Fluid Inclusion Analysis Research Articles

The Relationship between Fluid Evolution and Hydrocarbon Accumulation and Metallization in the Nanpanjiang-Youjiang Basin: Evidence from Calcite Petrography and Fluid Inclusions

In the process of diagenesis and burial of sedimentary basins, basin fluid activities participate in the process of hydrocarbon accumulation and metal mineralization. Understanding the evolution of basin fluid is of great significance in revealing the related hydrocarbon accumulation and mineralization. Paleo-reservoirs are closely associated with Carlin-type gold deposits in the Nanpanjiang-Youjiang Basin, South China. Calcite, the fluid activity product, is closely related to bitumen and gold-bearing pyrite. By integrating petrographic, cathode luminescence, and fluid inclusion analysis, as well as the relevant chronological results of predecessors, this paper attempts to establish the relationship between fluid evolution, hydrocarbon accumulation, and gold mineralization. Two types of calcite (black/gray and white) developed in the Banqi-Yata-Laizishan area, the Nanpanjiang-Youjiang Basin. Black/gray calcite is symbiotic with bitumen and features dark red colors in cathode luminescence. Many hydrocarbon inclusions developed along with fluid inclusion analysis at low homogenization temperatures (65.7~173.1 °C). Combining the previously reported U-Pb ages (~250–230 Ma) of this kind of calcite with some geochemistry data on the associated reservoir and gold deposit, this calcite records the consecutive hydrocarbon accumulation and Carlin-type gold mineralization from the Late Permian to the Late Triassic periods controlled by Indosinian tectonic movement. The white calcite featuring bright red in cathodoluminescence is symbiotic with gold-bearing pyrite and realgar, and the associated fluid inclusions have high homogenization temperatures (128.2~299.9 °C). Combined with regional tectonic background and isotopic chronology (~140–106 Ma), it seems to record the early Cretaceous Carlin-type gold mineralization controlled by the subduction of the paleo-Pacific plate in the late Yanshanian period.

Major period and process of hydrocarbon accumulation of deep reservoirs — A case study of Tuoputai area in Tarim Basin, China

Deep hydrocarbon reservoirs in China's Tarim Basin are characterized by multistage accumulation, but the major period and the process of hydrocarbon accumulation are still ambiguous. By integrating analyses of fluid inclusions, light hydrocarbon and diamondoid, and reservoir bitumen, we determined the contribution of oils charged in different periods and reconstructed the hydrocarbon accumulation process of Ordovician reservoirs in the Tuoputai area, which is significant to expand deep petroleum exploration in Tarim Basin. The results show that the abundance of yellow to yellowish-green (34.6%–42.1%) and bluish-green oil inclusions (42.1%–46.2%) is relatively high, followed by the blue oil inclusions (15.4%–23.1%). The Th values of the corresponding coexisting aqueous inclusions are mainly in 70–80 °C, 90–100 °C, and 120–130 °C, respectively. The hydrocarbon charging is mainly in the late Caledonian and the late Hercynian, followed by the Himalayan. However, oils charged in the late Caledonian experienced strong biodegradation with the tectonic uplift in the early Hercynian and were transformed into heavy oils and carbonaceous bitumen. The recharged oils have not been altered by biodegradation, thermal cracking, gas washing, and thermochemical sulfate reduction, the amount of oils charged in the Himalayan was relatively small. And the calculated maturity of oils, determined by vitrinite reflectance (Rc = 1.3%–1.5%), is similar to the thermal evolution degree of the Є1y source rock in late Hercynian (1.3%–1.6%) and far less than that in Himalayan (1.8%–2.0%). Large amounts of oils accumulated in the late Hercynian made a major contribution to the current reservoirs. Deep hydrocarbon reservoirs in Tuoputai area were mainly got accumulated when the formation was shallowly buried and then preserved with the deep burial.

Fluid evolution and ore genesis of the Tiantangshan granite-related vein-type Rb-Sn-W deposit, south China: constraints from LA-ICP-MS analyses of fluid inclusions

Fluid evolution and ore genesis of the Tiantangshan granite-related vein-type Rb-Sn-W deposit, south China: constraints from LA-ICP-MS analyses of fluid inclusions

Gas expansion caused by formation uplifting and its effects on tight gas accumulation: A case study of Sulige gas field in Ordos Basin, NW China

Gas expansion caused by significant exhumation in the Sulige gas field in the Ordos Basin since Late Cretaceous and its effects on hydrocarbon accumulation have been investigated systematically based on comprehensive analysis of geochemical, fluid inclusion and production data. The results indicate that gas volume expansion since the Late Cretaceous was the driving force for adjustment and secondary charging of tight sandstone gas reservoirs in the Sulige gas field of the Ordos Basin. The gas retained in the source rocks expanded in volume, resulting in gas re-expulsion, migration and secondary charging into reservoirs, while the gas volume expansion in the tight reservoirs caused the increase of gas saturation, gas-bearing area and gas column height, which worked together to increase the gas content of the reservoir and bring about large-scale gas accumulation events. The Sulige gas field had experienced a two-stage accumulation process, burial before the end of Early Cretaceous and uplifting since the Late Cretaceous. In the burial stage, natural gas was driven by hydrocarbon generation overpressure to migrate and accumulate, while in the uplifting stage, the gas volume expansion drove internal adjustment inside gas reservoirs and secondary charging to form new reservoirs. On the whole, the gas reservoir adjustment and secondary charging during uplifting stage is more significant in the eastern gas field than that in the west, which is favorable for forming gas-rich area.

Genesis of the Sanshiliuwan Sn-Pb-Zn-Sb deposit in the Xianghualing ore district (Southern Hunan, South China): Constraints from geology, fluid inclusion analyses, and geochronology

Tin (Sn) and antimony (Sb) deposits commonly formed in distinctive environments and thus the coexisting of Sn and Sb mineralization in a single deposit is rarely recognized worldwide and the genesis of this type of deposit remains poorly understood. The newly discovered Sanshiliuwan Sn-Pb-Zn-Sb deposit in the Xianghualing ore district of Southern Hunan has metal reserves of 44,000 t Sn, 291,000 t Pb + Zn, and 20,000 t Sb and average grades of 0.39 % Sn, 4.91 % Pb + Zn, and 1.00 % Sb, and thus offers an ideal window for prospecting the genesis of the Sn-Pb-Zn-Sb deposit. The orebodies in the deposit were mainly hosted in Cambrian and Devonian sequences and occurred as veins. Three mineralization stages were recognized including the cassiterite-sulfide, Pb-Zn-Sb sulfide, and carbonate. The fluid inclusions in the quartz from the cassiterite-sulfide stage (QtzI) have homogenization temperatures of 418–244 °C (typically 400–320 °C) and salinities in the range of 0.87–10.11 % NaClequiv. While the fluid inclusions in the quartz from the Pb-Zn-Sb sulfide stage (QtzII) have the homogenization temperatures of 253–141 °C (typically 250–180 °C) and salinities in the range of 0.18–9.86 % NaClequiv. The fluid inclusions in quartz from different mineralization stages have low Na+/K+ ratios in the range of 0.05–0.96 and clustered Cs/Rb ratios of 2–5, indicating that the ore fluids of the deposit were likely to have been exsolved exclusively from a magmatic source. Cassiterite U-Pb dating suggested that the Sanshiliuwan deposit formed ca. 155 Ma, which is well coincided with the concealed granites (formed ca. 156 Ma). Therefore, the genetic model for this deposit would demonstrate that evolved fluids from Jurassic granites invaded the faults and deposited Sn and Sn-Pb-Zn ore veins in the granite periphery. Thence, Sn-depleted but Pb-Zn-rich fluids interacted with the Sb-rich Cambrian sequence and leached it to create Sb-rich fluids. In response to fluid cooling and boiling, Pb-Zn-Sb-rich fluids then deposited Pb-Zn-Sb sulfide ore veins at favorable structural sites.

A quantitative model and controlling factors of secondary pore development for tight sandstone reservoirs in the carboniferous Benxi Formation, Ordos Basin, China

As the Ordos Basin is the largest gas-producing area in China, its central and eastern Paleozoic Carboniferous Benxi Formation is poised to become a significant source for increasing reserves and production within the basin. However, the extreme diagenesis, complex pore structure, and strong heterogeneity of this formation hinder the exploration of its “sweet spots”. The reasons are usually considered to be its large burial depth (>3000 m), differential uplift, highly acidic environment and strong hydrocarbon supply in the coal-bearing strata of this area. In this study, we performed qualitative and quantitative analyses of thin sections of Benxi Formation rocks, combined with scanning electron microscopy, X-ray diffraction, and fluid inclusion analyses to determine the development, type, and evolution sequence of secondary porosity from the diagenesis of the Benxi Formation. The main reservoir type of the Benxi Formation is characterized by secondarily dissolved pores (>0.5 μm) and micropores (<0.5 μm) within clay aggregates; the low porosity and permeability of <5.8% and <1 × 10−3 μm2 indicating a tight sandstone reservoir. Throughout the reservoir formation, its porosity has decreased by strong compaction and cementation, and also increased primarily by dissolution. Quantitative analyses of the thin-section porosity of the rocks based on the Scherer initial porosity recovery model and pore evolution parameters defined by Paxton and Ehrenberg showed 39.8% recovery of original porosity and a porosity increase due to dissolution of 1.0–5.4% (mean, 4.1%); this constituted 40–250% of the porosity prior to dissolution. The model-simulated results were closely correlated with the experimental measurements. The lithology, diagenetic fluid, and organic acids of the source rocks jointly controlled the development of secondary pores in the Benxi Formation. These findings indicate that the evolution and controlling factors of secondary pore development must be clarified to identify favorable areas for gas exploration. The results will provide a basis for predicting favorable reservoirs in the tight sandstones of the Benxi Formation in the central and eastern Ordos Basin.

Genesis of the Xinfang gold deposit, Liaodong Peninsula: Constraints from fluid inclusions, H-O-S-Pb isotopes, pyrite trace element concentrations, and chronology

The Xinfang gold deposit is situated in the southwest of the Liaodong Peninsula, and represents the first large-scale gold deposit hosted by the Xinfang Metamorphic Core Complex. We conduct pyrite and galena Rb-Sr isotope dating, multiple isotope compositions (H-O-S-Pb-Sr), fluid inclusions (FIs), and pyrite laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analysis to constrain the genesis of the deposit. Our fluid inclusion analyses show that the ore-forming fluids belong to a medium–low-temperature and moderate-low salinity NaCl-H2O-CO2 fluid system. Fluid boiling is the key factor for Au precipitation in the Xinfang gold deposit. In-situ sulfide S-isotope data (δ34S = −1.64 ‰ to 6.07 ‰) favor a magmatic source. The initial 87Sr/86Sr ratios of the sulfide samples range from 0.709556 to 0.710731, indicating that the ore-forming materials came from a mixture of the crust and mantle. The in-situ lead compositions of sulfides (206Pb/204Pb = 16.562–18.367, 207Pb/204Pb = 15.257–17.071, 208Pb/204Pb = 37.005–38.750) reveal that the ore-forming materials originated from the crustal source with a small amount from the mantle source. The δ18OH2O (−3.28 ‰ to 3.64 ‰) and δDH2O (−89.6 ‰ to − 81.3 ‰) values suggest that the ore-forming fluids came from magmatic water with a contribution of meteoric water. LA-ICP-MS trace element analyses reveal that stage II was the main gold mineralization stage, and Archean gneiss may have contributed ore-forming materials. The Co/Ni ratios suggest that the pyrite may have formed from hydrothermal fluids. Pyrite and galena samples from the quartz-polymetallic sulfide-gold vein yielded a Rb-Sr isochron age of 122.6 ± 2.1 Ma (Initial 87Sr/86Sr = 0.710255 ± 0.000059, MSWD = 1.2), which is interpreted to be the ore-formation age. In combining all available data results, we propose a magmatic-hydrothermal source for the Xinfang gold deposit and that the ore formation is related to the subduction of the Paleo-Pacific Plate beneath Eurasia.

An indication of salt caprock failed integrity in inclusion records: Dongpu depression, bohai bay basin, China

Salt rocks are widely distributed in the Paleogene strata of the Dongpu Depression, China, as major caprocks of the Shahejie Formation. Previous studies have suggested that the decline of formation pressure during the Dongying movement is accompanied by petroleum leakage, but they lack any direct evidence of failed integrity of the salt caprocks. Therefore, fluid inclusion analysis under both incident ultraviolet (UV) and transmitted light (TR) modes on halite samples were conducted in this study, supported by basin and fluid inclusion PVTX modeling to prove lack of integrity in the caprock, periodically. Our observation showed that various types of hydrocarbon inclusions exist in halite crystals, revealing multi-stage fracturing and healing processes inside the salt caprocks. Coupling the PVTX and basin modeling results, revealed that oil inclusions were trapped at around 6.6 Ma during the secondary hydrocarbon generation of the source rocks. Moreover, salt caprocks failed due to the overpressure that was formed during hydrocarbon generation. This was exacerbated by the following uplifting which ultimately led to the present-day normal reservoir pressure in the sampled depth. In addition, the fluorescence of oil inclusions in the same fracture gradually changed from orange to blue. This proved that analyzing the thermal maturity of trapped hydrocarbons within the inclusions, solely based on the fluorescence color, could be misleading and other submicron scale analytical methods should be employed to examine the inclusions. Overall, this study enabled us to investigate the caprock integrity via optical methods, which can eventually help us to avoid regions where hydrocarbon is lost vs. prolific areas in the formation understudy.

Metallogenic mechanism of the Houge’zhuang gold deposit, Jiaodong, China: Evidence from fluid inclusion, in situ trace element, and sulfur isotope compositions

The Houge’zhuang gold deposit, located in the Penglai–Qixia gold belt of the Jiaodong peninsula, is a representative auriferous quartz vein-style deposit. Pyrite is the most common and main gold-bearing mineral and shows complex textures in the Houge’zhuang gold deposit. Study of ore-related pyrite is of great significance for understanding the metallogenesis of this deposit, especially the gold precipitation mechanism. The present study applied systematic microscopic observation, fluid inclusion analysis, electron microprobes,in situ LA-ICP-MS trace-element analysis, and in situ sulfur isotope analyses. Three types of fluid inclusions were identified, among which the ore-forming fluids exhibited medium–low salinity and temperature, with the fluid inclusions mainly comprising H2O and CO2. Three types of pyrites were identified: 1) Py0, characterized by low concentrations of As and Au and low δ34S values (5.51–6.86‰). 2) Py1, found in the gold-quartz-pyrite veins and homogeneous in chemical composition with no obvious zonal growth but with notably more gold and chalcopyrite inclusions. Py1 contained medium and uniform concentrations of As and was Au-rich, with δ34S values ranging from 7.13 to 7.89‰ (mean 7.44‰). 3) Py2 contained arsenic-bearing pyrite and was found in quartz-polymetallic sulfide veins, with distinct As enrichment and As-rich rims of pyrite and growth zoning. Consequently, the primary ore-forming fluids passed through some arsenic and δ34S-rich sedimentary rocks, such as the Jingshan, Fenzishan, and Penglai groups. With the occurrence of stable water-rock interaction, the extracted fluids were enriched for As and δ34S. Furthermore, Au was closely associated with As, visible gold grains tended to occur in association with Py1 at stage II, and invisible gold was related to the Au-As-rich Py2 of stage III. The As-bearing pyrites shared a close spatiotemporal relationship with gold, playing an important role in the formation and exploration of high-grade gold deposits.

КОМПЛЕКСНОЕ ИССЛЕДОВАНИЕ РЕДКОЗЕМЕЛЬНЫХ КАРБОНАТИТОВ МАССИВА ВУОРИЯРВИ (КОЛЬСКИЙ РЕГИОН) И МОДЕЛЬ ИХ ФОРМИРОВАНИЯ

This paper reviews a series of articles devoted to rare-earth carbonatites of the Vuoriyarvi alkaline-ultrabasic complex (Murmansk region). The basis for these articles is a comprehensive study that involved petrographic-mineralogical and geochemical methods, analysis of fluid inclusions and a wide range of isotope systems (Rb-Sr, Sm-Nd, C, O, He, Ar), and the development of several original research methods. The result of these investigations is a consistent model for the accumulation, deposition, and subsequent dilution of the accumulated REE ores in carbonatites, applicable to the rocks of the Vuoriyarvi massif and other similar objects all over the world.

Fracture-filling cement in the Boquillas Formation: A proxy for the origin and maturation of hydrocarbons in South Texas

The Austin Chalk is a low porosity (3–10%) and low permeability (0.1–0.5 mD) reservoir that relies on fracture networks to store and produce hydrocarbons at commercial flow rates. Charged by the underlying Eagle Ford Formation, fracture networks provide critical permeability and fluid migration pathways from the Eagle Ford source to the Austin Chalk reservoir. In order to investigate these migration pathways, this study analyzes outcrops of the Eagle Ford Formation, located along U.S. 90, west of Del Rio (Texas), also known as the Boquillas Formation. These outcrops expose two sets of conjugate hybrid-shear fractures whose dilational segments are filled with calcite veins. Calcite veins microstructure suggest that they experienced repeated crack and seal events and contain abundant fluid inclusions. Fluids include aqueous solutions and liquid hydrocarbons. Micro-thermometry analyses of aqueous fluid inclusions suggest that they were entrapped at a depth of 1.8–2.7 km. Kinematic analysis suggest that the hybrid shear fractures predate oil generation and expulsion of the Eagle Ford Formation, allowing for migration of oil and gas in the fracture system immediately after generation. These results highlight the importance of these hybrid shear fractures as viable migration pathways for hydrocarbon migration in the Austin Chalk-Eagle Ford hybrid petroleum system.

The Middle Jurassic Sanhe Pb–Zn–Ag deposit in NE China: Constraints from geochronology, geochemistry, fluid inclusion and multi-isotope (S–Pb–He–Hf) systematics

The Sanhe is a typical medium-size Pb–Zn–Ag deposit in the Argun Massif, NE China. Its Pb–Zn–Ag orebodies are mainly hosted in the Jurassic Tamulangou Formation volcanic rocks. The orebodies occur as quartz vein clusters. The ore-forming quartz porphyry is LA–ICP–MS zircon U–Pb dated to be 165.2 ± 1.2 Ma, and the ore sphalerite yielded a Rb–Sr age of 162.5 ± 4.3 Ma. These ages indicate that the Sanhe represents the oldest Pb–Zn–Ag mineralization in the Argun Massif. Combined with published age data, the Pb–Zn–Ag mineralization of the Argun Massif occurred in three phases at ca. 160 Ma, 140 Ma and 130 Ma. The quartz porphyry shows right-inclining REE ((La/Yb)N = 22.92–40.69) patterns and negative Eu anomalies (δEu = 0.68–0.81), together with enrichments in K, La, Ce, Nd, Zr and Hf but depletions in Ba, Ta, Nb, Sr, P and Ti. Whole-rock geochemistry and Hf isotopes suggest that the quartz porphyry was derived from partial melting of juvenile lower crustal materials during the post–collision extension of the Mongol–Okhotsk orogenic belt. Sulfide S–Pb isotope compositions indicate that the ore-forming materials were derived from the deep magma, whilst He isotopes indicate that the ore fluids were dominantly crustal-derived. Fluid chemical compositions obtained from LA–ICP–MS single fluid inclusion analyses support that mixing of magmatic fluids with meteoric water was an important mineralization trigger at Sanhe. Our new data show that the involvement of unusually metal-rich fluids is a prerequisite to form abundant Pb–Zn–Ag mineralization at Sanhe.

Reservoir heterogeneity of an Eocene mixed siliciclastic-carbonate succession, northern Pannonian Basin

The quality of any reservoir is strongly controlled by diagenetic processes, especially of those that are comprised of mixed siliciclastic-carbonate sequences. An Eocene mixed siliciclastic-carbonate sequence forms reservoirs with diverse reservoir quality in the Hungarian Palaeogene Basin, which is found within the Pannonian Basin. Late Eocene transgression established extensive shallow-marine depositional environments strongly controlling the early diagenetic processes. Drill cores and cuttings were selected from several boreholes, representing a wide range of lithofacies to reveal the cause for reservoir heterogeneity. Petrographic, stable isotope and fluid inclusion analyses were performed to identify the diagenetic processes that controlled the porosity evolution of the reservoir, The early diagenetic calcite neomorphism caused lateral homogeneity, whereas the early meteoric water incursion enhanced reservoir properties via the development of dissolution pores, which in turn resulted in vertical reservoir heterogeneity. The deterioration of porosity started in the eogenetic realm by kaolinite formation and continued during mesodiagenesis as burial calcite, quartz, dolomite and minor illite cement precipitated in remaining pore spaces. Petrographic evidence, such as kaolinite transformation into dickite and fluid inclusion homogenisation temperatures highlight the role of temperature increase with burial, the effect of changing fluid chemistry during the evolution of the basin is documented by the chemical zonation of dolomite, the variation in salinities of the fluids enclosed fluid inclusions, as well as recurrent and prolonged periods of cementation with calcite, quartz and dolomite. Mesogenetic processes were accompanied by hydrocarbon emplacement and the formation of late-stage pyrite. The propagation and cementation of faults first contributed to increase and later to reduce fluid flow, respectively. These steps collectively resulted in a heterogeneous reservoir, characteristic of the Eocene sequence in the Hungarian Palaeogene Basin.

Geology, molybdenite Re-Os age, H-O-S-Pb isotopes, and fluid inclusion features of the Maodeng Mo-Bi-Sn-Cu deposit, Inner Mongolia

Maodeng deposit is a large-sized Mo-Bi-Sn-Cu deposit located in the Xinlinhot of Inner Mongolia in the southern segment of the Great Xing’an Range. The ore-forming elements in the deposit are found in the volcanic breccia of Lower–Middle Permian Dashizhai Formation and granite porphyry of the Alubaogeshan complex, constituting distinct regional mineralization zones such as hydrothermal vein-type of Sn-Cu orebodies in the upper parts and stockworks, veinlets and disseminated-type of Mo-Bi orebodies in the lower parts. This paper has conducted an integrated study using a comprehensive field survey, molybdenite Re-Os age, fluid inclusion, and isotopic (O, H, S, and Pb) analyses. The mineralization in the deposit can be divided into four stages, which are molybdenite-native bismuth-quartz (stage I), cassiterite-wolframite-topaz-quartz (stage II), chalcopyrite-sphalerite-galena-fluorite-sericite-quartz (stage III) and stibnite-pyrite-quartz-fluorite (stage IV). The Re-Os isochron age of four molybdenite samples was estimated as 139.2 ± 3.9 Ma, indicating the deposit was formed during the Early Cretaceous. The Maodeng deposit is characterized by three kinds of fluid inclusions (FIs): liquid-rich two-phase, vapor-rich two-phase, and daughter mineral-bearing three-phase FIs. The homogenization temperatures for the FIs of stages I through IV are 292–387 °C, 222–333 °C, 172–252 °C, and 122–197 °C, with calculated salinities of 4.2–43.6 wt%, 2.1–38.9 wt%, 1.9–33.6 wt% and 0.7–5.1 wt% NaCl equiv., respectively, showing the gradual reduction in temperature and salinity of hydrothermal fluids. Hydrogen and oxygen isotopic data of quartz indicate that the dominant origin of the ore-forming solutions from deep granitic intrusions followed by their dilution by limited meteoric water. The sulfides have δ34S values ranging from − 6 ‰ to − 0.49 ‰, and have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios similar to those of K-feldspars from the Alubaogeshan pluton, suggesting the primary origin of ore-forming materials of the Maodeng deposit are magmatic. Molybdenite and native bismuth may precipitate primarily by fluid boiling, chalcopyrite could deposit mainly by fluid mixing, and cassiterite precipitate mainly through fluid boiling and mixing.

Geological Investigation of Lead Mineralization Associated with Silicified Zone in Büyük Kızılcık (Göksun-K.Maraş) Region

Doğu Toroslar'da farklı ortam ve tektonik koşullarda oluşmuş birbirinden farklı istifler yüzeylemektedir. Bu istifler tabandan tavana Üst Jura-Kretase yaşlı peridotitler, bantlı gabro, izotrop gabro, levha dayk karmaşığından oluşan Kömürhan Ofiyolitleri ve Bodrum Napı’na ait metamorfik kaya birimleri yer almaktadır. Kömürhan Ofiyolitleri ve Bodrum Napı Geç Kretase yaşlı Baskil Granitoyidleri tarafından kesilmiştir. Baskil Granotoyidinin kesmiş olduğu birimler içerisinde cevherleşmeye ilişkin alterasyon zonları gözlemlenmektedir. Cevher mikroskopi çalışmalarında pirit, sfalerit, galenit, arsenopirit, manyetit ve hematit tespit edilmiştir. Pirit üzerinde yapılan izotop analizinde δ 34S değeri -3 %o olarak belirlenmiştir. Kuvars mineralleri üzerinde yapılan sıvı kapanım analizinde 69-127 °C arasında ve ortalama olarak 103 °C epitermal oluşum sıcaklığı belirlenmiştir. Çalışma alanında cevherleşmelerin birincil kökeni Miyosen’den itibaren bölgeyi etkileyen asidik karakterli volkanizmanın etkisiyle metamorfik istifler içerisindeki fay ve kırık çatlak sistemlerinde yerleşmiş sülfürlü yataklanma şeklinde oluşurken, ikincil cevherleşmelerin birincil sülfürlü minerallerin çözünerek tektonik olaylar sonucu oluşan süreksizlik zonlarında depolandığı düşünülmektedir.

Palaeo-thermal history of the Blanice Graben (the Bohemian Massif, Czech Republic): The origin of anthracite in a late-Variscan strike-slip basin

Coal petrology data and vitrinite reflectance modelling were combined with micro-geochemical and fluid inclusion analyses of coal mineralization and apatite fission track analysis (AFTA) of coal-bearing sediments to constrain the thermal history of the Blanice Graben, a narrow, nearly 150-km long, late-Variscan strike-slip basin filled with Permo-Carboniferous continental coal-bearing deposits. The vitrinite reflectance modelling and the ranking of coals, which increased along the meridional axis of the graben from bituminous coals (Rr ∼ 0.70%) to high-rank anthracites (Rr = 2.85–4.77%), provide evidence of maximum palaeo-temperatures ranging from about 120–145 °C to 310–390 °C. This corresponds to the diagenetic realm in the north of the graben that graded to high-grade anchizonal to low-epizonal metamorphic conditions in the south. The present level of coalification resulted from the Permian burial, 2500–5000 m thick, which aggregated under a high geothermal gradient (∼ 90 °C/km). In central and southern relics of the graben, however, specific microscopic features of coals and fluid inclusion and chlorite thermometry suggest that the anthracite rank was attained by the combined effect of a stratigraphic burial, a regional discharge of ∼300 °C hot, mineralized fluids that ascended along faults into buried coal-bearing strata and an intense Permian faulting and shearing of coal seams, which collectively contributed to the anthracitization process. The heat source, promoting high geothermal gradient and circulation of hot fluids through the coal-bearing sediments, was probably associated with a mantle magma chamber hidden deeply below the graben and manifested at current errosional level by ∼303 and ∼ 270 Ma old microdiorite dykes. Similar dramatic geothermal settings, characterized by elevated geothermal gradients, localized fault-controlled incursions of hot hydrothermal solutions, and rapid coalification of peat layers at relatively shallow depths of burial, dominated in many late Variscan coal-bearing basins during a period of post-collisional extension at ∼315–280 Ma. The AFTA method, applied to sedimentary apatites, provides evidence that following the climax of the Variscan orogeny, the Blanice Graben coal-bearing deposits were rapidly uplifted and, throughout the Mesozoic and Cenozoic times, they experienced only prolonged slow cooling (0.46–0.54 °C/My) and moderate burial temperatures of 50–70 °C. Its final ascent towards the present-day erosional surface occurred during a period of accelerated uplift that has affected the Bohemian Massif since the Oligocene.

In-situ cracking of oil into gas in reservoirs identified by fluid inclusion analysis: Theoretical model and case study

Natural gas formed by cracking of crude oil occurs in many basins globally. One of the key questions about such gas is whether it was produced in-situ—by cracking of crude oil in the reservoir itself—or whether it was generated elsewhere and accumulated in the reservoir subsequently. Fluid inclusions trapped in minerals can provide direct evidence regarding the genesis of natural gas, because these inclusions represent samples of the fluid present at the time of trapping, and/or resulting from subsequent modification of the trapped fluid. Here, we outline some theoretical aspects of the processes of cracking, trapping of fluid inclusions, and potential post-entrapment modifications, that lead to a model for identifying evidence for in-situ cracking of crude oil in reservoirs based on fluid inclusion assemblages (FIAs). This model divides the crude oil cracking process into three main stages, and considers the predicted evolution of fluids and possibility of forming FIAs. In the initial step, before cracking, only liquid oil and aqueous fluid are present in a reservoir, and two endmember types of inclusions may be formed. If cracking of crude oil occurs in the reservoir after these inclusions were trapped, then in-situ cracking of oil within the inclusions gives rise to systematic changes that can be interpreted through the lens of progressive thermal maturation, and hence provide direct evidence for cracking in the reservoir. In the second step, during cracking, the reservoir may host up to four fluid phases (residual oil, aqueous fluid, natural gas, and solid bitumen), and in theory as many as fourteen types fluid inclusions can be trapped; hence, complex FIAs comprising multiple fluid inclusion types can be considered evidence for trapping during in-situ cracking of crude oil in the reservoir. In the third and final step, after complete cracking, liquid oil has been exhausted and only natural gas, solid bitumen and aqueous fluid remain in the reservoir; at this stage, up to six fluid inclusion types can be trapped, which can be indicative of cracking. We applied this model to study the genesis of natural gas in the Sinian Dengying Formation reservoir of the Anyue Gas Field in the Sichuan Basin, southwest China. Six types of hydrocarbon-bearing primary fluid inclusions were observed in fluorite crystals formed during hydrocarbon accumulation. Notably, one type of inclusion showed consistent volumetric phase ratios of solid bitumen plus gas, suggesting that these inclusions originally captured crude oil, which subsequently underwent in-situ cracking in a closed system (within the inclusions) to generate fixed proportions of gas and solid bitumen. These FIAs are consistent with our model and suggest that the fluid inclusions trapped in fluorite can record the in-situ cracking of crude oil in the reservoir, and can therefore be used as direct evidence for the genesis of natural gas.

Pushing the limits: Resolving paleoseawater signatures in nanoscale fluid inclusions by atom probe tomography

New insight into the geochemistry of ancient environments can be gained through structural and chemical analyses of nanometer-scale features within minerals. Here, we present recent developments using atom probe tomography (APT) enabling direct visualization of nanoscale fluid inclusions trapped within pyrite (FeS2) and thereby chemical characterization of remnant seawater. Pyrite framboids (spherical clusters of nanocrystals) were sampled from the Middle Devonian Leicester Pyrite Member (New York). Scanning transmission electron microscopy shows low density features distributed within the pyrite (<4 nm in size). Three-dimensional chemical visualization and analysis by APT reveals these to be nanoscale fluid inclusions preserving the elemental signatures of the water column in which the framboids formed, as alkalis consistent with seawater including Na, K, Mg, and Ca are identified. Further, within the inclusions Mg/Ca is measured to be ∼0.6±0.2 – consistent with conditions for calcite-dominated seawater existing in the Middle Devonian. The results and approach developed in this study show the potential to reconstruct paleoenvironmental conditions from coupled elemental and structural analyses of nanoscale fluid inclusions.

From Fossil to Active Hydrothermal Outflow in the Back‐Arc of the Central Apennines (Zannone Island, Italy)

AbstractPost‐orogenic back‐arc magmatism is accompanied by hydrothermal ore deposits and mineralizations derived from mantle and crustal sources. We investigate Zannone Island (ZI), back‐arc Tyrrhenian basin, Italy, to define the source(s) of mineralizing hydrothermal fluids and their relationships with the regional petrological‐tectonic setting. On ZI, early Miocene thrusting was overprinted by late Miocene post‐orogenic extension and related hydrothermal alteration. Since active submarine hydrothermal outflow is reported close to the island, Zannone provides an ideal site to determine the P‐T‐X evolution of the long‐lived hydrothermal system. We combined field work with microstructural analyses on syn‐tectonic quartz veins and carbonate mineralizations, X‐ray diffraction analysis, microthermometry and element mapping of fluid inclusions (FIs), C, O, and clumped isotopes, and analyses of noble gases (He‐Ne‐Ar) and CO2 content in FIs. Our results document the evolution of a fluid system of magmatic origin with increasing mixing of meteoric fluids. Magmatic fluids were responsible for quartz veins precipitation at ∼125 to 150 MPa and ∼300°C–350°C. With the onset of extensional faulting, magmatic fluids progressively interacted with carbonate rocks and mixed with meteoric fluids, leading to (a) host rock alteration with associated carbonate and minor ore mineral precipitation, (b) progressive fluid neutralization, (c) cooling of the hydrothermal system (from ∼320°C to ∼86°C), and (d) embrittlement and fracturing of the host rocks. Both quartz and carbonate mineralizations show noble gases values lower than those from the adjacent active volcanic areas and submarine hydrothermal systems, indicating that the fossil‐to‐active hydrothermal history is associated with the emplacement of multiple magmatic intrusions.

Ore genesis of the Narenwula quartz-vein type W polymetallic deposit in the southern Great Xing’an Range W belt, NE China: Constraints from wolframite geochronology and individual fluid inclusion analysis

The Narenwula deposit, situated in the southern Great Xing’an Range W belt, is a large-scale quartz-vein type W polymetallic deposit in NE China. The mineralization is spatially associated with Late Jurassic monzogranite and granite porphyry. In this study, we present detailed description of the ore geology, wolframite in-situ U-Pb dating, and fluid inclusion analyses including microthermometry, laser Raman spectra, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) microanalyses to precisely constrain the timing of ore formation, origin and evolution of ore-forming fluids as well as metal precipitation mechanisms. In-situ U-Pb dating of hydrothermal wolframite yields a W mineralization age of 136.4 ± 2.0 Ma (1σ, MSWD = 1.3), which is ∼14 Ma younger than the zircon U-Pb ages of W-bearing granitoids, indicating the W-bearing granitoids are not genetically related to the W mineralization at Narenwula. The hydrothermal process can be divided into four stages: stage I is characterized by abundant wolframite (I), which is the dominant stage of W mineralization; stage II is featured with the development of wolframite (II) together with minor amounts of pyrite and chalcopyrite; stage III is the predominant stage of sulfide mineralization; and stage Ⅳ is characterized by lack of sulfide or wolframite and occurrence of quartz, carbonate, and fluorite. Cathodoluminescence analysis of quartz further revealed four generations of quartz, corresponding to the four hydrothermal stages. Four types of fluid inclusions were distinguished for quartz, including CO2-rich (C1-type), CO2-bearing (C2-type), liquid-rich (l-type), and brine (B-type) fluid inclusions. Fluid inclusion microthermometry shows a decrease in homogenization temperatures and salinities from the early to late stages. The ranges of the homogenization temperature for stages I to Ⅳ are 440–280, 320–240, 260–180, and 200–160 ℃, respectively, and the ranges of salinities for stages II to Ⅳ are 2.9–17.8, 1.4–10.2, and 0.4–6.7 wt% NaCl equiv., respectively. The salinities of fluid inclusions from stage I have a bimodal distribution in the ranges of 9.6–17.4 and 31.1–35.6 wt% NaCl equiv. Individual fluid inclusion LA-ICP-MS analyses suggest a magmatic source for the fluids of all stages, evidenced by the high Rb and Mn concentrations, high Rb/Na, K/Na, Cs/Na, Li/Na, and Zn/Na ratios, which are distinctly different from those of basinal brines. The Rb/Sr and K/Rb ratios of fluid inclusions are similar to those of Late Mesozoic highly fractionated W-mineralized granitoids in NE China, indicating that the metallogenic parent rocks at Narenwula are highly evolved. In combination with evidence of consistent Cs/Rb and Cs/(Na + K) ratios of fluid inclusions from all stages, we suggest that the mineralizing fluids originated from an underlying, geochemically uniform, and highly fractionated granitic magma. Fluid immiscibility and fluid-rock interaction (especially greisenization) are the main mechanisms for wolframite precipitation, whereas natural cooling of fluids may be subordinate. Fluid inclusion data provide evidence that the addition of meteoric water to the magmatic-hydrothermal fluid initiated at the sulfide mineralization stage, and the fluid dilution and cooling due to fluid mixing played an important role in the process of sulfide precipitation. This study highlights that W and base metals (e.g., Cu, Pb, Zn) in ore-forming fluids display different migration and precipitation progresses, the knowledge of which is important for elucidating multi-stage ore-forming processes of quartz-vein type W polymetallic deposits in NE China.