Fluid Inclusion Characteristics Research Articles

Hydrocarbon accumulation conditions and exploration targets of the Sinian Dengying Formation in the Southeastern Sichuan Basin

No breakthrough has been made in petroleum exploration of the Sinian Dengying Formation in the southeastern Sichuan Basin as the major factors controlling the hydrocarbon accumulation of the Dengying Formation have not been well established, including the development period and distribution of the platform margin, the effectiveness of the source-reservoir combination and the reliability of the Cambrian pre-salt structural trap. Based on the interpretation of seismic data and hydrocarbon-source correlation, the distribution of the platform margin of the Sinian Dengying Formation in the southeastern Sichuan Basin was mapped. Moreover, the gas source rocks of the Dengying Formation were clarified according to geochemical characteristics of fluid inclusions, and the pre-salt structures were further confirmed by seismic forward modeling and experiments of prestack depth migration. Results showed that the sedimentary facies of the 4th member of Dengying Formation in the southeastern Sichuan Basin was platform margin of gentle slope type. The platform margin reservoir of the Dengying Formation and the Lower Cambrian source rocks constitute a favorable source - reservoir combination. The carbon isotope characteristics of gas in fluid inclusions indicate that some of the oil/gas in the Dengying Formation were contributed by the Doushantuo Formation source rocks. As the difference in seismic velocities between carbonate rocks and gypsum/salt beds may cause a reduction in the amplitude of pre-salt structures, the low-amplitude structures identified on current seismic profiles are underestimated instead of overestimated. Accordingly, the Sinian structural traps are confirmed to develop in the deep pre-salt sequences in the low-steep anticline belt, southeastern Sichuan Basin. On the basis of the above new understanding, we propose two types of hydrocarbon migration modes for the Sinian Dengying Formation in southeastern Sichuan Basin, including lateral migration of hydrocarbons generated from the Cambrian Qiongzusi Formation and vertical migration of hydrocarbons derived from the Sinian Doushantuo Formation. Lithologic traps at the platform margin and pre-salt structural traps within the platform are favorable targets for natural gas exploration.

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.

Fluid inclusion characteristics of the Jurassic reservoir and hydrocarbon accumulation process in the eastern Kuqa Depression, Tarim Basin

There has been significant progress in hydrocarbon exploration of the eastern Kuqa Depression, Tarim Basin, whose Jurassic reservoir is a key area for abundant hydrocarbon with poorly understood accumulation mechanism. This study conducts an integrated investigation comprising petrographic observations, microthermometry measurements, and fluorescence spectroscopy analyses on fluid inclusions to reconstruct hydrocarbon accumulation history in the Jurassic reservoir. These conclusions are combined with biomarker characteristics to reveal the burial history and thermal evolution of the study area. The results show that (1) there were two episodes of oil charging that occurred at 6~4 Ma and 4~2 Ma, respectively. The QGF experiment response and yellow fluorescent oil inclusions indicated that low-maturity medium crude oil from the Triassic source rocks was expelled into the upper Ahe Formation. In the second charging stage, some mature oil generated from Jurassic source rocks was charged into the reservoir of the Kezilenuer and Yangxia formations, while the condensate from the Triassic source rocks was migrated upward to the Jurassic reservoir, consistent with smaller QGF-E intensity values and blue-white fluorescent petroleum inclusions. (2) There were two kinds of hydrocarbon accumulation models including the lower-generation and upper-storage and the self-generated and self-stored models. (3) It was noted that active faults were developed under the compression stress to provide channels for hydrocarbon vertical migration. Moreover, the early reservoir was destroyed resulting from the strata denudation and a large amount of crude oil was escaped to the surface. The findings of this study can help for better understanding of hydrocarbon accumulation mechanism in the eastern Kuqa Depression and provide theoretical guidance for further oil and gas exploration.

Superimposed hydrocarbon accumulation through multi-source and multi-stage evolution in the Cambrian Xixiangchi Group of eastern Sichuan Basin: A case study of the Pingqiao gas-bearing anticline

The Xixiangchi Group in eastern Sichuan Basin has great potential for natural gas exploration. However, there is a lack of in-depth studies of the hydrocarbon sources and the formation and evolution of gas reservoirs in this Group. Systematic investigation about the gas reservoir in Pingqiao anticline was consequently carried out in terms of characteristics of reservoir bitumen, the geochemical characteristics of natural gas, diagenetic minerals, and fluid inclusions. Based on this, combined with the reconstruction of the burial history, thermal evolution history and uplifting history of strata, and analysis of the regional tectonic settings, the hydrocarbon sources were identified and the formation and evolutionary processes of the gas reservoirs in Xixiangchi Group was revealed in this study. It was shown that the gas reservoirs have mixed gas sources from the shale source rocks in the Lower Cambrian Qiongzhusi Formation and in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation, and experienced several evolutionary stages, including the paleo-oil reservoir stage from the Late Siliurian to the Middle Permian, the paleo-gas reservoir stage from the Late Permian to the Early Cretaceous, and the superimposed accumulation and mixed-source gas reservoir stage since the Late Cretaceous. The mixed-source gas reservoir is formed by the adjustment of the Xixiangchi Group paleo-gas reservoirs and depressurization of the overpressure Wufeng-Longmaxi shale gas reservoirs and the charging of gas into the Xixiangchi Group reservoir of the Pingqiao anticline since the Late Cretaceous, which show obvious superimposed accumulation characteristics. There are different accumulation patterns in different geological periods. The accumulation pattern of the “old source - young reservoir” (i.e. hydrocarbons generated from older source rocks accumulating in younger reservoirs) dominates before the Late Cretaceous, and that of “juxtaposed young source - old reservoir” (i.e. hydrocarbons generated from younger source rocks accumulating in juxtaposed older reservoirs) dominates after the Early Cretaceous. Moreover, faults acted as critical vertical pathways for hydrocarbon migration during the evolution of the Xixiangchi Group gas reservoirs. This model provides new insights and theoretical basis for evaluation and mapping of the Xixiangchi Group play fairway in eastern Sichuan Basin.

Characteristics of fluid inclusions and fluid coupling mineralization of the Pengyang uranium deposit, Ordos Basin

The Pengyang uranium deposit, located in the Tianhuan Depression in the southwestern Ordos Basin, is a large-scale sandstone-type uranium deposit in aeolian sediments. The uranium-bearing rock series in the deposit is mainly sandstone and conglomerate from the Lower Cretaceous Luohe Formation, which is dominated by desert facies. Based on our investigation of the geological characteristics of this deposit, a systematic study of the type, characteristics of the ore-forming fluid and ore genesis was conducted. Through the petrographic study of inclusions, three stages of inclusions have been identified in the quartz overgrowth, carbonate cements, and secondary microfractures in the ore-bearing sandstone of the Luohe Formation, and the third-stage inclusions in the secondary microfractures or late calcite cements represent the ore-forming fluids. There are three types of inclusions captured from ore-forming fluids: liquid-dominated aqueous inclusions, liquid-dominated hydrocarbon inclusions, and natural gas inclusions. Their gaseous components are mainly CH4 and H2O, with a small amount of N2. The homogenization temperatures of liquid-dominated aqueous inclusions range from 58 to 147 ℃, and the salinity ranges from 0.35 to 6.88 wt%. The homogenization temperature of liquid-dominated hydrocarbon inclusions mainly exhibits two peaks (at 60–140 ℃ and 160–190 ℃). Our studies of carbon-hydrogen–oxygen isotopes indicate that the formation of uranium-bearing sandstone is closely related to the participation of organic matter such as oil and gas. Based on the comprehensive researches of fluid inclusions and isotope analysis, it is concluded that the ore-forming fluid of the Pengyang uranium deposit is a mixture of meteoric water and hydrocarbon fluid. The deep hydrocarbon fluid traveled upward along the structural fractures, acting as a reductive agent and participating in the oxidation–reduction reaction of the Pengyang uranium deposit. Although hydrocarbon fluid can bring some uranium from deep area, the uranium from the provenance area of the basin may be the main source of mineralization in the Pengyang uranium deposit. The Pengyang uranium deposit is a sandstone-type uranium deposit related to the reduction of oil and gas. Uranium mineralization is mainly caused by the coupling between the uranium-bearing oxygen-rich fluid infiltrated from the surface and the deep hydrocarbon fluid.

Hydrocarbon Accumulation Mechanism of Ordovician in the Halahatang Area, Tarim Basin—Evidence From Organic Geochemistry

As an important oil and gas-bearing area in the Tarim Basin, the Halahatang Area has great potential for resource exploration. However, the research on oil and gas sources and the filling period of the Lower Paleozoic Ordovician is still limited. In this study, the organic geochemical analysis of source rocks, the simulation technology of hydrocarbon generation evolution basin, the distribution characteristics of fluid inclusions, and the measurement of uniform temperature are used and combined with the stable isotope of natural gas and the characteristics of oil-source biomarkers, and the process of oil and gas accumulation is restored. The results show that: ①The Cambrian source rock is the main contributor to Ordovician crude oil in the Halahatang Area, and its crude oil shows the characteristics of lightweight, rich chain hydrocarbons, and poor aromatic hydrocarbons. The composition of C7 compound shows the advantage of n-heptane, which belongs to type I kerogen. The biomarker compounds show the characteristics of high content of long-chain tricyclic terpane and high Ts/Tm values, and the crude oil is considered to be a highly mature sapropelic kerogen source oil. ②The carbon isotopes of methane and ethane of Ordovician natural gas in the Halahatang Area are light (from −35.6 to −29.5‰), showing obvious characteristics of sapropelic gas. ③There are three stages of the oil and gas accumulation process in Ordovician reservoirs in the Halahatang Area. In the middle and late Caledonian periods, the source rock reached the hydrocarbon generation threshold and began to generate oil. The late Hercynian period was the main accumulation period, and the Himalayan period was dominated by dry gas filling. ④The crude oil generated in the Middle-Late Caledonian periods migrated along the faults and conductive layers and finally accumulated in the high parts of the structure in the north. The migration of oil and gas in the late Hercynian period was limited by the bitumen produced by early biodegradation. Since the asphalt plugging condensate gas has almost failed to cause gas invasion from the reservoir, the Ordovician reservoir still maintains the characteristics of crude oil-associated gas, forming an oil and gas reservoir with a gas cap.

Discussion on the Characteristics of Granite Fluid Inclusions Associated with Dry-hot Rock in Gonghe Basin and Its Application

Discussion on the Characteristics of Granite Fluid Inclusions Associated with Dry-hot Rock in Gonghe Basin and Its Application

Insights into the Ore Genesis of the Harla Gold Deposit in Eastern Tianshan, NW China: Evidence from Geology, Fluid Inclusions, and H-O-C-S-Pb Isotopes

The Harla gold deposit is located on the eastern segment of the Kanggur-Huangshan ductile shear belt in Eastern Tianshan, on the southern margin of the Central Asian Orogenic Belt. The orebodies show close spatial association with NEE-trending faults and are hosted in volcanic rocks of the Lower Carboniferous Xiaorequanzi Formation. Three mineralization stages were recognized: an early stage characterized by specularite–pyrite–quartz mineralization, an intermediate stage with pyrite–chalcopyrite–gold–quartz genesis, and a late stage defined by sulfide-poor calcite and quartz veins. Three types of fluid inclusions (FIs) were recognized: daughter mineral-bearing triphase FIs (S-type), liquid-dominated biphase FIs (LV-type), and liquid-only monophase FIs (L-type). S- and LV-type FIs entrapped in the early stage show similar homogenization temperatures (Th) at 216–229 °C and 199–222 °C. However, they display contrasting salinities of 32.2–33.4 and 6.7–8.9 wt.% NaCl eqv., respectively. LV-type FIs entrapped in the intermediate stage display Th of 163–191 °C, with salinities of 5.6–7.4 wt.% NaCl eqv. LV-type FIs occur in the late stage display Th of 135–258 °C and salinities of 3.7–5.6 wt.% NaCl eqv. Insights from hydrogen, oxygen, and carbon isotope analyses (δ18OH2O = −10.7 to 3.3‰, δDH2O = −88.1 to −61.4‰, δ13CH2O = −8.9 to −6.2‰), combined with FI characteristics, indicate that the ore-forming fluids were derived from a hybrid source of magmatic and meteoric water, with the latter becoming dominant in the later stage. Sulfide and lead isotopic compositions (δ34Spyrite = 0.2 to 2.5‰, 206Pb/204Pb = 17.709–18.273, 207Pb/204Pb = 15.483–15.569, 208Pb/204Pb = 37.533–37.754) suggest that ore-forming materials were mainly derived from wall rocks (Xiaorequanzi Formation rocks and plagiogranite). By integrating geological background work and fluid inclusion analyses, as well as isotope data, we conclude that the Harla gold deposit is an epithermal-type deposit.

Geochemical Characteristics of Fluid Inclusions and Isotopes in Zhazixi Sb-W Deposit

Zhazixi Sb–W deposit, where Sb and W coexist in different orebodies, features a unique metal association between the two minerals. This study aims to analyze the respective sources of Sb and W, through temperature measurement of fluid inclusions, laser-based Raman spectroscopy, as well as isotope test. The ore-forming fluid of Zhazixi Sb-W deposit was found to be a CO2-N2-H2O-NaCl system, which is rich in CO2 and N2. N2-rich inclusions and C, H, O isotopes were observed, indicating that the ore-forming fluid mainly comes from deep magmatic rocks, and secondarily from the metamorphic hydrothermal fluids. The detected S and Pb isotopes suggest that the ore-forming material stems from multiple sources. After comprehensive analysis of the geological features of the deposit and the local region, we believed that the Sb in Zhazixi Sb-W deposit stems from Banxi group, but W comes from the concealed rock body. The results provide a theoretical basis for the metallogenic research and prospecting direction of Xuefengshan metallogenic belt.

Hydrocarbon Charge History of the Upper Paleozoic, Ordos Basin as Revealed by Fluid Inclusions

The petrographic characteristics of fluid inclusions of the main gas-bearing intervals of the Upper Paleozoic in the Ordos Basin are analyzed by means of plane-polarized light, fluorescence microscopy, and microthermometry. With the burial history of the basin, the charging and enrichment of natural gas in low-permeability reservoirs are also studied through a comparison of parameters such as the homogenization temperature and freezing temperature of aqueous inclusions associated with hydrocarbon inclusions. A comprehensive analysis of the assemblage characteristics, components, and temperature of fluid inclusions of the Upper Paleozoic shows that the abundance, attitude, and composition of gaseous hydrocarbon inclusions can reflect petroleum enrichment in low-permeability reservoirs. The homogenization temperature and freezing temperature of aqueous inclusions associated with gaseous hydrocarbon inclusions show no obvious discontinuities, indicating that natural gas charging was a long and continuous process since the Late Triassic to Early Cretaceous (210–100 Ma) and natural gas accumulation mainly occurred during the Early Cretaceous (125–100 Ma).

Telluride-bearing Au-Ag mineralization in the Singu-Tabeikkyin gold District, Mogok metamorphic Belt, Myanmar: New constraints on an intermediate-sulfidation epizonal orogenic ore system

The Singu-Tabeikkyin gold district is located in the Mogok Metamorphic Belt in Myanmar. Gold mineralization in this district occurs in a set of mainly N − S trending quartz veins, which are associated with minor replacement zones and skarns. This study investigates the field geology, ore mineralogy, mineral chemistry, and fluid inclusion characteristics of three gold prospects (CG-1, CG-2, and CG-3) containing Au-Ag along with telluride assemblages. Gold has formed as two generations, and only the second gold mineralization stage is associated with major telluride assemblages. The tellurides are associated with chalcopyrite, a late generation of sphalerite, and electrum. The telluride phases identified in the study are hessite (Ag2Te), petzite (Ag2AuTe3), altaite (PbTe), tellurobismuthite (Bi2Te3), and tetradymite (Bi2Te2S), and unnamed Au-Ag telluride phases. Liquid-rich two-phase fluid inclusions have homogenization temperatures in the range of 174–310 °C, and salinities of 1–12 wt% NaCl equiv. The chlorite geothermometry in this study yielded a variable temperature range from as low as 250 °C to 380 °C, in general agreement with the fluid inclusion temperature range. The thermometric data, in conjunction with the analysis of telluride-sulfide phase stability, constrain the conditions of ore formation to logƒTe2 and logƒS2 values of − 17 to − 9.5, and of − 13.4 to − 12.5. The geological and mineralogical data and the reconstructed conditions of ore formation as well as high-grade metamorphic host rocks of gneiss, marble and calc-silicates suggest that the telluride bearing gold-silver mineralization in the Singu-Tabeikkyin gold district forms part of an intermediate-sulfidation epizonal orogenic Au system.

Genesis of the distal axis East Longjing-2 hydrothermal field on the ultraslow-spreading Southwest Indian Ridge: Evidences from fluid inclusions

Tectonic-related hydrothermal fields constitute about half of the hydrothermal activities on slow-ultraslow spreading ridges, and may form sulfides large in size and rich in precious metals. Ore-forming fluids are critical for element migration progressions, thus are key parameter to reveal their genesis and sulfide mineralization processes. However, previous studies of this kind of hydrothermal activities primarily focused on vent fluids which have transient effects or based on surface samples, while the ore-forming fluid evolution and related sub-seafloor mineralization process are still poorly constrained. In this study, we analyzed the characteristics of fluid inclusions in the stockwork mineralization of the East Longjing-2 hydrothermal field (ELHF-2) on the Southwest Indian Ridge. The results showed that the fluids evolved gradually from medium-high temperatures (260–315 °C, mean of 284 °C) and low salinities (0.70–3.70 wt.% NaCl eq.) in the disseminated pyrite mineralization stage (Stage I) to medium temperatures (239–261 °C, mean of 249 °C) and low salinities (1.05–3.85 wt.% NaCl eq.) in the chalcopyrite–pyrite–quartz veinlet mineralization stage (Stage II). Laser Raman specular analysis indicated the fluid inclusions were mainly composed of H2O. The low salinity (lowest 0.7 wt.% NaCl eq.) and various vapor volume (0.05–0.5) observed in these fluid inclusions suggested the ore-forming fluids were probably mixtures of the low-salinity vapor phase formed by phase separation with large amounts of seawater. The precipitation of the ore-forming elements was likely induced by temperature decreasing. We propose that hydrothermal circulation in ELHF-2 was a result of coupled high-permeability detachment fault and shallow gabbro intrusion. Our results suggest that high temperature hydrothermal activities could be developed on distal axis area on ultraslow spreading ridges.

Characteristics of fluid inclusions and hydrocarbon accumulation period of Huoshiling -Yingcheng Formations in Wangfu fault depression, Songliao Basin, China

With the advancement of technology and the improvement of the level of understanding, deep hydrocarbon exploration in the Songliao basin has become one of the key objects. Wangfu fault depression is a typical oil-gas fault depression with a high degree of deep hydrocarbon exploration in the southeast of the Songliao basin. Finding out the oil-gas accumulation period of the deep hydrocarbon exploration is of great practical significance. The aim of the study is to find out the period and time of deep hydrocarbon accumulation in Wangfu fault depression, so as to provide an important theoretical basis for deep hydrocarbon exploration. The objective is to reveal the process and track of deep hydrocarbon accumulation of Huoshiling -Yingcheng Formations based on the lithofacies characteristics and the homogenization temperature distribution characteristics of fluid inclusions, combined with the burial history and thermal history simulation of strata in Wangfu fault depression. The results show that the reservoir of Huoshiling-Yingcheng Formations in Wangfu fault depression have experienced at least two periods of oil and gas filling events and two periods of oil filling events. The first hydrocarbon filling event was formed in the middle of Quantou period (about 95.4–97.5 Ma). Being influenced by the hydrocarbon expulsion from the source rocks of Huoshiling Formation, a large-scale oil and gas migration was formed. The second filling event occurred in the early Qingshankou period (about 93.6-92.0 Ma). During this period, the lake of the Songliao Basin expanded extensively, and the oil migrated and recharged through the opening fault on a large scale. The third filling event occurred in the late Qingshankou period (about 88.9–90.8 Ma), during which the source rocks of Shahezi Formation were in the peak of hydrocarbon generation and a large number of hydrocarbon migration occurred. The last large-scale filling event occurred in the early Nenjiang period (about 85.8-84.0 Ma). Affected by the rapid expansion of the lake basin in this period, the fault fractures reopened, resulting in the secondary migration and filling of oil.

Thermobaric characteristics of fluid inclusions in subsalt rocks of the East of Precaspian syneclise and the Pre-Ural of Aktobe

This article presents the results of study of fluid inclusions in calcite and quartz of the Devonian, Carboniferous and Permian strata of the eastern part of the Precaspian syneclise and the Aktobe Preuralie of Pre-Ural trough. Were determined by microthermometry method the temperatures of melting and homogenization of fluids in crystals of rock and the types of hydrocarbons in inclusions. Interpretation of the measurement results made it possible to determine the most specific values of pressure and temperature, equal, respectively, 350-500 atm and 85°-130°C increasing up to 650 atm / 350°C and more towards the Urals. Accordingly, the geothermal gradient increases from west to east from 24°C/km to 40°C/km in the Aktobe Preuralie. The mineralization of fluid inclusions varies in the same direction from 10-12% to 3-6% of the weight equivalents of NaCl. These results demonstrate three types of gradients: the lowest in the western inner and platform parts of the Precaspian basin (24°C/km), the average in the Pre-Ural trough and the highest in the eastern suture with the Ural folded system. Accordingly, there is an increase in the geothermal background in the Aktobe Preuralie of Pre-Ural trough to 40°C/km.

Origin and Characteristics of the Shwetagun Deposit, Modi Taung-Nankwe Gold District and the Kunzeik and Zibyaung Deposits, Kyaikhto Gold District in Mergui Belt, Myanmar: Implications for Fluid Source and Orogenic Gold Mineralization

The Mergui Belt of Myanmar is endowed with several important orogenic gold deposits, which have economic significance and exploration potential. The present research is focused on two gold districts, Modi Taung-Nankwe and Kyaikhto in the Mergui Belt comparing their geological setting, ore and alteration mineralogy, fluid inclusion characteristics, and ore-forming processes. Both of the gold districts show similarities in nature and characteristics of gold-bearing quartz veins occurring as sheeted veins, massive veins, stockworks to spider veinlets. These gold deposits are mainly hosted by the mudstone, slaty mudstone, greywacke sandstone, slate, and slaty phyllite of Mergui Group (dominantly of Carboniferous age). The gold-bearing quartz veins generally trend from NNE to N-S, whereas some veins strike NW-SE in all deposits. The gold-bearing quartz veins are mainly occurred within the faults and shear zones throughout the two gold districts. Wall-rock alterations at Shwetagun are mainly silicification, chloritization, and sericitization, whereas in Kyaikhto, silicification, carbonation, as well as chloritization, and sericitization are common. At Shwetagun, the gold occurred as electrum grains in fractures within the veins and sulfides. In Kyaikhto, the quartz-carbonate-sulfide and quartz-sulfide veins appeared to have formed from multiple episodes of gold formation categorizing mainly as free native gold grains in fractures within the veins or invisible native gold and electrum within sulfides. At Shwetagun, the ore minerals in the auriferous quartz veins include pyrite, galena, and sphalerite, with a lesser amount of electrum, chalcopyrite, arsenopyrite, chlorite, and sericite. In Kyaikhto, the common mineralogy associated with gold mineralization is pyrite, chalcopyrite, sphalerite, galena, pyrrhotite, arsenopyrite, marcasite, magnetite, hematite, ankerite, calcite, chlorite, epidote, albite, and sericite. At Shwetagun, the mineralization occurred at a varying temperature from 250 to 335°C, with a salinity range from 0.2 to 4.6 wt% NaCl equivalent. The Kyaikhto gold district was formed from aqueous–carbonic ore fluids of temperatures between 242 and 376°C, low to medium salinity (<11.8 wt% NaCl equivalent), and low CO2 content. The ore-forming processes of the Shwetagun deposit in the Modi Taung-Nankwe gold district and the Kyaikhto gold district are remarkably comparable to those of the mesozonal orogenic gold systems.

Characteristics of fluid inclusions in the sandstone‐hosted Qianjiadian uranium deposit, southwest Songliao Basin, northeastern China: Implications for the nature and evolution of ore‐forming fluids

AbstractThe Qianjiadian deposit is a typical sandstone‐hosted uranium deposit that is hosted mainly in sandstone and siltstone of the Lower Cretaceous Yaojia Formation, located within the transition between the Kailu Depression and Jiamatu Uplift in the Songliao Basin, northeastern China. We studied the geological characteristics of this deposit, and analysed the mineralized sandstone by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) to identify the host minerals of fluid inclusions associated with uranium mineralization and describe their petrographic characteristics. In addition, this research investigated the origin of ore‐forming fluids and the relationship between petroleum fluids and uranium mineralization, based on the following findings. (1) EPMA and SEM data show that uranium minerals are hosted mainly in hydrothermal quartz (HQ) within sandstone cement, which indicates that uranium minerals co‐precipitated with HQ. (2) The fluid inclusions in the HQ show low homogenization temperatures (102.5–169.5°C) and low salinities (1.7–6.1 wt%). In contrast, aqueous inclusions in quartz overgrowths show lower temperatures (60.5–117°C) and higher salinities (4.2–8.7 wt%). (3) Petroleum fluids trapped in HQ homogenize fluid and those along healed microfractures in quartz overgrowth or plagioclase overgrowths at ~70–80°C, those trapped in present‐day organic inclusions at ~85–95°C, and at the boundary of detrital mineral grains show higher homogenization temperatures (~135–145°C). These results indicate that the hydrothermal quartz cement is associated with uranium mineralization, and the ore‐forming fluid of Qianjiadian sandstone‐hosted uranium deposit are characterized by low‐temperature, low‐salinity hydrothermal fluids, and the changes of temperature and salinity of hydrothermal fluids may not be intimately associated with uranium mineralization.

Zinc and cadmium isotopic constraints on ore formation and mineral exploration in epithermal system: A reconnaissance study at the Keyue and Zhaxikang Sb–Pb–Zn–Ag deposits in southern Tibet

The Keyue Sb–Pb–Zn–Ag deposit is a medium-sized deposit near the super-large Zhaxikang Sb–Pb–Zn–Ag deposit within the North Himalayan Metallogenic Belt (NHMB). These two deposits, separated only by a regional NS-trending normal fault with strike-slip properties, have similar ore-forming elements and mineral associations, ore characteristics, ore paragenetic sequence, wall rocks, and H–O–S–Pb isotopic and fluid inclusion characteristics. Herein, we establish Zn–Cd isotopic connections between these two proximal ore deposits. As a means to draw comparisons, we conducted Zn–Cd isotopic research of the Keyue deposit to compare it with that of the Zhaxikang deposit. Both the δ66Zn and δ114/110Cd values of sphalerite from the Keyue deposit display a temporally decreasing trend from stage 2 (δ66Zn: 0.15‰–0.58‰, average value = 0.29‰; δ114/110Cd: –0.23‰ to 0.38‰, average value = 0.05‰) to stage 3 (δ66Zn: –0.28‰ to 0.12‰, average value = –0.06‰; δ114/110Cd: –0.38‰ to –0.32‰, average value = –0.35‰), which was also found in the Zhaxikang deposit. A simple three-phase separation distillation models this temporally decreasing trend, which resulted from the Zn–Cd isotopic fractionation that is most likely related to vapor–liquid-solid dynamics of ore-forming fluid. This distillation model was further augmented by the general geochemical characteristics and fluid inclusion data. A congruence among several geological and geochemical evidence points to the genetic relationships between these two ore deposits that have the same metal origins and experience similar ore formation processes. A three-phase separation model has been established to imitate this evolution, which reveals substantial exploration potential for Zn–Cd at depths below 4100 m within the Zhaxikang orefield. In summary, Zn–Cd isotopes have the potential to trace metal sources, monitor fluid evolution, constrain ore formation, and provide insights into mineral exploration.

Hydrocarbon Accumulation Process and Mechanism in the Lower Jurassic Reservoir in the Eastern Kuqa Depression, Tarim Basin, Northwest China: A Case Study of Well Tudong 2 in the Tugerming Area.

The Lower Jurassic reservoir has recently made a significant breakthrough in petroleum exploration in the Tugerming area of the eastern Kuqa depression, Tarim Basin, northwest China. Therefore, it is significantly essential to simulate the hydrocarbon charging process and analyze the accumulation mechanism in the study area. In our study, we mainly combine three geochemical techniques to identify the origin of crude oils and fluid inclusion characteristics in the target well (Tudong 2), including gas chromatography–mass spectrometry (GC–MS), comprehensive two-dimensional GC-time-of-flight MS (GC × GC–TOFMS), fluid inclusion analysis, and quantitative fluorescence technique. Combined comprehensive experiments with a set of burial history and tectonic thermal evolution history realize the reconstruction of the hydrocarbon charging process in the Yangxia formation. The results show that Jurassic coal-bearing source rocks are primary hydrocarbon sources, and there are three hydrocarbon charging events in the Lower Jurassic reservoir. First, the mature oils have expelled into the reservoir during the early-middle period (15–10 Ma) of the Miocene Kangcun Formation, forming yellow fluorescent oil inclusions and most of the quantitative grain fluorescence (QGF) indexes exceed 4. Second, numerous condensate oils have charged into the reservoir in the period of late Miocene Kangcun Formation-early Kuqa Formation (9–6 Ma), accompanied by blue-white oil inclusions and QGF on extract intensity greater than 4. Finally, vast natural gas has accumulated in the reservoir since Kuqa Formation (5–0 Ma), resulting in the adjustment of ancient reservoir and residual bitumen. Interestingly, we find that the hydrocarbon accumulation mechanism is characterized by self-generation and self-storage in the Jurassic Yangxia Formation, and the study area has prominent characteristics of late accumulation. Thus, the evidence obtained from our analysis suggests favorable geological conditions for the formation of a large oil and gas reservoir in the eastern Kuqa Depression.

Fluid inclusion evidence for the magmatic-hydrothermal evolution of closely linked porphyry Au, porphyry Mo, and barren systems, East Qinling, China

Abstract The Xiong’ershan district in central China hosts broadly coeval porphyry Au (Qiyugou deposit), porphyry Mo (Leimengou deposit), and barren (Huashan pluton) systems. The key controls on the ore potential and different mineralization styles in these systems are not well understood, with first-order differences in fluid chemistry and melt sources being the main alternatives. The fluid inclusion characteristics of all three porphyry systems have been studied using an integrated approach that combines field geology, petrography, microthermometry, and laser ablation–inductively coupled plasma–mass spectrometry analysis of single fluid inclusions. The results permit a reconstruction of the magmatic-hydrothermal evolution of the ore-forming fluids, and to elucidate whether specialized hydrothermal fluids strongly enriched in ore metals (i.e., Mo, Au, Cu) were essential to form the economically significant deposits. The fluid compositions across the three hydrothermal stages from the Qiyugou Au deposit remain approximately the same over time, suggesting that progressive magma fractionation, fluid-rock reaction along fluid path, and mineral precipitation had a limited effect on fluid composition. The syn-ore stage fluids of the Leimengou Mo deposit are characterized by higher Cs/Na, Sr/Na, and B/Na, but lower K/Na and Cl/Na ratios, and also have salinities and homogenization temperatures distinct from the earlier fluids. This demonstrates that Mo mineralization was caused by a second pulse of fluid input from a highly fractionated felsic magma subsequent to the pre-ore stage. At the Huashan barren pluton, fluids from phase II have higher Cs/Na, B/Na, Li/Na, and Rb/Na ratios with lower homogenization temperatures than fluids occurring in porphyritic rocks of phase III, reflecting a higher degree of magma fractionation of this plutonic complex. The Huashan pluton does not host economic mineralization which is likely caused by the low ore metal tenor, inefficient fluid extraction from the melt, or the flat-roof geometry preventing accumulation of a large volume of fluid in the apical part. The Au tenor of the Qiyugou deposit was most likely contributed by mantle-derived material of higher Mg/Na, Fe/Na, Pb/Na, and Zn/Na ratios. Taken together, the metal charged magmatic-hydrothermal fluids, steeply dipping geometry, and small volume of the porphyry stocks all suggest that a much larger magma chamber feeding the porphyry systems should be present at deeper levels with good potential for Mo mineralization below the current level of exposure at Qiyugou deposit.

Geological and geochemical characterization of the Nanlia and Makorongo gold prospects, Mozambique Belt, northeastern Mozambique

Northern Mozambique hosts numerous occurrences of gold mineralization in metamorphic terranes, which are little known because of the insufficient studies. In this work, we describe the main geological and geochemical characteristics of the Nanlia and Makorongo gold prospects located in the Namuno district, northeastern Mozambique. We report the mode of occurrence of gold, petrography and mineralogy of the host rocks and ores, fluid inclusion characteristics and stable isotope data, to understand the genesis of gold mineralization in the study area. The gold mineralization in the Nanlia and Makorongo prospects is hosted by the Neoproterozoic Xixano Metamorphic Complex in the Mozambique Belt, which is composed of mafic granulite, amphibolite, paragneiss and marble. The host rock amphibolite consists mainly of amphibole, plagioclase, biotite and minor quartz, titanite and sulfides. The gold mineralization is associated with E-W to ENE-WSW trending quartz veins, oriented parallel to low angle to the foliation of the host rock amphibolite. The ore mineralogy consists of pyrite, pyrrhotite, chalcopyrite, magnetite, galena and sphalerite, with minor tellurides, xilingolite, electrum and native gold. Mineralized quartz veins in the Nanlia and Makorongo prospects host two types of fluid inclusions which locally coexist in the same assemblage: type 1, one-phase CO2 inclusions with final melting temperature between −58 and −52 °C; and type 2, two-phase H2O + NaCl (±CO2 ± N2 ± CH4) inclusions with homogenization temperature ranging from 246 to 370 °C and 239 to 382 °C in the Nanlia and Makorongo prospects, respectively, and salinity of 12–21 wt% NaCl eq. Pressure and temperature (P-T) conditions of ore-formation were estimated from fluid inclusions and sphalerite geobarometry at 420–620 °C and 160–280 MPa, and 330–440 °C and 150–200 MPa in the Nanlia and Makorongo prospects, respectively. The Nanlia and Makorongo gold prospects are classified as orogenic-type gold deposits. We suggest that gold mineralization was caused by an aqueous-carbonic fluid with low-moderate salinity originated by devolatilization during prograde metamorphism of underlying rocks. The gold mineralization post-dates the peak of metamorphism of the host rocks, thus formed during the retrograde metamorphism of the host rocks. The precipitation of gold in the Nanlia and Makorongo prospects was caused by fluid phase separation induced by pressure drop as the mineralizing fluid migrated to shallower depths.