Ore-forming Hydrothermal Fluids Research Articles

Origin and evolution of a porphyry-skarn-stratabound system: constraints from geology, magmatic-hydrothermal zircon U–Pb and molybdenite Re–Os geochronology, and in situ S isotopes of the Dongguashan Cu(–Au) deposit, eastern China

Intrusion-related stratabound deposits within Late Carboniferous carbonate units are an abundant and highly economically significant ore type in eastern China. In such deposits, stratabound mineralization commonly coexists with porphyry and/or skarn mineralization, and the vast majority of reserves (>90 %) are derived from extensive stratabound orebodies, which are easy to exploit and of great interest to geologists. However, it is unclear whether these deposits were formed via the Late Carboniferous seafloor sedimentary exhalation formation or the Early Cretaceous magmatic-hydrothermal processes. Moreover, the fluid and sulfur isotope variations within porphyry-skarn-stratabound systems have not been adequately constrained. We investigated the Dongguashan deposit, one of the largest and most representative stratabound Cu(–Au) deposits in eastern China. Three mineralization types were recognized based on field and petrologic studies: porphyry, skarn, and stratabound. In the stratabound mineralization zone, magmatic and hydrothermal zircons from laminated ores yielded concordant U–Pb ages of 137.3 ± 1.1 and 135.7 ± 1.4 Ma, respectively, while molybdenite samples from vein-like ores yielded Re–Os model ages of 137.8–133.1 Ma (isochron age, 135.9 ± 1.4 Ma). These ages were consistent with the zircon U–Pb age of quartz monzodiorite (137.8 ± 1.7 Ma). A total of 146 in situ S isotope analyses yielded δ34S values (1.9–7.4 ‰) indicating that the sulfur was derived from a magmatic source. Therefore, the Dongguashan deposit resulted from Early Cretaceous magmatism rather than submarine exhalative sedimentary processes. The fluctuations in S isotopes of sulfides across the three mineralization types are attributable to different factors (pH, temperature, and oxygen fugacity) during the evolution of hydrothermal ore-forming fluids. This study demonstrates the importance of geochronology of mineralization and in situ sulfur isotope analysis under robust geological and petrographic frameworks, which can yield information about the ore-forming processes and ore genesis.

The source and ore-forming processes of post-collisional Qulong porphyry Cu-Mo deposit in Tibet constrained by Mo isotopes

Porphyry deposits supply most of the world's Cu and Mo resources. Most porphyry deposits are developed in magmatic arcs above subduction zones. However, abundant Miocene porphyry Cu-Mo deposits have been found in the post-collision stage in Tibet, more than ∼30 Myr after the Indian-Eurasian continental collision. The magma source and the enrichment process of ore-forming elements for these post-collision porphyry deposits remain controversial. Here, we present Sr-Nd-Mo isotope compositions of a suite of mineralized and barren igneous rocks from the Rongmucuola pluton in the Miocene post-collisional Qulong porphyry Cu-Mo deposit in Tibet to reveal their magma sources and ore-forming processes. Our results indicate that the mineralized and barren igneous rocks in the Qulong deposit were derived from a cogenetic source with similar Sr-Nd isotope compositions (87Sr/86Sr(i) = 0.7049–0.7050 and εNd(t) = −0.24 to 0.20). However, the mineralized igneous rocks have large variations in δ98/95Mo values (−0.30‰ to 0.74‰; relative to NIST SRM 3134) and Mo/Ce ratios (0.01 to 4.84) compared to those of the barren igenous rocks (δ98/95Mo = −0.74‰ to −0.02‰; Mo/Ce = 0.01 to 0.04). In combination with the higher LOI values, Mo/Ce and Cs/Ta ratios and Cu concentrations and lower Ce/Pb ratios of the mineralized igneous rocks relative to the barren igneous rocks, we propose the mineralized igneous rocks were affected by ore-forming hydrothermal fluids originated from the exsolution of later co-genetic magmas, which significantly changed the isotopic and elemental compositions of intrusions and resulted in Mo mineralization. The fluid-rock interaction modeling supports this interpretation and successfully reproduce the observed δ98/95Mo and Mo/Ce compositions of mineralized igneous rocks. Our study indicates that the exsolved magmatic-hydrothermal fluids have heavy Mo isotopes, and the fluids with a slightly heavier Mo isotope composition have the greatest potential for mineralization. As the water-rock reaction progresses, leading to the precipitation of Mo-rich minerals, ore-forming fluids with a heavier Mo isotope composition may not have significant mineralization potential. This highlights that Mo isotope system is an effective tool to study the ore-forming processes of porphyry deposits.

An accurate model for the solubilities of anhydrite in H2O–NaCl solutions at temperatures up to 1073 K, pressures up to 14,000 bar, and mole fractions of NaCl up to 0.3

An accurate density-based solubility model is established for anhydrite in H2O–NaCl solutions, which is valid for temperatures up to 1073 K, pressures up to 14,000 bar, and mole fractions of NaCl (xNaCl) up to 0.3. The isobaric linear dependence of the mineral solubility logarithm on the solvent density logarithm should be a common phenomenon similar to isothermal linear dependence, particularly at relatively high pressures. The terms about P in the equations of the density-based model are rational and necessary to elevate the accuracy of mineral solubility models. This model reproduces the complicated changing trends of anhydrite solubility with temperature, pressure, and xNaCl. Compared to numerous experimental data, the average relative deviations of calculated anhydrite solubilities in water and the aqueous NaCl solution are 3.52% and 9.64%, respectively, which are within experimental uncertainties. According to this model, the anhydrite deposition would result from any salinity variation departing from the specified xNaCl in skarn- or porphyrite-type iron ore-forming hydrothermal fluids.

Paleoproterozoic hydrothermal overprinting over Neoarchean banded iron formation produced high-grade iron ores in the giant Gongchangling deposit of North China: Evidence from O–S–B isotopes

The Gongchangling iron deposit in North China hosts abundant high-grade (>50 wt% total Fe) magnetite ores and represents one of the largest BIF-hosted magnetite deposits worldwide. These high-grade magnetite ores are generally accepted to be formed by replacement of banded iron formation (BIF) (∼30 wt% total Fe), but the mechanism of iron enrichment remains controversial. Here we report new O–S–B isotope data of BIFs, high-grade iron ores, and related altered wall rocks, from the Gongchangling iron deposit, together with the geological and geochronological data to constrain the formation of the high-grade iron ores, and proposed new genetic model. The high-grade iron ores have positive δ56Fe and negative δ30Si values, similar to the BIFs, whereas the δ18O values of the high-grade iron ores and altered wall rocks are significantly lower than those of the BIFs. The δ34S values of pyrite from the high-grade iron ores are much higher than those of the BIFs. The δ11B values of tourmaline in the altered wall rocks of the high-grade iron ores are anomalously high, which is distinct from those of the BIFs and wall rocks, but similar to those of the evaporites from the Liaohe Group. At ca. 1.85 Ga, intense hydrothermal activity was initiated by post-orogenic extension and uplift with 18O-poor meteoric water circulated along crustal structures. Compositions of Na+, K+, Mg2+, Cl–, CO32–, 34S-rich SO42–, 11B-rich borate, and other soluble salts minerals were leached from the evaporitic rocks of the Liaohe Group, which driving the ore-forming hydrothermal fluids toward slightly alkaline and weakly oxidized as system matures. As the alkaline fluids migrated along the structures and fractures in the BIFs and reacted with the wall rocks, SiO2 was mobilized from the low-grade iron ores and consequently formed chlorite, garnet, and tremolite in the wall rocks due to reactions with clay minerals. These Si-rich hydrothermal fluids was also related to vein type quartz and massive quartz-bearing rocks. Most Fe was retained in situ and resulted in the formation of magnetite-rich iron ores. The SO42– in the hydrothermal fluids was reduced to form pyrite while B in the hydrothermal fluids reacted with the wall rocks to form 11B-rich tourmaline.

Structure-controlled mineralization resulted from Paleoproterozoic orogenesis in the Dahenglu Cu-Co deposit, NE China

Paleoproterozoic orogenic-related (or orogenic-like) ore deposits play a significant role in the metal and non-metal metallogenic systems in the North China Craton. This study focuses on detailed field structural of the Dahenglu Cu-Co deposit to reveal relationship between structural controls and enrichment process of cobalt (Co)-bearing hydrothermal fluids in the metasedimentary rock. In the context of Paleoproterozoic orogenesis, at least three phases of deformation were identified within the ore-bearing country rocks: an early DPl-1, the peak-orogenic deformation phase called DPl-2 and a final exhumation stage of deformation designated as DPl-3. Through meticulous structural analysis, microscopic examination of drilling cores, and the application of the TESCAN Integrated Mineral Analyzer, we have discerned that thrust faults, operating at both outcrop-scale and microscale, function as substantial conduits for the migration of ore-forming hydrothermal fluids. Additionally, fractures within thrust faults, the core of folds, and the high strain micro-scale shear zone between overturned folds provided ideal spaces for the precipitate of Co-bearing hydrothermal fluid. Based on the regional geological correlations, the second phase of deformation (DPl-2) is characterized by S2 foliation, crenulation cleavage, and thrust fault, which played a crucial role in the enrichment of cobalt in minerals such as siegenite and pyrite. This cobalt enrichment is attributed to the peak-orogenic stage, DPl-2, which likely occurred between 1.95 and 1.90 Ga. The ore-forming geodynamic was associated with Paleoproterozoic orogenesis, characterized by intense deformation and metamorphism. This conclusion aligns with cobalt enrichment observed in the Paleoproterozoic Trans North China orogen in the central North China Craton and the Paleoproterozoic Peräpohja belt in northern Finland. The structures generated by significant tectonic events, such as Paleoproterozoic orogenesis, represent key controlling factors of Co-rich ore deposits.

Estimates of chlorine isotope fractionation factors using density functional theory: Applications to ore-forming systems

Chloride is the most important anion in ore-forming hydrothermal fluids, and chlorine isotopes are therefore potentially sensitive tracers of the origin and evolution of ore-forming fluids. However, they remain a relatively under-utilized tool in ore deposit geochemistry due to the lack of knowledge of chlorine isotope fractionation during ore-forming processes. Using first-principles density functional theory, this study estimates chlorine isotope fractionation factors for various ore-forming processes. All metal-Cl complexes in hydrothermal fluids are enriched in 37Cl compared to chloride ions, but the change in δ37Cl of the fluid trapped in ore minerals is small after the destabilization of metal-chloride complexes during ore mineral deposition. In the precipitation of evaporite minerals from brine, the sequence of enrichment of the heavy Cl isotope (37Cl) is halite > carnallite > aqueous chloride > kainite > sylvite > bischofite. The results of this study agree with the experimental observations that progressive precipitation of halite from brine lowers the δ37Cl value of the residual fluid until the formation of K-Mg chlorides. In low-temperature deposits, δ37Cl values for fluid inclusions and minerals reflect those of the hydrothermal fluid provenance and the mixing of this fluids with saltwater or basinal brines. In high-temperature magmatic-hydrothermal ore deposits that undergo liquid–vapour phase separation, chlorine isotopes fractionate among phases of silicate melt, vapour and chloride-rich liquid. The considerable range in δ37Cl in fluid inclusions may also reflect fluid mixing and hydrothermal alteration. At ambient temperature, the δ37Cl values may reflect evaporative processes and, in the case of chemical weathering of metallic mineral deposits, may record the supergene enrichment of the metals. This study highlights the use of chlorine isotopes as a new tool for interpreting ore-forming processes.

An integrated approach for quantifying fluid inclusion data combining microthermometry, LA-ICP-MS, and thermodynamic modeling

The development of LA-ICP-MS and other microanalytical techniques for in situ analysis of single fluid inclusions has made it possible to quantify the elemental concentration of major elements, trace metals, sulfur and halogens in fluid inclusions from a wide range of geological environments. The established quantification methods typically interpret the LA-ICP-MS and microthermometric data (melting points of ice, salt and hydrate phases) in terms of simple binary or ternary model systems (e.g., H2O-NaCl and H2O-NaCl-CaCl2), which can be inaccurate for complex natural geofluids such as multi-solute basinal brines, magmatic-hydrothermal fluids or pegmatitic fluids. Another approach uses mass or charge balance constraints along with the elemental ratios obtained from LA-ICP-MS. This improves the quantification of more complex fluids to some extent, but because it is based on the equivalent concentration of NaCl from the NaCl-H2O binary system, it typically introduces systematic uncertainty in the concentrations of all elements analyzed. These limitations can be fundamentally overcome by combining microthermometric and LA-ICP-MS data with thermodynamic modeling. In this approach, elemental ratios (element/Na) are directly calculated from the LA-ICP-MS data and then the unknown Na concentration is calculated by modeling the chemical equilibrium between the aqueous solution and the last melting solid phase (e.g., ice, hydrohalite, sylvite) of the fluid inclusion using a Pitzer model for the aqueous solution. Comparison between modeled and experimental data of chloride systems (including salt-solution equilibria and synthetic fluid inclusion data) demonstrates excellent agreement over wide ranges of temperature and composition. Our new model, accompanied by a new approach for quantifying uncertainty, is more general than previous methods and makes it possible to quantify complex multi-element fluid inclusions from diverse geological environments including magmatic, metamorphic, sedimentary and various ore-forming hydrothermal fluids.

Cathodoluminescence and geochemical characteristics of Apatite: Implications for mineral exploration in the Xiongcun District, Tibet, China

Apatite grains from the No. 2 porphyry Cu–Au deposit in the Xiongcun district of Tibet, China, have a range of physical and compositional features. This study investigates the potential use of apatite associated with hydrothermal alteration as a tool for mineral exploration. In situ textural, cathodoluminescence (CL), and chemical analyses were performed on thin sections containing apatite from various alteration zones in the deposit. No clear variation was seen in apatite from rocks of the potassic, sodic–calcic, chlorite–sericite, phyllic, and propylitic alteration zones using visible light and scanning electron microscopy; however, distinct variations were observed in optical cathodoluminescence (OP-CL) (dark yellow to green, orange to green, yellow–brown to green, yellow-orange to brown, and light yellow–brown to green luminescence for the respective zones). The alteration-related differences in luminescence were a result of the interaction between Fe and Mn; the Na and Cl contents; and depletion in total heavy rare earth elements (ΣHREE) and Y. The variations in the REE contents of apatite from different types of altered rock record reactions with Cl- and F-rich fluids. The partitioning behavior of Cl between apatite and hydrothermal fluids is influenced by pressure, temperature, pH, and fluid composition, whereas the partitioning of F between apatite and fluids is dependent on the fluid composition and not the temperature. At high temperatures, Eu3+ is reduced to Eu2+, which results in negative Eu anomalies in apatite from the potassic-altered rocks and propylitic-altered rocks. Apatite grains in the sodic–calcic-altered rocks and chlorite–sericite-altered rocks show a similar enrichment in light REEs (LREEs) that resulted from REE and Y incorporation as follows: Na+ + (Y + REE)3+ = 2Ca2+. Apatite grains in the phyllic-altered rocks are enriched in the middle REEs. This study confirms that the luminescence, chemical composition (including Fe, Mn, Na, Cl, ΣHREE, and Y contents), and REE patterns of apatite can be affected by hydrothermal alteration; therefore, these results may be used to constrain the evolution of ore-forming hydrothermal fluids and may offer an effective tool for mineral exploration.

Review on the development of multi- and hyperspectral remote sensing technology for exploration of copper–gold deposits

Most Copper (Cu) and Gold (Au) deposits are accompanied by hydrothermal alteration, generally related to ore-forming hydrothermal fluids. Multispectral and hyper-spectral Remote Sensing (RS) technology is essential for Cu-Au mineral prospecting. This paper provides an overview of RS data and the research situation and development tendencies of RS exploration technologies related to Cu-Au deposits. Initially, RS geologists applied the Band Ratio (BR) and Principal Component Analysis (PCA) to extract hydroxyl and iron anomalies from Landsat RS data to characterize the hydrothermal alterations associated with Cu-Au deposits. In the era of advanced spaceborne thermal emission and reflection radiometry (ASTER), clay minerals, sulfate minerals, carbonate minerals, silica minerals, and iron minerals can be identified because of three Visible and near-infrared bands (VNIR), six shortwave infrared bands (SWIR), and five Thermal Infrared bands (TIR). Alteration of mineral assemblages can be carried out to map alteration zones, which are essential for calibrating Cu-Au alteration zones, such as the typical alteration zoning characteristics of Porphyry Copper Deposits (PCD). Because of its high spectral resolution, Satellite hyperspectral RS can quantitatively map surface alteration minerals to indicate the exploration of Cu-Au deposits. RS geologists have extensively researched matching libraries and field spectra for Alteration Mineral Mapping (AMM). However, subpixel mineral information caused by low spatial resolution and atmospheric factors has an unavoidable influence on satellite hyperspectral RS AMM, and it is essential to use low-altitude unmanned aerial vehicle (UAV) hyperspectral RS. UAV hyperspectral AMM combined with geochemistry and geophysics is a means of prospecting and exploring Cu-Au deposits.

The Role of Reductive Carbonaceous Surrounding Rocks in the Formation of Porphyry Mo Deposits

Porphyry Mo deposits are the most important type of Mo resource. They result from a high oxygen fugacity of the parent magma, which acts as an effective indicator for evaluating the mineralization. In the ore-forming system of porphyry Mo deposits, sulfur exists mainly as sulfate in highly oxidized magma but as sulfide in ores. What triggers the reduction in the mineralization system that leads to sulfide precipitation has not yet been determined. Most of the previous studies have focused on the origin and evolution of the ore-forming parent magma, and the effects of reductive surrounding rocks on porphyry mineralization have been ignored. In this study, a comprehensive geological–geochemical investigation and review have been performed on the typical porphyry Mo deposits, the Nannihu-Sandaozhuang, Yuchiling, and Shapingou deposits in China, and the Mt. Emmons deposits in America. Black carbonaceous sedimentary layers commonly surround porphyry Mo ores, which are widely altered and discolored during mineralization. CH4 is commonly present in fluid inclusions in the main mineralization stage, and the δ13CV-PDB values of calcite and fluid inclusions from the altered surrounding rocks and ore minerals are generally low and significantly different from those of marine sedimentary carbonate rocks, indicating that the involvement of reductive components from carbonaceous surrounding rocks might be key to the redox state transformation leading to mineral precipitation. On the other hand, the CH4 produced by the thermal decomposition of organic matter or carbonaceous reaction with H2O can diffuse into the ore-forming system along the structural fractures and reduce the SO42− in the ore-forming hydrothermal fluids to form sulfide precipitation without direct contact between the intrusion and the carbonaceous surrounding rocks. Moreover, the CH4 content controls the location of the orebody formation with the high content producing orebodies mainly in the porphyry intrusion, while the low CH4 content results in the orebodies mainly occurring at the contact zone between the porphyry and carbonaceous surrounding rocks. Compared to the magmatic stage of mineralization, the involvement of reductive components in the carbonaceous surrounding rocks during the hydrothermal stage is more favorable for forming giant/large Mo deposits. The highly oxidized porphyry with reductive carbonaceous surrounding rocks or Fe-rich volcanic rocks offers a new indicator for efficiently evaluating porphyry Mo mineralization.

Mineralogy and Mineral Chemistry of the Au-Ag-Te-(Bi-Se) San Luis Alta Deposit, Mid-South Peru

A mineralogical and mineral chemistry study was carried out in the San Luis Alta telluride-rich gold deposit, mid-south Peru, to contribute towards determining its formation and improving the ore processing. The San Luis mineralization is considered an intrusion-related gold deposit located in the Arequipa segment of the Coastal Batholith. The mineralization occurs in quartz veins hosted in diorites and granodiorites from the Tiabaya Super-Unit. These veins are sulfide-rich in the deep areas and contain abundant iron oxides. Sulfides are mainly pyrite with minor chalcopyrite and galena. Native gold and telluride minerals are abundant. Mineral chemistry was determined using an electron microprobe. The mineralogy of veins was classified into four stages. Gold occurs in the three last stages either in large grains, visible to the naked eye, or, more frequently, in grains of less than 10 µm. Gold appears as grains encapsulated in pyrite, Fe oxides, quartz and filling fractures. The first stage is characterized by the deposition of quartz and massive pyrite, which does not contain gold. During the second stage, hessite, calaverite, petzite and altaite are formed. Additionally, Bi-tellurides, mainly volynskite, rucklidgeite, kochkarite and tellurobusmuthine, are formed. Some of these minerals occur as blebs encapsulated in pyrite, suggesting that a Bi-Te-rich melt was formed from the ore-forming hydrothermal fluid and transported the Au and Ag elements. This stage was followed by a fracturing event and tellurobismuthite, tetradymite and montbrayite precipitated. In the last stage, a supergene replacement formed covellite, bornite and goethite. Te-Bi minerals do not appear in this stage, but selenium minerals occur in minor amounts. Chlorargyrite and iodargyrite occur and are associated with gold.

Pore Variation Characteristics of Altered Wall Rocks in the Huize Lead–Zinc Deposit, Yunnan, China and Their Geological Significance

The porosity and permeability of the rock surrounding lead–zinc deposits are key factors for controlling the migration and precipitation of ore-forming hydrothermal fluid. In this paper, the Huize super-large lead–zinc deposit was taken as the case study, and variations in the porosity and permeability of the wall rocks and their relationship with the orebody were analyzed by using CT scanning technology. The experimental results showed that the average pore radius of dolomite with a decreasing distance to the orebody ranged from 1.60 to 1.65 μm, increasing to 1.77~2.05 μm. The CT porosity increased from 2.76%–2.81% to 3.35%–3.99%. The average pore throat length decreased from 29.57–39.95 μm to 13.57–16.83 μm. In the research, it was found that the hydrothermal fluids rich in chemical elements changed the properties of the surrounding rocks. Temperature rise will lead to dolomitization of limestone and recrystallization of dolomite. This process led to an increase in the porosity of the wall rocks. During the formation of the orebody, the metal minerals in the hydrothermal fluid entered the pores of the rock. As a result, the pore radius and pore volume of the wall rocks were reduced, along with the pore throat radius and pore throat length. Therefore, the wall rock pores near the orebody were isolated from each other, and the permeability of the surrounding rock decreased. The variation characteristics for the porosity and permeability of the dolomite at various distances from the mine can be used to discover orebodies.

Mineralogical Evidence for Hydrothermal Uranium Mineralization: Discovery and Genesis of the Uranyl Carbonate Minerals in the BLS U Deposit, SW Songliao Basin, Northeast China

Diabase intrusions have been widely found in sandstone-type U deposits of the southwestern Songliao Basin, indicating diabase-related hydrothermal fluids might play an important role in this type of U mineralization. The first discovery of U-bearing carbonate minerals in the BLS U deposit provides an opportunity for understanding hydrothermal U mineralization and its relationship to diabase intrusions. U-bearing carbonate minerals occurred as thin shells generally ringing ankerite and then surrounded by colloidal pyrite through examination of scanning electron microscopy and energy dispersive spectroscopy. They can be interpreted as uranyl carbonate minerals, with the empirical formula of Ca2.7Fe0.9Mg0.4 (UO2) (CO3)5•9.6H2O, based on infrared absorption spectroscopy and electron microprobe. The formation of uranyl carbonate minerals is most likely related to the CO2-rich hydrothermal fluids from diabase intrusions according to its occurrence state, but the key factors are that the Ca-UO2-CO3 ternary complexes should have been produced in ore-forming hydrothermal fluids and adsorption of ankerite on ternary complexes. Thereby, a potential diabase-related hydrothermal U mineralization model for sandstone-type U deposits can be proposed. The ore-forming fluids that originated from diabase-related hydrothermal are formed through continuously extracting the adsorbed U6+ and dissolving the early U minerals. Then, U in the ore-forming hydrothermal fluids was migrated and transported probably either as uranyl carbonate ions or as Ca-UO2-CO3 ternary complexes. The former is easy to precipitate in the form of pitchblende dispersed in the fossil wood cells, ringing pyrite, and occurring along the edge of adsorbents at the site of reducing capacity change, while the latter will extremely inhibit the reduction of U6+, eventually leading to the precipitation of uranyl carbonate minerals ringing ankerite by adsorption. The above research results can provide mineralogical evidence for hydrothermal U mineralization in sandstone-type U deposits of the Songliao Basin.

Geochemistry and origin of hydrothermal apatite in Carlin-type Au deposits, southwestern China (Gaolong deposit)

Apatite can record the evolution of magmatic–hydrothermal ore systems, but it is unclear whether it is suitable for investigating low-temperature, sediment-hosted hydrothermal ore systems. To fill the knowledge gap. we investigated the Gaolong Carlin-type Au deposit in the Guangxi Zhuang Autonomous Region, southwestern China (Youjiang Basin). Results show that the hydrothermal apatite occurs in quartz–sulfide ores, has an intact crystal form and homogeneous texture, and is F-rich (2.77–3.67 wt%) and Cl-poor (<0.01 wt%). The apatite is enriched in rare earth elements (REEs; ΣREE = 739–2481 ppm) and has middle-REE-enriched chondrite-normalized REE patterns with positive Eu anomalies (δEu = 1.35–2.00). These REE features were inherited from a reducing hydrothermal system in which anion complexation was dominant. The apatite has high Sr contents (up to 0. 9 wt%) and relatively uniform initial 87Sr/86Sr ratios (0.709055–0.709208), suggesting that Sr was leached or derived from the deep continental crust. In situ U–Pb dating of 156.8 ± 8.3 Ma for apatite shows that the Late Jurassic–Early Cretaceous was an important time for the formation of Carlin-type Au deposits in the Youjiang Basin. The Au mineralization was associated with far-field tectonism and deeply sourced hydrothermal ore-forming fluids. Hydrothermal apatite can be used to fingerprint low-temperature, sediment-hosted hydrothermal ore systems.

Metal Source and Fluid Evolution in Xiaojiashan Gold Deposit in Northeastern Hunan, China: Implications of Rare Earth Elements, Fluid Inclusions, and Pyrite S Isotopic Compositions

The material source and the evolution of ore-forming hydrothermal fluids of Xiaojiashan gold deposits remain controversial. We carried out a mineralogical characteristics analysis, trace elements analysis, sulfur isotope composition analysis, and fluid inclusion microthermometry in order to explore the ore-forming sources, conditions, and process of this deposit. Gold mineralization can be divided into three stages: the quartz-pyrite stage, the quartz-polymetallic sulfide stage, and the quartz-ankerite stage. This gold deposit was probably formed under the following conditions: temperature of 122–343 °C and salinity of 0.8–11.4 wt% (NaCl). It was inferred that the ore-forming hydrothermal fluids were early metamorphic–hydrothermal (Stage I) and late magmatic–hydrothermal (Stages II and III), and were characterized by medium–low temperature and medium–low salinity based on fluid inclusion microthermometry and S isotope composition. The temperature and salinity of the ore-forming fluid decreased during mineralization, which was caused by the involvement of groundwater. The chondrite-normalized trace element patterns of the gold ores are similar to the host rocks of the Lengjiaxi Formation, indicating that the ore-forming materials were sourced from the Lengjiaxi Formation. The S isotopes indicated that the magmatic components also provided the ore-forming materials during Stages II and III.

In situ elemental and Sr-Nd isotopic compositions of hydrothermal apatite from the Shazhou U deposit in the Xiangshan complex: Implications for the origins of ore-forming fluids of volcanic related U deposits in South China

• In situ Sr-Nd isotopes of apatite are effective in tracing origins of ore forming fluids in the Shazhou U deposit. • The reducing U-poor fluid was derived from mantle-derived magma which formed the mafic dikes in wall rocks. • The oxidizing U-rich fluid originated from percolation of meteoric water in the adjacent red-bed basins. • The Late Cretaceous mafic magmatism and red-bed basins are two key factors controlling U mineralization in South China. The origin of ore-forming fluids of volcanic related U deposits remains controversial. This study reports in situ elemental and Sr-Nd isotopic compositions of apatite associated with U mineralization from the Shazhou volcanic related U deposit in South China, which provides a new perspective to constrain the origin and evolution of its ore-forming fluids. Hydrothermal apatites from the hematized rocks and U-mineralized rocks show distinct textures and chemical features, indicating two types of fluids involved in the U mineralization process. The hematitization fluid was oxidizing and enriched in U and Cl. A reducing fluid with lower Cl/F and U was introduced and mixed with the oxidizing hematitization fluid to cause U precipitation. Apatite from the hematized rocks has a lower 87 Sr/ 86 Sr (0.709246–0.714383) than that of apatite in U ore veins (0.710383–0.719318). The εNd(t) of apatite in the hematized rocks is from −7.6 to −9.4, whereas apatite in U ore veins has higher εNd(t) ranging from −1.2 to −8.4, close to that of mafic dikes intruded into the volcanic rocks. In situ Sr-Nd isotopes of apatite indicate the oxidizing fluid originated from the red-bed basins, whilst the origin of the reducing fluid was related to the mantle-derived mafic dikes. The reducing fluid has experienced assimilation by the metamorphic basement rocks to gain higher radiogenic strontium. This study demonstrates that in situ elemental and Sr-Nd isotopic compositions of apatite are effective in identifying different ore-forming hydrothermal fluids and constraining their origins and chemical evolution.

Molybdenum isotope variation mechanism and ore-genesis of Niutoushan Pb–Zn sulfide orebodies in the Xiangshan volcanic basin, South China

Niutoushan Pb–Zn sulfide orebodies are found in the western part of the Xiangshan volcanic basin; however, genesis of the mineralization is highly debated. To appropriately constrain the mineralization process and genesis of the Niutoushan Pb–Zn sulfide orebodies present in the volcanic rocks in the Xiangshan volcanic basin, we investigate Mo isotope compositions of sulfide minerals (pyrite and sphalerite) from the Niutoushan Pb–Zn sulfide orebodies and those of the volcanic and subvolcanic rocks (the least-altered porphyritic lava and granitic porphyry rocks) from the Xiangshan volcanic-subvolcanic complex.The δ98Mo values (normalized to NIST3134 of 0.25 ‰) of the Xiangshan volcanic-subvolcanic rocks, including the least-altered granitic porphyry (0.20 ‰–0.26 ‰) and porphyritic lava rocks (0.40 ‰–0.51 ‰), are similar to those of the bulk continental crust (0.1 ‰–0.35 ‰, Willbold and Elliott, 2017). The δ98Mo values of sulfides from the Niutoushan Pb–Zn sulfide orebodies (pyrite: −0.95 ‰ to 0.30 ‰, sphalerite: −0.94 ‰ to −0.59 ‰) are mostly lower than those of the continental crust and span a wide range. After examining potential factors that may cause δ98Mo variation, it is concluded that the wide δ98Mo range of the Niutoushan Pb–Zn sulfide orebodies resulted mainly from the hydrothermal fluid evolution through Rayleigh fractionation in the mineralization process than from changes in other factors of the fluid, such as temperature, redox, and fluid boiling process.Further, based on this conclusion, the δ98Mo value of the initial hydrothermal fluid finally forming the Niutoushan sulfide orebodies was estimated to be 0.46 ‰ by combining the lowest δ98Mo value of the sulfides and the Mo isotope fractionation between the fluid and molybdenite. The initial hydrothermal ore-forming fluid with such a δ98Mo value was most likely derived from the Xiangshan subvolcanic magmas. This study highlights the potential of Mo isotope composition as a novel tool for tracking origin and evolution of ore-forming hydrothermal fluids and metals in case of Pb–Zn sulfide mineralization associated with igneous activities.

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.

Caledonian Tin Mineralization in the Jiuwandashan Area, Northern Guangxi, South China

The Jiangnan orogenic belt is located between the Yangtze and Cathaysia blocks in South China and is one of the largest W–Sn–Nb–Ta ore belts worldwide. Mineralization occurred from the Proterozoic to Mesozoic, but Caledonian Sn mineralization has rarely been reported. The Jialong cassiterite–sulfide deposit is located in the western Jiangnan orogenic belt. It is hosted by the Sibao Group and in contact with the northeastern part of the Yuanbaoshan granite. The deposit was overprinted by the Sirong ductile shear zone. Here, we present cassiterite U–Pb and mylonitic granite muscovite 40Ar/39Ar ages for this deposit. The cassiterite and muscovite yielded concordant U–Pb and 40Ar/39Ar ages of 422–420 Ma, indicating that Sn mineralization occurred during the early Paleozoic and was spatially and temporally related to the ductile shear zone. The cassiterite is depleted in Nb (0.51–5.46 ppm), Ta (0.01–1.09 ppm), Ti (32.84–423.15 ppm), Sc (0.02–1.45 ppm), Hf (0–1.11 ppm), and other high-field-strength elements. Elements, such as Pb (0.01–8.11 ppm) and Sb (9.92–56.45 ppm), are relatively enriched in the cassiterite, which indicate the Jialong deposit was not directly related to magmatism. Shearing along the Sirong ductile shear zone occurred at 419.6 ± 3.8 Ma, concurrent with the formation of the Jialong Sn–Cu deposit. Moreover, cassiterite in the deposit exhibits obvious shear and brittle deformation, and dissolution and regrowth, suggesting that Sn mineralization was closely related to ductile shearing. The Sirong ductile shear zone and secondary shear structures had a key role in controlling the Sn orebody. The heat generated during tectonic deformation in the ductile shear zone may have produced the ore-forming hydrothermal fluids, and NW–SE-trending fractures in the strata provided the space for mineralization. Metamorphic hydrothermal fluids generated by Caledonian shear deformation extracted Sn from Sn-rich strata, which then migrated along interlayer fractures produced by shearing. A decrease in pressure and water–rock reactions led to the mineralization of Sn and other elements. This deposit is the first example of Caledonian and shear zone-related Sn mineralization identified in the Jiuwandashan area of northern Guangxi.

The Paleo-Tethys subduction and related mineralization in the SW Yangtze Block: Evidence from the Zhesang Carlin-type gold deposit

The Zhesang gold deposit is a Carlin-type gold deposit located in the “Golden Triangle” structural belt of Yunnan-Guizhou-Guangxi. Pyrite and arsenopyrite are important gold-bearing minerals of the deposit. In order to understand the typomorphic characteristics of pyrite and arsenopyrite and the enrichment mechanism of gold, morphological and structural features of these sulphides and the occurrence state of gold were investigated by using reflected light microscope and electron probes (EPMA). The results show that pyrites are characterized mainly by medium-fine broken pentagonal-dodecahedron crystal forms. Most of the arsenopyrites display needle and columnar crystal forms. Compositionally, pyrite is depleted in both Fe and S, whereas arsenopyrite is As-depleted but S-enriched. The w(Co)/w(Ni) of pyrite is between 1.731 and 3.215 (i.e., within the range of ratio of the hydrothermal pyrite, which is between 1 and 5). The crystal forms and compositional features of pyrite and arsenopyrite indicate that the Zhesang gold deposit is typical of medium-low temperature hydrothermal origin. The positive correlation between As and Au and the Au/As (mol%) ratio are lower than the solubility limit of Au in arsenic pyrite. Combined with previous studies, our data suggest that gold is likely to exist in the form of Au+ complex. Arsenic pyrite and arsenopyrite condensed Au in unsaturated ore-forming hydrothermal fluids relative to Au0 through surface voids and/or defects and the As content, forming a Zhesang Carlin-type gold deposit. Based on the age data of the Mesozoic polymetallic deposits in South China, it can be concluded that the age of the Zhesang gold deposit (215.3 ± 1.9 Ma), is consistent with the age of the main body of the Mesozoic polymetallic deposits (230–200 Ma) in South China Plate and the age of the granites (230–200 Ma) related to tungsten-tin mineralization in the Cathaysian Block. Therefore, we believe that the Zhesang gold deposit is the product of the subduction regime of the Paleo-Tethys Ocean in the Middle-Late Triassic.