Homogenization Temperatures Of Fluid Inclusions Research Articles

Petrographic and geochemical characteristics of the Middle−Upper Cambrian Xixiangchi dolomites in the central Sichuan Basin, southwest China: Implications for dolomite origins and dolomitization mechanisms

The Middle−Upper Xixiangchi Formation of the Sichuan Basin consists of mixed carbonates and siliciclastics. The carbonates are dominated by dolomitized lime-mudstones and dolo–grainstones of shallow warm-water environments. Petrographic examination of carbonates reveals two types of dolomite: (i) fabric retentive dolomicrite (D1, micritic to near-micritic, 4–50 μm) and (ii) fabric-destructive dolomite (eu-to subhedral crystals with cloudy cores and clear rims, 50–300 μm). The mean ∑REE values of D1 (15.3 ± 6.3 ppm) and D2 (14.6 ± 7.1 ppm) and the nearly identical mean PAAS-normalized patterns that coincide at some points, suggest an origin from similar parent fluids that were not dramatically changed by water-rock interaction through basinal sedimentary rocks. The estimated Sr/Ca molar ratios of D1 (0.0043–0.0055) parent fluids and their estimated δ18O values imply an origin from a mixture of meteoric and seawaters, which is also consistent with the petrographic evidence. The partially coinciding normalized REE pattern of D2 with that of the earlier D1 suggests that the latter was formed at a mid-burial setting rather than deep burial, which is consistent with higher homogenization temperatures of primary two-phased fluid inclusions retained in D2 (Th = 118.1 ± 5.3 °C). Therefore, D1 is suggested to have formed (1) during a penecontemporaneous period that resulted from a slight evaporative pumping effect within a near-surface diagenetic environment, and/or (2) during post-penecontemporaneous or penecontemporaneous seepage-reflux dolomitization, induced by sea-level fluctuations, during early stages of diagenesis at near-surface conditions. D2 formed likely through the recrystallization of D1 or a continuation of reflux dolomitization of the same precursor at mid-burial rather than deep burial settings.

Geneses of multi-stage carbonate minerals and their control on reservoir physical properties of dolomitic shales

The formation and distribution of carbonate minerals have important influences on reservoir development. The dissolution-precipitation effect plays an important role in controlling the reformation of reservoir physical properties. High-quality source rocks with abundant reserves exist in the Lower Permian Fengcheng Formation, Junggar Basin, China, and the basin reservoir contains a large amount of carbonate minerals. Multi-stage cementation and dissolution of these carbonate minerals have important influences on diagenesis-porosity evolution of the reservoirs. Because of the coupled influences of deep fluid transformation and volcanic activity in the diagenetic evolution of the Fengcheng Formation, it has experienced a complex evolution of carbonate minerals. In this study, through core observation, thin section, X-ray diffraction mineral composition, carbon and oxygen isotope, fluid inclusion homogenization temperature, and laser Raman and cathodoluminescence analysis, the material sources, development period, dissolution-precipitation reactions, and transformation mechanisms of carbonate minerals in the diagenetic evolution of the Fengcheng Formation reservoir were evaluated in detail. Finally, the controlling effect of multi-stage cementation of carbonate minerals on the reservoir physical properties was determined. The results showed that there are three stages of carbonate minerals in the Fengcheng Formation. The material source of the early carbonate was mainly terrigenous debris, and the formation temperature range was 30–70 °C. The material source of middle-late carbonate minerals was mainly decarboxylation of organic acid, and the formation temperature was 60–110 °C. In addition, some were generated through hydrothermal metasomatism at high ambient temperatures, up to 120 °C. In areas where early carbonate minerals are well developed, a large number of microcracks was more likely to form under the influence of tectonic movements, resulting in high-quality reservoirs. The formation of late carbonate minerals, which is the main reason for reservoir densification in the later stage of diagenesis, resulted in poor reservoir physical properties. This study provides information useful for predicting the distribution areas of favorable reservoirs in the Fengcheng Formation, and a theoretical basis and guidance for improving the accuracy of shale oil exploration and development.

Characteristics and Geological Significance of CO2-Rich Fluid Inclusions in Dakalasu No. 1 Pegmatite Dyke, Altay

The fluids in of pegmatite rare metal deposits are generally rich in rare metal elements and volatiles (B, P, F, H2O, CO2, etc.), and they have a high capacity for dissolving and migrating rare metals. The Dakalasu No. 1 rare metal pegmatite vein is located in northwest China’s Altay orogenic belt. Previous studies have indicated that it is a small- to medium-sized beryllium-niobium-tantalum deposit. It showed significant mineral assemblage zonations from the rim to the core, and the mineralizing fluids define a volatile-rich NaCl-H2O-CO2 ± CH4 system. In this contribution, beryl and quartz, which are widely developed in each mineral association and textural zone, were selected for fluid inclusion research through detailed petrographic investigation, microthermometry, and LA-ICP-MS analysis. Petrographic results show that at least three types of fluid inclusions are developed in each mineral textural zone. They are CO2-rich inclusions (type I), gas-liquid two-phase inclusions (type II), and daughter mineral-bearing inclusions (type III), respectively. Additionally, minor melt inclusions (type IV) are visible in the beryl from the rim zone. Microthermometric measurements showed that the homogenization temperature of fluid inclusions in the rim zone was concentrated between 242 °C and 293 °C, with an average of 267 °C, and the salinity was between 7.2–10.3 wt% NaCleqv, with an average of 8.6 wt% NaCleqv. In comparison, the temperature of the core zone was in the range of 225–278 °C, with an average of 246 °C, and the salinity focused between 6.0–7.7 wt% NaCleqv, with an average of 7.1 wt% NaCleqv. The quantitative analysis of individual inclusions by LA-ICP-MS revealed that Li, B, K, Zn, Rb, Sb, Cs, and As were relatively enriched in the rim zone. In contrast, the core zone showed a decreasing trend in trace elements such as Li, B, K, Rb, and Cs. The CO2 content in the fluid exhibited the same decreasing trend from the rim to the core zone, indicating that volatile components such as CO2 played an essential role in the migration and enrichment of rare metal elements. The melt-fluid immiscibility is likely to be a necessary mechanism for significantly enriching rare metals in the Dakalasu No. 1 pegmatite dyke.

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

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

Distal accumulation of leaked gas from deep overpressured zone：the case of the Yanan Sag, Qiongdongnan Basin, South China Sea

In the overpressured Yanan Sag of Qiongdongnan Basin, South China Sea, four structures contain varying gas volumes despite overall similarities in geological conditions. One notable difference, however, is their varying pressure characteristics. We investigated the nature and cause of the overpressure as well as its relation to natural gas migration-accumulation in the four structures. Well logs, drill-stem tests (DSTs), and mud weights, reveal two zones of overpressure in the YC21-1 and YC26-1 structures that contain no economic gas reservoirs. There is one zone of overpressure in the YC13-1 structure, whereas the YC13-4 structure is hydrostatically pressured. Cross-plot analysis of density and sonic logs indicates a mild to moderate overpressure section in the Neogene strata, interpreted as resulting from disequilibrium compaction and clay diagenesis.The Pliocene to Quaternary is considered to be a key period for strong overpressure formation in the Yanan Sag. Fast subsidence and burial may have induced intense gas generation by maturation and vertical compaction disequilibrium in the Oligocene strata of the central Yanan Sag. The resulting high pressure would have been transferred to the Oligocene reservoirs in the YC21-1 and YC26-1 structures in the slope belt. Fluid-inclusion homogenization temperature (Th) in combination with modeled burial curves indicates that gas filled the four structures during this period, suggesting that gas migration was associated with overpressure. Fluid-inclusion phase reflecting gas saturation and abnormal excursion of vitrinite reflectance (%Ro) suggest that leakage related to caprock fracturing resulted in gas accumulation failure in the high-pressure YC21-1 and YC26-1 structures. 13C depletion of methane in the YC13-1 and YC13-4 structures away from the sag center suggests that the current gases probably result from gas exsolution from formation waters. These gases are high-to over-mature, and thus were mainly derived from the deeper, central Yanan Sag rather than the local source rocks. Therefore, we infer that the gas leaked from the deep high-pressure zone in the sag center, then migrated in solution until a point at which PVT conditions would allow exsolution. Subsequently, the free gas would have migrated into the normally pressured reservoirs in the slope belts where it finally accumulated. Compared to the YC13-4 structure, the overlying Neogene mudstones in the YC13-1 structure are overpressured, which may serve to increase the seal capacity and explain the higher gas reserves in this area.

Diagenetic Evolution of Marine Carbonate Reservoirs: A Case Study from M51-2 Submember of Middle Ordovician Majiagou Formation in SE Ordos Basin, China

In the Yan’an gas field, southeast of Ordos Basin, the discovered natural gas reserves are 114 billion cubic meters. Highly productive gas reservoirs were formed in paleokarst. By using 164 samples from 27cores, the geochemical study of diagenetic evolution was carried out, and the relationship between different diagenesis environments was discussed by using scanning electron microscopy, carbon and oxygen isotope, and isotopic and fluid inclusion microthermometric observations. Results show that the lithology of reservoir is anhydrite-bearing mud powder crystal dolomite, powder crystal dolomite, and karst breccia dolomite; the filling includes dolomite, calcite, anhydrite, quartz, fluorite, and clay minerals; δ13C (‰ PDB) value is between −6.31‰ and 1.01‰, δ18O (‰ PDB) value ranges from −11.36‰ to −6.54‰, and the 87Sr/86Sr value ranges from 0.7077 to 0.7081. The overall carbon isotope of Majiagou Formation decreases first and then increases, and the strontium isotope evolution curve generally increases first and then decreases, indicating that the water environment of Majiagou Formation experienced a relatively high salinity concentration period and a relative desalinization period during deposition. The stratigraphic development shows a cyclical trend, responding to the multistage transgression and regression process. The homogenization temperature of fluid inclusions is 70–100°C, salinity is 0–3.5%, homogenization temperature of fluid inclusions is 70–100°C, salinity is >3.5%, homogenization temperature of fluid inclusions is 100–160°C, salinity is 0–3.5%, homogenization temperature of fluid inclusions is 100–160°C, salinity is >3.5%, homogenization temperature of fluid inclusion temperature is 160–200°C, and salinity is >3.5%, indicating that the fluid source in the sedimentation and diagenesis process is seawater, atmospheric freshwater, hydrocarbon fluid, and deep thermal fluid. The diagenetic environments are divided into marine diagenetic environments, metric freshwater diagenetic environments, and burial diagenetic environments.

Characteristics of the gold-bearing and barren quartz veins at the Zaylik-Sarilar epithermal deposit, Ahar-Arasbaran Zone, NW Iran: Evidence from mineralogy, alteration, texture and fluid inclusion

The Zaylik-Sarilar epithermal deposit is located 35 km southeast of Ahar in the Ahar-Arasbaran Zone in the Alborz-Azerbaijan Magmatic Belt, NW Iran. Exposed rocks in the Zaylik-Sarilar deposit are mainly composed of Eocene volcanic rocks and Miocene dacite-rhyodacite that are intruded by younger intrusions. The deposit occurs within a sequence of Eocene porphyritic andesite and andesitic lithic tuff. Mineralization occurs as open space filling, taking place as irregular veins, veinlets, and hydrothermal breccias. The mineralization zones can be categorized into two main groups according to sulfides and sulfosalts mineralogy, alteration assemblages, ore textures, and fluid inclusion data. The first group consists of gold-bearing quartz-sulfide ± breccia veins, silicified-phyllic ± argillic zones, and a few quartz-calcite veins. The second group is composed of barren milky (or white) quartz veins, silicified lithocaps, and argillic-kaolinite ± silicic zones. Pyrite, chalcopyrite, bornite, galena, sphalerite, and Au-Ag sulfosalts were identified in ore paragenesis of the gold-bearing veins. Pyrite is the main sulfide mineral in the barren quartz veins and silicified lithocaps. Gold and silver were enriched during oxidation and supergene alteration of the gold-bearing veins. Textural evidences from the gold-bearing veins, such as mosaic breccia monomicts, floating clast breccias, hydrothermal breccias, colloform-crustiform textures, ginguro bands, dendrite textures, and rarely lattice- and parallel-bladed calcite replacement textures, suggest that boiling occurred in the veins. Comb, cockade, reed molds, zonal, and feathery textures are common in the barren veins and silicified lithocaps. Fluid inclusion data show a decreasing trend in salinity and homogenization temperature (Th) from the gold-bearing quartz veins to the barren ones. The average salinity and homogenization temperature (Th) of fluid inclusions from the gold-bearing veins are 1.05 wt% NaCl equiv. and 297 °C, respectively; higher than the average salinity (0.68 wt% NaCl equiv.) and homogenization temperature (192 °C) of fluid inclusions from the barren quartz veins. Th versus salinity diagram suggests that boiling was the main mechanism of ore deposition in the gold-bearing quartz veins and silicified zones. These data propose that the barren quartz veins and silicified lithocaps were formed by cooling. A combination of the geologic, mineralogic, alteration, and fluid inclusion data from the Zaylik-Sarilar deposit represents an epithermal system of low-sulfidation type. Structurally, mineralization has been controlled through the NW-trending strike-slip principal displacement zone and it’s subsidiary Riedel fractures in response to a N-trending compression in the Arabia-Eurasia collision zone.

Thermal history of Carlin-type gold deposits in Yukon (Canada) as revealed by organic matter geothermometry, clumped isotope data, fluid inclusion microthermometry, and apatite fission-track analyses

Sediment-hosted gold deposits in central Yukon have most of the diagnostic characteristics of Carlin-type gold deposits in Nevada. This study combines organic matter geothermometry with fluid inclusion microthermometry, clumped isotope data (Δ47) for late ore-stage hydrothermal calcite, and apatite fission-track analyses to constrain the thermal evolution of Carlin-type gold zones in central Yukon. The Tmax parameter derived from pyrolysis analyses indicates that organic matter is overmature and records regional temperatures of > 150 °C. Calcite and fluorite associated with the waning stage of mineralization at ca. 74 Ma have mean fluid inclusion homogenization temperatures of 123–173 °C, with an average salinity of 4.8 wt.% NaCl equiv. These temperatures overlap values of 91–162 °C determined from calcite clumped isotope measurements and are similar to data from Carlin-type deposits in Nevada. Fluid mixing is suggested by a variation of the isotopic composition of the fluid in equilibrium with calcite, with the higher temperature end-member having higher δ18OH2O values. In thermal models based on Tmax, fluid inclusion microthermometry, clumped isotope measurements, and apatite fission-track data, a higher temperature event at approximately 220 °C is consistent with pyrobitumen reflectance measurements. This event is either related to maximum tectonic burial prior to mineralization or to the flow of acidic hydrothermal fluids early in the main ore-stage. Each analytical technique used in this study is independent of the others and records part of the thermal and chemical evolution of the Yukon Carlin-type gold deposits and their host rocks.

Study on the Tight Gas Accumulation Process and Model in the Transition Zone at the Margin of the Basin: A Case Study on the Permian Lower Shihezi Formation, Duguijiahan Block, Ordos Basin, Northern China

Recent discoveries of oil and gas have principally been located in the central part of the Ordos Basin, which is a petroliferous basin with the largest discovered reserves and annual production of tight sandstone gas in China. For tight sandstone gas reservoirs in the transition zone of the basin margin, the process of natural gas accumulation has remained relatively vaguely understood, because of the transitional accumulation of geological conditions such as structure, sedimentation, and preservation. In this study, thin-section identification and scanning electron microscopic observations of the reservoir core, measurement of the physical properties of the reservoir, microscopic petrography research and measurement of the homogenization temperature of fluid inclusions, digital simulations, and laser Raman spectroscopy analysis were combined to analyze the process of natural gas accumulation of the Permian Lower Shihezi Formation in Duguijiahan block, Hangjinqi area, northern Ordos Basin. The results showed that the Lower Shihezi Formation reservoir in the Duguijiahan block began gas charging in the southern part as early as the Early Cretaceous (130–128 Ma), and then gradually charged in the northern part. Three stages were identified in the digital simulations of gas charging, i.e., the breakthrough, rapid, and fully saturated stages. The initial porosity of the Lower Shihezi Formation reservoir ranged between 28% and 40%. Later, because of strong compaction and interstitial filling during burial, the sandstone porosity decreased rapidly, and densification (porosity < 10%) occurred in the mid–late Jurassic. This late tectonic uplift caused a continuous reduction in ground temperature, and diagenesis had a weak effect on pore transformation. The present porosity of the Lower Shihezi Formation reservoir basically inherited its characteristics in the late Early Cretaceous. The current average porosity of the reservoir is 8.58%, and the average permeability is 0.88 mD, and it can thus be characterized as a tight reservoir. The gas accumulation process of the Lower Shihezi Formation has three stages: (1) the depositional stage (C–P), corresponding to the depositional stage of the source-reservoir-cap combination in gas reservoir; (2) the natural gas accumulation stage (T–K1), corresponding to the period of rapid source rock maturation and natural gas charging step-by-step; and (3) the gas reservoir adjustment stage (K2–present), corresponding to the period of uplift and natural gas charging in the early stage that gradually migrated and accumulated northward along the fracture zone. Finally, the gas accumulation model in the transition zone at the margin of basin was established.

Fluid evolution and ore genesis of the A'gui Cu deposit in southern Great Xing'an Range, Northeast China: Evidence from fluid inclusion and C–H–O–S–Pb isotopes

The A'gui Cu deposit is located in the eastern slope of the southern Great Xing'an Range (SGXR), and it is a vein-type Cu deposit spatially and temporally related to the Cretaceous monzogranite which intruded Pingshan Formation. Vein-type Cu orebodies are mainly hosted in the NE and nearly EW faults. Previous studies on the A'gui deposit mainly focused on geological exploration, and there was no study on its fluid evolution and genesis. Therefore, we carried out conducted fluid inclusion and stable isotope (C–H–O–S–Pb) analysis to study the fluid evolution, fluid and ore-forming material sources and genesis of the A'gui deposit. According to the field investigations and mineral crosscutting relationships, four paragenetic stages were identified: quartz–pyrite–chalcopyrite–pyrrhotite–arsenopyrite ± magnetite (Stage I), quartz–pyrite–chalcopyrite (Stage II), quartz–chalcopyrite ± pyrite–sphalerite–galena (Stage III) and carbonate ± quartz (Stage IV). From Stage I to Stage II, the assemblage of fluid inclusions (FIs) in quartz is characterized by the development of daughter mineral–bearing three–phase FIs (SL–type), vapour FIs (V–type), vapour–rich two–phase aqueous FIs (LV–type) and liquid–rich two–phase aqueous FIs (VL–type). Only VL–type FIs appeared in the Stage III quartz and Stage IV calcite. The homogenization temperatures of FIs in stages I, II, III and IV are 329–390 °C, 255–336 °C, 166–244 °C and 120–157 °C, with salinities of 3.37–45.33 wt%, 3.53–39.76 wt%, 4.17–7.86 wt% and 3.37–7.15 wt% NaCl eqv., respectively. The fluid inclusion type assemblage suggested that obvious fluid boiling occurred in the Stage I and Stage II. Fluid boiling may be the reason for the precipitation of useful minerals. According to the HO isotope analysis of stages I–II quartz (δ18OH2O = −2.1 to 3.2 ‰, δDV–SMOW = −128.4 ‰ to −110.6 ‰), the fluid was originally magmatic water. From Stage III to Stage IV (δ18OH2O = −12.3 to −2.3 ‰, δDV–SMOW = −129.6 ‰ to −104.2 ‰), the HO isotope value is obviously close to the meteoric water line, indicating that meteoric water is mixed with evolved magmatic solutions. The ore–forming fluid of the A'gui deposit represents a medium–high temperature NaCl-H2O magmatic hydrothermal system. The C isotope compositions (δCV–PDB = −5.74 ‰ to −4.76 ‰) in stage IV indicate that the C in the fluid was derived from a magmatic source and was affected by meteoric water. In addition, the measured S isotope compositions in stages I–III of the hydrothermal fluids (δ34SV–CDT = 2.2 to 3.7 ‰) indicate that S mainly comes from granitic magma. Further, the Pb isotope (206Pb/204Pb = 18.276–18.367, 207Pb/204Pb = 15.52–15.556, 208Pb/204Pb = 38.157–38.193) in stages I–III indicate that the ore-forming materials are derived from the mixture of mantle and orogenic material. In summary, this study showed the A'gui is a typical magmatic hydrothermal vein-type Cu deposit that related to Cretaceous monzogranite formed under the joint constraints of Mongolia-Okhotsk Ocean and Paleo-Pacific Ocean tectonic system. Fluid boiling and mixing are the main ore-forming mechanism.

Key stage and model of hydrocarbon accumulation of Ordovician reservoir in Gucheng area, Tarim Basin, China

The heterogeneity of deep carbonate reservoirs is strong, it is significant for exploration and development to clarify the key stages of oil and gas accumulation. Taking the Ordovician in the Gucheng area of Tarim Basin as an example, this paper systematically investigated the key stage and model of hydrocarbon accumulation using the data of the cores, thin sections, cathode luminescence tests, laser ablation U-Pb isotope geochronometry, bitumen reflectance, and fluid inclusion tests, and seismic interpretation. (1) The Tarim Basin mainly develops three sets of effective source rocks, namely the Cambrian Yuertusi Formation, Cambrian Moheershan Formation, and Mid-Lower Ordovician Heituwa Formation, which are concentrated in the slope-basin facies of eastern Tarim Basin. (2) The Gucheng area is located in a favorable paleo-structural position. The pathway system composed of vertical faults and lateral unconformities occurs in the paleo-uplift, which is critical for hydrocarbon migration and accumulation. (3) The laser in-situ U-Pb dating, distribution and genesis of reservoir bitumen, burial-thermal maturation history, and the homogenization temperature of fluid inclusions suggest that the multi-stage tectonic thermal events of the Caledonian and Hercynian stages result in key adjustment and modification to deep hydrocarbon accumulation. Due to magmatic hydrothermal activities, the Caledonian primary oil reservoir massively evolved into oil-cracking gas reservoirs and residual paleo-oil reservoirs. (4) The paleo tectonic-fluid effect plays an important factor controlling the development of the Ordovician reservoirs. The evolution of the hydrocarbon accumulation in Gucheng area can be divided into three stages, namely formation of primary oil and gas reservoirs, oil-cracking gas reservoirs, and adjustment, destruction and reforming of gas reservoirs. To sum up, the western and northern parts of the paleo-uplift with favorable source-reservoir-cap rock assemblage and less effects of hydrothermal activities are the favorable zones for hydrocarbon exploration in the Gucheng area, the Tarim Basin.

Genesis of the Shibaogou Mo–Pb–Zn deposit in the Luanchuan ore district, China: Constraints from geochronology, fluid inclusion, and H–O–S–Pb isotopes

The Shibaogou deposit is located in the Luanchuan ore district within the East Qinling orogenic belt (EQOB), central China, which is a newly discovered Mo–Pb–Zn skarn deposit. The skarn and Mo–Pb–Zn ore bodies are mostly hosted in the contact zones between the Shibaogou porphyritic granite and carbonaceous sedimentary rocks from the Luanchuan and Guandaokou sets. A study combined of geochronology, fluid inclusion (FI), and stable isotopes was performed to constrain the mineralization age, source of ore materials, and the origin and evolution of the ore-forming fluids and their relationship with the subduction of the Paleo-Pacific Plate. The mineralization process includes skarn and quartz–sulfide episodes, which has four stages: skarn (I), quartz–molybdenite (II), quartz–galena–sphalerite (III), and quartz–calcite (IV). Molybdenite Re-Os dating suggests that the deposit was formed in the Late Jurassic (147.4 ± 7.2 Ma). Reportedly, there are five primary types of fluid inclusions: L-type, V-type, H-type, S-type, and C-type. In the skarn stage, coexisting H-type (35.58 wt%–46.05 wt% NaCl equiv.) and low-salinity V-type (0.35 wt%–5.7 wt% NaCl equiv.) fluid inclusions show similar homogenization temperatures, which suggests that fluid boiling occurred at 513–550°C and 580–650 bar (2.19–2.45 km). In the quartz–molybdenite stage, the homogenization temperatures of L-type, V-type, minor H-type, and S-type fluid inclusions indicate continued fluid boiling at 324–387°C and 180–250 bar (0.49–0.94 km). In the quartz–galena–sphalerite stage, a fewer number of coexisting V-type and L-type fluid inclusions in quartz shows different salinities with similar homogenization temperatures, indicating that they are trapped at 303–347°C and <150 bar in the boiling process (<0.56 km hydrostatic depth). The minor primary L-type fluid inclusions that have lower salinities of 0.88 wt%–11.34 wt% NaCl equiv were observed in quartz and calcite in the quartz–calcite stage; in addition, their homogenization temperatures are 103–247°C (typical post-ore conditions). This study found that the ore-forming fluids at the Shibaogou deposit were dominantly magmatic water at the early stage, with input of atmospheric water during fluid evolution, with δ18Ofluid values from −1.168‰ to 8.997‰ and δ18Dfluid values from −106.5‰ to −79.9‰, based on the O and H isotope data from garnet, quartz, and calcite. Furthermore, the S isotopic compositions were measured ranging from 0.8‰ to 14.7‰, and it demonstrated that the ore-forming fluid was mainly derived from magmatic sources. The relatively homogeneous Pb isotopic compositions are similar to those of Shibaogou granite porphyry, which demonstrated that the ore-forming materials were mainly derived from magmatic sources. Molybdenite was precipitated as a result of fluid–rock interactions and fluid boiling, and the galena and sphalerite were precipitated as a result of the decreasing temperature. The subduction of the Paleo-Pacific plate has a critical impact on the complex evolution of ore formation in the Shibaogou skarn deposit in EQOB.

A petroliferous Ediacaran microbial-dominated carbonate reservoir play in the central Sichuan Basin, China: Characteristics and diagenetic evolution

Some of the largest gas accumulations in the central Sichuan Basin, south China are hosted in the Ediacaran microbial-dominated dolomite reservoirs. However, recent exploration of the Secondary Member of the Ediacaran Dengying Formation (Z2dn2) has been hampered by an overall lack of understanding of the microbial-dominated dolomite reservoir and its diagenetic evolution process due to its prolonged burial history and complex diagenetic alternation. On the basis of detailed petrographic and geochemical analysis, we have conducted an in-depth investigation on the characteristics, diagenetic sequence and pore evolution process of the microbial-dominated dolomite reservoirs. The main reservoir comprises thromatolitic dolomite, stromatolitic dolomite and straticulate dolomite. Five generations of cement are identified in the microbial dolomite lithofacies. Multiple dissolution, cementation, differential dolomitization, recrystallization and silicification are recognized. Based on elemental geochemistry, carbon and oxygen isotope characteristics, as well as fluid inclusion homogenization temperatures, cements are attributed to six diagenetic stages. The coupling of high-energy deposition, early supergene karsting and dolomitization processes are found to be essential for developing some world-class favorable high-quality microbial-dominated dolomite reservoirs in Z2dn2 in the central Sichuan Basin.

Role of carbonaceous material in gold precipitation for orogenic gold deposits: A case study of the Bangbu gold deposit in southern Tibet, China

Carbonaceous material (CM) is widely distributed at the Bangbu orogenic gold deposit in vein-hosted and disseminated ores, as well as wall rocks. Despite being closely ore-related, the CM nature and genesis, as well as its possible role in gold metallogeny, remain unclear. Hence, we investigate the role of five ore-related CM types (CM1-4 and methane, which coexist with three pyrite generations of Py1-Py3) by integrating petrographic observation, Raman spectroscopy, LA-ICP-MS, fluid inclusion microthermometry, and thermodynamic modeling. CM1 only occurs in disseminated ores and is overprinted by large euhedral Py1 with median 5.28 ppm Au. Raman spectroscopy analysis shows that CM1 has undergone high-temperature regional metamorphism (∼510 °C). Geochemical modeling shows that Au precipitation in Py1 can be attributed to the fluid-rock interactions of CM1 and Au-bearing ore fluids, which future leads to the host rock alteration and CM1 consumption. CM2 occurs with large subhedral Py2 and hydrothermal sericite in quartz veins, and gold exists as native gold and as Au+ in pyrite. Py2 consists of Py2a with median 0.44 ppm Au and Py2b with median 49.80 ppm Au. Raman spectroscopy of CM2 experienced a temperature of ∼455 °C, much higher than the main ore stage fluid inclusion homogenization temperature (167–247 °C). Meanwhile, methane is commonly found in CO2-bearing fluid inclusions in the quartz closely associated with Py2 and CM2. Geochemical modeling shows that methane can be a more efficient reductant than CM2 and is capable of reducing Au-bisulfide to native gold. CM3 intergrows with fine Au-poor subhedral Py3 (median 1.00 ppm Au) in randomly-oriented “black sinuous” veins within vein-type ores. The CM3 formation temperature is calculated as ∼290 °C. During the sulfidation processes, H2S and CO2 from the ore fluids may have reacted with Fe-bearing minerals, precipitating CM3 and Py3. The loss of aqueous H2S would destabilize Au-bisulfide complexes, which is the secondary gold precipitation mechanism proposed for this deposit. CM4 occurs in the wall rocks and have similar structural features with structure-bound semi-graphite observed by petrology, indicating that the formation process is different from that of other CM types. This is supported by the Raman geothermometric results of CM4 (formation temperature ∼388 °C). Additionally, no spatial association was observed between CM4, auriferous sulfides, and native gold. In conclusion, our study proposed the most likely roles of CM in gold precipitation in this deposit, which bears important implications for better understanding of the processes that lead to the formation of high-grade ores.

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.

Diagenetic–Porosity Evolution and Reservoir Evaluation in Multiprovenance Tight Sandstones: Insight from the Lower Shihezi Formation in Hangjinqi Area, Northern Ordos Basin

Abstract The reservoir property of tight sandstones is closely related to the provenance and diagenesis, and multiprovenance system and complex diagenesis are developed in Hangjinqi area. However, the relationship between provenance, diagenesis, and physical characteristics of tight reservoirs in Hangjinqi area has not yet been reported. The Middle Permian Lower Shihezi Formation is one of the most important tight gas sandstone reservoirs in the Hangjinqi area of Ordos Basin. This research compared the diagenesis-porosity quantitative evolution mechanisms of Lower Shihezi Formation sandstones from various provenances in the Hangjinqi area using thin-section descriptions, cathodoluminescence imaging, X-ray diffraction (XRD), scanning electron microscopy (SEM), and homogenization temperature of fluid inclusions, along with general physical data and high-pressure mercury intrusion (HPMI) data. The sandstones mainly comprise quartzarenite, sublitharenite, and litharenite with low porosity and low permeability and display obvious zonation in the content of detrital components as a result of multiprovenance. Pore space of those sandstone mainly consists of primary pores, secondary pores, and microfractures, but their proportion varies in different provenances. According to HPMI, the order of the pore-throat radius from largest to smallest is central provenance, eastern provenance, and western provenance, which is consistent with the change tend of porosity (middle part>northern part>western part) in Hangjinqi region. The diagenetic evolution path of those sandstones is comparable, with compaction, cementation, dissolution, and fracture development. The central provenance has the best reservoir quality, followed by the eastern provenance and the western provenance, and this variation due to the diverse diagenesis (diagenetic stage and intensity) of different provenances. These findings reveal that the variations in detrital composition and structure caused by different provenances are the material basis of reservoir differentiation, and the main rationale for reservoir differentiation is varying degrees of diagenesis during burial process.

Crystallization processes and genesis of scheelite in a quartz vein-type W deposit (Xianghuapu, South China)

Scheelite is widely developed in quartz vein-type tungsten (W) polymetallic deposits, and CL images, REE patterns and SrO isotope distributions of this mineral provide important constraints regarding its precipitation mechanism. We undertook a systematic investigation of the geochemistry (using in-situ SrO isotopes, EPMA, and LA-(MC)-ICP-MS), CL images, and fluid inclusions along core-to-rim transects of single scheelite grains from the Xianghuapu W deposit (South China) to constrain the source of ore-forming materials and reconstruct the crystallization history of the deposit. The scheelite grains show heterogeneous signatures in CL images recording a three-stage (core–mantle–rim) crystallization history. The scheelites also show decreasing concentrations of REEs from core to rim (core mean = 20.1 ppm, SD = 13.2; mantle mean = 18.7 ppm, SD = 17.8; rim mean = 5.80 ppm, SD = 5.27), as well as contrasting REE patterns and MREE distributions. There are significant variations in concentration, from core to rim of the scheelite, of MREEs (core mean = 10.0 ppm, SD = 6.43; mantle mean = 4.89 ppm, SD = 5.33; rim mean = 0.79 ppm, SD = 0.68) and HREEs (core mean = 39.4 ppm, SD = 23.4; mantle mean = 22.3 ppm, SD = 20.3; rim mean = 4.74 ppm, SD = 4.07). The 87Sr/86Sr ratios and δ18Oscheelite values decrease from core (87Sr/86Sr = 0.740817–0.743807; mean 18Oscheelite = −6.2‰) and mantle (87Sr/86Sr = 0.740048–0.745267; mean δ18Oscheelite = −7.0 ‰) to rim (87Sr/86Sr = 0.733809–0.738398; mean δ18Oscheelite = −7.9 ‰). In addition, the homogenization temperatures of fluid inclusions from the scheelite grains also decrease from core (225–236 °C, mean = 230 °C, SD = 8.20) and mantle (184–217 °C, mean = 201 °C, SD = 11.2) to rim (153–178 °C, mean = 166 °C, SD = 7.04), but the salinity of fluid inclusions changes less (mean = 1.48 wt%, 0.84 wt%, and 0.75 wt%, respectively). The low salinity and δ18OH2O values of the fluid suggest a large amount of meteoric water was mixed into the metallogenic system, resulting in a pH increase and triggering scheelite precipitation. The high Sr contents and change in 87Sr/86Sr ratios reveal that W mineralization was mainly related to Jurassic granitic magmatism, and extensive fluid-rock interaction occurred during the scheelite crystallization process. From core to rim, the magmatic Sr decreased, while the proportion of Sr derived from the limestone wall rock increased. The inhomogeneous REE and SrO isotope distributions may be a ubiquitous feature of quartz vein-type W deposits in magmatic settings globally, and in situ LA-(MC)-ICP-MS analysis can assist in revealing the complex crystallization processes of scheelite in such deposits.

Spectral Characteristics, Rare Earth Elements, and Ore-Forming Fluid Constrains on the Origin of Fluorite Deposit in Nanlishu, Jilin Province, China

Nanlishu fluorite has had an annual output of nearly 10,000 tons since its discovery and mining in 1987. At present, few studies have been undertaken on the genesis of the Nanlishu fluorite deposit. Microscopy, Fourier transform infrared (FTIR) spectroscopy, Raman microprobe spectroscopy, and ultraviolet–visible (UV–Vis) spectroscopy were used to study the mineral composition, mineral structure, and spectral characteristics, and fluid inclusion and rare earth element (REE) geochemistry studies were conducted to discuss the characteristics and evolution of the ore-forming fluids and the mineralization process. The Nanlishu fluorite ore rock is composed mainly of fluorite, with minor amounts of quartz, pyrite, and apatite, and trace amounts of calcite. The purple in the fluorite is caused by the Ca colloids. The homogenization temperatures of fluid inclusions in Nanlishu fluorite vary from 116 to 179 °C, while the salinity ranges from 0.35 to 4.03 wt% NaCl equiv, and the density varies from 0.90 to 1.00 g/cm3. Ore-forming fluids were derived from heated meteoric water characterized by low temperature, low salinity, and low density. The total rare earth element (∑REE, excluding Y) content of the ore ranged from 8.072 to 27.230 ppm. The light/heavy REE ratio (LREE/HREE) varies from 0.971 to 1.513. The δEu value is 1.010–1.074 and the δCe value is 0.893–1.048. Compared with other deposits, the Nanlishu fluorite has lower ∑REE content, no obvious enrichment of LREEs or HREEs, and no anomalies in δEu and δCe. Water/rock reaction is the main mechanism for Nanlishu fluorite precipitation. The Nanlishu fluorite is a fault-controlled, low-temperature hydrothermal, fracture-filling deposit.

Hydrothermal zircon: Characteristics, genesis and metallogenic implications

Zircon is a principal source of zirconium and hafnium, as well as a key mineral both for petrogenetic modeling and age dating. Although zircon occurrences have been reported from hydrothermal mineral systems, detailed studies on zircon that crystallized entirely from saturated hydrothermal fluid are rare. Here we report on hydrothermal zircon crystals dated at about 1911 Ma that crystallized from boiling high-salinity H2O-CO2 hydrothermal fluid with fluid inclusion homogenization temperatures of 295 to 325 °C and the corresponding pressure of 71 bar. These zircon crystals were then overprinted by two stages of hydrothermal zircon veins at 1864 Ma and 528 Ma. The zircon veins crystallized from boiling hydrothermal fluids of moderate to low salinities with fluid inclusion homogenization temperatures and the corresponding pressures of 302 to 329 °C and 75 bar and 290 to 320 °C and 70 bar, respectively. The zircon crystals in our study show sharp and regular cathodoluminescent oscillatory zonation and low and consistent trace element concentrations and Th/U ratios that decrease with fluid salinity. The Th/U ratios of these zircons lie between the magmatic zircon values (>0.5) and metamorphic zircon values (≤0.1). The rare earth element concentrations and trace element spidergram patterns of the zircon samples do not record their hydrothermal origin. Importantly, the low concentrations of HfO2 (<1%), Y (<400 ppm) and Ti (<4 ppm) can discriminate hydrothermal zircon from igneous and metamorphic zircons. Hydrothermal zircon is considered as an effective tool for the dating of mineral deposits and also represents one of the main sources of critical elements such as zirconium and hafnium.

Implications of lithofacies and diagenetic evolution for reservoir quality: A case study of the Upper Triassic chang 6 tight sandstone, southeastern Ordos Basin, China

The Chang 6 tight sandstone reservoir of the Upper Triassic, developed in a large lacustrine shallow-water delta depositional environment, is the most important target for oil exploration and development in the basin. This study is undertaken with the purpose of investigating the influences of lithofacies, diagenetic fluid activity and diagenetic process on quality of the Chang 6 tight sandstone reservoir situated in the southeastern Ordos Basin, based on core observation and description, petrographic investigations, measurement of electron probe microanalysis (EMPA), homogenization temperature and laser Raman spectroscopy of fluid inclusions, laser microzone sampling carbon and oxygen isotope of carbonate cement, the X-ray diffraction (XRD), back scattered (BSE) image observation. Research result shows that three lithofacies are recognized. Each lithofacies has its initial petrological components that slightly differ from other ones, and this is in turn restricted the diagenetic evolution route and imposed diagenetic process and oil-charging intensity, which led to a different reservoir quality.Three carbonate cementation of microcrystalline calcite, sparry calcite I, sparry calcite II and ankerite cements enriched near the mudstone-sandstone interface, occurred in the Chang 6 reservoir which are unfavorable for the reservoir quality. The Chang 6 reservoir experienced three phases of diagenetic fluid activities. The fluid activity of the meteoric water occurred in the eodiagenetic stage offered a carbon source for the cementation of microcrystalline calcite (δ18OPDB: 13.3‰∼-10.0‰; δ13CPDB: 0.4‰∼2.1‰) and sparry calcite I (δ18OPDB: 19.8‰∼-13.4‰; δ13CPDB: 0.4‰∼0.8‰). The latter two fluids released by the thermal decarboxylation of organic matter during the two oil-charging stages, occurred in the mesodiagenetic stage, providing a carbon source for the formation of sparry calcite II (δ18OPDB: 21.8‰∼-18.7‰; δ13CPDB: 5.1‰∼-0.4‰) and ankerite (δ18OPDB: 21.5‰∼-16.8‰; δ13CPDB: 4.6‰∼-3.9‰). The dominant impact on reservoir quality is mechanical compaction, which destroys more intergranular pore than cementation, especially in the sandstone of lithofacies III (the porosity loss by compaction and cementation is 30.9% and 7.3%, respectively), compared to that of the lithofacies I sandstone (the porosity loss by compaction and cementation is 28.7% and 6.6%%, respectively) and the lithofacies II sandstone (the porosity loss by compaction and cementation is 28.8% and 8.5%, respectively). Laumontite cement formed during the eodiagenetic stage effectively resisted the intensity of mechanical compaction. The authigenic clay minerals in the three lithofacies are dominated by chlorite and illite, and most abundant in the lithofacies III. Primary intergranular pore can be effectively protected when the chlorite cement is less than 4%. The dissolution pores with average of 3.0%, 2.4% and 2.3% in the lithofacies I, lithofacies II and lithofacies III sandstone, respectively, formed during the mesodiagenetic stage improved the reservoir quality to a certain degree. Four models of diagenesis and pore evolution of the Chang 6 reservoir are summarized. The pattern I has the best reservoir quality due its fewer ductile component and more coarser grains, the weaker mechanical compaction and weaker cementation of laumontite and carbonate, as well as a stronger dissolution attributed to the intensified oil-charging during mesodiagenesis.