Ningwu Basin Research Articles

3D mineral prospectivity modeling using multi-scale 3D convolution neural network and spatial attention approaches

A significant proportion of recent mineral exploration has increasingly focused on the targeting of deep-seated orebodies. Mineral prospectivity modeling is one of the more important approaches that facilitates exploration targeting and the mitigation of risks associated with mineral exploration, particularly under cover. Recent advances in 3D mineral prospectivity modeling enable the effective extraction of predictive information from three-dimensional geological models, enabling more accurate exploration targeting of deep-seated orebodies. These advancements have synergized with deep learning approaches to improve the efficiency of mineral exploration based on nonlinear and multi-layer sensing attributes, effectively enabling the identification and extraction of key relationships between 3D predictive maps and mineralization. The main deep learning method used for 3D mineral prospectivity modeling is convolutional neural network (CNN) modeling. However, this research typically does not consider the multiscale features of geological structures, meaning further improvements can be made to this modeling approach. This paper introduces a multi-scale 3D convolutional neural network model (3D CNN) incorporating a spatial attention mechanism and an Inception module (MSAM-CNN) for 3D mineral prospectivity modeling. By integrating Inception modules and spatial attention mechanisms, the network's capability to identify multi-scale geological features and delineate key predictive areas is significantly enhanced compared to typical CNN approaches. This new approach provides further improvement in the accuracy and generalization capability of 3D mineral prospectivity modeling. To evaluate the effectiveness of this model, we undertook 3D mineral prospectivity modeling within the area of the Baixiangshan iron deposit, in the Ningwu Basin of the Middle-Lower Yangtze River Metallogenic Belt, China. The results show that the multi-scale 3D convolutional neural network model is remarkably robust and has good generalization capabilities. The approach can also can effectively delineate targets within the deep and peripheral areas of the deposit, providing targets for future exploration. The addition, performance indicators, ROC curve, and Capture-Efficiency curve generated during this modeling consistently demonstrate that the MSAM-CNN model outperforms Inception-enhanced CNN (M-CNN), CNN, Random Forest (RF), and Support Vector Machine (SVM) models. All of this indicates that MSAM-CNN approaches can effectively extract 3D spatial features within 3D predictive maps during 3D mineral prospectivity modeling better than other approaches that are commonly used, indicating that thius approach represents a promising tool for the accurate and precise identification of targets during future exploration for deep-seated mineralization.

Influence of rock properties and prediction on the methane storage capacity in marine-continental transitional shale and coal from northern China

Systematic understanding of geological controls on methane storage capacity in marine-continental transitional shale can help to predict the potential gas resources in such shale reservoirs. In the present study, seven transitional shale samples were first collected from three different basins (Qinshui Basin, Ningwu Basin, and eastern Ordos Basin) in Northern China and characterized for their composition, thermal maturity, pore structure and CH4 adsorption capacity. Their total CH4 storage capacities (including adsorbed and free gas) at pressures up to 10 MPa were determined using an ingenious method from previous studies. Based on this, we investigated the properties controlling gas adsorption and total gas storage in these shales. Meanwhile, several coal samples from the same strata unit were analyzed through analogous processes and treated as the naturally occurring extreme total organic carbon (TOC) cases of transitional shale during discussion. Our findings indicate that the transitional shale’s composition (typically represented by the TOC and total clay content) directly influences the pore development and then controls the CH4 adsorption, which is consistent with the previous findings for marine shale. Though the positive correlations between CH4 adsorption capacity and both TOC and total clay content were observed for the studied shale samples, the observations demonstrate the dominance of organic components in rock for gas adsorption when the transitional shale and coal samples are considered together. In addition, the positive correlation between clay minerals and CH4 adsorption may be overshadowed, even reversed. Based on the method used, the total gas-in-place (GIP) storage in these transitional shales shows a stronger positive correlation with total clay content than TOC content. This is mainly attributed to the meso- and macroporosity of clay minerals. For the studied shale samples, the relationship between thermal maturity and CH4 storage capacity can be represented by a quadratic polynomial function, which is likely due to considerable variation in rock composition. Furthermore, it is emphasized that predicting the total GIP in shale and coal reservoirs using these correlations should be approached with caution, regardless of the method (direct/indirect) used to determine the GIP.

Diagenesis and Diagenetic Mineral Control on Reservoir Quality of Tight Sandstones in the Permian He8 Member, Southern Ningwu Basin

The sandstone reservoirs of the He8 member within the Lower Permian Shihezi Formation are important targets for oil and gas exploration in the southern Ningwu Basin. This study utilized thin-section identification, scanning electron microscopy, and X-ray diffraction analysis to examine the petrological features and reservoir characteristics, and evaluate the impact of the mineral composition and diagenesis type on the porosity of the sandstone reservoir. Additionally, a multiple linear regression prediction model was developed to predict the distribution of promising sandstone reservoirs in the study area. The results of the analysis revealed that the sandstone of the He8 member is mainly composed of feldspathic lithic sandstone, followed by lithic sandstone. The main reservoir type is characterized by secondarily dissolved pores and micropores within kaolinite aggregates. The low porosity (ranging from 0.2% to 10.7%) and permeability indicate that the He8 member is a tight sandstone reservoir. This reservoir has undergone compaction, cementation, and dissolution diagenesis, and is presently in the stage of mesodiagenesis B. The rigid framework of quartz, the dissolution of feldspar grains, and the intergranular pores of kaolinite are significant contributors to reservoir quality and the main drivers of porosity. In this study, a multivariate linear regression model was developed based on the mineral content of quartz, feldspar, carbonate minerals, kaolinite, smectite, and rock fragments, which accurately predicts the porosity of the studied reservoirs. Based on this model, it was predicted that the north of the Jingle South sub-depression contains a favorable reservoir space in the tight sandstone reservoir of the He8 member. The findings of this study hold significant reference value in the quantitative characterization of tight sandstone reservoirs with similar depositional and diagenetic characteristics, and improving the prediction effect of favorable reservoirs.

Magma process of the ore-forming gabbrodiorite porphyry of the Meishan iron deposit, eastern China: Insights from the phenocrysts chemistry

Mesozoic iron deposits in the Ningwu volcanic basin in eastern China are genetically linked to the gabbrodiorite porphyries, which have long been regarded as the subvolcanic intrusive of the andesitic volcanics. Compared with abundant research conducted on the mineralization process, the magmatic process of the ore-forming intrusion has been poorly understood. This study performed petrographic and in situ analyses of representative phenocrysts from the ore-forming gabbrodiorite porphyry of the Meishan iron deposit to gain new insights into the magmatic process. The phenocrysts are composed of clinopyroxene and plagioclase, which commonly exhibit disequilibrium features, such as reverse zoning and sieved textures, suggesting that the gabbrodiorite porphyry evolved in an open-system andesitic-magma reservoir into which mafic magma was repeatedly recharged. The zoned clinopyroxene phenocrysts show consistent trace-element distribution patterns and a narrow range of 87Sr/86Sr of 0.7037–0.7049, which suggest that the recharged mafic magma was the primitive basaltic magma and crystal fractionation dominated the evolution from basaltic magma to andesitic magma. Pressure estimations based on clinopyroxene phenocryst compositions indicate that the magma reservoirs were emplaced at the uppermost mantle. Comparison of clinopyroxene phenocryst compositions between the Meishan gabbrodiorite porphyry and published andesitic volcanics shows distinct rare earth element distribution patterns, which don't support a volcanic–subvolcanic relationship between the gabbrodiorite porphyry and andesitic volcanics. Estimated pressures indicate that the primitive magma of andesitic volcanics evolved in lower crust reservoir, which was much shallower than the reservoirs of the ore-forming gabbrodiorite porphyry. Therefore, the Meishan gabbrodiorite porphyry was not the subvolcanic intrusive of the andesitic volcanics and probably represented an independent magmatic pulse. This understanding suggests that the exploration target of iron deposits in the Ningwu basin is not necessarily limited to the contact zone between gabbrodiorite porphyry and andesitic volcanics.

Evolution of pore structure in organic shale with type III kerogen and high kaolinite content in Ningwu Basin

Evolution of pore structure in organic shale with type III kerogen and high kaolinite content in Ningwu Basin

Mineral Composition and Its Control on Nanopores of Marine-Continental Transitional Shale from the Ningwu Basin, North China.

There is a large difference between the sedimentary environment and maturity of organic matter between marine shale and marine-continental transitional shale. It is of great significance to discuss the effect of inorganicminerals on the pores for marine-continental transitional shale gas exploration. In this study, scanning electron microscopy (SEM), low temperature liquid nitrogen adsorption and Xray diffraction (XRD) were conducted on eight marine-continental transitional shale samples from the Ningwu Basin, Shanxi Province, China. The pore structure differences in the different minerals were discussed, and the relationship between the mineral content and pore parameters was analysed. The results show that the mineral composition of shale is dominated by clay minerals, quartz, carbonate minerals and a small amount of pyrite. The clay minerals content is between 39.5% and 77.0%, with an average of 59.9%. The quartz content ranges from 21.8% to 47.8%, with an average of 31.9%. The carbonate minerals content in shale is between 0.6% and 23.9%, and the average is 6.3%. The clay minerals are composed of mixed illite-montmorillonite layer, kaolinite and chlorite. The content of mixed illite-montmorillonite layer is between 13.8% and 27.4%, with an average of 20.4%. The kaolinite content ranges from 57.0% to 86.2%, with an average of 76.0%. The content of chlorite is between 0 and 15.6%, with an average of 5.7%. The types of pores are mainly intergranular pores and interlaminar pores, which are mostly presented as slit and parallel plates. The mixed illite-montmorillonite layer contributes more to the specific surface area, which is favourable for shale gas adsorption. The pores in kaolinite are more developed than those of the mixed illite-montmorillonite layer, but the pore diameter is relatively large. The quartz granule has a complete crystal type, and intergranular pores with a large pore size are often developed at the mineral contacts. Compared with clay minerals and quartz, the pore development in the carbonate minerals is relatively poor and develops more micro-fractures. The pyrite contributes a certain number of intergranular pores and mold pores.

Apatite (U–Th)/He thermochronometry of iron deposits in the central Ningwu volcanic basin, eastern China: Implications for exhumation and preservation

Apatite (U–Th)/He thermochronometry (AHe) was used to constrain the exhumation history of the Taocun and Washan iron oxide deposits, located in the central Ningwu volcanic basin of the Lower Yangtze River Belt (LYRB) in eastern China. The porphyritic gabbro–diorite host rock yields zircon U–Pb ages of ~130 Ma, which are different to the apatite (U–Th)/He ages from this study. The Taocun iron deposit was emplaced below the apatite helium partial retention zone (PRZ) and its AHe ages (17.8 ± 1.0 to 19.5 ± 1.2 Ma) reveal a rapid exhumation-induced cooling event that began at ~23 Ma. The Washan iron deposit, which is hosted within a cryptoexplosion breccia, was probably emplaced in the apatite helium PRZ. It yields two groups of AHe ages: an older group (58.5 ± 3.3 and 57.8 ± 3.5 Ma) and a younger group (27.0 and 22.8 Ma). The youngest AHe age from the Washan iron deposit (22.8 Ma) records the same rapid exhumation-induced cooling event that is recorded in the Taocun deposit AHe ages. This event was related to structural inversion in eastern China that began during the early Miocene (~23 Ma). This inversion marked the end of widespread extension that started in the Early Cretaceous, and it was accompanied by the formation of structures related to Neogene and Quaternary regional compression. Given the relatively recent timing of the exhumation, the overall amount of exhumation of the central Ningwu Basin is relatively minor. We integrated our results with the porphyrite iron deposit model and existing information on the Xuejin detachment fault. It seems that iron deposits of the deepest (Gushan) sub-type of the porphyrite iron deposit model may still be preserved at depth in the central Ningwu Basin.

The three-dimensional electrical structure and metallogenic prospect of the Ning (Nanjing)-Wu (Wuhu) basin and the southern adjacent area in eastern China

The Ning (Nanjing city)-Wu (Wuhu City) volcanic basin, which is famous for its large-scale, high-grade ore deposits, is a typical iron ore district in the middle-lower reaches of the Yangtze River Metallogenic Belt (MLRYMB). The southern part of the basin is primarily covered by Quaternary strata. Few in depth studies have been conducted on this area. Due to the increasing demand for mineral resources and the controversy over regional tectonic models, some fundamental geological issues still need to be addressed. These questions include defining the characteristics of the geological structures and the mineralization prospects. Therefore, a three-dimensional (3D) magnetotelluric survey was conducted in this area to obtain a deep 3D electrical model. It was found that the Yangtze fault (CJF) and the main thrust fault (MTF) in the upper crust have the same deep (middle crust to upper mantle) setting and part of the thrust tectonics of the metallogenic belt. The southern boundary of the Ning-Wu Basin is located on a fault belt (boundary fault, BF) with a northeast-east (or east-west) strike influenced by the MTF in the southern part of the study area. Bounded by the BF and MTF, the southeastern part of the study area exhibits signs of uplift. With the recent discovery of a large porphyry copper deposit to the southeastern part of the study area, which is located in an uplift structure similar to the one south of the Ning-Wu Basin, this local uplift could have potential mineralization prospects based on a previous mineralization theory developed for this belt.

Geology and transitional shale gas resource potentials in the Ningwu Basin, China

We analyzed the tectonic evolution characteristics, sedimentary environment, geochemical characteristics, petrological characteristics, and gas-bearing properties of three mudstone sections of the Lower Paleozoic in Ningwu Basin, NE China, and determined the geologic characteristics and resource potential of the transitional facies shale gas. Geochemical analysis of the organic carbon content, kerogen macerals, and vitrinite reflectance of the shale samples showed that the total organic content was generally over 2.0%, the main organic type was type III, and the vitrinite reflectance values (Ro) were between 1.20 and 1.90%. Thus, the mudstones are good shale gas source rocks. The thickness of the three mudstone sections was approximately 30–70 m, and the average porosity was 3.10%. The pore types were diverse with good reservoir capacity. The shale gas resources of the Carboniferous-Permian transitional facies estimated by the volumetric method were approximately 2798.97 × 108–4643.09 × 108 m3. Through a comparison with shales in SW China, where shale gas has been successfully exploited, we determined the preferred criteria for favorable shale gas areas, as well as favorable areas for shale gas enrichment.

An evolving magmatic-hydrothermal system in the formation of the Mesozoic Meishan magnetite-apatite deposit in the Ningwu volcanic basin, eastern China

The Meishan iron deposit contains 338 Mt of iron-ore reserves at 39% Fe and represents the largest magnetite–apatite deposit in the Ningwu Basin of eastern China. Controversy has long existed about whether this deposit had a hydrothermal or iron-oxide melt origin. Iron mineralization is genetically related to plutons that are composed of gabbro-diorite, which were emplaced at 130 ± 1 Ma. These rocks have SiO2 contents of 51.72–54.60 wt%, Na2O contents of 3.47–4.04 wt%, K2O contents of 2.02–2.69 wt%, and K2O/Na2O ratios of 0.51–0.73. These rocks are enriched in LILEs and LREEs and depleted in Nb, Ta, and Ti, which indicates that the magma originated through partial melting of an enriched lithospheric mantle source in a subduction environment. A pattern of decreasing initial Sr isotopic ratios and increasing εNd(t) values with time in Early Cretaceous magmatic rocks of the Ningwu Basin may indicate incorporation of increasing proportions of asthenospheric mantle material into the source magma, which is consistent with the processes of lithospheric thinning and asthenospheric upwelling in eastern China related to Mesozoic subduction of the Paleo-Pacific Plate. Two stages of magnetite are found in the gabbro-diorite: (1) early-crystallized magnetite as euhedral–subhedral crystals in larger clinopyroxene crystals, and (2) later-crystallized magnetite and accompanying ilmenite grains in the voids between plagioclase and clinopyroxene crystals. The formation of magnetite before clinopyroxene, combined with the results of Fe–Ti oxide geothermometry and analysis of magnetite V content, indicates that the oxygen fugacity of the source magma was greater than ΔFMQ +2.2 at an early stage (>640 °C) but decreased to ΔFMQ −2.66 as abundant magnetite crystallized at a later stage (∼489 °C). The early crystallization of magnetite at a high oxygen fugacity does not support a Fenner evolution trend for the primitive magma and diminishes the likelihood of liquid immiscibility, which could have generated an iron-rich melt, and is thus inconsistent with an iron-oxide melt origin for the Meishan iron deposit. The δ34S values of pyrite (6.6–15.1‰) and anhydrite (15.6–16.9‰) in the deposit and the occurrence of evaporites under the volcanic rocks likely indicate that the iron ores and alteration rocks of the Meishan deposit were formed by the circulation of fluids of evaporitic origin driven by heat from the hypabyssal gabbro-diorite intrusives. In the late magmatic stage, oxygen fugacity decreased to a reducing range, triggering the reduction of sulfate to reduced sulfur and leading to local gold and pyrite mineralization.

Provenance analyses of early Mesozoic sediments in the Ningwu basin: Implications for the tectonic–palaeogeographic evolution of the northcentral North China Craton

ABSTRACTThis paper reports results from detrital zircon U–Pb geochronology, Hf isotopic geochemistry, sandstone modal analysis, and palaeocurrent analysis of the early Mesozoic strata within the Ningwu basin, China, with the aims of constraining the depositional ages and sedimentary provenances and shedding new light on the Mesozoic tectonic evolution of the northcentral North China Craton (NCC). The zircons from early Mesozoic sandstones are characterized by three major populations: Phanerozoic (late Palaeozoic and early Mesozoic), late Palaeoproterozoic (with a peak at approximately 1.8 Ga), and Neoarchaean (with a peak at approximately 2.5 Ga). Notably, three Phanerozoic zircons in the Early Triassic Liujiagou Formation were found to have positive εHf(t) values and characteristics typical of zircons from the Central Asian Orogenic Belt (CAOB). Therefore, the CAOB began to represent the provenance of sediment in the sedimentary basins in the northern NCC no later than the Early Triassic (261 Ma), implying that the final amalgamation of the NCC and CAOB occurred before the Early Triassic. The U–Pb geochronologic and Hf isotopic results show that the Lower Middle Triassic sediments were mainly sourced from the Yinshan–Yanshan Orogenic Belt (YYOB), and that a sudden change in provenances occurred, shifting from a mixed YYOB and CAOB source in the Middle Jurassic to a primarily YYOB source in the Late Jurassic. The results of the sandstone modal analysis suggest that the majority of the samples from the Lower Middle Jurassic rocks were derived from either Continental Block or Recycled Orogen sources, whereas all the samples from the Upper Jurassic rocks were derived from Mixed sources. The change in source might be ascribed to the southward subduction and closure of the Okhotsk Ocean and the resulting intense uplift of the YYOB during the Late Jurassic. This uplift likely represents the start of the Yanshan Orogeny.

Nanopore Characteristics of Late Paleozoic Transitional Facies Coal-Bearing Shale in Ningwu Basin, China Investigated by Nuclear Magnetic Resonance and Low-Pressure Nitrogen Adsorption

Nanopore Characteristics of Late Paleozoic Transitional Facies Coal-Bearing Shale in Ningwu Basin, China Investigated by Nuclear Magnetic Resonance and Low-Pressure Nitrogen Adsorption

Geochronology and geochemistry of the Fe ore-bearing Zhonggu intrusions of the Ningwu Basin: Implications for tectonic setting and contemporaneous Cu-Au mineralization in the Middle–Lower Yangzte Metallogenic Belt

The Zhonggu ore field is located in the southern Ningwu volcanic basin, the largest iron ore district within the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) of eastern China. This area has produced more than 2000Mt of iron ore although comparatively little research has been done on the ore-bearing intrusions in this region despite the significance of the mineralization in this area. This in turn means that the petrogenesis and the links between these intrusions and the metallogenesis in this area remain unclear. This study focuses on the six intrusions that are associated with almost all of the iron deposits within the Zhonggu ore field, namely the Gushan porphyritic gabbrodiorite, the Longshan gabbrodiorite, the Hemushan diorite, the Baixingshan diorite, the Zhongjiu diorite and the Taipingshan monzonite. All of these rocks are either high- to medium-K and calc-alkaline or are shoshonitic, although the former dominates over the latter. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb dating yielded ages of 132.6±1.6 to 131.6±1.6, 130.7±2.2 to 129.4±1.9, 131±2.0, and 132.3±2.1Ma for intrusions associated with the Longshan, Taipingshan, Baixiangshan and Zhongjiu deposits, respectively, indicating that all of these intrusions were emplaced in the Early Cretaceous (132.6–129.4Ma). This magmatism and mineralization was contemporaneous with the second stage (135–127Ma) of magmatic activity that has been documented elsewhere within the MLYRMB. The mineralized intrusions associated with the Zhonggu iron ore field have similar chondrite-normalized rare earth element (REE) and primitive mantle-normalized multi-element variation patterns that are characterized by light REE (LREE) and light ion lithophile element (LILE; Rb, Ba, K, and Sr) enrichments and heavy REE (HREE) and high field strength element (HFSE; Nb, Ta, Zr, and Hf) depletions. These intrusions also have total REE (ΣREE) concentrations that are consistent with the evolution of the magma from mafic-intermediate to felsic. The magmas that formed these intrusions were derived from an enriched region of the mantle and underwent contamination by the assimilation of continental crustal or lithospheric material. The magmatism within the Zhonggu iron ore field formed during an Early Cretaceous period of lithospheric thinning and extension. In addition, the arc affinities of the intrusions in this region indicative of a relationship to the subduction of the Paleo-Pacific Plate, a process that was most likely the main factor in the generation of the mineralization within the MLYRMB. The Fe mineralization-related intrusions in the study area are also distinctly different from intrusions in this region associated with Cu–Au mineralization, most likely as a result of the heterogeneous distribution of slab-derived fluids in this period over time. This research also highlights the exploration potential of this region, indicating that the 132–129Ma monzonites and diorites in this area and beyond should be considered prime targets for Fe exploration but are also unlikely to be related to Cu–Au mineralization.

Effect of Fracturing Fluid on Coalbed-Methane Desorption, Diffusion, and Seepage in the Ningwu Basin of China

Summary The production process of coalbed methane (CBM) includes and is controlled by desorption, diffusion, and seepage processes acting conjointly. A coal reservoir commonly has a low permeability. Well stimulation is applied to increase the CBM production, during which contact between the fracturing fluid and the coal seam is unavoidable. Developed microfractures, large specific surface area, and high capillary pressure of coal generate serious damage easily caused by fracturing fluid, which can affect the production of CBM. Therefore, we presented a multiscale mass-transfer damage-evaluation method dependent on the coal-reservoir microscopic-pore structure and CBM migration-output mechanism that includes the capacity of CBM desorption/diffusion/seepage. The No. 9 coal in the Ningwu Basin of China and the associated fracturing fluid at the site are the object of our research. A comprehensive evaluation experiment on the damage caused by fracturing fluids on CBM desorption/diffusion/seepage was conducted to optimize the fracturing fluid in construction sites. Combining the methods of mechanical testing, scanning electron microscopy (SEM), low-temperature N2 adsorption, wettability, solution ratio, and infrared spectrum, we investigated the effect of fracturing fluids on CBM desorption/diffusion/seepage. Results show that the CBM desorption rate and the diffusion coefficient of samples treated by the fracturing fluid decrease compared with those treated by formation water. Compared with formation water, the fracturing fluid can decrease the mechanical strength of coal and strengthen its sensitivity to stress. Microcracks will seal more under the action of geostress, producing a narrower flow channel and causing the seepage capacity to eventually decrease. The results of our analysis suggest that the fracturing fluid can change the pore connectivity, wettability, and surface structure of coal, as well as affect the migration capacity of CBM. This study provides a theoretical guidance for fracturing-fluid optimization during the operation of CBM-well fracturing.

Geological, geochronological, and H–O isotopic constraints on the genesis of the Tongjing Cu–Au deposit in the Ningwu basin, east China

The Tongjing Cu–Au deposit is a medium-sized deposit within the Ningwu volcanic basin, east China, and is hosted by Cretaceous volcanic rocks of the Dawangshan and Niangniangshan Formations. The veined and lenticular Cu–Au orebodies are spatially and temporally related to the volcanic and subvolcanic rocks of the Niangniangshan Formation in the ore district. The wall-rock alteration is dominated by silicification, siderite alteration, carbonation, sericitization, chloritization, and kaolinization. On the basis of field evidence and petrographic observations, two stages of mineralization are recognized: (1) a siderite–quartz–sulfide stage (Stage 1) associated with the formation of chalcopyrite and pyrite in a quartz and siderite gangue; and (2) a quartz–bornite stage (Stage 2) cutting the Stage 1 phases. Stage 1 is the main mineralization stage. Quartz that formed in Stage 1 has δ18OH2O values of −4.3‰ to 3.5‰ with δD values of fluid inclusion waters of −97.1‰ to −49.9‰, indicating that the ore-forming fluids were derived from early magmatic fluids and may have experienced oxygen isotopic exchange with meteoric water during Stage 1 mineralization.LA–MC–ICP–MS zircon U–Pb dating of the mineralization-related nosean-bearing phonolite and nosean-bearing phonolitic brecciated tuff at Tongjing yields ages of 129.8±0.5Ma and 128.9±1.1Ma, respectively. These results are interpreted as the crystallization age of the volcanic rocks of the Niangniangshan Formation. A hydrothermal sericite sample associated with Cu–Au mineralization at Tongjing yields a plateau 40Ar–39Ar age of 131.3±1.3Ma. These results confirm a genetic link between the volcanism and associated Cu–Au mineralization. The Tongjing Cu–Au deposit in the Ningwu basin is genetically and possibly tectonically similar to alkaline intrusion-related gold deposits elsewhere in the world.

Geochemical constraints on the origin of Late Mesozoic andesites from the Ningwu basin in the Middle–Lower Yangtze Valley, South China

An integrated study of zircon U–Pb ages, Hf–O isotopes, whole-rock major-trace elements and Sr–Nd–Hf isotopes was carried out for Mesozoic andesites from the Ningwu basin in the Middle–Lower Yangtze Valley, South China. The results not only provide insights into the origin of anorogenic magmatism in the Middle–Lower Yangtze Valley but also shed light on the petrogenesis of continental arc andesites in general. The Ningwu andesites are mainly composed of trachyandesite and trachyte, with subordinate of basaltic trachy-andesites. They exhibit variable and low MgO contents of 0.1–3.7 wt.% and low Mg# values of 1.6–51.2, high K2O contents of 2.0–9.9 wt.%. They are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Furthermore, these rocks exhibit relatively enriched whole-rock Sr–Nd–Hf–Pb isotope compositions, with initial 87Sr/86Sr ratios of 0.7050–0.7082, negative εNd(t) values of −8.1 to −1.8, and negative εHf(t) values of −6.7 to 0.4. Zircon U–Pb dating yields consistent ages of 128±2 to 133±2Ma for magma emplacement through volcanic eruption. The zircon exhibits slightly high δ18O values of 5.7–7.8‰ and variable εHf(t) values of −5.8 to 0.7. All andesites can be categorized into two groups in view of their distinct geochemical compositions, an integrated interpretation of which allows deciphering of their petrogenesis. Group I andesite was produced by a relatively larger extent of partial melting of a more fertile and enriched mantle source that contains a greater amount of phlogopite, whereas Group II andesite was generated by a relatively smaller extent of partial melting of a less fertile and enriched mantle source that contains a less amount of phlogopite. In either case, these two mantle sources are less ultramafic in lithochemistry than normal mantle peridotite due to its reaction with metasediment-derived hydrous felsic melts. This metasomatism would take place during subduction of the Cathaysia oceanic crust beneath the Yangtze Craton in the Neoproterozoic, whereas the Mesozoic mafic magmatism in the Middle–Lower Yangtze Valley is essentially caused by reactivation of the fertile and enriched mantle sources due to lithospheric extension in response to the subduction of Pacific plate beneath the eastern China continent. Therefore, the geochemical composition of andesites is primarily governed by their mantle sources that were generated by reaction of the mantle wedge peridotite with hydrous felsic melts originated from the subducted terrigenous sediment in Andean-type subduction zones, though such sources may be partially melt either immediately following generation or at a later time. Late-stage magma evolution processes such as fractional crystallization and magma mixing would have further modified the lithochemical composition of andesites but they are not the governing factors for their trace element and radiogenic isotope compositions.

Three-dimensional mineral prospectivity modeling for targeting of concealed mineralization within the Zhonggu iron orefield, Ningwu Basin, China

The Zhonggu iron orefield is one of the most important iron orefields in China, and is located in the south of the Ningwu volcanic basin, within the middle and lower Yangtze metallogenic belt of eastern China. Here, we present the results of new 3D prospectivity modeling that enabled the delineation of areas prospective for exploration of concealed and deep-seated Baixiangshan-type mineralization and Yangzhuang-type mineralization within the Zhonggu orefield; both of these deposits are Kiruna-type Fe-apatite deposits but are hosted by different formations within the Ningwu Basin. The modeling approach used during this study involves 3 steps: (1) combining available geological and geophysical data to construct 3D geological models; (2) generation of 3D predictive maps from these 3D geological models using 3D spatial analysis and 3D geophysical methods; (3) combining all of the 3D predictive maps using logistic regression to create a prospectivity map. This approach integrates a large amount of available geoscientific data using 3D methods, including 3D geological modeling, 3D/2D geophysical methods, and 3D spatial analysis and data integration methods. The resulting prospectivity model clearly identifies highly prospective areas that not only include areas of known mineralization but also a number of favorable targets for future mineral exploration. The 3D prospectivity modeling approach showcased within this study provides an efficient way to identify camp-scale concealed and deep-seated exploration targets and can easily be adapted for regional- and deposit- scale targeting.

Cretaceous Cu–Au, pyrite, and Fe-oxide–apatite deposits in the Ningwu basin, Lower Yangtze Area, Eastern China

The Cretaceous Ningwu volcanic basin of the Middle and Lower Yangtze River Valley metallogenic belt of eastern China hosts numerous Fe-oxide–apatite, Cu–Au, and pyrite deposits. The mineralization in the Ningwu basin is associated with subvolcanic rocks, consisting of gabbro–diorite porphyry and/or pyroxene diorite. However, the mineralization is associated with subvolcanic and volcanic rock suite belonging to the Niangniangshan Formation in the Tongjing Cu–Au deposit, including nosean-bearing aegirine–augite syenites, quartz syenites, and quartz monzonites. The zoning displayed by the alteration and mineralization comprises: (1) an upper light-colored zone of argillic, carbonate, and pyrite alteration and silicification that is locally associated with pyrite and gold mineralization, (2) a central dark-colored zone of diopside, fluorapatite–magnetite, phlogopite, and garnet alteration associated with fluorapatite–magnetite mineralization, and (3) a lowermost light-colored zone of extensive albite alteration. The Cu–Au and pyrite orebodies are peripheral to the Fe-oxide–apatite deposits in this area and overlie the iron orebodies, including the Meishan Cu–Au deposit in the northern Ningwu basin and the pyrite deposits in the central Ningwu basin. The δ34S values of sulfides from the Fe-oxide–apatite, Cu–Au, and pyrite deposits in the Ningwu basin show large variation, with a mixed sulfur source, including magmatic sulfur and/or a mixture of sulfur derived from a magmatic component, country rock, and thermochemical reduction of sulfate at 200–300°C. The ore-forming fluids associated with iron mineralization were derived mainly from magmatic fluids, and the late-stage ore-forming fluids related to Cu–Au and pyrite mineralization may have formed by the introduction of cooler meteoric water to the system. The Fe-oxide–apatite, Cu–Au, and pyrite deposits of the Ningwu basin formed in an extensional environment and are associated with a large-scale magmatic–hydrothermal system.

The role of evaporites in the formation of magnetite–apatite deposits along the Middle and Lower Yangtze River, China: Evidence from LA-ICP-MS analysis of fluid inclusions

Numerous magnetite–apatite deposits occur in the Ningwu and Luzong sedimentary basins along the Middle and Lower Yangtze River, China. These deposits are located in the contact zone of (gabbro)-dioritic porphyries with surrounding volcanic or sedimentary rocks and are characterized by massive, vein and disseminated magnetite–apatite±anhydrite mineralization associated with voluminous sodic–calcic alteration. Petrologic and microthermometric studies on multiphase inclusions in pre- to syn-mineralization pyroxene and garnet from the deposits at Meishan (Ningwu basin), Luohe and Nihe (both in Luzong basin) demonstrate that they represent extremely saline brines (~90wt.% NaClequiv) that were trapped at temperatures of about 780°C. Laser ablation ICP-MS analyses and Raman spectroscopic studies on the natural fluid inclusions and synthetic fluid inclusions manufactured at similar P–T conditions reveal that the brines are composed mainly of Na (13–24wt.%), K (7–11wt.%), Ca (~7wt.%), Fe (~2wt.%), Cl (19–47wt.%) and variable amounts of SO4 (3–39wt.%). Their Cl/Br, Na/K and Na/B ratios are markedly different from those of seawater evaporation brines and lie between those of magmatic fluids and sedimentary halite, suggesting a significant contribution from halite-bearing evaporites. High S/B and Ca/Na ratios in the fluid inclusions and heavy sulfur isotopic signatures of syn- to post-mineralization anhydrite (δ34SAnh=+15.2 to +16.9‰) and pyrite (δ34SPy=+4.6‰ to +12.1‰) further suggest a significant contribution from sedimentary anhydrite. These interpretations are in line with the presence of evaporite sequences in the lower parts of the sedimentary basins.The combined evidence thus suggests that the magnetite–apatite deposits along the Middle and Lower Yangtze River formed by fluids that exsolved from magmas that assimilated substantial amounts of Triassic evaporites during their ascent. Due to their Fe-oxide dominated mineralogy, their association with large-scale sodic–calcic alteration and their spatial and temporal associations with subvolcanic intrusions we interpret them as a special type of IOCG deposits that is characterized by unusually high contents of Na, Ca, Cl and SO4 in the ore-forming fluids. Evaporite assimilation apparently led to the production of large amounts of high-salinity brine and thus to an enhanced capacity to extract iron from the (gabbro)-dioritic intrusions and to concentrate it in the form of ore bodies. Hence, we believe that evaporite-bearing sedimentary basins are more prospective for magnetite–apatite deposits than evaporite-free basins.

Re‐Os Dating of Pyrite From the Taocun Fe Deposit, Ningwu Basin, Lower Yangtze River Valley, Eastern China

Re‐Os Dating of Pyrite From the Taocun Fe Deposit, Ningwu Basin, Lower Yangtze River Valley, Eastern China