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[Display omitted] • Fluid overpressure plays a key important role in the formation of skarn iron deposit. Skarn iron deposits are important resources for iron-rich ores and their genesis has been a topic of debate. The Nanminghe iron deposit is a typical skarn iron deposit in Wu'an, Hebei Province, China which provides a good example to investigate the metallogenic mechanism. In this paper, we present a detailed study of the geology, petrography, and geochemical characteristics of magnetite in the Nanminghe iron deposit. Two types of magnetite can be identified in the massive ores: Mag1 is euhedral with radial cracks developed in the margin. It is a low-Si magnetite, with SiO 2 content ranging from 0.07% to 0.92%, with an average value of 0.49%. The SiO 2 content of Mag2 is from 0.90% to 3.29%, and an average value of 2.03%. The Mag2 is associated with quartz, goethite, and apatite. Geochemical characteristics of Mag1, together with the ore textures suggest its crystallization from an iron magma. There are significant differences in the geochemical characteristics between Mag1 and Mag2. Mag2 is rich in Nb, Ta, high field strength elements, large ion lithophile elements and has higher silica content. The Nb/Ta of Mag2 is higher than that of the original mantle. These characteristics as well as La/Ta vs Th/La, Th/U vs Sm/Nd and Nb/Ta vs Nb features show that Mag2 is formed in the environment of supercritical fluids. The δ56Fe of Mag1 is 0.157‰-0.172‰, with an average value of 0.166‰, and the δ56Fe of Mag2 is 0.119‰-0.160‰, with an average value of 0.139‰. The Fe isotope of Mag2 is lighter than Mag1. There is a negative correlation between δ56Fe and SiO 2 of Mag1 and Mag2, suggesting that the lighter δ56Fe of Mag2 was caused by injection of Si-rich fluids. We propose a genetic model where we envisage that the iron-rich melt formed in the deep magma chamber by reaction of dioritic rocks and carbonates. Mag1 formed by cooling of the iron-rich magma, following which Si-rich supercritical fluid were injected into the deep chamber resulting in fluid overpressure. The iron-bearing melt-fluid migrated upward along the magmatic conduit, when Mag2 formed. Our study provides insights into the formation mechanism of skarn iron deposits. [ABSTRACT FROM AUTHOR]

[Display omitted] • The genetic stage of siderite nodules of Late Paleozoic coal measures in North China epicontinental Sea Basin was identified. • The material source of siderite nodules in the study area was identified. • The genetic mechanism of siderite nodules in the study area was clarified. • The genetic model of siderite nodules in epicontinental sea basin coal measures was established. Siderite is a mineral resource and a climate sensitive mineral. Siderite nodule layers are widely developed in coal measures within the marine-ontinental transitional facies of North China epicontinental sea basin, yet their formation environment and genetic mechanism are poorly understood. In this paper, taking the siderite nodule layers in the coal measures of the Late Paleozoic Taiyuan Formation in Zibo area of North China as an example, the formation environment and genetic mechanism of siderite nodules in the transitional facies of the epicontinental sea basin are studied through detailed petrological, sedimentological and geochemical analysis. It is found that the siderite nodules in the research area were formed in a tidal flat-lagoon environment. The siderite nodules are formed in the synsedimentary stage, the original information of chemical composition and characteristics of the nodules being largely retained. Most of the carbon in the siderite nodules originates from inorganic carbon from marine carbonate rocks, with the remainder originating from inorganic carbon formed by dissimilatory iron reducing bacteria (DIR) degrading organic matter. Meanwhile, most of the iron in the siderite nodules originates from hydrothermal fluids, and the remainder from terrigenous sediments. The content of iron input from hydrothermal fluids and terrestrial sediment represents a trade-off relationship. The two genetic mechanisms of the siderite nodules are as follows: 1) Chemical genetic mechanism: The siderite nodules are mainly formed by the combination of low-valence iron (Fe2+) and inorganic carbon in seawater through reduction with iron input from hydrothermal fluids as the main source. 2) Biochemical genetic mechanism: Some is formed by the combination of low-valence iron (Fe2+) and inorganic carbon with high-valence iron (Fe3+) as an oxidant to degrade the organic matter and convert organic carbon into inorganic carbon through DIR. On this basis, a genetic model of the Late Paleozoic coal measures siderite nodules in the epicontinental sea basin is established. [ABSTRACT FROM AUTHOR]

[Display omitted] • The mineralized fluids are characterized by weakly acidic, reduced, and low temperature. • W/R interaction and CO 2 degassing caused the precipitation of carbonate minerals. • Mixing of metals and BSR-originated S2− resulted in the sulfide precipitation. • The origin of the Banbanjie deposit is similar to that of the MVT deposits. This paper investigates the origin of the newly discovered Devonian carbonate-hosted Banbianjie Ge-Zn deposit in South China. This deposit is rich in Ge (>900 t @ 110 ppm Ge) and contains 7.7 Mt of sulfide ores at a mean grade of 5.1 wt% Zn. The mineralization is stratabound or veined and is structurally controlled by the E-W trending Huangsi fault. Sulfide ores are dominated by sphalerite and dolomite with subsidiary pyrite, marcasite, and calcite, and minor galena. The mineralization forms massive, veined, brecciated, and disseminated structures and the hydrothermal minerals present in the deposit have colloform, fibrous, granular, and acicular textures. The hydrothermal dolomite has positive Ce (δCe = 1.13–1.20) and negative Eu (δEu = 0.43–0.68) anomalies that are indicative of formation in a weakly acidic, reduced, and low-temperature environment. The δ13C PDB and δ18O SMOW values of the dolomite range from −0.97‰ to −0.24‰ and +13.89‰ to +21.84‰, respectively, indicating that the dissolution of carbonate wall rocks was involved in the formation of hydrothermal dolomite. Theoretical C-O isotopic calculations further suggest that the hydrothermal dolomite formed as a result of water/rock (W/R) interaction and CO 2 degassing. The δ34S values of the sulfides (−11.23‰ to −4.95‰) are markedly lower than those of the Devonian seawater sulfate and sulfate from evaporates in the Devonian strata (+21.9‰ to +25.9‰), suggesting that the reduced sulfur was derived from evaporates within the ore-hosting sedimentary rocks by bacterial sulfate reduction (BSR). The Banbianjie sulfides have an upper crustal Pb isotopic affinity, as demonstrated by their 206Pb/204Pb (18.335–18.343), 207Pb/204Pb (15.725–15.735), and 208Pb/204Pb (38.288–38.326) ratios, suggesting the metals were derived from metamorphosed basement rocks. These data suggest that: i) the Banbianjie deposit formed as a result of a combination of weakly acidic, reduced, and epigenetic low temperature mineralized fluids, trap structures, and favorable lithologies; ii) W/R interaction and CO 2 degassing caused the precipitation of hydrothermal dolomite, whereas mixing of metals and S2− was the main formation mechanism of the sulfides; and iii) the Banbianjie Ge-Zn deposit is a typical Mississippi Valley-type (MVT) deposit. [ABSTRACT FROM AUTHOR]

[Display omitted] • A complete Fe isotopic composition and comprehensive overview of the evolution of magnetite for the Huogeqi deposit has been presented by this paper. • The magnetite ore at the Huogeqi deposit has characteristics typical for a hydrothermal skarn deposit. • Hydrothermal magnetite and Cu-Pb-Zn skarns at the Huogeqi deposit probably formed from similar hydrothermal fluids derived from Mesozoic granite. The Huogeqi deposit is a major Cu–Fe–Pb–Zn deposit in China. Numerous studies have been conducted on the genesis of Cu–Pb–Zn sulfides of this deposit; but detailed researches on the genesis of magnetite ore bodies and its relationship to sulfide ores are few. This study utilizes field and microscopic techniques to showcase the hydrothermal characteristics of magnetite ore and tremolite rocks, including net-veined magnetite ore, which cuts across carbonate rock and quartzite, and intrusive contacts between magnetite-bearing tremolite rocks and quartzite, carbonaceous slate, and carbonate rock. Furthermore, calcite with metasomatic relict texture could be widely observed near magnetite and tremolite. Magnetite-bearing tremolite rocks with low contents of trace elements, rare earth elements, and obvious positive anomalies of Eu differed from the mica schist and carbonaceous slate, but were consistent with the carbonate rocks, revealing that the magnetite ore and tremolite rocks have a hydrothermal origin and are closely related to carbonate rocks. The Fe isotopic compositions of 22 magnetite and 17 wall samples were determined using MC-ICP-MS methods. δ56Fe values of magnetite-bearing tremolite rocks varied from −0.57 to −0.15‰ (average, −0.38‰), which differed from those of the mica schist (0.27–0.33‰), quartzite (−0.12 to 0.57‰), and carbonaceous slate (0.30–0.38‰), but were similar to the carbonate rocks (−0.38 to −0.51‰). δ56Fe values of magnetite varied from −0.48 to 0.36‰ (average, −0.11‰), which differed from banded-iron formation and magmatic magnetite deposits but were like skarn deposits. The δ56Fe value of the initial ore-forming fluid was −0.09‰ at 350 °C, which is consistent with the fluid that likely originated from the fluid exsolution of granite. According to theoretical calculations, δ56Fe values of the residual fluid after precipitation of magnetite ores ranged from −1.01 to −0.94‰, and that for pyrite precipitation ranged from −0.02 to 0.07‰, which is consistent with the measured Fe isotopic data of pyrite. Hence, we suggest that the hydrothermal magnetite and Cu–Pb–Zn sulfides of the Huogeqi skarn deposit could be derived from similar hydrothermal fluids. [ABSTRACT FROM AUTHOR]

The study results of Zn, He-Ar and Sr-Nd of the Tongkeng deposit show that Zn source mainly comes from the Cretaceous Longxianggai granite magmatic fluid, classified into the type of post-magmatic hydrothermal filling-metasomatism. He-Ar isotope study shows that the mantle-sourced substances involved in the mineralization are related to that of the Devonian Liujiang Formation exhalative sedimentary sequences. • The Longxianggai granite belongs to the typical remelted S-type granite. • The mantle-sourced substances in ores are from the Devonian exhalative sediments. The Zn most likely derived from the Cretaceous Longxianggai granite. The Tongkeng tin-polymetallic ore deposit in Nandan County, Guangxi Zhuang Autonomous Region, southern China, is the representative deposit of the largest scale in the famous Dachang tin-polymetallic ore field. It consists of the upper tin-polymetallic ores and the lower newly found zinc-copper ores with the Sn metal reserve of 680,000 tons (average grade 1%), 4,500,000 tons of Zn (average grade 4%), and 80,000 tons of Cu (average grade 0.2%). The orebodies occur as layers and veins within the Middle to Late Devonian siliceous rocks and carbonate rocks. In this paper, the isotopic compositions of Zn, He-Ar and Sr-Nd of the Tongkeng deposit have been studied systematically. The results show that the isotopic composition of Zn is quite homogenous with δ66Zn = 0.22 ~ 0.34‰ in sphalerite samples of various colors from brown to black, collected from different orebodies (upper tin-polymetallic and lower zinc-copper ores) in the Tongkeng deposit. It indicates that Zn source mainly came from magmatic fluid, as the calculated δ66Zn (0.42 ~ 0.54‰) of ore-forming fluid is almost same as that of the Longxianggai granite (0.41 ~ 0.55‰), but quite different from that of the marine carbonate (average 0.91‰). He-Ar isotope study of pyrite shows that the exhalative substances might have been involved in the mineralization, with 3He/4He = 1.2 Ra ~ 2.5 Ra in the upper tin-polymetallic ores and 3He/4He = 0.8 Ra ~ 1.9 Ra in the lower zinc-copper ores in the Tongkeng deposit, which is different from that of the Longxianggai granite (3He/4He = 0.7 ~ 0.8 Ra), but close to that of the Devonian Liujiang Formation exhalative sedimentary sequences with 3He/4He = 1.5Ra. The Longxianggai granite has I Sr = 0.719 (in average), εNd(t) = -7.9 ~ -9.6 and Nd model age T 2DM = 1.54 ~ 1.68 Ga. Based on the geochemical characteristics of granite and Sr, Nd and O isotopes, the source rocks of the Longxianggai granite may be mainly derived from the continental crust, which is a typical remelted S-type granite. Based on the results of this study and the previous data in the literature, it is concluded that the mineralization in the Dachang ore field is closely related to the Cretaceous Longxianggai granite in time and space with the main metal substances of Zn most likely derived from the magmatic fluid, classified into the type of post-magmatic hydrothermal filling-metasomatism. The admixing of the early Devonian spouting sediments in the mineralization was evidenced in the study of source trace and has led to the longtime dispute over the ore genesis. [ABSTRACT FROM AUTHOR]