Huanglong Formation Research Articles

Genesis of Danping bauxite (Northern Guizhou, China) and associated lithium: Evidence from LA-ICP-MS analysis of Al-bearing minerals

Lithium (Li), as an essential element of green new energy technology, is a global strategic resource. Sedimentary Li is primarily associated with bauxite, with a local grade of >1000 ppm, and has a huge utilization prospect. However, fine particles limit the study of the occurrence of Li. The Danping bauxite deposit, located in northern Guizhou, China, which accompanying considerable Li resources. In this study, in situ elemental concentration analyses were conducted by laser ablation-inductively coupled plasma-mass spectrometry. The analysis results show that Li is mainly enriched at the bottom and top of the aluminiferous rock series, and the Li content at the top is higher (615.98–3289.98 ppm). Li content is positively correlated with Mg and Fe content, indicating that chlorite is a potential Li-bearing mineral. K content of <10,000 ppm, K content is positively correlated with Li content, indicating that illite may also be a Li-bearing mineral. Kaolinite, transformed from montmorillonite, may also have a high Li content. Ga content is always positively correlated with Al content, indicating that Ga mainly occurs in diaspores. Bauxite has similar Nb/Ta and Zr/Hf ratios to the underlying Hanjiadian Formation shale, as well as the same positive Ce anomalies (negative anomalies in Huanglong Formation), indicating that the ore-forming material of Danping is primarily derived from the underlying Hanjiadian Formation.

Distribution and enrichment mechanism of critical metals in the Danping bauxite deposit, northern Guizhou (SW China)

Critical metals are indispensable for the global scientific and economic development, and their shortage has drawn attention to critical metal exploration in bauxite-related rocks. Bauxite deposits, especially karst-type ones, are usually enriched in critical metals like Li and REE (rare-earth elements La-Lu and Y). In this study, we analyzed the bauxite ore-bearing rocks at Danping (Guizhou, SW China) with a combination of XRF, ICP-MS, XRD, and SEM-EDS techniques, with the aims to investigate the distribution characteristics and enrichment mechanisms of the critical metals (e.g., Li, Ga, Zr, Sc, and REE). The bauxite ore layer at Danping is unconformably contacted with the underlying Huanglong Formation (Fm.) limestone or the Hanjiadian Fm. mudstone/shale, and the overlying Qixia Fm. limestone or Liangshan Fm. carbonaceous shale. Lithium is mainly enriched in the top and bottom of the bauxitic clay strata (abbrev. OBC and UBC, respectively). Gallium is enriched in all the bauxite layers, OBC and UBC. Zirconium is moderately enriched in the bauxite layer, whilst Sc is more enriched in the UBC and its nearby wallrocks (e.g., compact bauxite). REE are locally enriched in/around the basement, with the total REE content of the bauxite ore host being 33.3 to 3082 ppm. In situ analysis indicates that the bauxite-bearing rocks contain REE minerals, including bastnasite and xenotime. Moderate to strong lateritization likely plays a vital role in the different enrichments of critical metals, especially in the bauxite and kaolinitic bauxite zone. Physicochemical conditions of the mineralization are very complex. The Li enrichment may have been closely linked to the clay formation and enrichment, especially during the pre-lateritization and late-leaching stages. REE are mainly enriched in the lateritization stage, and then percolated downward via acidic leaching, where differentiation and re-enrichment occur. When REE migrate to an alkaline environment (caused by Huanglong Fm. limestone), bastnaesite precipitated readily, eventually forming REE-rich claystone and concentrated along the alkaline barrier of carbonate rock.

The origin and ore-forming processes of the Qixiashan Pb–Zn–Ag deposit, South China: Constraints from LA–ICP–MS analysis of pyrite and sphalerite

The Qixiashan Pb–Zn–Ag deposit, located in the Nanjing–Zhenjiang area of the Middle–Lower Yangtze Metallogenic Belt (MLYB), is one of the largest PbZn deposits in Eastern China. The middle Carboniferous carbonates of the Huanglong Formation are the main host rocks of orebodies. Whether this deposit is associated with magmatic activity or sedimentary exhalative deposit is still debated due to the poor constraints on the origin of the ore materials and metallogenic processes. To address this, we analyzed the compositions of pyrite and sphalerite from different mineralization stages and depths, and the distributions of minor and trace elements within the pyrite and sphalerite crystals, using in situ laser ablation–inductively coupled plasma–mass spectrometer and scanning electron microscope analyses. Four hydrothermal stages can be identified: the pre-ore pyrite stage (stage I); Pb–Zn–Py stage (stage II); Pb–Zn–Ag stage (stage III); and carbonate stage (stage IV). The sphalerite in the deposit is typically enriched in Fe, Mn, In, and Sn, and depleted in Ge and Cd. These results are characteristic of magmatic hydrothermal activity and indicate the mineralization was related to late Yanshanian magmatic activity. The Co and Ni contents of pyrite, and the Fe, In, and Sn contents of sphalerite decrease from the early to late stages, suggesting that the temperature of the hydrothermal fluid decreased continuously. Variations in the Se content of pyrite suggest that the fO2 increased from stage I to stage II, then decreased during stage III. Detailed SEM and elemental maps show that stage-II sphalerite (Sph-1) and pyrite (Py-2) are oscillatory zoned, which we interpret to reflect pressure fluctuations and repeated local fluid phase separation during stage II. We also infer from the zoning and subsequent formation of Py-3 that magmatic hydrothermal activity increased from stage I to II, then decreased from stage II to III. In addition, changes in the In, Sn, and Fe contents of sphalerite can be useful indicators of the upward migration of ore-forming fluid and can help during prospecting for related deep magmatic rocks.

Constrains of physical properties and geochemical characteristics of country rock on skarn tungsten mineralization: a case study of the Longjiaoshan-Fujiashan skarn tungsten deposit in the Middle-Lower Yangtze River Metallogenic Belt

Previous research on skarn tungsten deposits has mainly focused on causative rocks and skarn minerals. The effect of the physical and mechanical properties and geochemical characteristics of the country rock on the mineralization of skarn tungsten deposits urgently needs to be further studied. A new skarn deposit, Longjiaoshan-Fujiashan, was discovered in the Edong orefield of the Middle-Lower Yangtze River Metallogenic Belt in the contact zone between the causative intrusive rock and the country rocks. The Longjiaoshan-Fujiashan deposit is an ideal research object for the influence of country rock on skarn mineralization. Based on rich ore-hosting and lean ore-hosting strata in the Longjiaoshan-Fujiashan deposit, we conducted analyses of major and trace elements and organic carbon content, specific surface area, porosity, and permeability. Combined with previous studies, we concluded that the difference among the levels of major and trace elements and organic carbon of the strata could be negligible. The conclusion demonstrates that the influence of geochemical properties of limestones on mineralization is small, whereas the physical properties of limestones are likely the main factor influencing mineralization. The pore-type of rich ore-hosting strata (Qixia Formation and Maokou Formation) are fractured pores and organic pores, whereas those of the lean ore-hosting strata (Huanglong Formation and Dabu Formation) are intercrystalline and intercrystalline pores. The average specific surface areas of the rich ore-hosting and lean ore-hosting strata of the limestone samples were 0.1575 and 0.0167 m2/g, respectively. The lower specific surface area is probably not conducive to the adsorption of gas in the ore-forming hydrothermal fluid, which reduces the reaction rate. Similarly, a high pore volume is conducive to the full reaction of the ore-forming hydrothermal fluid with the country rock and the formation of the orebody. The pore diameter of the rich ore-hosting limestone strata in the Longjiaoshan-Fujiashan deposit was in the range of 40–110 μm, and showed “multiple peaks with one main peak and multiple secondary peaks.” The country rock with the reported pore size characteristics is likely to benefit from ore formation. The alteration scale of high porosity and high permeability country rock (i.e., rich ore-hosting strata: 9.0989 MDarcy; lean ore-hosting strata: 2.9699 MDarcy) are conducive to the formation of large skarn-type mineralization. In conclusion, compared with the geochemical properties of the country rock (major and trace elements and organic carbon content), the physical properties of the country rock (specific surface area, porosity, and permeability) have an obvious influence on the mineralization. Determining the influence of different physical property parameters of the country rock on the mineralization process of skarn is of great research value.

The source, host minerals, and enrichment mechanism of lithium in the Xinmin bauxite deposit, northern Guizhou, China: Constraints from lithium isotopes

Most bauxites in China can be classified as the paleoweathering crust sedimentary type (PCS type) and are normally enriched with lithium (Li). However, the source, host minerals, and enrichment mechanism of Li in bauxites are poorly understood. This study takes the Xinmin bauxite deposit, a typical PCS type in northern Guizhou, China, as a case to trace the Li source and its enrichment using Li isotopes. The Xinmin bauxite deposit is a typical PCS type deposit with high Li contents (average = 756 × 10−6) and heavy Li isotopic compositions (δ7Li) (average = 0.83‰). The Hanjiadian Formation’s silty shale (S1–2hj) and the Huanglong Formation’s limestone (C2h) are the parent rocks of the Xinmin bauxite deposit. The Dazhuyuan Formation (P1d), as bauxite-bearing rocks of the Xinmin bauxite deposit, was formed via diagenesis from the parent rocks’ weathering products. According to the Li isotope fractionation mechanism under chemical weathering, the Dazhuyuan Formation (P1d) should have a lighter δ7Li than parent rocks. However, the δ7Li of the Dazhuyuan Formation (P1d) is heavier than that of parent rocks, indicating that the Dazhuyuan Formation’s original δ7Li was altered by adding external-source Li characterized by heavy δ7Li. Considering the sedimentary environment and Li isotopes, this study proposes that the Li carried by surface runoff characterized by heavy δ7Li is the main source of Li in the Xinmin bauxite deposit. The correlations between clay minerals vs. Li contents and δ7Li show that illite and kaolinite may be the major host minerals of Li in the Xinmin bauxite deposit. Furthermore, the positive correlations of Li vs. Na2O and MgO indicate that smectite is another possible Li host mineral. Combined with the formation process of a PCS type bauxite deposit, this study proposes that Li enrichment occurs during the primary bauxitic material formation stage. During this stage, the Li carried by surface runoff converged into the coastal restricted basin where the primary bauxitic materials were deposited. After small-scale transgression, retention water formed in the basin with the rise in the groundwater table. The retained water evaporated under a hot and dry climate, and Li carried by surface runoff accumulated in the basin and was finally adsorbed by clay minerals such as illite, kaolinite, and smectite in the primary bauxitic materials. This mechanism can significantly increase Li contents and δ7Li in the PCS type bauxite deposit. Additionally, this mechanism implies that the migration process of Li in surface runoff during the formation of primary bauxitic materials determined the present Li contents and δ7Li of a PCS type bauxite deposit, mainly controlled by the sedimentary environment and hydrologic and climatic conditions at that time.

Role of paleo-uplift in the scale formation of intra-platform carbonate mound-bank body reservoirs in the Sichuan Basin

Intra-platform carbonate mound-bank body reservoirs is an important type of oil and gas reservoirs in the Sichuan Basin, and paleo-uplift plays an important role in the scale formation process of scale intra-platform mound-bank body reservoir. So far, however, the control action of paleo-uplift on the formation and distribution of intra-platform mound-bank body reservoirs has not been understood clearly enough, which restricts the oil and gas exploration progress of this type of reservoirs. In order to provide the basis for oil and gas exploration deployment, this paper analyzes the formation and evolution of four paleo-uplifts developed in the cratonic stage of the Sichuan Basin. On this basis, the genetic mechanisms and characteristics of scale intra-platform mound-bank body reservoirs are studied, and the characteristics and distribution of large-scale intra-platform reservoirs in the main series of strata are analyzed. And the following research results were obtained. First, the four paleo-uplifts developed in the Sichuan Basin plays an important constructive role in the formation of scale intra-platform mound-bank body reservoirs. Their control actions mainly include sedimentation and diagenesis. Along with the sea level change, they control the sedimentation scale and distribution range of intra-platform mound-bank body. They control the interlayer karstification and quasi-syngenetic dolomitization of intra-platform mound-bank body. And together with the late tectonic movement, they control the weathering crust karstification and fracturing of intra-platform mound-bank body. Second, Gaoshiti-Moxi paleo-uplift mainly controls the formation of the large-scale intra-platform mound-bank body reservoirs in the Sinian Dengying Formation and the Cambrian Longwangmiao Formation, Leshan-Longnüsi paleo-uplift mainly controls the formation of the scale intra-platform mound-bank body reservoir in the Cambrian Xixiangchi Formation and the Carboniferous Huanglong Formation, while Luzhou and Kaijiang paleo-uplifts mainly control the formation of the scale intra-platform mound-bank body reservoirs in the Triassic Jialingjiang Formation and Leikoupo Formation. Third, six sets of scale intra-platform reservoirs are developed in the Sichuan Basin. The intra-platform mound-bank body reservoirs of Dengying Formation and Longwangmiao Formation are mainly distributed in the core of paleo-uplift, and the other four sets are mainly distributed in the slope of paleo-uplift. In conclusion, these research results provide the geological basis for the prediction of scale intra-platform mound-bank body reservoirs in the Sichuan Basin and they are of important theoretical and practical significance.

Study on the distribution of sulfur-bearing formations in the Sichuan Basin and its damage to tunnel engineering

The sulfur-bearing formations in the Sichuan Basin are widely distributed, whose harmful gases have caused serious threat to tunnel engineering. Based on the existing data collected, the research shows that the main sulfur-bearing formations in the Sichuan Basin include the Triassic Leikoupo Formation, Jialingjiang Formation, and Feixianguan Formation, the Permian Longtan Formation, the Carboniferous Huanglong Formation, the Silurian, the Cambrian and the Sinian. Sulfur-containing gas is closely related to the active degree of tectonic movement in the area where it is located, and is mainly manifested in three aspects: the generation, migration and storage of sulfur-containing gas. According to the tectonic distribution, depth and concentration of the Sichuan basin, the hazard grade of the sulfur-bearing strata in the basin was classified. The Permian Longtan Formation and the Carboniferous Huanglong Formation can be divided into Level IV sulfur-containing strata. The Triassic Leikoupo Formation, Jialingjiang Formation, Feixianguan Formation, and the Silurian strata can be divided into Level III sulfur-containing strata, meanwhile the Cambrian and Sinian strata can be divided into Grade II. The result of sulphur-bearing strata hazard classification was applied to the Yuelongmen Tunnel of Chenglan Railway, and the on-site inspection results were consistent with the application analysis results.

Spatial Distribution Prediction of Oil and Gas Based on Bayesian Network with Case Study

Effectively predicting the spatial distribution of oil and gas contributes to delineating promising target areas for further exploration. Determining the location of hydrocarbon is a complex and uncertain decision problem. This paper proposes a method for predicting the spatial distribution of oil and gas resource based on Bayesian network. In this method, qualitative dependency relationship between the hydrocarbon occurrence and key geologic factors is obtained using Bayesian network structure learning by integrating the available geoscience information and the current exploration results and then using Bayesian network topology structure to predict the probability of hydrocarbon occurrence in the undiscovered area; finally, the probability map of hydrocarbon-bearing is formed by interpolation method. The proposed method and workflow are further illustrated using an example from the Carboniferous Huanglong Formation (C2hl) in the eastern part of the Sichuan Basin in China. The prediction results show that the coincidence rate between the results of 248 known exploration wells and the predicted results reaches 89.5%, and it has been found that the gas fields are basically located in the high value area of the hydrocarbon-bearing probability map. The application results show that the Bayesian network method can effectively predict the spatial distribution of oil and gas resources, thereby reducing exploration risks, optimizing exploration targets, and improving exploration benefits.

Oncoids and Its Significance from the Base of Huanglong Formation in Langping， Guangxi

Oncoids and Its Significance from the Base of Huanglong Formation in Langping， Guangxi

Formation of the Qixiashan Pb–Zn deposit in Middle-Lower Yangtze River Valley, eastern China: Insights from fluid inclusions and in situ LA-ICP-MS sulfur isotope data

The Qixiashan Pb–Zn deposit, Jiangsu Province (1212011220678), is hosted in Huanglong Formation limestones and is one of the largest Pb–Zn deposits in the Lower Yangtze region, China, which is an important Cu–Pb–Zn metallogenic belt. Here we report in situ sulfur isotopic analyses of sulfides using LA-ICP-MS method, together with fluid inclusion data. Three types of pyrite mineralization were identified in the host rocks and ores, corresponding to three genetic stages: (1) fine-grained framboidal or colloform pyrite hosted in limestone (Py1); (2) fine-grained euhedral–subhedral pyrite (Py2) associated with brown sphalerite (Sph1), occurring as massive sulfide ores; and (3) coarse-grained euhedral–subhedral pyrite (Py3) associated with yellow–brown sphalerite (Sph2) in breccia-type or massive ores. Stages 2 and 3 constitute the main Pb–Zn-forming event. Depending on the stage in which they were formed, the sulfides exhibit different sulfur isotope values, implying different ore-forming processes. The Stage 1 Py1 was formed by bacterial sulfate reduction and yields extremely low δ34S values, in the range −14.39‰ to −3.08‰. Stage 2 Py2 and Sph1 have δ34S values ranging from 6.5‰ to 9.4‰ and from 5.0‰ to 8.3‰, respectively. Stage 3 Py3 and Sph2 have δ34S values of 1.3‰ to 3.2‰ and −0.05‰ to 2.5‰, respectively. The δ34S value of pyrite is greater than that of sphalerite for each coexisting pyrite–sphalerite pair, indicating that sulfur isotope equilibrium was reached between these minerals. The total δ34S values and formation temperatures of the ore fluids in Stages 2 and 3 were 9.4‰ and 173 °C–279 °C, and 2.3‰ and 195 °C–328 °C, respectively. The high total δ34S in Stage 2 ore fluids was a result of the interaction of magmatic fluids with marine sulfur, whereas the low total δ34S in Stage 3 ore fluids reveal that these fluids were derived from magmatic water only. Two-phase liquid-rich aqueous inclusions in Stage 2 sphalerite (Sph1) yield homogenization temperatures and salinities of 182 °C–284 °C and 0.9–8.8 wt% NaCl equivalent, respectively. Such inclusions in Stage 3 sphalerite (Sph2) yielded higher homogenization temperatures (207 °C–306 °C) and higher salinities (0.4–12.3 wt% NaCl equivalent), suggesting an increase in magmatic–hydrothermal activity from Stage 2 to 3. Such high-salinity fluids may be produced by magmatic water exsolving from a concealed intrusion beneath the Qixiashan ore district. On the basis of ore fluid temperatures and total δ34S values, sulfides in Stage 2 (δ34S = 9.4‰, T = ~250 °C) are inferred to have precipitated under conditions of pH 2–6 and log fO2 −38 to −33, and sulfides in Stage 3 (δ34S = 2.3‰, T = ~350 °C) are inferred to have precipitated under conditions of pH 2–6 and log fO2 −31 to −27. The textural, fluid inclusion, and sulfur isotopic evidence indicate that the Qixiashan Pb–Zn orebody is a hybrid deposit (rather than a SEDEX or magmatic–hydrothermal deposit) that formed by syngenetic sedimentary deposition and hydrothermal overprinting.

卧龙河气田石炭系黄龙组储层分布特征研究

卧龙河气田石炭系黄龙组储层分布特征研究

Constraints on the Genesis of the Jiande Polymetallic Copper Deposit in South China Using Fluid Inclusion and O-H-Pb Isotopes

ABSTRACT The Jiande copper deposit is located in the Qin-Hang metallogenic belt, South China. The deposit is dominated by “stratiform-like” ores, which are hosted in the dolomite of the Upper Carboniferous Huanglong Formation. These ore bodies were previously proposed to be Carboniferous sedimentary exhalative (Sedex) style mineralization, but they also appear to be related to the Late Mesozoic granodiorite porphyry at Jiande. Three stages of mineralization can be observed. The prograde skarn minerals garnet and diopside were formed in the pre-ore stage. The “stratiform-like” ores, with minor quartz-polymetallic veins, were formed in the main mineralization stage. The post-ore stage is characterized by quartz-calcite ± pyrite veins. Fluid inclusions in quartz from the pre-ore skarn, the main stage of mineralization, and post-ore quartz–calcite ± pyrite veins were studied. Fluid inclusion petrography shows that two-phase liquid-rich (Type I), two-phase vapor-rich (Type II), and halite-bearing (Type III) fluid inclusions can be identified in the studied hydrothermal quartz samples. Primary Type II and Type III fluid inclusions only occur in quartz from the the main stage, whereas Type I fluid inclusions are present in all three stages of hydrothermal quartz. Type I fluid inclusions in pre-ore stage have homogenization temperatures of 290-368°C and salinities of 2.6-8.8 wt.% NaCl equiv. Type II and coexisting Type III fluid inclusions in the main stage share similar homogenization temperatures of 293 to 334 °C and 290 to 326 °C,but have two contrasting salinity ranges of 1.2 to 2.2 wt.% and 31.87 to 38.16 wt.% NaCl equiv, respectively. The coexistence of Type II and Type III fluid inclusions and their similar homogenization temperatures but contrasting salinities suggest that fluid boiling processes occurred. Type I fluid inclusions in the post-ore quartz–calcite veins have homogenization temperatures of 202-278 °C and salinities of 0.2-6.5 wt.% NaCl equiv. The hydrogen and oxygen isotopes (δD ranges from -78 ‰ to -61 ‰, δ18OH2O calculated from quartz are from 8.1 ‰ to 10.6 ‰) of fluid inclusions in quartz imply that ore fluids were principally derived from magmatic water. The lead isotopes of sulfide from the ores are close to the orogenic Pb evolution curve and are also similar to those of the Jiande Jurassic igneous rocks, suggesting that the metals were sourced mostly from the Jurassic igneous rocks. The skarn alteration, inferred boiling fluid inclusion assemblages, stable isotopic features of magmatic fluids, and metal source imply that the main stages of mineralization in the Jiande polymetallic copper deposit is of skarn-type, related to Jurassic granodiorites.

Rare earth element characteristics of the carboniferous Huanglong Formation dolomites in eastern Sichuan Basin, southwest China: Implications for origins of dolomitizing and diagenetic fluids

Marine dolostones of Carboniferous Huanglong Formation constitute major gas reservoir rocks in eastern Sichuan Basin. However, the investigation with respect to sources of dolomitizing and diagenetic fluids is relatively underexplored. The current study attempts to investigate the REE characteristics of dolomites using seawater normalization standard, and therefore discusses the origins of dolomitizing and diagenetic fluids, on the basis of continuous 47.33-m-long core samples from the second member of Huanglong Formation (C2h2) in eastern Sichuan Basin. Low Th, Sc, and Hf concentrations (0.791 × 10−6, 4.751 × 10−6, and 0.214 × 10−6, respectively), random correlation between total REE concentration (ΣREE) and Fe or Mn abundance, and seawater-like Y/Ho ratios (mean value of 45.612) indicate that the carbonate samples are valid for REE analysis. Based on petrographic characteristics, four dolomite types are identified, including micritic-sized dolomite (type Dol-1), fine-to medium-sized dolomite (type Dol-2), medium-to coarse-sized dolomite (type Dol-3), and coarse-to giant-sized saddle dolomite (type Dol-4). Dol-1 dolomites, characterized by positive Ce anomaly (mean value of 6.398), light REE (LREE) enrichment, and heavy REE (HREE) depletion with mean LREE/HREE ratio of 12.657, show micritic calcite-like REE patterns, indicating seawater origin of their dolomitizing fluids. Dol-1 dolomites were formed in sabkha environment whereas the dolomitizing fluids originated from evaporative brine water due to their micritic crystal sizes and tight lithology. Dol-2 dolomites, particularly subtype Dol-2a barely developing vuggy porosity, also show micritic calcite-like REE patterns, suggesting their dolomitizing fluids were seawater or seawater-derived fluids. This inference is confirmed by low Fe and Mn concentrations, which range from 651 μg/g to 1018 μg/g (mean value of 863 μg/g) and 65 μg/g to 167 μg/g (mean value of 105 μg/g), respectively, whereas homogenization temperatures (Th, mean value of 103 °C) indicate that Dol-2 dolomites were formed under burial environment. Dol-3 dolomites, in form of cements of Dol-2 dolomites, show similar REE patterns to their host minerals (i.e., Dol-2 dolomites), indicating their parent source was possibly derived from Dol-2 dolomites. Dol-3 dolomites have high Fe and Mn concentrations with mean values of 3346 μg/g (ranging from 2897 μg/g to 3856 μg/g) and 236 μg/g (ranging from 178 μg/g to 287 μg/g), respectively, indicating the involvement of meteoric water. Meanwhile, it confirms that the dissolution in Dol-2 dolomites was caused by meteoric water leaching. Positive Eu anomalies (mean value of 1.406) in Dol-4 dolomites, coupled with high homogenization temperatures (mean value of 314 °C), suggest that Dol-4 dolomites precipitated from hydrothermal fluids. High Fe and Mn concentrations (mean values of 2521 μg/g and 193 μg/g, respectively) in Dol-4 dolomites likely results from interactions of hydrothermal fluids with deep burial clastic rocks.

Depositional System of the Carboniferous Huanglong Formation, Eastern Sichuan Basin: Constraints from Sedimentology and Geochemistry

AbstractUsing analyses of the lithology, sequences, paleoenvironment, and tectonic setting, the depositional system of the Carboniferous Huanglong Formation in the eastern Sichuan Basin was identified. The lithological characteristics of the Lower Member, Middle Member, and Upper Member were analyzed and classified. Before the use of carbon, oxygen, and strontium isotopes in the analysis, all of the geochemical data were tested for validity. On the basis of the Z values obtained from carbon and oxygen isotopes, the paleoenvironments of the three members were elucidated. Lower Member was dominantly an enclosed marine environment with intense evaporation and little freshwater input into the sea. Middle Member developed in a semi‐enclosed to normal marine environment with many rivers. Upper Member was formed in a normal marine environment. The east Sichuan Basin was enclosed by paleouplifts before the deposition of the Huanglong Formation, forming a relatively enclosed depositional setting. Paleogullies developed in the Silurian strata that underlie the Carboniferous rocks; these paleogullies can be identified. On the basis of a comprehensive analysis, we propose that the Huanglong Formation developed in a platform system. Four microfacies were identified: supratidal flat, dolostone flat, grain shoal, and shelf microfacies. The high‐permeability and high‐porosity characteristics of the grain shoal microfacies are favorable for hydrocarbon accumulation, while the supratidal flat and shelf microfacies developed very few high‐quality reservoirs. The paleogullies, in which increased amounts of grain shoal microfacies developed, controlled the distribution of high‐quality reservoirs.

A hydrothermal origin for the large Xinqiao Cu-S-Fe deposit, Eastern China: Evidence from sulfide geochemistry and sulfur isotopes

The Xinqiao Cu-S-Fe deposit in the Tongling ore district, Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB; Eastern China), is located along the northern margin of the Yangtze Craton. The stratiform- and skarn-type Xinqiao mineralization comprises five stages, namely the early skarn (Stage I, garnet and diopside), late skarn (Stage II, epidote-dominated), iron oxides (Stage III, hematite and magnetite), colloform pyrite (Stage IV) and quartz-sulfides (Stage V). There are three pyrite types at Xinqiao, i.e., colloform (Py1; Stage IV), fine-grained (Py2, from Py1 recrystallization; Stage V) and coarse-grained (Py3; Stage V) pyrites.Scanning Electron Microscope (SEM) imagery for Py1 reveals that they are cubic microcrystalline pyrite aggregates, and the EDS and XRD data indicate that some Py1 contain minor siderite impurities. Electron Microprobe Analysis (EMPA) and LA-ICP-MS geochemical data demonstrate that the three pyrite types have relatively high Fe/S ratios and distinctly high Mn, Cu and As concentrations. Compared to Py2 and Py3, Py1 has higher Pb, Bi and Ag, but lower Co, Ni, Se, Cd, Te and Au. Ratios of Fe/S (0.837 to 0.906), Se/Te (2.39 to 14.50) and Co/Ni (0.67 to 4.67) of the Xinqiao pyrites resemble typical hydrothermal pyrites. δ34SCDT of Py1 (−0.6‰ to 2.7‰, average 0.58‰), Py2 (1.8‰ to 2.5‰, average 2.1‰) and Py3 (1.9‰ to 4.4‰, average 3.5‰) are close to those of the Xinqiao skarn-type orebodies (1.3‰ to 4.1‰), but distinct from those of the Upper Carboniferous Huanglong Formation limestone (−9.5‰ to −15.4‰), suggesting that the three pyrite types (especially Py1) were genetically linked to the Yanshanian (Jurassic-Cretaceous) magmatic-hydrothermal events, with Py1 probably reflecting rapid crystallization during fluid mixing. We interpret that the Xinqiao stratiform mineralization may have been associated with the Jitou quartz diorite stock, as may be the case also for the skarn-type mineralization hosted in the contact between the Yanshanian Jitou stock and the Lower Permian Qixia Formation limestone. Overall, the Xinqiao Cu-S-Fe mineralization may have been generated by the Jurassic-Cretaceous tectono-thermal event in Eastern China.

Geochemical characteristics and geological significance of shale gas from the Lower Silurian Longmaxi Formation in Sichuan Basin, China

The shale of the Lower Silurian Longmaxi Formation has become a key target for shale gas exploration and development in China recently. The origin of carbon and hydrogen isotopic reversal in shale gas is discussed in this paper based on the analysis of the chemical components and stable isotope composition of natural gas in the Carboniferous Huanglong Formation sourced from Longmaxi Formation and Longmaxi shale gas. The shale gas was mainly composed of hydrocarbon gas in which the content of CH4 was in the range of 95.52%–99.59%, C2H6 0.23%–0.72%, and C3H8 0.0%–0.03%. The drying coefficient (Cl/Cl–5) was more than 0.99, indicating typical dry gas. The values of δ13C1, δ13C2 and δ13C3 ranged from −37.3‰ to −26.7‰, −42.8‰ to −31.6‰, and −43.5‰ to −33.1‰, respectively. The carbon isotope values indicated that the hydrocarbon gas was an oil-type gas. However, the reversal of carbon and hydrogen isotopes of hydrocarbon gases in shale gas occurred, i.e., δ13C1 > δ13C2, δD1 > δD2. The geochemical characteristic of high thermal maturity and carbon isotope reversal in the Longmaxi shale gas was similar to those of the Fayetteville shale gas in U.S. The Longmaxi shale gas is a mixture of gases from decomposition of kerogen at high thermal maturity and cracking of soluble organic matter retained within the shale, suggesting there is an abundance of shale gas to support high productivity.

川东地区石炭系典型古岩溶剖面露头表征

石炭系黄龙组古岩溶储层是川东地区重要的天然气储集类型之一，位于华蓥山地区的溪口剖面出露完整，岩溶现象明显，为川东地区石炭系难得的岩溶型露头剖面。通过野外精细观测、室内薄片鉴定分析，该剖面岩溶岩系发育，主要见有网缝镶嵌状、角砾支撑状和基质支撑状岩溶角砾岩，分别代表不同成因的岩溶改造，角砾成分均为云质，角砾间及角砾内的溶蚀孔洞缝发育，常充填地表水下渗携带的砂质、碳泥质充填物和大气淡水胶结物。根据溪口剖面岩溶角砾岩类型等古岩溶标志的识别，结合大量钻井取芯资料观察，石炭系黄龙组可识别出地表残积岩溶相、洞穴跨塌堆积相、地下暗河搬运充填相、基岩溶蚀破裂相、基岩溶蚀孔洞相和基岩溶蚀交代相6种岩溶相类型，不同岩溶相由不同类型岩溶岩及充填物构成。结合古岩溶储层与岩溶旋回期次和溶蚀特征的关系，该剖面可划分地表溶蚀段、下部溶蚀段和底部溶蚀段，其中下部溶蚀段为主要的古岩溶储层发育段。 Paleokarst reservoirs of Carboniferous Huanglong Formation are regarded as important reservoirs for natural gas accumulations in eastern Sichuan Basin. Xikou section in the Huayingshan region has been considered as the most typical outcrop section due to the developed karst rock series. It is demonstrated by field observation and indoor thin section analysis that karst rock series are significantly developed, which primarily consist of breccias of net-seam mosaic form, breccia-supported form, and matrix-supported form, corresponding to karst structures with dif-ferent causes. Breccias are commonly dolomitic, and there develop large amounts of solution holes, caves and seams, which are usually filled with sandy and carbonaceous fillings brought by the surface water infiltration as well as cements of atmospheric fresh water. Based on identification of paleokarst symbols such as the karst breccia types in Xikou section combined with comprehensive observation and analysis of numerous drilling cores, this study classifies paleokarst facies into six categories, namely surface residual karst facies, cave collapse and accumulation facies, under-ground river transporting and filling facies, bedrock solution holes and caves facies, and bed rock solution and replacement facies, respectively. They are composed of different types of karst rocks and fillings. Then, three segments could be divided in Xikou section according to the relationships of paleokarst reservoirs, karst cyclicity, and dissolution characteristics, corresponding to the surface, lower and bottom dissolution parts, respectively. Among them, the lower dissolution part is the main part where develop paleokarst reservoirs.

Origin of dedolomite in the Carboniferous of Eastern Sichuan basin and its geological significance

Resulting in dense lithology and poor physical properties, dedolomitization has detrimental effects on reservoir characteristics, so there has been little systematic research reported on the dedolomitization in the Carboniferous Huanglong Formation from the Eastern Sichuan basin. Through well drilling, well logging, thin-section authentication, and geochemistry tests, we studied for the first time the growth characteristics, regularities of distribution, genesis, and geological significance of dedolomitization in rock with typical fabric. From this study, we arrived at three conclusions: (1) the microstructure type divisions of dedolomite are as follows: A. a large micrite core and clear calcite rim, B. dolomite crystal, C. gypsum crystal, and D. rhombohedral pores; (2) rocks had low porosity and high density after dedolomitization in Eastern Sichuan, which is detrimental to reservoir development; and (3) micrite–microlite crystal dolomite and micrite–microlite crystal gypsum dolomite are the protoliths before dedolomitization. Sabkha is the primary sedimentary environment, in which the sabkha brine was kept within the layer and would become the main fluid of dolomitization in the high-quality dolomite reservoir from Eastern Sichuan. Analyzing the instances of dedolomitization has important geological significance in some ways, such as recovering the protolith, recovering the ancient environment, and discussing the relationships between diagenetic fluid properties and the reservoir’s development.

The Origin of Paleokarst in the Huanglong Formation of the Eastern Sichuan Basin: Evidence from δ13C, δ18O and87Sr/86Sr

Karst rocks from the Huanglong Formation exposed at the margin of the Eastern Sichuan Basin can be divided into four types: slightly corroded, moderately corroded porous, intensely corroded brecciated and intensely corroded and replaced secondary calcic karstic rocks. The carbon, oxygen and strontium isotope compositions of the various karst rocks are analyzed systematically and compared to rocks without karst corrosion. The results indicate that (1) the Huanglong Formation in the eastern Sichuan Basin was a restricted bay supplied and controlled by freshwater in which mud-micrite and mud-dolomicrite exhibit low δ13C and δ18O values and high 87Sr/86Sr ratios; (2) all types of karstic rocks in the paleokarst reservoirs of the Huanglong Formation in the research area are affected by atmospheric freshwater with the δ13C and δ18O values and 87Sr/86Sr ratios in the original formation approaching those of atmospheric freshwater, which reflects ancient hydrological conditions, fluid properties, isotopic source and the fractionation effect; (3) the intensely corroded and replaced secondary limestone is affected by a variety of diagenetic fluids, often reflected by δ13C and δ18O values, while the 87Sr/86Sr ratios exhibit the strong degree of the corrosion; (4) after comparing the 87Sr/86Sr ratios of each type of karst rock, the diagenetic fluids are determined to be mainly atmospheric freshwater, and depending on the strength of corrosion, and the low 87Sr/86Sr ratio fluids in the layer will participate in the karst process. The carbon, oxygen, and strontium isotopes of different karstic reservoirs can provide meaningful geochemical information for forecasting and evaluating the development and distribution rules of the Huanglong Formation at the margin of the eastern Sichuan Basin in time and space.

Geology, isotope geochemistry and ore genesis of the Shanshulin carbonate-hosted Pb–Zn deposit, southwest China

The Shanshulin Pb–Zn deposit occurs in Upper Carboniferous Huanglong Formation dolomitic limestone and dolostone, and is located in the western Yangtze Block, about 270km west of Guiyang city in southwest China. Ore bodies occur along high angle thrust faults affiliated to the Weishui regional fault zone and within the northwestern part of the Guanyinshan anticline. Sulfide ores are composed of sphalerite, pyrite, and galena that are accompanied by calcite and subordinate dolomite. Twenty-two ore bodies have been found in the Shanshulin deposit area, with a combined 2.7milliontonnes of sulfide ores grading 0.54 to 8.94wt.% Pb and 1.09 to 26.64wt.% Zn. Calcite samples have δ13CPDB and δ18OSMOW values ranging from −3.1 to +2.5‰ and +18.8 to +26.5‰, respectively. These values are higher than mantle and sedimentary organic matter, but are similar to marine carbonate rocks in a δ13CPDB vs. δ18OSMOW diagram, suggesting that carbon in the hydrothermal fluid was most likely derived from the carbonate country rocks. The δ34SCDT values of sphalerite and galena samples range from +18.9 to +20.3‰ and +15.6 to +17.1‰, respectively. These values suggest that evaporites are the most probable source of sulfur. The δ34SCDT values of symbiotic sphalerite–galena mineral pairs indicate that deposition of sulfides took place under chemical equilibrium conditions. Calculated temperatures of S isotope thermodynamic equilibrium fractionation based on sphalerite–galena mineral pairs range from 135 to 292°C, consistent with previous fluid inclusion studies. Temperatures above 100°C preclude derivation of sulfur through bacterial sulfate reduction (BSR) and suggest that reduced sulfur in the hydrothermal fluid was most likely supplied through thermo-chemical sulfate reduction (TSR). Twelve sphalerite samples have δ66Zn values ranging from 0.00 to +0.55‰ (mean +0.25‰) relative to the JMC 3-0749L zinc isotope standard. Stages I to III sphalerite samples have δ66Zn values ranging from 0.00 to +0.07‰, +0.12 to +0.23‰, and +0.29 to +0.55‰, respectively, showing the relatively heavier Zn isotopic compositions in later versus earlier sphalerite. The variations of Zn isotope values are likely due to kinetic Raleigh fractional crystallization. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of the sulfide samples fall in the range of 18.362 to 18.573, 15.505 to 15.769 and 38.302 to 39.223, respectively. The Pb isotopic ratios of the studied deposit plot in the field that covers the upper crust, orogenic belt and mantle Pb evolution curves and overlaps with the age-corrected Proterozoic folded basement rocks, Devonian to Lower Permian sedimentary rocks and Middle Permian Emeishan flood basalts in a 207Pb/204Pb vs. 206Pb/204Pb diagram. This observation points to the derivation of Pb metal from mixed sources. Sphalerite samples have 87Sr/86Sr200Ma ratios ranging from 0.7107 to 0.7115 similar to the age-corrected Devonian to Lower Permian sedimentary rocks (0.7073 to 0.7111), higher than the age-corrected Middle Permian basalts (0.7039 to 0.7078), and lower than the age-corrected Proterozoic folded basement (0.7243 to 0.7288). Therefore, the Sr isotope data support a mixed source. Studies on the geology and isotope geochemistry suggest that the Shanshulin deposit is a carbonate-hosted, thrust fault-controlled, strata-bound, epigenetic, high grade deposit formed by fluids and metals of mixed origin.