using fluid inclusions, which generally requires that fluid inclusions trapped a single homogeneous phase and experienced no mass and volume changes after entrapment (Roedder's Rules). However, heterogeneous entrapment and post-entrapment modifications have been frequently identified in fluid inclusion studies, making the recovery of primary fluid properties from fluid inclusion data challenging or impossible. In this study, we constructed a specific tool, FlincPro, for the calculations of ideal and nonideal Fluid inclusion Properties in the H2O-NaCl system (ideal here refers to fluid inclusions that adhere to Roedder's Rules). Multiple modules have been developed to quantify the variations of fluid inclusion properties caused by heterogeneous entrapment, volume contraction/expansion, and water loss. The developed software illustrates possibilities in the variation of fluid inclusions in a specific assemblage with the input of a certain starting point.We found that compositions of the vapor endmember fluid are more significantly affected by heterogeneous entrapment than the liquid endmember, as the latter may have several orders of magnitude higher salinity and density. Homogenization pressure and temperature of halite-bearing fluid inclusions trapped on the halite liquidus will increase sharply with the addition of halite, but the homogenization behavior will not be changed. Volume contraction/expansion are effective ways to change the homogenization behaviors, especially for high salinity fluid inclusions. Water loss is not likely to cause the homogenization behavior change from total homogenization by vapor disappearance to total homogenization by halite dissolution for a halite-bearing fluid inclusion without additional volume contraction. Low-salinity and low-density fluid inclusions with total homogenization temperature around the critical temperature of water are least affected by post-entrapment modifications. Therefore, the interpretation of fluid inclusions formation depends on more parameters, e.g., petrographic observations, density and composition variations in fluid inclusion assemblages, and geological information, than only microthermometric data.

Fluid Spontaneous Imbibition Under the Influence of Osmotic Pressure in Deep Coalbed Methane Reservoir in the Ordos Basin, China

Summary The flowback rate of a hydraulic fracturing fluid is related to coalbed methane (CBM) production in gas wells. The deep (&amp;gt;2000 m) CBM reservoir in the Ordos Basin has an extremely high salinity (&amp;gt;200 000 mg/L), which results in a very low flowback rate of fracturing fluid. The mechanism underlying the extremely low flowback rate of the fracturing fluid remains unclear. This study experimentally simulated two patterns of osmotic pressure variation that exist at a hydraulic fracturing site: the processes of injection of a low-salinity fracturing fluid into a high-salinity reservoir and a high-salinity fracturing fluid into a low-salinity reservoir. Low-field nuclear magnetic resonance (NMR) technology was used to monitor dynamic fluid migration and fluid distribution in the coals. Results showed that osmotic pressure is a driving force for spontaneous imbibition when the salinity of the fracturing fluid is lower than that of the reservoir water, and more fluid enters the coal as the osmotic pressure increases. This causes the displacement of the high-salinity fluid already present in the micropores by the low-salinity fracturing fluid. In high-salinity deep coal seams, both osmotic pressure and capillary forces cause the spontaneous imbibition of the fracturing fluid from fractures into pores, promoting CH4 desorption, alleviating the water-blocking effect, and enhancing the filtration loss of the fracturing fluid. In contrast, the injection of a high-salinity fluid into the reservoir with a low-salinity brine (LSB) creates an osmotic pressure difference that prevents fluid imbibition. In shallow, low-salinity coal seams, the injection of high-salinity fracturing fluids can result in high flowback rates. Therefore, these two injection schemes are significant for an understanding of the role of osmotic pressure in deep CBM extraction and serve as valuable guides for optimizing the selection of the fracturing fluid and improving its effective flowback.

Genesis of the Yi’nan Tongjing Gold–Copper Skarn Deposit, Luxi District, North China Craton: Evidence from Fluid Inclusions and H–O Isotopes

The Luxi district presents an exceptional research area for the investigation of the significant role played by magma exsolution fluids in the mineralization process of Au–Cu deposits. A particularly noteworthy occurrence within this region is the Yi’nan Tongjing Au–Cu skarn deposit, situated in the central-southern part of the Luxi district. This deposit primarily occurs in the contact zone between the early Cretaceous Tongjing complex and the Proterozoic to Cambrian sequences. The ore formation process observed in this deposit can be categorized into three distinct stages: (I) thermal metamorphism, (II) prograde alteration, and (III) retrograde alteration. The retrograde alteration stage is further divided into four sub-stages: late skarn (III-1), oxide (III-2), sulfide (III-3), and late quartz-calcite (III-4). It is primarily during the III-3 sub-stage that gold mineralization occurs. Petrographic analysis has identified three types of fluid inclusions (FIs) within garnet, quartz, and calcite grains. These include liquid-rich two-phase aqueous FIs, vapor-rich two-phase aqueous FIs, and halite-bearing multi-phase FIs. The homogenization temperatures of fluid inclusions from stages II, III-3, and III-4 range between 430–457 °C, 341–406 °C, and 166–215 °C (first to third quartiles), respectively. The garnet samples from stage II exhibit hydrogen and oxygen isotope compositions (δ18OH2O = 6.8‰ and δD = −73‰) that are indicative of a typical magma source. However, the hydrogen and oxygen isotopes of sub-stages III-1, III-2, and III-3 (δ18OH2O = 7.32‰ to 9.74‰; δD = −107‰ to −81.9‰) fall below the magma water box while the hydrogen and oxygen isotope values of III-4 (δ18OH2O = −5.3‰ to −0.9‰ and δD = −103.8‰ to −67‰) tend to move towards the meteoric water line. Furthermore, the ore-forming fluid displays characteristics of a mixture between the crustal and mantle fluids. The Tongjing complex occurred along a weakened fault zone, initiating a process of thermal metamorphism upon contact with the wall rock. This thermal metamorphism resulted in the formation of diverse assemblages, including hornfels, reaction skarns, and skarnoids. Subsequently, the upward movement of ore-forming fluids triggered exsolution which led to the establishment of a high-temperature, medium-salinity NaCl–H2O system with a single phase at depths ranging from 1–3 km. This marked the formation of the prograde alteration stage. Afterward, the ore-forming fluid underwent water–rock interactions and the admixture of meteoric water at a depth of 1–2 km. These processes facilitated phase separation, commonly referred to as boiling, resulting in the transformation of the ore-forming fluid into higher salinity fluids and lower-density gases. This evolutionary transition ultimately induced the precipitation and liberation of gold and copper from the fluid.

Timing and origin of skarn-, greisen-, and vein-hosted tin mineralization at Geyer, Erzgebirge (Germany)

AbstractThis contribution presents new insights into the origin and age relationships of the Geyer tin deposit in the Erzgebirge, Germany. Tin mineralization occurs in skarns, greisen, and in cassiterite-bearing fluorite-quartz veins. Skarn alteration replaces marble layers of the Cambrian Jáchymov Group and occurs in two clearly distinct stages. The first skarn stage forms skarnoid textured assemblages of clinopyroxene, garnet, and wollastonite with no tin phases recognized. Garnet U-Pb ages of this skarn stage (~322 Ma) relate the earlier skarn stage to the emplacement of the Ehrenfriedersdorf granite (~324 to 317 Ma). The second stage of skarn alteration is marked by the occurrence of malayaite and cassiterite associated with garnet recording ages of 307 to 301 Ma. Greisen- and skarn-hosted cassiterite-bearing veins provide U-Pb ages in the range of 308 to 305 Ma, relating greisenization and vein formation to the same magmatic-hydrothermal event as the second skarn stage. This suggests that tin mineralization at Geyer is related to a distinctly younger magmatic-hydrothermal event, clearly postdating the Ehrenfriedersdorf granite, which was previously assumed as the source of the tin-rich fluids. Fluid inclusions show salinities in the range of 1.0 to 31.5 % eq. w(NaCl±CaCl2) and homogenization temperatures between 255 and 340 °C. Cassiterite-associated fluid inclusions show indications for heterogeneous entrapment and dilution of hydrothermal with meteoric fluids. Dilution of high-salinity fluids with low-salinity fluids and cooling of the system was probably a decisive process in the precipitation of cassiterite in the Geyer Sn system.

The origin of fluorite deposits in the Bou-Izourane district (Central High Atlas of Morocco) and its relationships with the Tamazeght magmatic complex

Fluorite deposits in the Bou-Izourane district of the High Atlas in Morocco are distributed over seven known mineralized sites near the Middle Eocene Tamazeght magmatic complex and hosted in sedimentary rocks (limestones and marls) of the Lower Jurassic (Bou-Izourane, Takkat, Bou-Kharouba and Tabja deposits), magmatic rocks (Bou-Imetssene and Meghsan deposits) and skarns (Tamazeght and Bou-Imetssene deposits). This paper examines the origin of the mineralizing fluids and the relationship between the Tamazegth complex and fluorite deposits. Fluorspar occurs in veins oriented according to three families (N20, N50-80 and N120), in karst cavities and as stratabound deposits. Three stages are identified in the mineralization: (1) fluorite emplacement associated with the dissolution of the host rocks; (2) fracturing and sealing by calcite and finally (3) silicification associated with the partial dissolution of fluorite and calcite. Fluid inclusions in fluorite hosted in marls and carbonates of the Lower Jurassic have low salinities (1.5 to 3 wt% equiv. NaCl) and homogenization temperatures between 91 °C and 159 °C. Fluid inclusions in the Tabja, Tamazeght and Meghsan fluorites exhibit the same salinity range, but with increased homogenization temperatures, ranging from 99 °C to 241 °C, with a vapour phase containing CO2. Secondary solid daughter phases, such as nahcolite, calcite and strontianite are identified. Only Bou-Imtessene fluorite has high salinity fluid inclusions (∼18 wt% equiv. NaCl) and intermediate homogenization temperatures between 126 °C and 187 °C. These high local salinities and the presence of nahcolite and secondary carbonates in the fluid inclusions of the Tabja, Tamazeght and Meghsan fluorites may reflect the involvement of late-magmatic fluids. The δ18O isotopic composition of the fluid that precipitated calcites from Tabja (highest Th) and Bou-Izourane (lowest Th) indicates a meteoric origin. Fluorite has a broadly similar REE distribution to that of the Tamazeght carbonatites, suggesting that REE are derived from: i) carbonatites by hydrothermal alteration (e.g., Bou-Izourane, Bou-Kharouba and Takkat), and ii) carbonatitic (late-magmatic) fluids (e.g., Tabja, Tamazeght, Meghsan and Bou-Imtessene). Our data shows that fluorite mineralization in the Bou-Izourane district and the spatially associated middle Eocene Tamazeght magmatism have a direct genetic relationship through the contribution of late-magmatic F-rich fluids in close proximity to the complex. Fluorite mineralization farther from the complex is formed by per descensum circulation of meteoric waters along fractures resulting in fluorine leaching. The Tamazeght complex acts as a heat source and driving mechanism for fluid flows. The dissolution and recrystallization reactions are made possible by a strong geothermal gradient persisting after the complex was emplaced. The upwelling of these hydrothermal fluids into the calcium-rich host rocks (limestone and marl) leads to the precipitation of fluorite.

The relation of regional earthquakes to an intraplate volcano in the northern Hainan Island, China from magnetotelluric imaging

Volcanic activity can cause regional earthquakes before or during an eruption, while violent earthquakes are capable of changing static and dynamic stress-fields that may consequently accelerate or trigger volcanic eruption. It is thus important to consider the volcano and regional earthquake as an integrated system, which has implications for disaster prevention and may enable early warning. In this work, two magnetotelluric (MT) profiles were deployed across the active volcanic and seismic region in the northern Hainan Island, China. Data were inverted to generate a three-dimensional resistivity structure beneath the two profiles. The resistivity model shows a shallow (< 5 km) low-resistivity (< 10 Ωm) zone corresponding to Paleogene and Neogene sediments, and two conductors (10–30 Ωm) at depths of 5–12 km. The conductors in the middle-upper crust are attributed to the accumulation of high-salinity fluids supplied by deeper magmas. The north-south trending small earthquakes (< M3) are mainly distributed in high-resistivity basement, independent of the volcanic plumbing system, which are attributed to small-scale rupture in higher tensile rocks. The volcanic plumbing system appears to extend below the Qiongzhou Earthquake (M7.5 in 1605), indicating the possibility of distal volcano-tectonic earthquake.

Big data mining on trace element geochemistry of sphalerite

The determination of ore genesis is a main challenge in ore deposit research. Advanced and rapid analytical techniques have given rise to the accumulation of massive amounts of geoscientific data. Therefore, it is imperative to conduct data mining on ore geochemical data to efficiently extract useful metallogenic information. In this contribution, 4095 sets of sphalerite trace element data from 86 deposits of different origins (VMS, MVT, porphyry, epithermal, SEDEX, and skarn) were compiled and analyzed. Factor analysis revealed the effects of physicochemical conditions on sphalerite trace element compositions. Specifically, the high Mn-Fe-Cu-Co-In but low As-Ga-Ge-Sb-Pb concentration of sphalerite is commonly related to decreasing pH or increasing temperature; high sulfur fugacity favors the entry of Fe and Co (but not Ga or Sn) into the sphalerite lattice; and high salinity causes enrichment in Cd-Ga-Ge-Mn, but depletion in Ag-Cu-In-Sb. Multivariate statistical analysis reveals that the Mn-Ge-Sn contents in sphalerite have good potential to differentiate among ore genetic types, the Mn-Ge-In are used to determine between magmatic-hydrothermal and non-magmatic-hydrothermal deposits. Furthermore, machine learning models demonstrate high accuracy of sphalerite trace element data in distinguishing ore types, i.e., 93.02 % (random forest) and 92.82 % (gradient boosting), and its reliability was validated by receiver operating characteristics. Additionally, the blind tests by machine learning on sphalerite trace elements indicate that the Qingshuitang deposit (in the Qin-Hang metallogenic belt, South China), have been an MVT deposit, which is also supported by its low-temperature and high-salinity fluids, wallrock alterations (esp. bariteization), and the obvious age distinction between mineralization and local magmatism. This study highlights that machine learning and multivariate statistical analysis on sphalerite trace-element data can differentiate metallogenic origin and conditions.

Geology, geochemistry, fluid inclusion data, stable isotope characteristics, and ore genesis of the Barout Aghaji gold deposit, NW Zanjan, Iran

The Barout Aghaji gold deposit is located ∼90 km northwest of Zanjan, within the Takab-Takht-e-Soleyman subzone of the Sanandaj-Sirjan metamorphosed-deformed zone. Ore-bearing quartz veins are hosted by Neoproterozoic amphibolite and Eocene to Oligocene granitic gneisses. Oligo-Miocene Upper Red Formation unconformably overlies the amphibolite and granitic gneisses. Field observations and petrographic studies show that two deformation stages occurred in this area. The first deformation stage was ductile, producing mylonitic and proto-mylonitic microstructures, but the second one was brittle, represented by sheeted quartz veins and veinlets. In the first stage, barren milky quartz veins occurred containing minor sulfide minerals, but dark to light gray ore-bearing quartz veins and veinlets are formed in the latter stage. The mineralized veins appear as massive microcrystalline quartz cut by sheeted quartz veins with comb, druse, and crustiform textures. The gold-bearing quartz veins contain as much as 3% sulfide minerals. Pyrite is the main sulfide mineral and is associated with minor chalcopyrite. Sulfides are commonly altered to hematite, goethite, and rarely malachite. Hydrothermal alteration around the quartz veins consists of silicification, pyritization, and sericitization. The whole-rock geochemistry of the collected samples from the granitic gneisses and quartz veins shows that Au is enriched in the quartz veins (average of 114 ppb) relative to host rocks (average of 22.5 ppb). Au shows strong positive correlations with As, Ba, Mo, Pb, Sc, Tl, Ag, and negative correlations with Cu, Bi, Se, and Te in the granitic gneisses. It also shows strong positive correlations with S, Hg, Th, Co, Bi, Pb, and Ag and negative correlations with P, V, Te, W, Sc, Zn in quartz veins. Four types of primary fluid inclusions were identified, including type I, two-phase aqueous-rich fluid inclusions (liquid > vapor; LV); type II, two-phase vapor-rich fluid inclusions (gas > liquid; VL); type III, three-phase fluid inclusions containing CO2 with clathrate formation (L1L2V); and type IV three-phase fluid inclusions (aqueous, vapor, and solid; LVS). The homogenization temperatures of fluid inclusions in auriferous quartz veins range from 199 −446 with a mode of 270–300 °C. Salinities range from 0.8 to 49.02 wt% NaCl Equiv. with two distinct populations at 0.8–8.5 and 31.1–49.02 wt% NaCl Equiv. The large variations in the homogenization temperatures and salinities can be attributed to the cooling and isothermal mixing of fluids. The δ34S values for four pyrites separated from auriferous quartz veins range from +2.9 to +7.1‰, with an average of 4.5‰. δ34S values of fluids in equilibrium with pyrite were calculated from +3.5 to +7.3‰, with an average of 5.4‰, indicating a metamorphic source for the sulfur using temperatures estimated from the fluid inclusion study. The Field observations, vein textures, mineralogy, ore geochemistry, fluid inclusion studies, and sulfur isotope data indicate that gold mineralization in the Barout Aghaji area has many similarities to orogenic and intrusion-related gold deposits, such that low salinity fluids derived from metamorphic rocks are mixed with high salinity fluid inclusions possibly derived from granitic gneisses during syn to post tectonic magmatism.

Comparative Experiments on the Role of CO2 in the Gold Distribution between Pyrite and a High-Salinity Fluid

Experimental studies were conducted to identify the physical and chemical features of gold’s behaviour in hydrothermal processes linked to ore formation and involving CO2 in oxidized deposits. With the aid of the autoclave method, in a temperature range of between 200 and 400 °C, the isochoric dependences of the PVT parameters of concentrated sulphate chloride fluids were plotted, both in the presence and absence of CO2. Our experiments established that concentrated sulphate–chloride fluids (22 wt % Na2SO4 + 2.2 wt % NaCl) that lack CO2 are characterized by a wide supercritical temperature range, with homogenization temperatures of between 250 and 325 °C. In the presence of CO2, the same type of fluids showed heterogenization at a molar fraction of XCO2 = 0.18 (t = 192 °C, P = 176 bar). The process of homogenization for these low-density and high-salinity fluids was impossible at temperatures between 375 and 400 °C and at pressures between 600 and 700 bar. The behaviour of gold was studied during its interaction with a basic composition fluid of sulphate–chloride. We applied the autoclave method under the conditions of a simultaneous synthesis of pyrite and gold dissolution (metallic Au), at a temperature of 340 °C and at a pressure of 440 bar. High Au concentrations (up to 4410 ppm of Au in CO2-bearing fluids) were attained at high gold solubilities (up to 13.5 ppm in the presence of CO2), owing to the process of Au reprecipitation within the pyrite phase. We did not detect Au in the pyrite when we used the XRD or SEM methods, which suggested that it might be present as invisible gold. High values of the distribution coefficient (KD = CAu(solid)/CAu(solution)) in the fluids lacking (KD = 62) and bearing CO2 (KD = 327) empirically confirmed the possibility that gold concentrates in pyrite in structurally non-binding forms.

A fluid inclusion and stable-isotope study of hydrothermal vein mineralization, Schwarzwald district, Germany

A combined fluid inclusion and stable isotope study has been carried out on over 180 individual samples from 89 post-Variscan hydrothermal veins (Pb-Zn-Cu-bearing fluorite-barite-quartz veins, Co-Ni-Ag-Bi-U-bearing barite-fluorite-quartz veins and barren barite-fluorite-quartz veins) from the Schwarzwald district, Germany. The salinities of fluid inclusions in post-Variscan primary fluorite, calcite, barite and quartz are in the range of 22–25 wt.% equivalent (eqv.) NaCl, and the eutectic temperatures range between –57 and –45°C, indicating the presence of H2O-NaCl-CaCl2 fluids. Homogenization temperatures vary from 130 to 180°C. A low-salinity fluid (0 to 15 wt.% eqv. NaCl) was observed in some late stage fluorite, calcite and quartz samples, which were trapped similar temperature, range of high salinity fluids.&#x0D; Raman microprobe analyses show that the only detectable volatile in the vapour is CO2. Almost all δ18O (n=86) measurements of quartz from the fluorite-bearing post-Variscan veins range between +11.1 and +20.9 ‰. The calculated δ18OH2O values are between –11.0 and +4.4 ‰, using known quartz-water fractionation and fluid inclusion homogenization temperatures. The δ18OH2O values of directly extracted fluid inclusion water of fluorites range from –11.6 to +1.1 ‰, very consistent with the calculated values. The δD values of fluid inclusion water in calcites (extracted from primary and late calcite samples) lie in a narrower range between –26 and –15 ‰. The extracted fluid inclusion water from quartz samples has significantly more variable δD values between –63 and +9 ‰. The δ13C and δ18O values of fluid inclusion gas (CO2) range between –21.4 and –6.7 ‰ and between –16.3 to –7.1 ‰, respectively.&#x0D; Calculations for fluorite-barite-quartz veins combining oxygen isotope equilibria with microthermometric data result in quartz precipitation temperatures of 120–170°C at pressures between 0.3 to 0.5 kbar. The δ18OH2O and δD data, particularly the observed wide range in hydrogen isotopic compositions, indicate that the hydrothermal mineralization formed through large-scale mixing of a basement-derived saline NaCl-CaCl2 brine with meteoric water.

Geochemistry of magnetite from the Mamupu Cu polymetallic deposit, Yulong belt, Tibet: Implications for magnetite genesis, stages and mechanism of formation

The Mamupu copper polymetallic deposit is characterized by multistage magnetite, but the genesis and fluid evolution process of this deposit are still obscure. Three types of magnetite were identified in Mamupu based on comprehensive textural observations and paragenesis. Mag1 occurs as subhedral to euhedral crystals, and smooth Mag1a and porous Mag1b can be distinguished under Backscattered Electron (BSE). Mag2 is characterized by 120° triple junction textures and has been replaced by hematite. Mag3 is commonly replaced by hydrothermal fluids and exhibits well-developed oscillatory zoning and core (Mag3a)-rim (Mag3b) textures. The geochemistry of Mamupu magnetite suggests that they are of hydrothermal origin. Mag1 has undergone extensive fluid-rock interactions, whereas the sharp contacts with Mag1a and Mag1b and the microporosity of Mag1b imply that the Mag1b was generated by the coupled dissolution reprecipitation of the Mag1a grains. This process may be triggered by low-temperature and high-salinity fluid produced by intensive water-rock interaction between ore-bearing magmatic-hydrothermal fluid and carbonate surrounding rock of the Jiaoga Formation. The 120° triple junction textures of Mag2 indicate that it was formed during a fluid-assisted recrystallization process, and Mag2 replaced by hematite represents the increase in oxygen fugacity. From Mag3a to Mag3b, the contents of V and Mn increased while the contents of chalcophile elements decreased, suggesting a decrease of oxygen fugacity and the precipitation of sulfide, which represents the beginning of the sulfide stage. Moreover, petrography and geochemistry data of magnetite represented that the breccia ore of Mamupu belongs to skarn mineralization. The geochemical characteristics of magnetite show that Cu-Au and W-Mo in Mamupu precipitated in different stages, representing the transformation from a relatively oxidized environment in the early stage to a relatively reduced environment in the late stage. These results reveal that trace elements in magnetite can be utilized to define the genesis and evolution of the skarn deposit. In addition, it can also restrict the precipitation mechanism of ore-forming elements in the deposits. However, detailed textural observations should be carried out before experimental research.

Diagenesis of the Paleogene Sandstones in the DN2 Gas Field, Kuqa Foreland Basin and Its Link with Tectonics

AbstractWe investigated the diagenesis of sandstones from DN2 Gas Field of Kuqa Foreland Basin (KFB) to infer timing of fluid migration and discuss link between fluids and tectonics. Textures and chemical composition of authigenic minerals, fluid evidence from fluid inclusions and formation water measurements were used to fulfill the aim. Eodiagenesis occurred with the participation of meteoric waters and connate waters. Mesodiagenesis are related to high salinity fluids, which were attributed to originate from overlying Neogene Jidike Formation evaporite (main minerals including halite, anhydrite, glauberite, carnallite and thenardite). The onset of high salinity fluid migration was inferred to occur during the late Miocene (12.4–9.2 Ma) by using integration of Th measured in the present study and K‐Ar dating of authigenetic illites from previous study. The period is consistent with the crucial phase (13–10 Ma) witnessing the rapid uplift of southern Tianshan Mts and the stage when calcite and anhydrite veins formed in the studied strata. We thus argue that diagenesis related to high salinity fluids occurred as a response to Tianshan Mts's rapid uplift and related tectonic processes. The flow of high salinity fluids was probably driven by density gradient and channeled and focused by fractures formed contemporaneously.

Magnetotelluric image of a hyper-extended and serpentinized rift system

Magnetotelluric (MT) data can image the Earth's electrical resistivity down to the mantle but are rarely used for investigation of offshore rifted margins. In such settings, the lower crust and upper mantle are altered by distinct tectono-thermal processes but often display similar seismic velocities and densities. By integrating resistivity models from MT data, we aim at resolving such ambiguity. Here, 3D inversion of long period, marine MT data (1 – 3000 s) is performed on 104 receivers located along two, ∼300 km long transects in the SW Barents Sea. The resolving power of MT data is assessed with synthetic tests in an archetypal rift system where ample crustal thickness variation occurs. The results highlight that our MT data sense the transition from necking to hyper-extended domain where the crust (<10 km) is not recovered by 3D inversion. In the Bjørnøya Basin – the northernmost member of a hyper-extended Cretaceous basin chain in the NE Atlantic – we combine seismic interpretation and MT inversion models to assign resistivity properties at two depth intervals: (1) 0.1-1 Ωm within Lower Cretaceous marine shales buried at 10-15 km depth (2) 1-10 Ωm within the uppermost mantle. Based on a fluid-rock model, we emphasize that seawater as a sole pore fluid phase is not conductive enough to explain such high bulk conductivities at both intervals. A 25% serpentinization of mantle rocks can account for a fivefold rise in salinity of the residual fluid and is compatible with bulk resistivity, density, and seismic velocities in the Bjørnøya Basin. Such high-salinity fluid can ascend and mix with seawater in pore spaces of the sediments, supporting our model of saline fluid circulation in hyper-extended basins. In conclusion, electrical resistivity models can disambiguate interpretation of deep structures in rifted margin by detecting saline fluids from partial serpentinization, intermixing with seawater in overlying marine sediments.

Constraints on Diagenetic Fluid Source and Genesis in Tight Dolostone Reservoir of Submember Ma55 of Ordovician Majiagou Formation in Northwestern Ordos Basin, China: Evidence from Petrology and Geochemistry

Diagenetic fluids is one of the most important reservoir modifiers, their differences can result in various petrography, storage capacity, and geochemical characteristics, so clarifying the diagenetic fluids is vital for understanding dolostone origin. Dolomitization is an important genetic type of dolostone reservoir, the fluid properties differ in different dolomitization, which may lead to reservoir storage capacity changes. Therefore, the identification of dolomitization fluid properties is critical for deeply understanding of dolomitization process and predicting storage capacity. Two types of dolostone are developed in submember Ma55 of Majiagou Formation in northwestern Ordos Basin, with obviously different petrological characteristics and reservoir properties. On the basis of petrological studies such as core and casting thin section observation and cathodoluminescence analysis, the diagenetic fluid properties of these two dolostone are characterized by geochemical analytical methods such as major and trace element tests. The results show that Type-1 dolostone is mainly composed of micritic dolomite, showing micritic structure and algae-rich lamina structure, and accompanied by evaporite minerals and moldic pores. This type of dolostone has a various Mn content, weak to medium cathodoluminescence intensity, high contents of TiO2, Al2O3, K2O+Na2O, Li, and U, and lower content of TFe2O3. The type-2 dolostone is composed of fine-grained dolomite with obvious residual texture of primary limestone, clear brim, and cloudy center structure, accompanied by the existence of intergranular pores. Most of this kind of dolostone have medium-strong cathodoluminescence, higher TFe2O3 content and lower TiO2, Al2O3, K2O+Na2O, Li, and U contents. Moreover, both the two types of dolostone have similar Fe/Ca and Mn/Ca ratios, and a low and concentrated CaO content, whose composition is similar to that of stoichiometric dolomite. The comprehensive analysis shows that the diagenetic fluid of Type-1 dolostone is mainly a high salinity fluid existed in plaster and calcareous sediments in a near surface environment with low temperature. The diagenetic fluid of Type-2 dolostone may be a high salinity brine formed by evaporation and concentration of seawater with normal salinity. The research results will provide a significant theoretical basis for the evaluation of dolostone reservoir quality and the prediction of favorable areas of reservoir distribution.

Spontaneous imbibition characteristics of shale oil reservoir under the influence of osmosis

The spontaneous imbibition (SI) process in shale oil reservoirs is not only influenced by capillary force, but also by the osmotic pressure between the fracturing fluid and formation water in the nanopores media. In this study, experimental methods are used to investigate the mechanisms of osmosis in the SI, taking into account the presence of initial formation water in shale oil reservoirs. To investigate the effect of osmosis, SI experiments were performed on the fine-grained felsic shale of the Qikou sag of Dagang oilfield. Low-field NMR testers and high-precision electronic balances are utilized for the measuring of oil–water migration. The results show that, when Sw ≠ 0, high-salinity fluid SI can be divided into four stages: initial imbibition stage, drainage stage, secondary imbibition stage and stationary stage; when Sw = 0, there is no drainage stage of high-salinity fluid SI; when Sw ≠ 0 or Sw = 0, low-salinity fluid SI can be called the “osmosis-enhanced SI”; and we have found that “newly formed pores or microfractures” as well as reducing salinity can promote SI. This article presents a systematic study of SI of shale oil reservoirs under the influence of osmosis, which provide useful information for reservoir numerical simulation and development program design.

Fluid–Rock Interactions in Geothermal Reservoirs, Germany: Thermal Autoclave Experiments Using Sandstones and Natural Hydrothermal Brines

As renewable energy, geothermal can contribute substantially to the energy transition. To generate electricity or to harvest heat, high-saline fluids are tapped by wells of a few kilometres and extracted from hydrothermal reservoirs. After the heat exchanger units have been passed by, these fluids are reinjected into the reservoir. Due to the pressure and temperature differences between the subsurface and the surface, as well as the cooling of the fluids in the power plant, unwanted chemical reactions can occur within the reservoir, in the borehole, and within the power plant itself. This can reduce the permeability of the reservoir as well as the output of the geothermal power plant. This study aims to simulate real subsurface reactions using batch and leaching experiments with sandstone or sandstone powder as solid phase, and deionised water or natural brine as liquid phase. It is demonstrated that fluid composition changes after only a few days. In particular, calcite, aragonite, clay minerals, and zinc phases precipitate from the natural brine. In contrast, in particular minerals containing potassium, arsenic, barium, and silica are dissolved. Due to the experimental set-up, these mineral reactions mainly took place on the surface of the samples, which is why no substantial changes in petrophysical properties could be observed. However, it is assumed that the observed reactions on the reservoir scale have a relevant influence on parameters such as permeability.

Geochemical Characteristics and Fluid Properties of the Qixia Formation Dolomites of the Middle Permian in the Shuangyushi Block, NW Sichuan Basin, China

On the basis of petrology, in combination with analysis of the geochemical characteristics of major and trace elements, REE (rare earth elements), carbon isotopes, oxygen isotopes, strontium isotopes and fluid inclusions, the geochemical characteristics and fluid properties of the Qixia Formation dolomites of the Middle Permian Qixiain the Shuangyushi block in the NW Sichuan Basin were systematically studied. The analysis results of the samples show that the Qixia dolomites are mainly composed of medium, coarse, medium-coarse crystalline and dolomites with polymodal crystal size distribution, and cloudy centers and clear rims. They generally show dark purple cathodoluminescence (CL) colors in the centers and orange red CL colors in the rims. The Qixia dolomites are relatively highly ordered (0.79–0.95, averaging 0.90). The CaO and MgO contents show a negative correlation. They are characterized by higher Mg/Ca ratio (0.89–0.92, averaging 0.91), high Fe (94–2,991 ppm, averaging 622 ppm), high Mn (26–185 ppm, averaging 102 ppm), high Na (210–374 ppm, averaging 277 ppm) contents, low Sr (53–218 ppm, averaging 93 ppm) content and lower ∑REE (1.10–5.56 ppm, averaging 2.08 ppm) concentrations. The REE patterns of the dolomites are similar to those of the calcites (C1), and are characterized by light REE (LREE) enrichment, heavy REE (HREE) depletion (LREE/HREE = 5.38–9.58, averaging 7.10), and Eu (0.65–4.19, averaging 1.70) and Ce (0.99–9.52, averaging 2.61) positive anomalies. They have consistent δ13C (1.98–4.27‰ VPDB, averaging 3.01‰ VPDB) and δ18O (−9.54‰–−4.65‰ VPDB, averaging −7.09‰ VPDB) values with the coeval seawater. The salinity index Z is larger than 120, with diagenetic temperature higher than normal temperature. The average value of 87Sr/86Sr is positive (0.707150–0.712273, averaging 0.708669), while some samples are consistent to those of the coeval seawater. Their homogenization temperatures of fluid inclusions (96.3°C–105.0°C, averaging 101.1°C) exceed 100°C, which is equivalent to the buried paleogeothermal temperature, and their salinity (12.3%–14.3%, averaging 13.5%) is 3–5 times that of normal seawater. Based on the regional geological setting and previous research achievements, the dolomitized fluids in the Qixia Formation are probably derived from seawater or marine fluids sealed in pores under shallow-burial environment. The dolomites experienced late alternation by deep high-salinity hydrothermal fluids, and were formed before the Middle-Late Triassic Period.

The mechanism of phreatomagmatic maar-diatreme eruption in the Lei-Hu-Ling volcanic area of northern Hainan Island, China: Insights from magnetotelluric and magnetic data

A maar-diatreme forms in response to the phreatomagmatic eruption caused by violent interaction between magma and externally derived water. Experimental work on phreatomagmatic eruptions has yielded considerable knowledge on maar-diatremes. However, the mechanism and process at natural scale remain poorly understood, especially when surface deposits are present. This paper presents a magnetotelluric and magnetic study of the Late-Pleistocene maar-diatreme volcanoes in the Lei-Hu-Ling area of northern Hainan Island, China. The regional 3D electrical structures show shallow high resistivities below craters and a low resistivity layer overlying the deep high-resistivity basement. The local magnetic structure of maar Luo-Jing-Pan reveals a cylinder-like diatreme with high susceptibility. The shallow low-resistivity zone, which coincides with the Fushan depression controlled by the Changliu-Xiangou Fault, is associated with the accumulation of high-salinity fluids with high porosity. The considerable volume of water stored in the Fushan depression and the sedimentary basement ridges beneath maars could have enabled a high level of magma-water interaction. This indicates a fault-controlled type for the phreatomagmatic eruption in the Lei-Hu-Ling volcanic area. The explosion depth is at least 600 m for Ru-Huang-Ling and Luo-Jing-Pan maar-diatreme volcanoes, indicating deep phreatomagmatic eruptions with less contribution to surface deposits. In addition, the explosion energy of the Ru-Huang-Ling maar with a diameter of 1900 m is about 7–8 times that of Luo-Jing-Pan maar with a diameter of 900 m.