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Sulfate is traditionally considered to have retrograde solubility in aqueous solutions. However, our recent hydrothermal diamond-anvil cell (HDAC) experiments have shown that the solubility of Na2SO4 changes from retrograde to prograde in the presence of silica, leading to the formation of sulfate-rich solutions at high temperatures, in line with observations on natural geofluids. In this study, we use synthetic inclusions of fused silica capillary capsules containing saturated Na2SO4 solutions and Na2SO4 crystals to quantitatively investigate the solubility of Na2SO4 at different temperatures in the Na2SO4-SiO2-H2O system. Sulfate concentrations were measured using Raman spectroscopy and calibrated using Cs2SO4 solutions with known concentrations. The solubility of crystalline Na2SO4 dropped slightly when heated from 50 to 225 °C and dramatically from 225 to 313 °C. At 313 °C, the Na2SO4 crystals began to melt, forming immiscible sulfate melt coexisting with the aqueous solution, with or without solid Na2SO4. With the formation of sulfate melt, the solubility of Na2SO4 was reversed to prograde (i.e., solubility increased considerably with increasing temperatures). The solubility of Na2SO4 in the measured solution was significantly higher than that predicted in the absence of SiO2 over the entire temperature range (except for temperatures around 313 °C). This indicates that the presence of SiO2 greatly changes the dissolution behavior of Na2SO4, which may be caused by the formation of a sulfate–silicate intermediates such as Si(OH)4SO42−. Considering that most crustal fluids are silica-saturated, the solubility curve of Na2SO4 obtained in this study can better reflect the characteristics of geofluids when compared to that of Na2SO4-H2O binary system. At temperatures of 313–425 °C, the solubility of Na2SO4 increases with temperature following the function Csulfate = –3173.7/T + 5.9301, where Csulfate and T represent the solubility of Na2SO4 in mol/kg H2O and temperature in Kelvin, respectively. As an application, this temperature-solubility relationship can be used to evaluate the sulfate contents in fluid inclusions that contain sulfate daughter minerals, based on the temperature of sulfate disappearance obtained from microthermometric analysis. The sulfate concentrations of the ore-forming fluids of the giant Maoniuping carbonatite-related rare earth element (REE) deposit (southwest China) were calculated to be 4.67–4.81 m (mol/kg H2O). These sulfate concentrations were then used as internal standards to calibrate the previously reported semi-quantitative results of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of REE-forming stage fluid inclusions at this deposit. The calculated Ce concentrations in the REE-mineralizing fluid range from 0.42 to 0.49 wt%. The high fluid REE contents suggest that the sulfate-rich fluids are ideal solvents for REE transport. A mass-balance calculation was carried out to evaluate the minimal volume of carbonatite melt that was required for the formation of the giant Maoniuping REE deposit. The result indicates that the carbonatite dikes in the mining area are enough to provide the required fluids and metals, and thus a deep-seated magma chamber is not necessary for ore formation.

China dominates the world REE (rare earth elements) resources, 98% of which are from carbonatite-related REE deposits. Extensive studies of this type of REE deposits are available, but it is still poorly understood how carbonatite magmas were generated and how REEs were transported and precipitated in such magmas. On the basis of previous studies, this paper synthesizes the spatio-temporal distribution of this type of REEs deposits in China and their major features, including characteristics of ore-forming fluids, and discusses existing genetic models regarding the origin of causative magma, and mechanics of REE enrichment, transportation and precipitation. In China, carbonatite-related REE deposits occur in the craton margin environments with four major REE metallogenic belts: the Mesoproterozoic Bayan Obo-Langshan belt (e.g., the Bayan Obo deposit), the early Mesozoic eastern Qinling-Dabie belt (e.g., the Miaoya deposit), the late Mesozoic Zibo-Laiwu-Weishan belt (e.g., the Chishan deposit), and the Cenozoic Mianning-Dechang belt (e.g., the Maoniuping deposit). In these belts, REE-hosting carbonatites were emplaced in continental rift zones or along large-scale strike-slip faults at the Precambrian craton margins. The causative magmas were originated from partial melting of enriched lithospheric mantle sources, which may have undergone metasomatism by subducted slab-related fluids/melts, and may have also asthenospheric material involvement. These magmas may have formed through melt immiscibility and fractional crystallization from primary carbonated silicate melt. The melt immiscibility may include unmixing between alkaline silicate and carbonate melts, or multi-phase melt immiscibility of carbonate melt, mafic or ultramafic melt, phosphomagnesite melt, and alkaline silicate melt. The immiscibility and/or crystal fractionation of mafic mineral can lead to the formation of carbonate melts rich in REEs. The newly discovered lithofacies zonation at Bayan Obo, with a biotitite outer zone, implies a magmatic evolution of a silica-rich parent magma. The low REE concentrations and LREE/HREE ratios of biotitite indicate possible LREE enrichment in the evolved carbonatite magma by biotite fractional crystallization. The crystallization of carbonate minerals during the upwelling and emplacement of carbonatite magmas, may further enrich late-stage carbonatite melts and carbonatite fluids in LREE. This process can better explain the REE mineralization dominantly in the late-stage of carbonatite magmatism or post-magmatic fluid stage. The melt, melt-fluid and fluid inclusion results for Bayan Obo, Maoniuping and Chishan indicate SO42− and CO2-rich carbonatite fluids. The microthermometry and SEM/EDS results for primary melt-fluid inclusions hosted in fluorite from Maoniuping indicate that carbonatite fluids were extremely rich in SO42−, and underwent an unmixing process between sulfate melt and aqueous fluid. Both inclusion and experimental results indicate that the sulfate has a very high solubility in SiO2-bearing aqueous fluids, inconsistent with the SiO2-free system. The geochemical thermodynamic results also confirm the SO42− -dominated agent for REE transportation. The unmixing process between sulfate melts and aqueous fluids would lead to extensive decrease of SO42− in the fluid, and bulk REE precipitation. The rapid evolution of carbonatite fluids prevents long distance migration of carbonatite fluids, resulting in all REE mineralization within, or proximal to the carbonatite bodies. The strong mineralization would occur in the central zone or the shallow level and then would become weaker outwards and downwards of carbonatite bodies.

In situ Raman spectra of aqueous K2WO4-HCl, K2WO4-HCl-NaCl, and K2WO4-CO2-NaCl solutions were collected at elevated temperature (T, 100–400°C) and constant pressure (P) of 30 MPa. The stretching vibration band of the W=O bond (v1) was analyzed to reveal the species of tungsten (W) responsible for the hydrothermal transport of W during mineralization. Results showed that monomeric tungstates with v1(W=O) bands at ~930 and 950 cm-1 are the dominant W species in weakly alkaline (room temperature, pH=7.7) to near-neutral (room temperature, pH=7.2) solutions under the investigated T‐P conditions. Overall, the stability of polymeric tungstate species with v1(W=O) bands at ~965–995 cm-1 decreases with rising temperature and was not detected at ≥300°C in moderately acidic solution (room temperature, pH=4.9). However, increased fluid acidity and salinity obviously enlarged the temperature stability field of polymeric tungstate species. In highly acidic solution (room temperature, pH=1.4), polymeric tungstates are the only stable W species even at 400°C. In the presence of 1.9 mol/kg NaCl, polymeric tungstate(s) can persist to at least 350°C in moderately acidic solution. Considering that 300–350°C is the major W-mineralizing T range and W-mineralizing fluid is generally characterized by a moderately acidic nature, we propose that, in addition to monomeric tungstates, polymeric tungstates can also be important W species in some natural geological fluids that are responsible for the mineralization of W. Future studies of the W mineralization mechanism should take this issue into account.

The presence of sulfate-rich fluids in natural magmatic hydrothermal systems and some carbonatite-related rare earth element (REE) deposits is paradoxical, because sulfate salts are known for their retrograde solubility, implying that they should be insoluble in high-temperature geofluids. Here, we show that the presence of quartz can significantly change the dissolution behavior of Na2SO4, leading to the formation of extremely sulfate-rich fluids (at least 42.8 wt% Na2SO4) at temperatures >∼330 °C. The elevated Na2SO4 solubility results from prograde dissolution of immiscible sulfate melt, the water-saturated solidus of which decreases from ≥∼450 °C in the binary Na2SO4-H2O system to ∼270 °C in the presence of silica. This implies that sulfate-rich fluids should be common in quartz-saturated crustal environments. Furthermore, we found that the sulfate-rich fluid is a highly effective medium for Nd mobilization. Thermodynamic modeling predicts that sulfate ions are more effective in complexing REE(III) than chloride ions. This reinforces the idea that REEs can be transported as sulfate complexes in sulfate-rich fluids, providing an alternative to the current REE transport paradigm, wherein chloride complexing accounts for REE solubility in ore fluids.

Mantle-derived carbonatites provide a unique window in the understanding of mantle characteristics and dynamics, as well as insight into the assembly and breakup of supercontinents. As a petrological indicator of extensional tectonic regimes, Archean/Proterozoic carbonatites provide important constraints on the timing of the breakup of ancient supercontinents. The majority of the carbonatites reported worldwide are Phanerozoic, in part because of the difficulty in recognizing Archean/Proterozoic carbonatites, which are characterized by strong foliation and recrystallization, and share broad petrologic similarities with metamorphosed sedimentary lithologies. Here, we report the recognition of a ~1.85 Ga carbonatite in Chaihulanzi area of Chifeng in north China based on systematic geological, petrological, geochemical, and baddeleyite U-Pb geochronological results. The carbonatite occurs as dikes or sills emplaced in Archean metasedimentary rocks and underwent intense deformation. Petrological and SEM/EDS results show that calcite and dolomite are the dominant carbonate minerals along with minor and varied amounts of Mg-rich mafic minerals, including forsterite (with Fo > 98), phlogopite, diopside, and an accessory amount of apatite, baddeleyite, spinel, monazite, and ilmenite. The relatively high silica content together with the non-arc and OIB-like trace element signatures of the carbonatite indicates a hot mantle plume as the likely magma source. The depleted Nd isotopic signatures suggest that plume upwelling might be triggered by the accumulation of recycled crust in the deep mantle. As a part of the global-scale Columbia supercontinent, the Proterozoic tectonic evolution of the North China Craton (NCC) provides important insights into the geodynamics governing amalgamation and fragmentation of the supercontinent. The Paleo-Mesoproterozoic boundary is the key point of tectonic transition from compressional to extensional settings in the NCC. The newly identified ~1.85 Ga carbonatite provides a direct link between the long-lasting supercontinental breakup and plume activity, which might be sourced from the “slab graveyard,” continental crustal slabs subducted into asthenosphere, beneath the supercontinent. The carbonatite provides a precise constraint of the initiation of the continental breakup at ~1.85 Ga.

The giant Dongshengmiao is the third largest Pb–Zn deposit in China (Wang et al. 2014), with total metal reserves of 4.83 Mt Zn (@ 2.85%), 0.96 Mt Pb (@ 0.67%) and 0.11 Mt Cu (@0.86%; Long 2009). The enrichment of massive pyrite accompanying Zn–Pb–Cu mineralization also makes it possible to make sulfur acid through mining pyrite.

The eastern part of CAOB consists of multistage Paleozoic oceanic magmatic arc terranes, fragments of microcontinents, ophiolite complexes, and considered to be an accretionary orogenic belt (Fig. 1.3; Sengor et al. 1993; Hart et al. 2002; Xiao et al. 2003; Shao et al. 2007). According to the classic tectonic model of porphyry deposits (Sillitoe 1972; Mitchell 1973; Bowen and Gunatilaka 1977), porphyry deposits are expected in this tectonic setting.

In this book, we introduced the geochemical and geological characteristics, the genesis and ore-forming geodynamic backgrounds of four orogenic-type base metal deposits in the northern margin of the North China Craton.

The Jiashengpan Zn–Pb deposit is located in the Langshan-Zhaertai area, with metal reserves of 1.68 Mt Zn and 0.19 Mt Pb at an average grade of 3.95% and 1.35%, respectively.

The Langshan-Zhaertaishan metallogenic belt lies on the western part of the northern margin of the North China Block and is regarded as important for copper, lead, and zinc resources.

The HighPGibbs program is designed to calculate thermodynamic equilibrium of fluid-rock minerals and solid solutions up to depths of lithospheric mantle. It uses the Gibbs free energy minimization ...

The Betare Oya and Woumbou–Colomine–Kette gold districts, located in the East Metallogenic Province of Cameroon, are characterized by a complex Neoproterozoic history of metamorphism, polyphase deformation, and widespread crustal anatexis and magmatism. Controversy exists on the timing of the metamorphic and magmatic events, particularly within the area of the Lom Belt, and accurate absolute ages and even geodynamic chronology pertaining to the tectono-magmatism are yet to be well constrained. New LA-ICP-MS U–Pb zircon ages, Lu-Hf isotope data, and whole-rock geochemistry of granitoids and metavolcanic-metasedimentary rocks from the Woumbou–Colomine–Kette and Betare Oya districts better constrain the tectonic evolution of eastern Cameroon. The U–Pb age data of 646.5 ± 2.0 Ma, 643.7 ± 1.8 Ma, 641.3 ± 4.4 Ma, 641.4 ± 3.5 Ma, and 620.9 ± 2.6 Ma revealed that the granitoids from the Woumbou–Colomine–Kette district were emplaced during two main magmatic events at ca. 650–635 Ma and ca. 620 Ma. The zircon Hf(t) values between −13.80 and −6.20 with HfTDM model ages of 1.9–2.4 Ga, coupled with geochemical signatures show that granitoids were generated by partial melting of Paleoproterozoic supracrustal metasedimentary units and meta-igneous rocks during subduction and orogenic convergence. Metamorphic zircons from the actinolite-chlorite schist of the Lom Belt gave ages between ca. 655 and 585 Ma, implying that the metavolcanic-metasedimentary rocks of the belt were deposited prior to 655 Ma, then metamorphosed and deformed between ca. 655 and 585 Ma. Two age peaks of detrital zircons were measured at ca. 1035 Ma and ca. 784 Ma with the eHf(t) values of –14.60 and –18.37 and +2.97 and +8.60, respectively. These data, coupled with those from previous studies, suggest that the Lom Belt may represent a suture zone between the Central and Southern Cameroon domains with an early rifting at ca. 784 Ma. In general, our U–Pb ages indicate compressional to transpressional deformational phases in the two districts at ca. 660–635 Ma (D1), 630–610 Ma (D2), and 600–580 Ma (D3). The latter D3 transpressional event was also associated with the formation of the widespread occurrences of orogenic gold, although the precise timing of gold deposition needs to be constrained.

The Betare Oya district has a substrate composed of the Neoproterozoic metavolcanic-metasedimentary rocks of the Lom Belt in eastern Cameroon. The district is well-known for alluvial gold mining activities, however, the primary gold mineralization has received little attention. In the current study, we newly report geological, ore mineralogy, fluid inclusion microthermometric, laser Raman spectroscopy, and stable isotope data from auriferous veins of the Lom Belt to characterize the style of gold mineralization and to constrain the origin of the ore-forming fluids. The auriferous quartz veins are laminated, fractured, N- to NE-trending, and spatially associated with the Betare Oya Shear Zone. Fieldwork coupled with microscopic examination and the textural relationships of ore minerals revealed two stages of mineralization. The first stage is characterized by the presence of pyrite, sphalerite, galena, chalcopyrite, pyrrhotite, hematite, petzite, hessite, wolframite, electrum, and gold, while the second stage is characterized by the presence of a later deposition of galena and pyrite, as well as minor greenockite. The gangue minerals are quartz, sericite, muscovite, chlorite, calcite, ankerite, and barite, whereas the supergene assemblage (stage 3) includes goethite, hematite, covellite, and enargite. Two fluid inclusion assemblages containing three types of inclusions are identified in mineralized quartz veins. Assemblage 1 consists of gold-related types 1 and 2 with daughter minerals consisting of nahcolite, magnesite, and arsenolamprite (black native arsenic), whereas assemblage 2 consists of post-gold type 3 fluid inclusions. Type 1 has H2O-NaCl-CO2±N2±CH4 primary fluid inclusions (FI), with a TmCO2 ranging from −59.8 to −56.6 °C, salinities from 0.5 to 10.8 wt% NaCl eq., densities from 0.87 to 1.00 g.cm−3, and total homogenization temperatures between 280 and 360 °C. Type 2 contains CO2 (±H2O-NaCl)±N2±CH4 FI that exhibit TmCO2 ranging between −60.0 and −56.7 °C, ThCO2 from 13 to 25 °C, and densities between 0.73 and 0.85 g.cm−3. Type 3 shows H2O-NaCl FI with salinities between 0.2 and 10.1 wt% NaCl eq., densities between 0.82 and 0.98 g.cm−3, and total homogenization temperatures from 160 to 235 °C. Measured δ18O for gold-bearing quartz (+11.5 to +16.0‰), δD from FI (−50.6 to −21.8‰), δ13C from FI (−5.8 to −5.5‰), and δ34S from galena and pyrite grains (+5.3‰ and +8.2‰, respectively) suggest a metamorphic source as most likely for the ore-forming fluids and sulfur, although a mantelic CO2 contribution cannot be ruled out. The gold deposition probably took place by fluid-rock interaction and fluid unmixing at ∼310 °C and at a depth of about 6-9 km. The ore-forming fluid was a low salinity (∼6.2 wt% NaCl eq.) H2O-NaCl-CO2±N2±CH4 solution. These data revealed that the Lom Belt gold mineralization is best classified as a mesozonal orogenic gold deposit.

Sulfate-bearing geofluids are widespread in many geological environments and play important roles in ore-forming processes. Recent experiments provide evidences that in silica-rich fluids, Na2SO4 solubility exhibits a complex temperature dependence, first decreasing from 25 to 313 °C and then increasing from 313 to 425 °C. The fluid-borne silica have critical implications for sulfate-bearing hydrothermal systems and formations of rare earth element deposits. However, whether other sulfate salts share the same characteristics remains unknown. In this study, fused silica capillary capsules (FSCCs) containing K2SO4-saturated solutions and K2SO4 crystals are used to quantitatively investigate the solubility of K2SO4 at different temperatures in the K2SO4-SiO2-H2O system. In the K2SO4-SiO2-H2O system, K2SO4 shows prograde solubility from 50 to 400 °C, and does not melt through the whole investigated temperature range. The relationship between temperature and the solubility of K2SO4 can be described by a simple linear function with R2 = 0.991: Csulfate = 4.1307 × 10−3 × T + 0.5532, where Csulfate and T represent the solubility of K2SO4 in m (mol/kg H2O) and temperature in °C, respectively. These results show that the solubility of K2SO4 can be enhanced by 3 to 5 times at 300–400 °C by the presence of silica, compared with that in silica-absent system, even without the formation of sulfate melt. Considering that quartz is ubiquitous in the Earth's crust, the solubility of sulfate in natural geofluids may reach a much higher level than previously expected. The temperature dependences of the solubilities of K2SO4 and Na2SO4 in silica-saturated fluids were used to constrain the compositions of the mineralizing fluids of the synmetamorphic hematite-quartz veins at Ouro Fino (SE Brazil) and the Maoniuping REE deposit (SW China), based on the previously reported microthermometric results of fluid inclusions that contain sulfate daughter minerals. For the hematite-quartz vein at Ouro Fino, the sulfate concentration was calculated to be 8.9–19.2 wt%, which is in line with previous laser ablation measurements (~15.3 wt%). For the Maoniuping REE deposit, the total sulfate concentration was determined to be 16.3–27.4 wt%, and the REE concentration of the ore-fluid was further estimated to be 0.30–0.44 wt%.

Classification algorithms were constructed based on pyrite trace elements using two machine learning methods, support vector machine (SVM) and artificial neural network (ANN), to discriminate the genesis of pyrites sampled from sedimentary rock, orogenic and volcanic hosted massive sulfide (VHMS) deposits. The classifiers were trained with a dataset including published trace element compositions of 2104 pyrite samples from 92 mineral deposits or stratigraphic units. Cross validations were conducted to evaluate the performances of the classifiers on unknown samples, using a variant of the k-fold method. Each time, all pyrite samples from one of the 92 deposits/strata were set aside as the testing set, and the classifiers were trained with samples of the remaining 91. Then, the performances of the classifiers were evaluated based on whether it can correctly determine the genesis of testing deposit/stratum. The circulation was repeated 92 times for each one of the deposits/strata in the dataset, and 91 of them were correctly classified by the SVM-based classifiers, and 90 by the ANN-based. The trained classifiers were then applied to reveal the genesis of the Jiashengpan Zn-Pb deposit, which is characterized by two stages of pyrite formation: early-stage fine-grained massive and late-stage coarse-grained hydrothermal pyrite. The trained algorithms show that the early-stage is compositionally similar to sedimentary pyrite, while the late-stage has affinity to those from orogenic deposits, consistent with geological and geochemical features revealed by previous studies. This study sheds light on the power of machine learning in decoding the geochemical data of pyrite, which can well record the history of ore formation.

The magmatic complex in Uradzhongqi, Inner Mongolia, is located in the western segment of the northern margin of the North China Craton (NCC). The dominant components in the complex include syenogranite, monzogranite, granodiorite, diorite and gabbro. Mafic microgranular enclaves (MMEs) are common in syenogranite and granodiorite. Zircon U–Pb dating shows that the ages of these rocks range from 283 to 270 Ma, suggesting an early Permian emplacement. The syenogranite and monzogranite are peraluminous I-type granites, exhibiting conspicuous negative Eu anomaly, enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in high field strength elements (HFSE). The granodiorites, diorites and MMEs are metaluminous in composition, show high Al2O3, MgO and Fe2O3T contents and weak negative Eu anomaly, as well as LREE and LILE enrichment and HFSE depletion. The gabbros show weak positive Eu anomaly and slight REE differentiation. The Sr–Nd isotope compositions show that the source of mafic magma was depleted mantle (DM) with possible involvement of enriched mantle II (EM II), whereas the felsic magma was derived from the Archean lower crust. Petrographic observation and analytical results of mineralogy, geochronology, geochemistry and Sr–Nd isotopes indicate that the main petrogenesis of these magmatic rocks is the mixing of underplating mafic magma and felsic magma. Tectonically, the complex pluton was formed within a post-collisional regime, and the underplating in this area provides another piece of evidence for the vertical growth of the western segment of the northern margin of the NCC.

In hydrothermal ores, gold is commonly hosted by sulfide minerals due to its affinity for reduced sulfur. In contrast, the occurrence of gold in oxide minerals remains poorly documented and incompletely understood. In this contribution, we demonstrate that magnetite can be a significant host mineral for gold. Skarn ores from the giant Beiya Au deposit (323 t Au at 2.47 g/t), southwest China, contain abundant nanometer- to micronsized gold hosted by magnetite. A range of bismuth minerals (e.g., native bismuth, maldonite, and bismuthinite) occur within the same assemblage. Textural and mineralogical studies as well as thermodynamic modeling indicate that Bi melts scavenged Au from hydrothermal fluids during magnetite growth. Fluctuating f O 2 ( g ) at the surfaces of crystallizing magnetite led to the precipitation of Bi-Au melt, ongoing gold refinement, and entrapment of gold and bismuth minerals within magnetite. Sulfidation of Bi, triggered by increased sulfur content and/or decreased temperature, further promoted gold precipitation. Bismuth melts are interpreted to have played a crucial role in Au enrichment from gold-undersaturated fluids at Beiya. The results imply that magnetite-rich systems are also potential targets for Au exploration and extraction, particularly in cases where coexisting Bi minerals are observed in abundance.

The contiguous region between Guangxi, Guizhou, and Yunnan, commonly referred to as the Golden Triangle region in SW China, hosts many Carlin-type gold deposits. Previously, the ages of the gold mineralization in this region have not been well constrained due to the lack of suitable minerals for radiometric dating. This paper reports the first SIMS U–Pb age of hydrothermal rutile crystals for the Zhesang Carlin-type gold deposit in the region. The hydrothermal U-bearing rutile associated with gold-bearing sulfides in the deposit yields an U-Pb age of 213.6 ± 5.4 Ma, which is within the range of the previously reported arsenopyrite Re–Os isochron ages (204 ± 19 to 235 ± 33 Ma) for three other Carlin-type gold deposits in the region. Our new and more precise rutile U-Pb age confirms that the gold mineralization was contemporaneous with the Triassic W–Sn mineralization and associated granitic magmatism in the surrounding regions. Based on the temporal correlation, we postulate that coeval granitic plutons may be present at greater depths in the Golden Triangle region and that the formation of the Carlin-type gold deposits is probably linked to the coeval granitic magmatism in the region. This study clearly demonstrates that in situ rutile U–Pb dating is a robust tool for the geochronogical study of hydrothermal deposits that contain hydrothermal rutile.

This research was funded by a Monash Research Accelerator Grant to A. Tomkins and by a Society of Economic Geologists (SEG) student research grant to A. Rielli. O. Nebel was supported through an ARC DECRA fellowship (DE120100513).

The Qiaoxiahala deposit is a significant Paleozoic Fe-Cu-Au deposit in the northern margin of East Junggar, NW China, which has been confirmed to form in a basin inversion setting (dying back-arc basin). The composition of noble gases (Ar, Kr, and Xe) and halogens (Cl, Br, and I) extracted from fluid inclusions, hosted in quartz, calcite, and epidote from various mineralization stages of Qiaoxiahala, are investigated to trace the sources of ore fluids and discuss the ore genesis. Significant multistage mineralization at Qiaoxiahala is identified as magnetite mineralization (stage III), magnetite-pyrite mineralization (stage IV), and chalcopyrite mineralization (stage V). Syn-ore fluid inclusions show salinities varying from 13.2 to 15.3 wt% (NaCl eq.) associated with elevated I/Cl ratios (214 × 10−6 to 3950 × 10−6), uniformly low Br/Cl ratios (0.78 × 10−3 to 1.30 × 10−3), and low 40Ar/36Ar ratios (

The geochemistry of basaltic rocks is widely used to investigate the tectonic setting of magmatism. The limitation of traditional two-dimensional tectonic discrimination diagrams is mainly risen from the fact that they can only simultaneously use the information of two (x-y plots) or three (ternary diagrams) elements (or element ratios) for discrimination. This obstacle can be overcome with the assistance of machine learning method, which shows great performances in classification of multidimensional datasets. In this study, we present a neural network-based model that uses whole rock major and trace elements to discriminate basaltic rocks (SiO2 45–55 wt%) from a wide range of tectonic settings, including continental arc basalt (CAB), island arc basalt (IAB), intra-oceanic arc basalt (IOAB), mid-ocean ridge basalt (MORB), oceanic plateau basalt (OPB), oceanic island basalt (OIB), continental flood basalt (CFB) and continental rift basalt (CRB). Using a modified method of cross validation, it is estimated that the model can discriminate the tectonic setting with an average accuracy of ~86%, and ~98% in discriminating the major tectonic regimes (arc, spreading center, or within-plate magmatism). This discrimination model was programed as a stand-alone Microsoft Excel spreadsheet that can be directly used by pasting the whole rock data into it. The discriminator was then applied to investigate the geodynamic background of the Paleoproterozoic (~1.75 Ga) Xiong'er volcanism in the southern margin of the North China Craton (NCC). It has long been debated whether this magmatism took place in a continental arc or within-plate rift environment. The discrimination result shows that both the Xiong'er Group volcanic rocks and coeval intrusive rocks have CFB affinities, indicating that they were products of mantle plume activity and defines a large igneous province (LIP) in a within-plate setting. This, along with previous studies, constrains the breakup of NCC, a part of the Columbia supercontinent, not later than ~1.79 Ga, and supports the idea that the fragmentation of the Columbia was triggered by mantle plume impingement.

The original ore-fluid of the Huogeqi Cu–Pb–Zn deposit in Inner Mongolia, northern China, was enriched in heavy oxygen isotopes with δ18O values ranging from 9.9 to 11.4 per mil, which is characteristic of the metamorphic devolatilization of pelitic rocks. The δD values determined by direct measurement of syn-ore hydrothermal tremolite range from − 116 to − 82 per mil, lying between the domains of typical metamorphic fluid and meteoric water, which is in equilibrium with organic matter. Oxygen and hydrogen isotope ratios indicate that the ore-fluid was derived from deep-sourced metamorphic fluid and interacted with organic-rich shale during fluid migration, which is consistent with the fluid evolution history revealed by a previous fluid inclusion study. Sulfides in the deposit are characteristically enriched in heavy S isotopes, with an average δ34S value of 13.4 ± 6.2 per mil (1σ, n = 103). The S-isotope ratios are identical to stratabound sulfides generated through the non-bacterial reduction of Neoproterozoic marine sulfate (with δ34S values of ~ 17 per mil). Previous studies on lead isotopes of sulfides revealed that the ore-forming metals (Cu, Pb, and Zn) at the Huogeqi deposit were also remobilized from a stratabound source. This source was syngenetically elevated in its Cu-, Pb-, and Zn-sulfide content as a result of submarine hydrothermal activities forming sulfide-rich layers within a rift tectonic setting. The Fe isotope ratios for sulfides are consistent with those of an intercalated iron-formation within the ore-hosting rocks, suggesting that the Fe in the sulfides was derived from local host rocks during sulfide precipitation and the Fe-rich rocks are favorable lithological units for high-grade mineralization. The heterogeneous sources of ore-fluid, S, ore-forming metals, and Fe are explained by a multistage genetic model, which is supported by the geological characteristic of the deposit. The enriched sulfides were subsequently remobilized and enriched by metamorphic devolatilization during the Permian and Triassic periods. The metamorphic ore-fluid ascended along a shear zone and interacted with organic-rich shale. Sulfides eventually precipitated within the shear zone at a shallower crustal level, especially where the shear zone intersected Fe-rich host rocks. This multistage genetic model has implications for mineral exploration. Greenschist to amphibolite facies terranes containing thick Neoproterozoic rift sequences are ideal regions for potential Cu–Pb–Zn mineralization. In particular, intercalated volcanic rocks within the rift sequences are indicative of high heat-flow and are ideal for the development of submarine hydrothermal systems. The primary structures hosting mineralization and ore shoots in the Huogeqi area are jogs in the shear zones. In addition, Fe-rich lithological units, such as iron-formations, are ideal hosts for high-grade ore.

Scavenging of gold during metamorphic devolatilization is a widely accepted model for fluid and metal sourcing in orogenic gold deposits. In order to further constrain this process and quantify the capacity of autogenous fluids to release metals from pelites, we investigated the behaviors of S, Au, As, Cu, Pb, and Zn during pelite metamorphic devolatilization using thermodynamic modeling within the Al–As–Au–Cl–Cu–Fe–H–K–Mg–Na–O–Pb–S–Si–Ti–Zn system over a P–T range of 350–650 °C and 0.8–5 kbar. The model revealed that S, Au and base metals are predominantly released via partitioning into the fluid phase during reactions that liberate H2O: the dehydration of chlorite and muscovite, and to a lesser extent, the replacement of pyrite by pyrrhotite. Negligible sulfur is liberated during the pyrite–pyrrhotite transition, because the excess sulfur reacts with Fe in chlorite and muscovite to form pyrrhotite. The sulfidation of chlorite/muscovite releases water, so that a significant amount of Au can be liberated from S-rich pelites at the pyrite to pyrrhotite transition: up to 0.5 ppb Au (as a proportion of bulk rock) can be stripped from a pelite containing 1 wt.% sulfur, whereas only trace amounts of base metals can be mobilized under these conditions. Chlorite dehydration is the most important process in metal extraction; up to 2 ppb Au, 1.5 ppm Cu, 1 ppm Pb and 2 ppm Zn (as proportions of bulk rock) can be extracted from a pelite by autogenous fluids upon crossing the greenschist–amphibolite facies boundary. In comparison, an average pelite contains ∼3 ppb Au, indicating that most Au within an average pelite can be scavenged as a result of the breakdown of chlorite. Prograde metamorphism is an efficient mechanism for generating Au-bearing ore fluids: most Au can be extracted during chlorite dehydration from a source rock of average pelitic composition. In contrast, only a small portion of base metals can be released in autogenous fluids, and therefore only minor enrichment in base metals is expected within large orogenic gold deposits in metamorphic terrains, which is consistent with observations globally.

The Raman shift of the ν3(SiO4) mode of zircon at ~1008 cm−1 has been observed to vary with the pressure (P) and temperature (T) of the environment. The relationship between the ν3(SiO4) peak position and P-T was calibrated in previous work and used to develop a pressure sensor for high P-T experiments, up to 1000 °C and 10 GPa, to an accuracy within ±50 MPa. This pressure measurement method is valid for zircon grains for which the ν3(SiO4) Raman shift occurs specifically at or close to 1008.17 cm−1 at room temperature (25 °C). However, the peak position of ν3(SiO4) for some zircon grains may shift to a lower or higher wavenumber under ambient conditions, mainly due to the chemical composition of the grains. This may cause substantial deviation in the measured pressures. In the present study, we recalibrated the pressure sensor by determining the position of the ν3(SiO4) peak for zircon grains with different chemical compositions at given P-T conditions and developed a model applicable to both natural and synthetic zircon samples. All the zircon samples considered were found to have a universal ν-T relationship when the ν3(SiO4) wavenumbers under ambient conditions were calibrated to the same position. Based on this relationship, the temperature-induced wavenumber shift of ν3(SiO4) can be expressed as ΔνT (cm−1) = 8.00 × 10–9 × (T3 – T03) – 2.00 × 10–5 × (T2 – T02) – 2.78 × 10–2 × (T – T0), where T and T0 are the target and room temperatures, respectively. In addition, the pressure-induced wavenumber shift of ν3(SiO4) can be calculated using ΔνP (cm−1) = Δν – ΔνT, where Δν is the total frequency shift between the target and ambient P-T conditions. By the linear regression of the experimental results, the global slope (∂ν/∂P)T was determined to be 5.91 × 10–3 cm−1/MPa. In addition, high P-T experiments performed in isochoric systems using a hydrothermal diamond anvil cell (HDAC) showed that the cross-derivative term (∂2ν/∂P∂T) was equal to zero, which is the same as the previous study. The revised method was used to determine the pressures inside a fused silica capillary capsule (FSCC) containing a NaCl solution. The pressures determined from the zircon Raman spectra (PZr) were compared with those calculated using the equation of state (PEoS) of a NaCl-H2O system. The determined PZr and PEoS values agreed well for temperatures lower than ~500 °C. However, PZr was significantly lower than PEoS at higher temperatures due to the thermal expansion of the FSCC, i.e., the expansion of the sample chamber under the high internal pressures when the silica glass softened at high temperatures. The revised method was programmed in a Microsoft EXCEL spreadsheet and is presented in the Appendix B.

Measuring homogenization temperature (Th) is a basic work in fluid inclusion studies. However, the Th of some fluid inclusion is hard to obtain using the traditional microthermometric method, in particular those that is easy to decrepitate upon heating. Such inclusions are common when the host minerals have well-developed cleavages (e.g., calcite and fluorite) and the fluids are enriched in volatile components such as CO2 or CH4. In theory, upon heating of H2O-NaCl and H2O-CO2-NaCl inclusions that tends to homogenize to the liquid phase, their bubble volumes (Vg) will decrease with increasing temperatures (T) following cubic and quartic polynomial Vg-T functions, respectively. A method that can predict inclusion Th without heating it to homogenization is proposed in this study. The core idea is to retrieve the Vg-T function of a given inclusion based on a series of (Vg, T) data points that were measured before total homo genization (or decrepitation of the inclusion), and then calculate the Th based on the function (the temperature at which Vg equals to zero). To test the performance of this method, a total of 14 synthetic and natural fluid inclusions were heated to homogenization (at Th, real) and their changes in Vg were recorded with increasing temperature. It is assumed that the inclusions get decrepitated before homogenization, and their homogenization temperatures were predicted (Th, predicted) using the method proposed by this study. If the inclusions were “decrepitated” at temperatures 40 °C below the Th, real, the differences between the predicted and real homogenization are mostly smaller than 3 °C. This approach provides a quantitative estimation of the Th for inclusions that are easy to decrepitation, and is much more accurate than directly using the temperature of decrepitation as an approximation of Th. This method was programed in two Microsoft EXCEL worksheets that are available in Supplementary Materials for Th calculations of H2O-NaCl and H2O-CO2-NaCl inclusions, respectively.

Knowledge on hydrothermal tungsten (W) species is vital towards a better understanding of tungsten transport and mineralization mechanisms. In this study, in situ Raman spectra of a 0.005 – 0.1 mol/kg (m) K2WO4 solution containing CO2, HCl, and NaHCO3 were collected at 50–400 °C and 20–60 MPa. The spectra for the symmetric stretching vibration mode of the W O bond, v1(W O), were analyzed to investigate the hydrothermal tungstate species. Results showed that carbonate/bicarbonate do not associate with tungstate to form carbonic tungstate species. Nevertheless, the presence of CO2 can increase the fluid acidity, which favors the formation of polymeric tungstate species at O) modes of these species are centered at ∼930 cm-1 and 950 cm-1. Based on the above observations, we simulated the mineralization process in the context of fluid-rock interactions using tungstate and alkali tungstate ion pairs as the only aqueous W species. The thermodynamic simulations showed that (a) the timing of mineralization mainly depends on the W concentration in the initial mineralizing fluid and the availability of Ca2+, Fe2+ and Mn2+, with higher W concentrations generally favoring higher temperature mineralization; (b) highly W-enriched fluid is not essential for W mineralization, while extremely low contents of Fe, Mn and Ca in the magma are useful to maintain the mobility of aqueous W until favorable host rocks are encountered; and (c) a “hydrogen reservoir” effect was identified for dissolved CO2. The presence of CO2 can promote the extraction of Fe(II) from the pelitic host rocks, thereby facilitating a high-grade vein-type W mineralization. At O) modes are centered at ∼965 – 995 cm-1, are important hydrothermal W species along with monomeric tungstates. Therefore, polymeric tungstate species should be considered in future thermodynamic modeling of W transport and mineralization at

Permian granitoid emplacement represents one of the most important tectonothermal events in the northern margin of the North China Craton (NCC). In this study, we collected geochronological and geochemical data of the regional Permian granitoid in the northwestern margin of the NCC, and investigated the Dongshengmiao pluton, using it as an example to constrain the regional granitoid petrogenesis and its geodynamic settings. The Dongshengmiao pluton contains porphyritic granite and quartz diorite. LA-ICP-MS zircon U-Pb dating results have constrained the granitoid emplacement to be ca. 287‒275 Ma. The Dongshengmiao granitoids have a SiO2 range of 58.4‒76.5%, moderate to high alkali content (Na2O + K2O = 5.16–7.94%), and are rich in large-ion lithophile elements (LILEs; e.g. Rb, Ba) and depleted in high-field strength elements (HFSEs; e.g. Nb, Ta, Ti). The zircons in quartz diorite have eHf(t) values of −15.6 to −11.1 with two-stage Hf model ages (TDM2) of 1997–2281 Ma, suggesting that the magma was derived...

Sulfur and chlorine are the two most important ligands accounting for metal transport in the upper crust. In this study, four metal- and sulfur-saturated model fluids with varying salinities and redox states were simulated in the Fe-Cu-Pb-Zn-Au-S-C-H-O system, over a wide pressure-temperature (P-T) range (50–650 °C, 0.8–5.0 kbar), in order to compare the roles of chloride and bisulfide complexing for metal transport at the light of the latest available thermodynamic properties. The range in simulated Zn and Pb concentrations of the model fluids compares well with those of natural hydrothermal fluids, suggesting that the model can be used to evaluate hydrothermal ore-forming processes in Nature. The modeling reveals two different modes of Cu, Pb and Zn complexing in sulfur-saturated hydrothermal solutions. At lower temperature, chloride complexes are the predominant Cu, Pb and Zn species in sulfide-saturated systems, as expected from previous studies. However, hydrosulfide Cu, Pb and Zn complexes predominate at higher temperature. The predominance of bisulfide complexing for base metals at high temperature in sulfur-saturated systems is related to the prograde dissolution of pyrite and/or pyrrhotite, which results in a rapid increase in sulfur solubility. Metals transport as chloride or bisulfide complexes determines the modes of metal enrichment. In chloride-complexing dominated systems (e.g., Mississipi Valley Type deposits), low sulfide solubilities mean that the ore fluids cannot carry both reduced sulfur and metals, and ore precipitation is triggered when the ore fluid encounters reduced sulfur, e.g., via fluid mixing or via sulfate reduction. In contrast, in fluids where bisulfide complexing is predominant, cooling and desulfidation reactions are efficient mechanisms for base metal sulfide precipitation. Since both Au and base metals (Cu, Pb and Zn) are predicted to be transported as hydrosulfide complexes in high-temperature primary magmatic fluids in equilibrium with sulfide minerals, high-salinity is not a necessity for magmatic hydrothermal deposits such as porphyry- and skarn-style deposits.

The greenschist facies volcano-sedimentary sequence of the Bainaimiao Group in central Inner Mongolia hosts the Bainaimiao Cu–Mo–Au deposit and the Bainaimiao lode gold deposit. But the ore classification and genetic process of the Bainaimiao Cu–Mo–Au deposit is still disputed. In this study we have selected the interbedded metamorphosed volcanic rocks of the Bainaimiao Group for LA-ICP-MS zircon U–Pb analysis. 17 analyses form a tight cluster on the concordia and yield a weighted mean 206 Pb/ 238 U age of 465 ± 1 Ma (95% confidence level, MSWD = 0.95, n = 17), which represents the best estimate of the crystallization age of the volcanic rocks of the Bainaimiao Group. Biotite from the greenschist of the Bainaimiao Group yielded a well-defined plateau with an age of 429.2 ± 4.1 Ma. The inverse isochron age is 429.1 ± 4.2 Ma with an elevated ( 40 Ar/ 36 Ar) i ratio of 299.9 ± 22.7, which represents the time of regional greenschist facies metamorphism. Previous studies showed that the Re–Os isochron age of the molybdenite from the Bainaimiao Cu–Mo–Au deposit is 445.0 ± 3.4 Ma and the mineralized granodiorite porphyry yielded a weighted mean 206 Pb/ 238 U age of 445 ± 6 Ma. These data indicate that the Bainaimiao volcanic rocks were formed at about 465 Ma and the porphyry Cu–Mo–Au mineralization took place at about 445 Ma. Volcanic rock emplacement is 20 Ma older than the Cu–Mo–Au mineralization indicates it is a porphyry deposit rather than VMS deposit. The dynamic setting for the porphyry mineralization is the Paleo-Asian Oceanic plate subduction along the northern margin of the North China Craton in early Paleozoic. Subsequently, the collision between the Bainaimiao arc and the North China Craton led to regional metamorphism and deformation at about 430 Ma and the old porphyry system may have been overprinted during this metamorphic process.

The giant Dongshengmiao Zn–Pb–Cu deposit is located in the Langshan district, northern China. The ores are hosted within a Proterozoic rift sequence, which underwent lower greenschist facies metamorphism and shear deformation during development of Early Cretaceous intraplate orogenic belt. Northwest-dipping thrust faults, which share similar orientations and dip angles with the orebodies, are well developed in the mining area. Syngenetic stratabound sulfides were formed during the Proterozoic rifting event, but syngenetic ore textures have seldom been preserved except for some pretectonic fine-grained pyrite. Petrological observation, 39 Ar/ 40 Ar geochronology, combined with previous isotopic and fluid inclusion studies indicates that significant Zn–Pb–Cu remobilization took place as a result of thrust faulting associated with metamorphic devolatilization of ore-hosting rocks at ca. 136 Ma, coeval with the intraplate orogeny and regional crustal shortening. Sulfides were redistributed in shear structures or along grain boundaries of ore-hosting carbonates, and Fe-rich carbonates were ideal sites for Zn–Pb–Cu precipitation.

Huogeqi is a shear zone-hosted epigenetic deposit within the greenschist-amphibolite facies of the Mesoproterozoic Langshan Group in the Langshan district on the northern margin of the North China Craton (NCC). Copper mineralization in the Huogeqi deposit was formed in two stages: a main-stage controlled by the shear zone and characterized by brittle-ductile ore-forming structures and a lower greenschist facies mineral assemblage, and a late stage characterized by open space-filling textures and low-temperature minerals. Based on microthermometric and Raman microprobe analysis, the main-stage Cu-bearing fluid was mesothermal, low-salinity and H2O–CH4-dominant, and was generated by an interaction between a deep-crustal metamorphic fluid and graphite-bearing host rocks. This interaction resulted in a more CH4-rich fluid, which was more amenable to be immiscible. We showed that immiscibility of the H2O–CH4 fluid occurred due to temperature decrease, prior to the main-stage Cu mineralization; Cu was finally precipitated from the resultant H2O-rich aqueous fluid. Main-stage Cu mineralization temperature was obtained using various methods: 310–370 °C by intersection of isochors of coexistent CH4 and aqueous inclusions; 364 ± 41 °C on average by pressure correction of the homogenization temperatures of aqueous inclusions; and 362 ± 26 °C using the chlorite geothermometer. Pressure during Cu-deposition fluctuated between lithostatic and hydrostatic at depths of ca. 10–12 km, but it seemingly had no effect on the mineralization process. The late-stage Cu-bearing fluid was a low temperature, low salinity, H2O of meteoric origin.

The Bainaimiao Cu–Mo–Au deposit, located at the northern margin of the North China Craton (NCC) in central Inner Mongolia, is a large Cu-dominant deposit. Orebodies are hosted in strongly deformed and metamorphosed granodiorite porphyry intrusions or in EW-trending shear zones in greenschist and sericite felsic schist of the Bainaimiao Group. Mineralization can be divided into early, middle and late stages, characterized by quartz–pyrite, polymetallic quartz–sulfides and carbonate–quartz, respectively. Hydrothermal activity led to pervasive silicification, K-feldspar, biotite and chlorite–epidote–carbonate alteration. Eight samples from Cu–Mo-bearing quartz veins and massive sulfide ores in greenschist were collected from the VIII orebody in the North ore belt and molybdenite separates were selected for Re–Os analyses. The concentrations of Re and 187 Os range from 216.6 to 1051 ppm and 2302 to 4913 ppb, respectively. All samples give Re–Os model ages between 441.2 and 445.9 Ma and yielded an isochron age of 445.0 ± 3.4 Ma (MSWD = 0.26), with an initial 187 Os of − 4 ± 17 ppb. A granodiorite porphyry sample for SHRIMP U–Pb dating was collected from the underground of the XIII orebody of the North ore belt. Spots from 14 zircon grains have 206 Pb/ 238 U values between 426.2 and 462.9 Ma and form a tight cluster on Concordia yielding a weighted mean 206 Pb/ 238 U age of 445 ± 6 Ma. This is considered to be the best estimate of crystallization age of the granodiorite porphyry. The Re–Os isochron age of the molybdenite is consistent with the SHRIMP U–Pb age of the granodiorite porphyry, which indicates that the deposit is a porphyry system of Paleozoic age. According to geodynamic setting and geological history of the northern margin of the NCC, the deposit was probably related to magmatic activity of the Middle Paleozoic Ondor Sum subduction–accretion complex-Bainaimiao arc. During the Devonian, the Bainaimiao arc was accreted to the northern margin of the NCC. Collision between the Bainaimiao arc and NCC led to regional metamorphism of the ore-hosting terrane and ore metals were remobilized in this process. Gold mineralization was probably overprinted onto the earlier Cu–Mo mineralization.