Fluid Inclusions Research Articles

Magmatic−hydrothermal evolutionary processes in highly evolved granitic systems: Insights from zircons of the Baishitouquan pluton, NW China

As a robust accessory mineral in igneous rocks, the mineralogical and geochemical characteristics of zircon can record the lithological differentiation and magmatic−hydrothermal evolution of highly evolved granitic systems. The F-Rb−rich, highly evolved Baishitouquan pluton of NW China exhibits gradual lithological changes from leucogranite, amazonite-bearing granite, and amazonite granite in the lower levels of the pluton to topaz-bearing amazonite granite, topaz albite granite, and pegmatite in the upper levels. In this study, three types of zircon grains were identified in five lithological zones based on textural and chemical characteristics. Type I zircon, which mostly occurs in leucogranite and amazonite-bearing granite, exhibits oscillatory zoning in cathodoluminescence images and experienced low degrees of radiation damage (0.21−0.68 × 1015 α-decay events/mg), which is indicative of its magmatic origin. Type II zircon, which mostly occurs in amazonite granite and amazonite pegmatite, exhibits textures that are indicative of hydrothermal alteration (e.g., spongy texture, porosity, and microcracks), and has elevated concentrations of some cations, such as Ca and Al. Type II zircon contains a higher concentration of non-formula elements, including rare earth elements (REEs), and Hf, Th, and U, than Type I and III zircons. Additionally, Type II zircon exhibits a significant M-type lanthanide tetrad effect and experienced varying levels of radiation damage (3.75−11.72 × 1015 α-decay events/mg). These characteristics suggest that Type II zircon has a hydrothermally altered origin. Type III zircon, which is restricted to the topaz-albite granite, has the smallest crystal size among all types of zircon grains, shows a euhedral to anhedral mottled appearance, and is characterized by patchy, cloudy, or irregular zoning, with numerous fluid inclusions. This type of zircon contains higher concentrations of Ti (110−1030 μg/g) than other types of zircon grains. Additionally, this type of zircon experienced limited radiation damage (2.18−3.69 × 1015 α-decay events/mg), and has a smooth surface and homogeneous internal textures. These characteristics suggest that Type III zircon is the product of fluid interaction with hydrothermally altered Type II zircon. Accordingly, this type of zircon crystallized directly from a Zr-saturated hydrothermal fluid during the later stages of magmatic−hydrothermal evolution. These contrasting textural and compositional features of the three types of zircon grains are indicative of three stages of magmatic−hydrothermal evolution of the Baishitouquan pluton: magmatic, magmatic−hydrothermal transition, and hydrothermal. These magmatic and hydrothermal processes were involved in the enrichment, transport, and precipitation of rare metals, such as Rb. Accordingly, this contribution demonstrates that the textures and chemistry of zircon grains can serve as petrogenetic indicators for assessing magmatic−hydrothermal evolution and rare-metal mineralization in highly evolved granitic systems. Furthermore, this study presents a model of the magmatic−hydrothermal evolution of F-rich, highly evolved granitic systems through the lens of zircon.

Hydrothermal alteration of the surface volcanic rocks at the Acoculco geothermal field, Mexico: a multi-parametric approach

AbstractThe studies on hydrothermal alteration-induced effects in surface and subsurface rocks provide useful information in the characterization and exploitation of a geothermal reservoir. Generally, these studies are based on traditional, and reliable methods like petrography (primary and secondary minerals, and grade of alteration), and geochemistry (mobility of elements, changes in mass and concentration of elements, and fluid inclusions). Recently, apart from these established methods, some methods based on the geochemical (Chemical Index of Alteration, CIA; Weathering Index of Parkar, WIP; Loss on Ignition, LOI; and Sulfur, S) and rock magnetic properties (magnetic susceptibility, χlf; and percentage frequency-dependent susceptibility, χfd%) are also being applied in the identification of whether a rock is an altered or a fresh one. The Acoculco Geothermal Field (AGF), Mexico, is characterized by high temperature and very low permeability, and it is considered a promissory Enhanced Geothermal System. The following changes are observed in the rocks as a result of an increase in hydrothermal alteration: (1) an increase in CIA, LOI, and S values, and a decrease in WIP; (2) an increase in quartz and quartz polymorph minerals (silicification), and clay minerals (argillization); and (3) decrease in χlf values. At AGF, the most altered surface acid rocks are characterized by entirely quartz and its polymorphs, and clay minerals. The present study also indicates the applicability of the binary plots of major elements (felsic vs mafic component) and rock magnetic parameters (χlf vs. χfd%). The rock with χfd% value of 2–10 and χlf value < 0.5 × 10–6 m3 kg−1 indicate the presence of single domain and stable single domain grains, which in turn suggests that it is an altered rock. These methods are simple to apply, rapid, reliable, and have the potential to become effective tools for the identification of hydrothermally altered rocks during the initial stage of geothermal exploration.

Geology, fluid inclusions and C−O−S−Pb isotopic compositions of the Chahmileh Pb-Zn deposit, Central Iran: Implications for ore genesis

Abstract The Chahmileh Pb–Zn deposit, located northwest of the Central Iran Zone, is a sediment-hosted Pb–Zn deposit in the ‘Yazd-Anarak Metallogenic Belt’. It is hosted in Middle Triassic carbonate rocks and is mainly controlled by NW-trending faults. The main ore minerals are galena and sphalerite with minor chalcopyrite, pyrite, and quartz, dolomite, along with minor calcite and baryte as gangue minerals. Cerussite, hemimorphite, wulfenite, mimetite, smithsonite, malachite and iron oxy-hydroxides are the main non-sulphide ore minerals. The main styles of mineralization are vein-veinlet, breccia, disseminated and replacement in association with silicification and dolomitization. Microthermometry of fluid inclusions in dolomite and quartz indicates that the ore precipitated from a warm to hot basin-derived saline fluid. Dolomite samples have δ13CVPDB and δ18OVSMOW values of −0.99 to +1.99‰ and +20.74 to +25.48‰, respectively, and are plotted in the marine carbonate rocks field. These isotopic values suggest that CO2 in the hydrothermal fluids mainly originated from marine carbonate rock. The δ34S values range from +6.3 to +8.2‰ for galena, +5.9 to +6.2‰ for sphalerite, +1.4 to +3.4‰ for chalcopyrite and +15.0 to +17.4‰ for bayite are compatible with a predominant thermochemical sulphate reduction process, and with sulphur sourced from Triassic seawater. Galena samples have a homogeneous Pb isotopic composition that is indicative of a continental crustal reservoir as the main source of lead and probably for the other ore metals. Based on geology, mineralogy, texture and fluid characteristics, the Chahmileh deposit is classified as a carbonate-hosted Mississippi Valley-type deposit.

Inference of fracturing zones and degrees of fluid content in the Las Tres Virgenes volcanic complex based on an analysis of seismic anisotropy

AbstractAnisotropy strength in the Tres Vírgenes Volcanic Complex, Baja California Sur, Mexico, is analyzed employing 558 seismic events collected from 2009 to 2013. It was possible to delineate zones and volumes with the highest fracture densities, which are mainly located between the El Viejo and El Azufre volcanoes and around the La Reforma–El Azufre fault system, near some other mapped faults in the area (e.g., El Azufre, El Partido, El Volcán, El Viejo 1, and El Viejo 2 faults); likewise toward the La Virgen volcano and around the La Virgen-El Campamento and El Volcán faults. Individual delay times reached values of up to 0.16 s and an anisotropy percentage of up to 10.3%, with a pervasive anisotropy observed from at least a hypocentral distance of 3.5 km. High fracturing levels are observed from a depth of 7.0 km. Differences between splitting delays and the dominant frequency peaks obtained from the fast S phases allowed considering fracture systems with different degrees of fluid contents. Fractures with minor fluid contents were assumed for delay times higher than 0.03 s with lower dominant frequency peaks (< 1.0 Hz). Higher concentrations of fluid inclusions were assumed for splitting delays higher than 0.03 s but with larger dominant frequency peaks (> 1.0 Hz). Fractures systems chemically sealed or impermeable sealing caps were assumed for low splitting delays (< 0.02 s) with low dominant frequencies (< 1.0 Hz). These different fracture systems seem to be observed at least from 5- to 6-km depth intervals. Likewise, an analysis of the fast polarization directions with respect to different depth ranges (spanning from 3.0 to 8.0 km) has allowed observations of a strong NW–SE regional fracture system accompanied by minor NE–SW fracture systems. However, noteworthy variations from NW–SE to NE–SW, N–S, and E–W in fast polarization directions in rose diagrams have been preferentially observed for those seismic events deeper than 4–5 and 5–6 km in some areas, which could be indicating the location of magmatic bodies that probably caused the reorientation on fracture systems by changes in the local stress field. These magmatic bodies might be supported by a decrease in the dominant frequency peaks (lower than 1.0 Hz), percentage of anisotropy (from 0.1 to 2.5%), and S-wave velocities (from 1.0 to 2.7 km/s), which seem to be located from the 4.0-km depth but more concentrated from the 5–6-km depth interval.

Three-stage hydrocarbon accumulations in the Middle Permian in the Central Sichuan Basin

The Middle Permain Maokou Formation (P2m) is a new region of natural gas exploration in the Sichuan Basin, is characterized by bioclastic limestone with localized dolomitization, and karst fractured-vuggy reservoirs. Currently, on the gas source, hydrocarbon accumulation process and control factors in the Sichuan Basin during the Permian are lacking. To bridge this gap, herein, we identified the filling sequence minerals inside the pores/vugs, along with the oil charge of the Maokou Formation using drill cores, thin sections, oil inclusion analysis, and U-Pb dating of calcite cements. The results showed that the reservoir space of the Maokou Formation was predominated by the residual dissolved pores/vugs, fractures, and dissolved fractures. The pores/vugs underwent four stages of mineral filling by very fine-fine (-crystalline, CC1) calcite → fine-medium calcite (CC2: from 256.4 ± 1.7 to 244.1 ± 6.3 Ma) → fibrous calcite (FC; ∼183.9 ± 8.2 Ma) → coarse-macro calcite (CC3; ∼171.5 ± 5.3 Ma). Combined with the homogenization temperature and salty of fluid inclusion, we considered that three stages of oil charge were present in the Maokou Formation reservoirs. The first stage involved the formation of paleo-oil reservoirs during the Late Permian to Early Triassic, corresponding to the high-maturity aqueous inclusions in CC2, with a homogenization temperature of 106.7°C–137.8°C. At that time, the oil generation from the Lower Cambrian Qiongzhusi Formation rocks peaked, and the generated hydrocarbons migrated upward into the Maokou Formation through the strike-slip faults in the basin center. The second stage involved the formation of paleo-oil reservoirs during the Early Jurassic. The Permian source rocks reached the oil generation window with hydrocarbon expulsion, which was consistent with the oil inclusions in FC. The third stage involved the formation of paleo-gas reservoirs during the Middle Jurassic to Early Cretaceous, corresponding to the high-density methane inclusions and bitumen inclusions occurring in CC3, with the homogenization temperature peaking at 151.9°C–178°C. The natural gas in the Middle Permian of the Central Sichuan Basin is predominantly sourced from the Lower Cambrian Qiongzhusi Formation mudstone and partially from the source rocks of the Middle Permian, indicating a significant source-reservoir conduit of the strike-slip faults in the basin center. The findings provide considerable baseline data to advance further research in the Sichuan Basin.

Formation Conditions of Ignimbrites of the Khangar Volcano (Kamchatka)

Abstract —The study of minerals, melt inclusions, as well as natural glasses showed that two different melts contributed to the formation of ignimbrites of the Khangar Volcano. The first, providing the information on melt inclusions in plagioclase and quartz phenocrysts, represents the state of magma in a deep source. The other type of melt is responsible for the formation of glasses and microcrystals of feldspars in fiamme. Experimental and analytical studies of melt inclusions showed that crystallization of most plagioclase and quartz phenocrysts from ignimbrites of the Khangar Volcano occurred at temperatures of 840–960 °C and pressures up to 1.1 kbar, from the melt with water contents up to 3.23 wt.%, under the conditions of magma chamber. The presence of syngenetic primary melt and fluid inclusions in plagioclase and quartz phenocrysts from ignimbrites of the Khangar Volcano indicates phase separation (“boiling”) of the melt with mass formation of СО2 microbubbles in magma. The other type of melt is secondary relative to magmatic systems of the Khangar Volcano and is formed by sintering and melting of tuffogenic volcanoclastic material. This melt contributed to the formation of fiamme in the examined ignimbrites. Based on the study of glasses and microcrystals of feldspars in fiamme, it was found that crystallization of oligoclase occurred at temperatures of 770–840 °C in the melt between the spherules (with water content up to 2.91 wt.%). Sanidine crystals grew over spherules at lower temperatures, 680–760 °C.

Diagenetic evolution and origin of cements of the Cretaceous carbonates in the Akal Block, southeast Gulf of Mexico

The Cretaceous succession of deep marine facies (580 m-thick) of the Akal Block, southeast of the Gulf of Mexico, is dominated by lime mudstones that have been extensively affected by dolomitization. Petrographic examination, including cathodoluminescence (CL), and microthermometry reveal three phases of dolomitization that occurred as replacement and pore-/fracture-filling cements. These are (1) an early replacement dolomicrite (D1, <50 μm) with dull orange CL, (2) eu-to subhedral dolomite (D2, 60 μm–150 μm) with distinctive cloudy cores (dull CL) and clean edges (non-luminescent to dull CL), and intercrystalline porosity (6 %), and (3) anhedral vug-/fracture-filling dolomite (D3, 200 μm–500 μm) exhibiting CL zonation. The mean δ18O values of those dolomites show a trend of slight decrease from D1 to D3 (D1 = 0.0 ± 1.3‰ VPDB, D2 = −0.2 ± 1.1‰ VPDB, D3 = −1.2 ± 1.6‰ VPDB), which reflects the influence of progressive burial. The low Sr contents (101 ± 29 ppm) of D1, along with its near-micritic grain size and estimated δ18OD1fluid of parent dolomitizing fluid (−3.6‰–1.9‰ SMOW), support early-stage dolomitization at low temperature of near-surface conditions from solutions of mixed meteoric and seawaters. The higher estimated δ18Ofluid values of D2 and D3 (3.0–13.9‰ SMOW) are expected for fluids in burial settings of high temperatures. The homogenization temperatures of the primary two-phase fluid inclusions of D2 (Th = 99.1 ± 7.9 °C) and D3 (Th = 89.5 ± 11.2 °C) suggest precipitation at deeper burial settings, which is also consistent with their lower Sr contents (80 ± 14 ppm and 98 ± 33 ppm, respectively). The lower mean Th value of D3 than that of D2 likely reflects the influence of tectonic uplift that affected the Akal Block. The shale-normalized (REESN) patterns of the dolomites mimic that of modern seawater and those of D1 and D2, in particular, coincide with that of the lime mudstones (C1), thus suggesting that C1 was their precursor and also D3 originated almost from the same parent fluid that its geochemical composition evolved with progressive burial. The latest fracture-filling calcite cement (C2) postdated D3. The investigation of the products of burial diagenesis, such as fracturing, dissolution, and dolomitization events and their relationship with the tectonic uplift of the Akal Block allows a better understanding of the factors controlling the quality of Cretaceous reservoir rocks.

Genesis of the Yu'erya gold deposit, East Hebei, China: Insights from geology and fluid inclusions

The Yu'erya gold deposit is a large‐scale gold deposit in eastern Hebei. However, the ore genesis is controversial because the gold orebody is closely related to biotite granite in time and space. The hydrothermal processes of the Yu'erya gold deposit can be divided into early quartz (Q1)‐pyrite stage, middle quartz (Q2)‐sulphide stage and late quartz (Q3)‐calcite stage. The Q1 is subdivided into Q1‐1 and Q1‐2 generations by SEM‐CL images, and Q2 is subdivided into Q2‐1 and Q2‐2 generations. Three types of fluid inclusions (FIs), including aqueous‐carbonic type (C‐type), aqueous‐rich type (W‐type) and vapour‐rich type (V‐type), were observed in the Yu'erya gold deposit. The Q1‐1 is dominated by C‐type inclusions, and the other quartz is dominated by W‐type inclusions. Laser Raman analysis shows that the initial ore‐forming fluids mainly belong to the CO2‐H2O‐NaCl system and gradually transferred to the H2O‐NaCl system. The C‐type inclusions of Q1‐1 show the salinity of 3.1–10.1 (av. 6.9 ± 1.7) wt.% NaCl eqv., and the total homogenization temperature (Th) of 305–391°C. The homogenization temperature decreases gradually from the early to the late stage, with insignificant change in salinity, indicating that fluid cooling is the main precipitation mechanism. Trapping pressures of C‐type FIs are estimated at 206–284 MPa and 203–211 MPa for the early and middle stages, corresponding to the mineralization depths of 7.2–9.9 km and 7.1–7.4 km, respectively. The FIs in the early stage indicate that the initial ore‐forming fluids are characterized by low salinity and CO2 richness, which are different from those of the intrusion‐related gold deposits, but consistent with the orogenic gold deposits.

High-precision microcline 40Ar/39Ar dating by combined techniques

K-feldspar is one of the most common minerals applied for 40Ar/39Ar dating because of its high K content and ubiquitous presence. Excess 40Ar in K-feldspar resulting in unreliable ages has long been reported, however, its occurrence and trapping time are still inconclusive. Combined argon extraction techniques, including laser step heating, and step crushing, as well as succedent furnace incremental heating, have been applied to six microcline samples from two hydrothermal ore deposits in South China, to address these issues. Two microcline samples (09SZ37 & 39) measured by all the above three gas extraction techniques yield flat age spectra and concordant inverse isochron ages respectively. However, one sample (09NL10) produces a flat spectrum but with abnormally low radiogenic 40Ar, and the other three samples obtain uneven age spectra, by laser heating. All six samples measured by step crushing yield quite similar age spectra characterized with declining staircases for the first several steps, and followed with very flat plateaus of high-precision plateau ages for the subsequent later crushing steps. The declining sections with abnormally old ages indicate that excess 40Ar (and obvious atmospheric argon) was trapped in the microcracks and the secondary fluid inclusions. The data points corresponding to age plateaus well define inverse isochron ages which are in consistence with their plateau ages respectively. Furthermore, the crushed powders of all six samples by further furnace step heating also yield very flat age spectra, whose plateau ages are well concordant with those obtained by the foregoing step crushing experiments. The trapped excess 40Ar caused the total fusion ages of laser heating 0.7–17.8% older than their true ages. Mineral grains of ca. 0.10 mm may effectively remove excess 40Ar in microcracks and secondary fluid inclusions, proposed for direct 40Ar/39Ar step heating. Stepwise crushing, combined with furnace heating, is not only very useful to date fluid inclusions, but also robust to remove excess 40Ar and obtain more precise true ages for K-bearing minerals, e.g., K-feldspar, amphibole, tourmaline, and so on.

Dynamic coupling between cementation and porosity evolution of Chang 8 tight sandstone reservoir, Ordos Basin: Insights from in-situ microanalysis

Understanding cement paragenesis is crucial to the study of the porosity evolution in low-permeability sandstone reservoirs. Carbon and oxygen isotope analysis with micro-drilling sampling, electron probe microanalysis, and laser Raman spectroscopy microanalysis was used to clarify the origin of cements and their relationship with reservoir quality of the Chang 8 tight sandstone reservoir in the Longdong area of the Ordos Basin. The predominant cements are chlorite, illite, kaolinite, quartz, and carbonate, which occur in 3 forms, grain rim, overgrowth, and filling of intergranular pore and intragranular dissolved pore. The quantitative paragenesis of the cements is established based on the formation temperature of cements obtained through fluid inclusion, oxygen isotope, chemical composition and Raman parameters. The main source of carbon for carbonate cements is the decarboxylation of organic matter in the adjacent mudstone interlayer. The conversion of smectite to illite and the dissolution of feldspar and previous carbonates provide Ca2+, Mg2+, and Fe3+ ions for carbonate cements and authigenic chlorite, and the authigenic chlorite rim continuously grows from the grain surface outward. The porosity reduction by the cements mainly occurs during the main hydrocarbon filling period (80–120 °C), and the reservoir belongs to the type of “hydrocarbon accumulation prior to significant porosity reduction”. This study provides a new approach for studying the temporal and spatial evolution of clastic rock diagenesis.

Coupling of source rock gas expulsion intensity and reservoir quality in time and space-a key to determine gas accumulation in Upper Paleozoic tight sandstones of the Mugua gas field, Ordos Basin, China

Research on gas accumulation mechanisms and variations in gas saturation is limited in the newly established Mugua gas field in the Ordos Basin. Understanding these aspects is vital for reduce drilling uncertainty and exploration risk. This study employed a variety of methods, including: (1) the use of organic geochemical examinations to determine the gas expulsion intensity of hydrocarbon source rocks; (2) fluid inclusion analyses to determine the time of gas accumulation; (3) a combination of optical microscopy, scanning electron microscopy observations, pore-simulation and permeability tests, mercury-intrusion capillary pressure experiments, and nuclear magnetic resonance studies to assess the properties of the reservoir quality and identify the tight sandstone rock type; (4) wireline logging and interpretive data analyses to research the gas saturations features; and (5) establish reservoir quality evolution model to systematically study this problem. Integrating fluid inclusion homogenization temperatures with burial and thermal histories indicates that gas accumulation in the Mugua gas fields occurred throughout the Early Jurassic to Late Cretaceous period. Five diagenetic facies have been identified: dissolution, quartz-cemented, clay mineral filling, tightly compacted, and carbonate-cemented facies. The reservoir quality evolution model reveals that the five diagenetic facies experienced densification before the period of gas accumulation. Therefore, for gas accumulation in tight sandstones, it is essential that mature and gas-generating source rocks, provide sufficient expulsion forces to drive gas into the rocks. The mechanism of gas accumulation in the Mugua gas field's tight gas sandstone reservoirs involves the process occurring in sandstones that are either near or interbedded with source rocks, under high pore-fluid pressure generated by gas production. Compared to other Upper Paleozoic gas fields in the Ordos Basin, the gas expulsion intensity in the Mugua gas field is lower, ranging from 6.12–8.39 × 108 m3/km2. Due to the low gas expulsion intensity, the Shihezi 8 Member's tight sandstone reservoirs, situated away from the source rock, typically show low gas saturation. Conversely, the dissolution and quartz-cemented facies within the Benxi, Taiyuan, and Shanxi Formations, intimately interbedded with hydrocarbon source rocks, exhibit good reservoir quality. They offer ideal conditions for gas accumulation, making them prime candidates for exploration and development.

Physico-chemical constraints associated with hypogene hydrothermal alteration and supergene oxidation at the farallón negro intermediate-sulfidation epithermal Au-Ag deposit, NW Argentina

The Farallón Negro deposit is located in the Farallón Negro Volcanic Complex (FNVC; ∼9.40 to 4.88 Ma) in NW Argentina and includes intermediate-sulfidation, epithermal Au-Ag mineralization in the Alto de la Blenda deposit. Detailed mineralogical and geochemical investigations of the Esperanza-Esperanza Sureste, Cecilia veins, both hosted in the Alto de la Blenda monzonite, and the Laboreo vein, hosted in the volcaniclastic Morada Central andesitic breccia, reveal four hypogene vein stages. Each of these four stages are comprised of a texturally early quartz, followed by a sulfide-sulfosalt and a late Mn-carbonate sub-stage. Stage 1 is characterized by a quartz, base metal and Mn-carbonate sub-stage showing high δ18OVSMOW (δ18OQuartz = 13.7 ± 0.4 ‰ and δ18OMn-Calcite = 11.6 ± 0.2 ‰) and low δ13CVPDB values (δ13CMn-Calcite = –4.7 ± 0.2 ‰). Stage 2 includes most gold and silver minerals in a tennantite-tetrahedrite-sphalerite-pyrite-galena ± electrum sub-stage. Fluid inclusions trapped in vein-anhydrite, only associated with stage 2, record the highest temperatures (TEntrapment = 327–335 °C) and highest salinity (3.8NaCleq.wt.%) when compared to all other stages. Stage 2 quartz (δ18OQuartz = 9.9 to 11.9 ‰) and carbonate display lower δ18OVSMOW (δ18OMn-Calcite = 7.9 to 10.3 ‰), as well as lower δ13CVPDB values (δ13CMn-Calcite = -5.2 to 4.7 ‰) than all other stages. Stages 3 and 4 consist of barren quartz and carbonate sub-stages that show lower temperatures (TEntrapment = 187–262 °C) and higher δ18OVSMOW (δ18OQuartz = 13.4 to 13.9 ‰ and δ18OMn-Calcite = 10.7 to 11.8 ‰) and δ13CVPDB values (δ13CMn-Calcite = –3.5 to −3.9 ‰) when compared to earlier stages.Petrographic, sulfide mineral chemistry, microthermometric, and stable isotope data combined with distinct alteration mineral assemblages allow the reconstruction of a model for the Alto de la Blenda deposit. During the hypogene hydrothermal fluid flow stage, around 7.16 Ma, at ∼ 2.2 km depth (PHYDROSTATIC = 220 bars), the high temperature alteration mineralogy, low fluid salinities and 18O and 13C isotope compositions indicate cooling of ascending magmatic-hydrothermal fluids. These fluids were derived from a contracted magmatic vapor phase that exchanged with unexposed sedimentary rocks of the Sierras Pampeanas. Hypogene quartz-R3 illite/smectite-chlorite-calcite-pyrite alteration in the andesitic-breccia and quartz-illite-R3 illite/smectite-chlorite-calcite-pyrite alteration in the monzonite are temporally and spatially to the hydrothermal vein minerals deposited between 187 and 335 °C. During the post-hydrothermal supergene weathering event, around 2.70 Ma, sulfuric acids generated by meteoric fluids and supergene oxidation of hypogene sulfides, particularly pyrite, descended along normal fault zones and related vein and fracture networks to at least 450 m depth below the surface level, forming the supergene alteration assemblage of dioctahedral smectite-kaolinite-Mn-/Fe-oxides/-hydroxides-gypsum ± interstratified chlorite/smectite ± interstratified R1 illite/smectite in both wall rock types at temperatures < 50 °C.The dominant hypogene alteration assemblage, present in the monzonite and volcaniclastic andesitic breccia, may be indicative of deeper-seated, transitional magmatic-hydrothermal systems (>1 km depth), generally controlled by fluid cooling, as is the case at Farallón Negro. Pervasive supergene alteration can conceal these systems but the identified supergene alteration assemblage of dioctahedral smectite-kaolinite-interstratified chlorite/smectite-interstratified R1 illite/smectite that replaces the hypogene alteration assemblage along the permeable fault, vein and fracture zone network may present a useful vectoring tool towards concealed, deeper-seated, epithermal Au-Ag mineralization in arid climates.

Charging dynamic, resistance and accumulation process of tight sandstone oil of the Paleogene Es3 member in Linnan Sag, Jiyang Depression

The study of the coupling relationship between the dynamic and resistance of oil charging is the key to revealing the process of tight oil accumulation. The Paleogene Es3 member of Linnan sag, Jiyang Depression was taken as an example, basin simulation, fluid inclusion analysis, and porosity inversion were comprehensively utilized to recovery the residual fluid pressure and charging resistance during the critical oil accumulation periods. Combined with the physical modeling of tight oil charging, the charging process and characteristics of tight oil under dynamic and resistance coupling were reproduced and the effects of dynamic and resistance on the oiliness of tight reservoirs were analyzed. The comprehensive analysis indicates that there were early and late periods of oil accumulation in early Dongying period and late Guantao-Quaternary period. The reservoirs had not been densified during the early oil accumulation period. The residual pressure of the formation was between 1.0∼4.4MPa and the reservoir charging resistance was generally small, so that the dynamic was generally greater than the resistance. But the oil saturation of the reservoirs was low due to the small amount of hydrocarbon generation. During the late oil accumulation period, the source rocks entered the peak period of hydrocarbon generation and expulsion and the residual pressure of the formation was between 4.2∼12.1MPa. The reservoirs had been close to or completely densified. The dynamic, resistance and dynamic and resistance difference all increased significantly. Under the stronger dynamic, the oil saturation of reservoirs rapidly increased, ultimately achieving tight oil enrichment. The results of this study provide insights into the dynamic process of tight oil accumulation and the distribution of oil in tight reservoirs.

Fluid inclusion records of oil-cracked wet gas in Permian carbonate reservoirs from the Eastern Sichuan Basin, China

Oil-cracked gas is an important gas resource in petroliferous basins. In theory, oil-cracked gas should be rich in C2+ (i.e., wet gas). However, gas reservoirs of oil cracking origin in the Sichuan Basin (South China) are mainly composed of dry gas. Fluid inclusions record hydrocarbon generation and evolution. Obviously, identifying wet gas inclusions is the key to solving the above controversy. Here, we calibrate the Raman spectroscopic detection limits of ethane and propane (0.007 and 0.016 MPa, respectively) and demonstrate that Raman spectroscopy is a powerful method to identify C2+-bearing inclusions. Based on this technique, C1–C2–C3-bearing wet gas inclusions (C1/ƩCi = 0.90–0.93) are identified in calcite cements of the Permian Changxing Formation carbonate reservoir in the Eastern Sichuan Basin. These inclusions are direct geological evidence of oil-cracked gas because the calcite cement precipitated after the formation of pyrobitumen. Furthermore, reconstruction of the temperature-pressure-composition evolutions of calcite-hosted (Changxing Formation) and quartz-hosted gas inclusions in the Ordovician Wufeng–Silurian Longmaxi Formation black shale indicates that high pressure can effectively retard the cracking of C2+ gas hydrocarbons. Differences in gas composition and δ13CCH4 signature between reservoir gas and inclusion gas lead us to conclude that large-scale oil cracking occurred during the Early–Middle Jurassic in the Permian Changxing carbonate reservoir. Nevertheless, oil-cracked gas escaped during the evolution of the paleo-gas reservoir. Formation of the current dry gas reservoir can be attributed to the continuous mixing of late-stage kerogen pyrolysis gas.

Diagenesis and pressure evolution in Ediacaran carbonate rocks in the North Slope (Penglai area) of the central Sichuan Basin (SW China)

The Penglai area is an emerging giant gas field in the Sichuan Basin supporting the carbon neutrality goal. To accelerate the exploration, this study combines petrographic observations using optical and cold cathodoluminescence microscopy, fluid inclusion microthermometry, Raman spectroscopy, 1D basin modeling and fluid inclusion PVTX modeling techniques to reveal the paleo-fluid P-T evolution in the second member of the Dengying Formation (Ediacaran age). The results firstly showed a diagenetic sequence which includes three phases of dolomite (1–3), bitumen, quartz (core and overgrowth) and fluorite. Then, ten paleo-fluid episodes were identified based on microthermometric histograms of different mineral phases. Finally, by integrating the Raman results, the hydrostatic and lithostatic gradients calculated by the constructed 1D basin model, and the existing equation of states, the homogenization pressure of fluid inclusions from each fluid episode was determined. By assuming the system was saturated by CH4 as suggested by Raman results, the homogenization pressure equals to the trapping pressure. The P-T trajectory indicates that the second member of the Dengying Formation experienced two sever pressure accumulation stages. One is between fluid episode #3 and #4 recorded by dolomite 3, due to oil generation, and the other is from fluid episode #5 in dolomite 3 to episode #7 in quartz overgrowth to episode #8 in fluorite due to oil cracking to gas. Also, two pressure releasing periods were recorded by fluid episode #5 in dolomite 3 and #9 and #10 in fluorite, caused by both basin uplifting and fluid pressure reaching the lithostatic pressure. This study provides insights into fluid pressure evolution in deeply buried carbonate reservoirs. The understandings can support further geoenergy utilization and reduce exploration risks.

Heavy halogen impact on Raman water bands at high pressure: Implications for salinity estimations in fluid inclusions

We present a new experimental dataset on the impact of the heavy halogens chlorine, bromine and iodine on the Raman water bands concerning pressure and their concentration at room temperature. These experiments were conducted at ambient temperature, with variations in halogen concentration and pressure ranging from 0 to 1.4 GPa.The strength of the Raman water band shift change increases with the ionic size from chlorine, over bromine, to iodine. Our experiments further demonstrate that increased pressure diminishes the impact of the halogen shift change to a varying extent for each of the three halogens. This finding can have significant implications for the salinity calculation of fluid inclusions in minerals such as quartz or olivine. Particularly in the low salinity range, the concentration can be markedly underestimated if the pressure effect is neglected. For experiments in diamond anvil cells involving halogens dissolved in water, the change in Raman water band shifts can serve either as a new tool to monitor pressure, or to monitor the salinity.

Coarsening of quasi two-dimensional emulsions formed by islands in free-standing smectic films.

We study the coarsening behavior of assemblies of islands on smectic A freely suspended films in ISS microgravity experiments. The islands can be regarded as liquid inclusions in a two-dimensional fluid in analogy to liquid droplets of the discontinuous phase of an emulsion. The coarsening is effectuated by two processes, predominantly by island coalescence, but to some extend also by Ostwald ripening, whereby large islands grow at the expense of surrounding smaller ones. A peculiarity of this system is that the continuous and the discontinuous phases consist of the same material. We determine the dynamics, analyze the self-similar aging of the island size distribution and discuss characteristic exponents of the mean island growth.

On the issue of the formation temperatures of sellaite and fluorite in the Suran deposit (South Urals)

For the sellaite-fluorite Suran deposit, the currently available data on the temperatures of homogenization and decompression of fluid inclusions are summarized and critically reviewed. It is shown that the conclusions of previous researchers on fluorite formation temperatures based on these data are contradictory. Possible causes of the current situation are discussed. The results of studying sellaite, various fluorite and quartz generations by vacuum decompression are presented. It is concluded that the initial temperatures of mineral-forming fluids could reach 360–400°C.

Coevolutionary Diagenesis in Tight Sandstone and Shale Reservoirs within Lacustrine-Delta Systems: A Case Study from the Lianggaoshan Formation in the Eastern Sichuan Basin, Southwest China

Tight sandstone and shale oil and gas are the key targets of unconventional oil and gas exploration in the lake-delta sedimentary systems of China. Understanding the coevolutionary diagenesis of sandstone and shale reservoirs is crucial for the prediction of reservoir quality, ahead of drilling, in such systems. Thin-section description, scanning electron microscopy (SEM), X-ray diffraction (XRD), fluid inclusion analysis, porosity and permeability tests, high-pressure mercury intrusion (HPMI) measurements and nuclear magnetic resonance tests (NMR) were used to reveal the coevolutionary diagenetic mechanisms of a sandstone and shale reservoir in the Lianggaoshan Formation of the Eastern Sichuan Basin, China. The thermally mature, organic-matter-rich, dark shale of layer3 is the most important source rock within the Lianggaoshan Formation. It started to generate abundant organic acids at the early stage of mesodiagenesis and produced abundant hydrocarbons in the early Cretaceous. Porewater with high concentrations of Ca2+ and CO32− entered the sandstone reservoir from dark shale as the shale was compacted during burial. Potassium feldspar dissolution at the boundary of the sandstone was more pervasive than at the center of the sandstone. The K+ released by potassium feldspar dissolution migrated from the sandstone into mudstone. Grain-rimming chlorite coats occurred mainly in the center of the sandstone. Some silica exported from the shale was imported by the sandstone boundary and precipitated close to the shale/sandstone boundary. Some intergranular dissolution pores and intercrystal pores were formed in the shale due to dissolution during the early stages of mesodiagenesis. Chlorite coats, which precipitated during eodiagenesis, were beneficial to the protection of primary pore space in the sandstone. Calcite cement, which preferentially precipitated at the boundary of sandstone, was not conducive to reservoir development. Dissolution mainly occurred at the early stage of mesodiagenesis due to organic acids derived from the dark shale. Calcite cement could also protect some primary pores from compaction and release pore space following dissolution. The porosity of sandstone and shale was mainly controlled by the thickness of sandstone and dark shale.

Geochemistry, Mineralization, and Fluid Inclusion Study of the Bayan-Uul Porphyry Au-Cu-(Mo) Deposit, Central Mongolia

The Bayan-Uul porphyry Au-Cu-(Mo) deposit occurs within the Mongol–Okhotsk Orogenic Belt, which is a part of the Central Asian Orogenic Belt. To understand geotectonic, petrogenesis, mineralization, and ore-forming fluid evolution of the Bayan-Uul deposit, we report petrographic and geochemical analyses of host rocks, mineralogy of ores, and fluid inclusion characteristics. Based on petrographic and mineralogical analyses, Cu, Mo, and Au mineralization occurs as disseminated and sulfide-bearing quartz–tourmaline veins hosted within granodiorites, monzodiorites, and diorite porphyry and tourmaline breccia. Four main alteration assemblages are identified: potassic, phyllic, argillic, and quartz–tourmaline alteration. The ore mineralogy of quartz–tourmaline veinlets are classified into A-type veinlets (quartz + tourmaline + chalcopyrite + magnetite + pyrite ± electrum), B-type veinlets (quartz + tourmaline + molybdenum + chalcopyrite + pyrite), and C-type veinlets (quartz + tourmaline + pyrite ± chalcopyrite). Fluid inclusions are found in quartz–tourmaline veinlets consisting mainly of liquid-rich two-phase (L-type), vapor-rich two-phase (V-type), and solid-bearing multi-phase (S-type) inclusions. The homogenization temperatures for the fluid inclusions in A-type, B-type, and C-type veinlets range from 215 to 490°C, 215 to 500 °C, and 160 to 350 °C and their salinity varies from 5.4 to 43.5 wt.%, 16 to 51.1 wt.%, and 3.4 to 24.1 wt.% NaCl equivalent, respectively. Coexistance of (L-type), (V-type), and (S-type) inclusions support fluid boiling. The δ18O values of ore fluids from different mineralizing A-, B-, and C-type veins are 8.7‰, 10.9‰, and 8.4‰, respectively, and the δ34S values of sulfide minerals range from −1.4‰ to 5.3‰, which indicates magmatic origin.