Middle-Lower Yangtze River Metallogenic Belt Research Articles

Contrasting Tectonomagmatic Conditions for Coexisting Iron Oxide-Apatite Deposits and Porphyry and Skarn Cu ± Au Deposits in the Middle-Lower Yangtze River Metallogenic Belt, China

Abstract Porphyry Cu ± Mo ± Au and iron oxide-apatite (IOA) deposits rarely occur in spatial and temporal proximity in Phanerozoic arc-related settings, and the formation of these mineral deposit types in an evolving arc setting remains poorly understood. Specifically, the roles of magma composition and the tectonic regime remain the subject of some debate. Here, we systematically estimated the P-T-fO2 conditions and H2O-S-Cl contents for dioritic to granodioritic source magmas for porphyry and skarn Cu ± Au (150–135 Ma) and IOA deposits (~130 Ma) that formed in transpressional and transtensional settings in the Middle-Lower Yangtze River metallogenic belt, China. Our estimates show that, compared to IOA deposits, the porphyry- and skarn-related magmas were relatively felsic, cooler, and more hydrous. These geochemical features are consistent with the tectonic transition from subduction to slab rollback of the paleo-Pacific plate in the East Asia continental margin at <135 Ma and concomitant crustal extension and steepening of the regional geothermal gradient. Apatite data reveal that the silicate melts associated with the porphyry and skarn Cu ± Au and IOA deposits had comparable predegassed S concentrations (~0.13 ± 0.06 wt % vs. ~0.16 ± 0.09 wt % on average), but that IOA-related melts contained higher predegassed Cl/H2O ratios (~0.11 ± 0.03 vs. ~0.04 ± 0.03 for porphyry- and skarn-related magmas) that decreased by one order of magnitude after magmatic degassing. Magmatic fO2 estimated using zircon and amphibole, reported in log units relative to the fayalite-magnetite-quartz (FMQ) redox buffer, gradually increased during cooling of the porphyry- and skarn-related magmas (ΔFMQ +0.7 to +2.5) at 950° to 800°C and decreased to ΔFMQ +1 at 700°C owing to fractionation of Fe2+-rich minerals and subsequent S degassing, respectively. In contrast, the magmatic fO2 values for the IOA-related source magmas varied significantly from ΔFMQ –1.5 to ΔFMQ +2.5 but generally show an increasing trend with cooling from 970° to 700°C that probably resulted from variable degrees of evaporite assimilation, fractionation of Fe2+-rich minerals, and Cl degassing. These results are consistent with the hypothesis that Cl enrichment of the IOA-related source magmas played a determinant role in their formation. We propose that the porphyry and skarn Cu ± Au deposits in the Middle-Lower Yangtze River metallogenic belt formed in a transpressional setting in response to paleo-Pacific flat-slab subduction that favored storage and evolution of S-rich hydrous ore-forming magmas at variable crustal levels. A subsequent extensional setting formed due to slab rollback, leading to rapid degassing of Cl-rich IOA-related magmas. For the latter scenario, assimilation of evaporite by mafic to intermediate magmas would lead to an enrichment of Cl in the predegassed magmas and subsequent exsolution of hypersaline magmatic-hydrothermal fluid enriched in Fe as FeCl2. This Fe-rich ore fluid efficiently transported Fe to the apical parts of the magma bodies and overlying extensional normal faults where IOA mineralization was localized. The concomitant loss of S, H2O, and Cu with Cl by volcanic outgassing may have inhibited sulfide mineralization at lower temperatures.

Numerical modeling of the mineralizing processes within the Shaxi porphyry-type Cu-Au deposit, Eastern China: Influence and restriction from physical and chemical characteristics of host rocks

The Shaxi porphyry-type Cu-Au deposit is located in the northwestern outer margin of the Luzong volcanic basin, which is situated in the Middle-Lower Yangtze River Metallogenic Belt (MLYRMB), Eastern China. It is a large porphyry-type Cu-Au deposit with confirmed copper resources exceeding 1 Mt and gold resources exceeding 40 tons. Previous studies have extensively investigated the genesis, controlling structures, geochemical characteristics, fluid evolution, alteration zone, and ages of mineralization and host rocks in this deposit. However, research on the influence of the physical and chemical properties of the host rocks on the mineralizing processes has not been explored in detail. This study employs the finite element method to apply numerical modeling of multiple physical and chemical fields, interlinking heat transfer, fluid flow, material migration, mineralization reaction, and their coupling with porosity and permeability of rock, to investigation of the influence of different porosities, permeabilities, and chemical properties of the host rocks on the mineralizing processes in the Shaxi deposit. The results show that the porosity and permeability of the host rocks play a significant role in the mineralization of porphyry-type systems. The different porosity and permeability of the intrusions and strata control the depth, the mineralization intensity, the morphology of the ore bodies, and the spatial location of the mineralization center. The chemical properties of the strata significantly restrict the mineralization intensity and horizontal spatial rather than affecting the depth of mineralization. Furthermore, the formation of skarn-type deposits may require a longer period of time than that of porphyry-type deposits under the same mineralization conditions and ore grade. This paper also indicates that the numerical modeling method employed in this study can be used to predict the mineralization depth and spatial location of deep concealed porphyry-type deposits, aiding research in deep-seated mineral exploration.

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

Nowadays, mineral exploration has increasingly focused on the targeting of deep-seated orebodies. Mineral prospectivity modeling is one of the important approaches facilitating exploration targeting and mitigating 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, enhancing the accurate identification of the deep-seated orebodies. Notably, these advancements have synergized with deep learning approaches to improve the efficiency of mineral exploration based on their nonlinear and multi-layer sensing attributes, effectively identifying and extracting key relationships between the 3D predictive maps and mineralization. Currently, the main deep learning method used for 3D mineral prospectivity modeling is convolutional neural network (CNN) models. However, the related research has not considered the multiscale features of geological structures, it can be made further improvements on this regard. 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 concentrate on key predictive areas is significantly enhanced. This leads to further improvement in the accuracy and generalization capability of 3D mineral prospectivity modeling. To evaluate the effectiveness of this model, we carry out a case study on 3D mineral prospectivity modeling in the Baixiangshan iron deposit within the Ningwu Basin of the Middle-Lower Yangtze River Metallogenic Belt, China. The results show that the multi-scale 3D convolutional neural network model exhibits remarkable robustness and generalization capabilities. It can effectively delineate targets within the deep and peripheral areas of the deposit, providing Indications for future exploration. The addition, performance indicators, ROC curve, and Capture-Efficiency curve consistently demonstrate that the MSAM-CNN model outperforms the Inception-enhanced CNN (M-CNN), CNN, Random Forest (RF), and Support Vector Machine (SVM) models. All of these indicate that MSAM-CNN can extract the 3D spatial features within 3D predictive maps better during 3D mineral prospectivity modeling, positioning it as a promising tool for yielding more reliable targets for deep-seated mineralization in future exploration.

Cd–Pb isotopic characteristics within the Hehuashan carbonate-hosted Pb–Zn deposit, Middle-Lower Yangtze River Metallogenic Belt: Constraints for ore-forming metal sources and implications for exploration

The Hehuashan carbonate-hosted Pb–Zn deposit is a large-scale deposit situated in the Middle-Lower Yangtze-River Metallogenic Belt (MLYRMB) in China. It contains 0.99 million tons of Zn at an average grade of 1.5 % and 0.41 million tons of Pb at an average grade at 1.9 %, accompanied by critical metals of Cd and Ga. The ores are mainly composed of breccia-type and vein-type in the deposit. Though previous chemical compositions of sphalerite showed that Cd is enriched in both two types of mineralization and the sulfide sulfur isotopes research showed that the reduced sulfur is derived from the marine sulfate reduction, the source of ore-forming metals is still unclear. In this research, we studied the non-traditional stable isotope of Cd and the radioactive isotope Pb of the Hehuashan deposit to discuss the ore-forming metal sources and provide genesis constraints to the Hehuashan Pb–Zn deposit. The sphalerite Cd isotopic compositions are relatively concentrated but the Zn/Cd ratios are scattered in the Hehuashan deposit, of which the early-stage sphalerite δ114/110Cd values ranged from 0.21 ‰ to 0.30 ‰ with an outlier of 0.09 ‰, mean of 0.22 ‰ and a wide range of Zn/Cd ratios (78 to 430), and the late-stage sphalerite δ114/110Cd values ranged from 0.09 ‰ to 0.15 ‰ with two outliers of −0.05 ‰ and 0.52 ‰, mean of 0.17 ‰ with also a wide range of Zn/Cd ratios (50 to 286), both stages of Cd isotopic compositions are overlapped with the ranges of diorite and OIB & MORB, implying the Cd is might be derived from the dioritic intrusions of the Hehuashan deposit district, and the varied Zn/Cd ratios imply that the limestone also provided some of ore-forming metals to the both two ore-forming stages. Additionally, the Pb isotopic compositions of galena in the two mineralization stages exhibit a concentrated range (206Pb/204Pb = 18.2806 – 18.4190, 207Pb/204Pb = 15.6262 – 15.6484, and 208Pb/204Pb = 38.5710 – 38.6529) and all the Pb isotopic data points of the Hehuashan deposit are plotted in the right-hand side of the geochron and the overlapped field of marine sediments, Tongling dioritic intrusions, and Lower Triassic limestones in the Tongling area in the diagrams of (207Pb/204Pb)i vs. (206/204Pb)i and in the field (208Pb/204Pb)i vs. (206/204Pb)i, implying that the Pb is mainly derived from the magma of dioritic intrusions and marine sediments. Subsequently, the sulfide Cd–Pb isotopic compositions show that the ore-forming metals in the Hehuashan deposit are mainly originated from the dioritic intrusions and marine sediments. The plots of deposit genesis classification show that the Cd isotopic compositions in the Hehuashan deposit are unsuitable to the plots of δ114/110Cd vs. Zn/Cd ratios, but imply the metals were derived from multi sources. Finally, the Cd-Pb studies of the Hehaushan Pb–Zn deposit show that the earlier Lower Triassic carbonate formations and its contact with dioritic rocks should be ideal exploration targets for Pb–Zn and associated Cd sources in the Tongling mining area.

Re–Os isotopes and geochemistry of pyrite from the Dongguashan Cu(-Au) deposit, Eastern China: Implications for porphyry-skarn-stratabound mineralization systems

Stratabound deposits are widely distributed along the Middle-Lower Yangtze River metallogenic belt (Eastern China). The stratabound mineralization system in these stratabound deposits remains enigmatic as to whether it is associated with late Mesozoic intrusions or late Carboniferous sedimentary exhalative processes. Dongguashan is a typical stratabound deposit and is characterized by pyrite that formed in porphyry-skarn-stratabound orebodies, which makes it ideal for comparing porphyry, skarn, and stratabound mineralization systems by pyrite study. We classify pyrite types according to occurrence and generation. Specifically, the pyrite in the porphyry orebodies is composed of Py-p1 (potassic zone) and Py-p2 (propylitic and phyllic zone). Py-s, among garnet grains, occurs in the skarn orebodies. From top to bottom, the pyrite in the stratabound orebodies is composed of macrocrystalline pyrite (Py-m) in the massive mineralization zone, Py-l in the laminated mineralization zone, and coarse-grained pyrite (Py-v1) and microcrystalline pyrite (Py-v2) in the vein-like mineralization zone. Co, Ni, Se, As, and Au are most likely to occur as solid solutions in the Dongguashan pyrite. Pb and Bi are likely distributed in Dongguashan pyrite homogeneously as nanoinclusions. The Zn, Cu, and Mo in Dongguashan pyrite indicate that these elements occur as micromineral inclusions. The positive correlation between S and Fe, high Co/Ni ratios (mostly > 2), and coexistence with hydrothermal minerals imply a magmatic-hydrothermal origin for the Dongguashan pyrite. Py-m yields a Re–Os isochron age of 135.8 ± 2.8 Ma (MSWD = 1.8), which is interpreted as the stratabound ore-forming age. Combined with previous results, the porphyry-skarn-stratabound mineralization is the product of the same hydrothermal fluid system related to Early Cretaceous magmatic activity.

Non-magmatic sulfur source of hydrothermal colloform pyrite: Insights from the Jinshandian iron skarn deposit, Daye district, eastern China

Colloform pyrite occurs widely in many ore deposits in the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in eastern China, but its genesis is still debated. In this study, we present detailed textures and geochemical compositions of newly identified colloform pyrite (Py1) and late coarse-grained pyrite (Py2) from the Jinshandian iron skarn deposit in the Daye district to decipher the genesis of the pyrite grains. Colloform pyrite is composed of nanometer-sized (<1 μm) grains, showing typical colloform textures, and occurs as interstitial fillings between magnetite grains. In contrast, Py2 commonly has a large grain size (up to 200 μm) and is usually distributed along the margins of colloform pyrite. These two types of pyrite have relatively high Co/Ni (1.24–13.36) ratios. δ34SCDT values of Py1 (17.22‰–20.63‰, mean 19.42‰) and Py2 (18.96‰–19.95‰, mean 19.43‰) are close to those of gypsum (anhydrite) from evaporites in the Daye district, indicating the incorporation of evaporite-derived materials into the formation of colloform pyrite. On the basis of the textures, trace-element compositions, and sulfur isotope characteristics of the colloform pyrite, it is inferred that this pyrite from the Jinshandian iron deposit is of hydrothermal origin. The sulfur isotope composition of colloform pyrite from Jinshandian is heavier relative to other magmatic-hydrothermal colloform pyrites, which indicates colloform pyrite could formed in diverse hydrothermal fluids (magmatic to no-magmatic fluid). Our study further confirmed that hydrothermal colloform pyrite has widely occurred in Fe and Cu-Au deposits associated with the Cretaceous magmatism in the MLYRMB.

Correlation between the surface morphology of the intrusions and the formation of mineralization within skarn deposits: A numerical simulation study of the Qiaomaishan skarn Cu deposit, Middle and Lower Yangtze River Metallogenic Belt, China

Skarn deposits are high grade hydrothermal deposits distributed throughout the world, containing multiple mineral resources. However, the formation of skarn deposits is extremely complex, hindering our in-depth understanding of these deposits with high economic value. The Qiaomaishan Cu deposit is a typical skarn deposit located in the Liqiao-Tongshan orefield, Middle-Lower Yangtze River Metallogenic Belt (MLYRMB). This study applied numerical simulation approach to quantitatively describe the formation of copper mineralization within the Qiaomaishan skarn deposit, and explored the correlation between the surface morphology of intrusion and the formation of mineralization. This numerical model links five main processes related to ore formation within hydrothermal ore-forming system and their complex coupled relationship, namely chemical reactions, heat conduction, rock deformation, fluid migration, and materials diffusion. Results show that rough surface morphology of the intrusions is an ore controlling factor within the Qiaomaishan skarn deposit. The uneven surface of the intrusions greatly affects the time spent in cooling, thereby affecting the generation and spatial distribution of chalcopyrite. The concave part of the surface of the intrusions lives a longer cooling period (1000–2500 years) than the convex part (110–160 years) from 310 to 260 °C, which directly causes the difference in the temporal-spatial distribution of chalcopyrite. Results also show that the factors involved in prospectivity modeling must undergo strict conditional independence analysis, and numerical simulation method can be used to effectively achieve this goal. However, there are also some problems unsolved, such as difficulties caused by lack of data and oversimplified ore-forming chemical reactions. Future development of both geological research and modification of the law of mass action will undoubtedly improve the accuracy and practicability of numerical simulation and our understanding of the Qiaomaishan as well as other skarn deposit, providing information for mineral deposit research from the perspective of computational geoscience.

Origin of the Yinshan Pb-Zn-Ag Deposit in the Edong District Section of the Middle–Lower Yangtze River Metallogenic Belt: Insights from In-Situ Sulfur Isotopes

The investigation into the enigmatic origin of Pb-Zn mineralization within the Middle-Lower Yangtze River Metallogenic Belt has long been marred by a paucity of prior studies. Seeking to alleviate this dearth of knowledge, our study meticulously scrutinizes the Yinshan Pb-Zn-Ag deposit nestled within the Edong district of the belt, endeavoring to cast an illuminating spotlight upon its enigmatic genesis. We identify two distinct epochs: (1) the pre-mineralization pyrite epoch (Epoch I) mainly characterized by colloform and massive pyrite, and (2) the hydrothermal mineralization epoch (Epoch II) which can be further divided into three stages: pyrite-arsenopyrite (stage 1), galena-sphalerite (stage 2), and vein pyrite (stage 3). We conduct in-situ sulfur isotope analyses on sulfide minerals from both epochs, revealing δ34S values ranging from −0.5 to 4.8‰ for Epoch I and varying from 2.2–4.9‰ (stage 1), 1.1–3.0‰, 4.2–7.1‰ (stage 2), and 2.1–3.8‰ (stage 3) for Epoch II. Integrating our sulfur isotope data with the geological characteristics of the deposit, we infer that Pb-Zn mineralization was related to a granite of ~130 Ma age. Additionally, our study suggests the possibility of coexisting Mo mineralization beneath the Pb-Zn mineralization. Our findings contribute to a better understanding of the origin of Pb-Zn mineralization in the Middle-Lower Yangtze River Metallogenic Belt.

Distribution of Co, Se, Cd, In, Re and other critical metals in sulfide ores from a porphyry-skarn system: A case study of Chengmenshan Cu deposit, Jiangxi, China

Chengmenshan Cu deposit is a large porphyry-skarn copper deposit, located in the Jiutui ore district, Middle–Lower Yangtze River metallogenic belt, Eastern China. The deposit is enriched in tellurium, selenium, rhenium and cadmium; however, the occurrence and distribution of these metals are unclear. In this study, we report cobalt, selenium, cadmium, indium, rhenium and other critical metals in pyrite, chalcopyrite, and sphalerite from sulfide ores of Chengmenshan deposit. In-situ LA-ICP-MS trace element analysis and mapping of minerals were conducted to determine their trace element concentrations and reveal the distribution and occurrence of critical metals in different sulfides. Three ore types have been identified in Chengmenshan deposit, named: porphyry ore, skarn ore and strata-bound ore. Based on mineral assemblages, ore fabrics, and crosscutting relationships between the veins, Chengmenshan deposit is divided into three mineralization stages: pre-ore stage of skarn mineralization, syn-ore stage of sulfide mineralization, and post-ore stage of carbonation. The pyrite and sphalerite can be divided into two generations: Py1, Py2, and Sp1, Sp2, respectively. The critical metals exhibited an uneven distribution and closely related to sulfide minerals. Telluride minerals in the deposit mainly include hessite, tetradymite, joseite. Rhenium is mainly enriched in molybdenite (128 to 496 ppm, average 276 ppm); cobalt is mainly enriched in pyrite, and the value of the three types of ore in the early stage (Py1) higher late stage (Py2); Se in chalcopyrite is slightly higher than pyrite and sphalerite; Cadmium is mainly enriched in sphalerite (up to 3000–6000 ppm), and is relatively low in pyrite and chalcopyrite; Indium is distributed in chalcopyrite and sphalerite, and the values of sphalerite are higher than chalcopyrite; the Ga distribution was sphalerite > chalcopyrite > pyrite. The enrichment and distribution of Co, Cd, In in Chengmenshan porphyry-skarn deposit system were mainly control by the fluid temperature. The Cu-Mo (Re), Cu-Fe (Co, Se) and Cu-Pb-Zn-Ag (Te, Cd, In, Tl) mineralizations constitute Chengmenshan porphyry-skarn mineralization system, and show a Mo-Re to Cu-Co-Se-Te-Ga to Cu-Pb-Zn-Ag-Te-Cd-In pattern from granite to distal stratum in space.

3D convolutional neural Network-based 3D mineral prospectivity modeling for targeting concealed mineralization within Chating area, middle-lower Yangtze River metallogenic Belt, China

The Chating area is situated within the Middle-Lower Yangtze River Metallogenic Belt, China. Several concealed skarn and porphyry-type deposits have been discovered in this area, indicating high potential for hosting hydrothermal deposits. However, due to the complex geological structure, exploration risks significantly increase with increasing depth. To overcome this challenge, three-dimensional mineral prospectivity modeling (3DMPM) has begun to be widely applied for mapping the prospectivity of deep-seated and concealed mineralization. However, most previous studies on 3DMPM were based on shallow supervised machine learning models and dimensionality-reduced 3D predictive maps. Although these models have shown good results, they may lose spatial correlation within the 3D predictive maps and fail to explore nonlinear correlations between the 3D predictive maps and mineralization. Meanwhile, 3D geological models are the most important basis of the 3DMPM, however, in the past, few studies have incorporated the optimization of the 3D geological models into the process of 3DMPM. Therefore, this paper initially builds and optimizes 3D geological models through implicit 3D geological modeling and “total litho-inversion” approach. Subsequently, the 3D predictive maps are generated by employing various 3D methods, which are further integrated using a 3D convolutional neural network (3D CNN) model to identify highly prospective areas for mineralization. The results show that the highly prospective areas identified by the 3DMPM include not only the training data but also other mineral deposits that have previously been discovered within the study area. In addition, compared with the Logistic Regression model (LR), Support Vector Machines (SVM), and Radom Forest (RF), the 3D CNN performs better prediction capabilities due to its enhanced ability to capture the correlations between 3D predictive maps and multiple types of mineral deposits. It suggests that the 3DMPM based on the 3D CNN model has commendable predictive capabilities in identifying prospective mineralization areas, and some new highly prospective areas can be considered as priority areas for future exploration of concealed mineralization within the Chating Area.

Palladium, platinum, selenium and tellurium enrichment in the Jiguanzui-Taohuazui Cu-Au Deposit, Edong Ore District: Distribution and comparison with Cu-Mo deposits

The Jiguanzui-Taohuazui Cu-Au deposit is located in the Edong ore district, Middle–Lower Yangtze River metallogenic belt, eastern China. The deposit is palladium, platinum, selenium and tellurium enriched; however, the distribution of these metals is unclear. Three mineral assemblages of ore in the deposit have been identified, namely: a magnetite-bornite-chalcopyrite-(hematite) assemblage (Mt-Bn-Cp-Hm), a chalcopyrite-pyrite assemblage (Cp-Py), and a pyrite-chalcopyrite-(sphalerite) assemblage (Py-Cp-Sph). Forty-eight bulk ore assay results show high concentrations of up to 66.9 ppb for Pd, 5.9 ppb for Pt, 150 ppm for Se and 249 ppm for Te. The high temperature Mt-Bn-Cp-Hm assemblage (530–380 °C) is enriched in Pt and Pd, whereas the Py-Cp-Sph assemblage in the marble-replacement ore (300–220 °C) hosts the major Se and Te mineralization. Palladium, Pt, and Se are mostly hosted in sulfide minerals, whereas Te is hosted in tellurides and Bi-Te-S sulfosalt minerals. Building on previous experimental and thermodynamic calculations, we propose the major controls on the Pd and Pt distribution in the deposit are temperature and salinity, whereas the Se and Te mineralization is promoted by the precipitation of major sulfide phases such as pyrite, chalcopyrite and sphalerite.A comparison of the ores from the Jiguanzui-Taohuazui Cu-Au and Tongshankou Cu-Mo deposits in the Edong ore district shows that the Cu-Au deposit has higher PGE and Te, but similar Se concentrations. This scenario is consistent with the average grades and bulk ore contents of these elements from global (oxidized) porphyry (±skarn) Cu deposits. This suggests that the saturation of magmatic sulfides in the magma chamber as a result of higher proportion of crustal S-rich and/or reduced material contamination can be detrimental for PGE and Te enrichment processes, and thus, Cu-Au porphyry (±skarn) deposits have more potential for higher Pd and Te concentrations than the Cu-Mo deposits.

Critical metal enrichment in carbonate-hosted Pb-Zn systems: Insight from the chemistry of sphalerite within the Hehuashan Pb-Zn deposit, Middle-Lower Yangtze River Metallogenic Belt, East China

Carbonate-hosted Pb–Zn deposits frequently contain significant amounts of critical metals such as Cd, Ga, Ge, Tl, and In. However, the characteristics of critical metal enrichments and the geochemical controls of the critical metal enrichments present within different types of sphalerite in individual deposits remain poorly constrained. Textural observations and in-situ laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) and associated elemental mapping allows the investigation of the controls on the incorporation of critical metals into sphalerite within the Hehuashan carbonate-hosted Pb–Zn deposit, East China, which contains 639.36 t Cd at an average grade of 0.0020 % and 161.23 t Ga at an average grade of 0.0161 %. Two ore-formation stages (early and late) and five textural types of sphalerite are present in the deposit, namely fine-grained brown sphalerite (FCS), irregular sphalerite (IS), oolitic sphalerite (OS), banded sphalerite (BS), and coarse-grained sphalerite (CCS), all of which are present within breccia-, vein-, and massive ores. The FCS, IS, OS, and BS formed during early stage mineralization whereas the CCS formed during the later stages of mineralization. Large variations of Cd, Ga, Ge, Tl, and In concentrations are present within these sphalerites with an average Cd of 5997 ppm in OS, 14003 ppm in FCS, 962 ppm in IS, 2565 ppm in BS, and 5250 ppm in CCS; Ga of 1047 ppm in OS, 5 ppm in FCS, 3 ppm in IS, 15 ppm in BS, and 33 ppm in CCS; Ge of 166 ppm in OS, 21 ppm in FCS, 530 ppm in IS, 13 ppm in BS, and 4 ppm in CCS. The Cd within this sphalerite is incorporated by direct substitution between Cd2+ and Zn2+ whereas Ga concentrations are correlated to monovalent cation concentrations (Cu and Ag) and Ge concentrations are not correlated to monovalent cation concentrationss, indicative of an unusual vacancy-related substitution of Ge for Zn. Trace and minor element distribution variations and the calculated temperatures of formation of the deposit suggest that the ore-forming fluids associated with the system evolved significantly over the multiple stages of mineralization recorded within the study area. In addition, the deposit records two critical metal enrichment events associated with the early and late stages of mineralization within the Hehuashan Pb–Zn deposit, respectively; an early event associated with enrichments in Cd, Ge, and In, and a late event associated with enrichments in Cd and Ga, indicating that carbonate-hosted Pb–Zn deposits should be considered significant potential sources for critical metals such as Cd, Ga, Ge. However, the concentrations of these critical metals vary within the different types of sphalerite present in in individual Pb–Zn deposits, suggesting that thhe Hehuashan deposit, and other similar deposits within the MLYRMB, may have undergone at least two critical metal enrichments events that should be targeted for future critical metal exploration.

Three-Dimensional Mineral Prospectivity Modeling for Delineation of Deep-Seated Skarn-Type Mineralization in Xuancheng–Magushan Area, China

The Middle–Lower Yangtze River Metallogenic Belt is an important copper and iron polymetallic metallogenic belt in China. Today’s economic development is inseparable from the support of metal mineral resources. With the continuous exploitation of shallow and easily identifiable mines in China, the prospecting work of deep and hidden mines is very important. Mineral prospectivity modeling (MPM) is an important means to improve the efficiency of mineral exploration. With the increase in resource demands and exploration difficulty, the traditional 2DMPM is often difficult to use to reflect the information of deep mineral deposits. More large-scale deposits are needed to carry out 3DMPM research. With the rise of artificial intelligence, the combination of machine learning and geological big data has become a hot issue in the field of 3DMPM. In this paper, a case study of 3DMPM is carried out based on the Xuancheng–Magushan area’s actual data. Two machine learning methods, the random forest and the logistic regression, are selected for comparison. The results show that the 3DMPM based on random forest method performs better than the logistic regression method. It can better characterize the corresponding relationship between the geological structure combination and the metallogenic distribution, and the accuracy in the test set reaches 96.63%. This means that the random forest model could provide more effective and accurate support for integrating predictive data during 3DMPM. Finally, five prospecting targets with good metallogenic potential are delineated in the deep area of the Xuancheng–Magushan area for future exploration.

Knowledge-driven based three-dimensional prospectivity modeling of Fe–Cu skarn deposits; a case study of the Fanchang volcanic basin, anhui province, Eastern China

Mineral exploration at depth is becoming increasingly important as a result of the decades of discovery and mining of surficial or near-surface mineralization, leading to targeting of deeper-seated mineralization and mineral systems. The numerous challenges posed by this transition to deeper exploration can be partly overcome by the application of 3D prospectivity modeling. This study provides an outline of this type of prospectivity modeling and focuses on the under-explored Fanchang volcanic basin, an area prospective for skarn-type Fe–Cu mineralization within the Middle–Lower Yangtze River Metallogenic belt (MLYRB) of eastern China. The approach used utilizes a 3D geological model of the Fanchang volcanic basin that enables the identification and extraction of favorable elements for mineralization using 3D spatial and mathematical morphology analysis methods. A 3D distance field method was also used to obtain distance field information for the favorable ore-forming factors delineated during this study. The limited exploration to date in the study area means that there is a lack of specific information (e.g., location, size, grade) for the mineralization in this region. As such, knowledge-driven Index Overlay with Multi-Class (IOMC), Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS), and Fuzzy Comprehensive Evaluation (FCE) approaches were used to generate 3D prospectivity models for the Fanchang volcanic basin. The resulting weight coefficients relating to favorable ore-forming factors were quantified using an Analytic Hierarchy Process (AHP) approach, yielding random consistency ratio (CR) values of 0.0125. These are all less than a value of 0.1, which would indicate inconsistencies between the matrix values and values for the entirety of the hierarchy, indicating that our calculated weighting coefficients are precise and accurate. Comparison of the 3D prospectivity modeling results of the three methods using a prediction area (P–A) plot indicates that 88% of the known mineralization and mineralized occurrences within the basin were located within an area of only 12% of the study area considered prospective during modeling using the IOMC method. In comparison, TOPSIS and FCE modeling positively identified 82% (18% of the study area) and 91% (9% of the study area) of the mineralization in the study area, respectively. These results indicate that these approaches are viable tools for exploration targeting in relatively unexplored areas without specific orebody size, location, and grade data, and the FCE method appears more efficient and reliable than the other two methods in predicting locations to explore for Fe–Cu skarn mineralization. The approaches outlined during this study are widely applicable for exploration for other mineralizing systems in under-explored regions elsewhere and the specific high prospectivity targets identified during this study and the links between exploration indicators and high prospectivity regions also provide key areas and insights for both future modeling and mineral exploration within the Fanchang volcanic basin and the wider MLYRB.

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.

REE and cathodoluminescence features of scheelites from the Tongshankou Cu–Mo deposit in Eastern China

The Tongshankou porphyry-/skarn-type Cu–Mo deposit is located in the Edong ore district, the largest district of the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in Eastern China. From the ore-causative granodiorite porphyry and skarn at Tongshankou, three types of scheelite are recognized (Sch1 to Sch3): Sch1-subhedral–anhedral, occurring in the chalcopyrite ores within the porphyry; Sch2-granular, occurring inside garnet in the endoskarn and Sch3-subhedral–anhedral, occurring in calcite veins in the exoskarn. Sch2 and Sch3 have different internal texture according to CL images: Sch2-Dark core and bright rim, with the heterogeneous dark core; Sch3-Dark core and narrow dark rim, showing patches of different brightness in core. In addition, three REE types have been differentiated: 1) REE-Type1: LREE-enriched, HREE-depleted, and negative Eu anomalies patterns (Sch1), which is related to the porphyry system; 2) REE-Type2: No apparent fractionation between LREE and HREE, with positive Eu anomalies (Sch2), which is related to the endoskarn; 3) REE-Type3: Higher LREE and lower HREE with extremely positive Eu anomalies (Sch3), which is related to the exoskarn. The negative MoO3 vs. WO3 correlation for the Tongshankou scheelites suggests Mo-W substitution in the scheelite crystal lattice. No humped pattern (MREEs-enriched) of REE, lower Na content and the lack of correlation between Na and REE at Tongshankou excluded the Na substitute into the Ca site. The positive correlation with Nb and REE for Sch 1 and 3, suggests equation [2] and [3] should be more plausible mechanism to generate the Tongshankou scheelite REE patterns.

A 3D Investigation of Geological Structure and Its Relationship to Mineralization in the Nanling-Xuancheng Ore District, Middle-Lower Yangtze River Metallogenic Belt, China

A 3D Investigation of Geological Structure and Its Relationship to Mineralization in the Nanling-Xuancheng Ore District, Middle-Lower Yangtze River Metallogenic Belt, China

Comparative studies on stratabound and skarn-type deposits, Tongling region, lower Yangtze River metallogenic Belt: Constraints from pyrite geochemistry

The Xinqiao and Baoshantao deposits in the Tongling ore-cluster region, Middle-Lower Yangtze River Metallogenic Belt (MLYRMB) are typical stratabound and skarn Cu-Au deposits, respectively. The genetic model of skarn deposits has been fully studied and well established, while the genesis of stratabound deposits remains ambiguous. Two principal viewpoints include a late Carboniferous seafloor exhalative formation and Yanshanian magmatic-hydrothermal origin. In this study, three types of pyrite samples are recognized and collected, they are defined as Py A, Py B and Py C, characterized by fine-grained and anhedral, medium-grained and subhedral, and coarse-grained and euhedral, respectively. EPMA, LA-ICP-MS, and LA-MC-ICP-MS were used to detect major elements, trace elements, and S isotopes of these different pyrites. Ratios of Fe/S (0.99 ∼ 1.05, in mol %), Co/Ni (mostly greater than 1) suggest that both the Xinqiao and Baoshantao deposits were formed in the Yanshanian magmatic-hydrothermal system. The Xinqiao stratabound deposit was formed in a more oxidizing environment and have relatively lower δ34SCDT value (2.57 ∼ 5.95‰) than the Baoshantao skarn deposit (4.97 ∼ 9.57‰), indicating that the ore-forming contribution of strata in Xinqiao deposit is dominated. Combined with regional geological background, it is believed that the ore-hosting strata as well as migrations of magmatic-hydrothermal fluids are two important factors, affecting differences of formation and mineralization between the two types of deposits. Our results directly provide important enlightenments for understanding regional metallogenic mechanism and application of pyrite geochemical exploration.

Geological features and ore-forming mechanisms of the Chating Cu–Au deposit: A rare case of porphyry deposit in the Middle–Lower Yangtze River metallogenic belt

The Chating Cu–Au deposit is an important porphyry deposit located in the Middle–Lower Yangtze River Metallogenic Belt (MLYMB) of eastern China. Drill core logging and ore petrographic observations were systematically employed to recognize the geological features of the deposit. Several important and peculiar geological characteristics are revealed as fellows: (1) the ore-bearing quartz diorite porphyry emplaced within carbonate-dominated strata rather than clastic strata and volcanic rocks, (2) the ore-hosting location confined in the whole cryptoexplosive breccia pipe inside the porphyry stock, and (3) large-scale and barren marbles, marbled limestones, and hornfels almost encircling the porphyry stock and sporadic and barren skarns scattered within the porphyry stock. The fluid-inclusion study at the deposit reports a wide range of homogenization temperatures (161.4 °C–454.2 °C) and salinities (0.2–54.7 wt% NaCl eq) for the ore-forming fluids, and two fluid boiling events as suggested by the coexistence of halite-bearing liquid-rich inclusions and vapor-rich inclusions. In contrast with classical porphyry deposits, the Chating deposit has similar characteristics in the close relationships between the mineralization and the porphyry stock, the hydrothermal alteration zonation, and the fluid evolution process, while the wall-rock strata and ore-hosting position mark outstanding differences of the Chating deposit. The comprehensive geological and geochemical research in this study has been integrated to explore the ore-forming mechanisms of the deposit. The carbonate wall-rock strata were baked by early magma to form low-permeability thermal metamorphic shield at the contact zone, which prevents the migration and loss of the ore-forming fluids and avoid hydrothermal metasomatism for skarn ores. After that, the cryptoexplosions open up the porphyry system and promote the magmatic hydrothermal fluids mixing with the meteoric water, which successively induce the ore-forming fluids boiling and further cause ore-forming materials unloading and precipitation in the cryptoexplosive breccia pipe.

Titanite Geochemistry and its Implications for Cu Ore Genesis: Insight from Three Diorite Intrusions in the Tongling Ore District of the Middle–Lower Yangtze River Metallogenic Belt

Titanite Geochemistry and its Implications for Cu Ore Genesis: Insight from Three Diorite Intrusions in the Tongling Ore District of the Middle–Lower Yangtze River Metallogenic Belt