Texture and chemical evidence from apatite for hydrothermal fluid evolution and uranium mobilization in the Daliang deposit, South China

The Dachang Sn-polymetallic ore district in South China is the second largest tin district in the world with a tin reserve of over one million tonnes. Zn-Cu skarn and stratiform, massive, and vein Sn-Pb-Zn ores are all present in this district. This has led to a debate as to whether the Sn orebodies were formed by Cretaceous magmatic-hydrothermal replacement or Devonian submarine exhalative-hydrothermal sedimentation. Here, we present a systematic investigation of the major, trace element, and boron isotopic compositions of different types of tourmaline in the Dachang ore district. Tourmaline disseminated in the Longxianggai granite and pegmatite veins belongs to the schorl series and has high contents of Li, Zn, and Ga. The δ11B value of primary magma of the Longxianggai granite is estimated to be about −13‰, close to the global average δ11B value (−11‰) for S-type granites. Tourmaline from quartz-tourmaline veins in the Longxianggai granite has similar chemical composition to the magmatic tourmaline and likely formed from hydrothermal fluids exsolved from the evolved granitic melt. The δ11B value of the initial hydrothermal fluids is also calculated to be about −13‰. Tourmalines from the skarn and sulfide ores in the Lamo deposit have higher Mg/(Mg+Fe) and lower Na/(Na+Ca) ratios and higher contents of Be, Ge, Sr, and Sn than magmatic tourmaline. These patterns likely reflect input of elements derived from the host Devonian limestone. The δ11B values of the hydrothermal fluids are estimated to be between −13 and −10‰, suggesting evolved magmatic-hydrothermal fluids related to the Longxianggai granite. Tourmalines from the stratiform and vein ores in the Changpo-Tongkeng deposit are extremely Mg-rich and mostly belong to the dravite series. They have high contents of Sc, V, Cr, Sr, and Sn and show positive Eu anomalies. The δ11B values of these B- and Sn-rich fluids are estimated to be between −15 and −10‰, suggesting that the fluids also have a magmatic-hydrothermal origin. These fluids are most likely derived from the same granitic magma source, but may have interacted with the Devonian volcanic rocks.

Six uranium tailings samples from the shallow depth of two large-scale uranium tailings ponds in South China, which were affected by pedogenesis, were analyzed to determine the occurrence modes of uranium. The results demonstrated that an average of nearly 80% of the uranium in the samples was mobile and potentially mobile, indicating that this type of uranium tailings could significantly threaten their ambient environment when their contents of uranium were relatively high. In this sense, the possibility of the in-situ immobilization of active uranium in the uranium tailings was discussed. The experimental data indicated that the active uranium could be immobilized through its reaction with phosphate, which was theoretically feasible and promising in curbing uranium pollution.

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

The Nanpanjiang basin hosts the world’s second-largest concentration of Carlin-type gold deposits. To decipher the origin and evolution of hydrothermal fluid, this study conducted Sm–Nd dating, in-situ trace element, and C-O-Sr isotopic analyses on three types of calcite samples from the giant Lannigou gold deposit in the Nanpanjiang basin, SW China. The type-I calcite, intergrown with Au-bearing arsenian pyrite, has an Sm–Nd isochron age of 213 ± 7 Ma (MSWD = 0.81), indicating that gold mineralization occurred in Late Triassic. The type-II calcite, which coexists with high-maturity bitumens and cut through the main-stage gold orebodies, yields an Sm–Nd age of 188 ± 14 Ma (MSWD = 0.34), representing a post-ore hydrocarbon accumulation event. The type-I and type-II calcite samples have low REE contents (5.28–51.6 ppm) and exhibit MREE-enriched and LREE-/HREE-depleted patterns. Combined with their identical C-O-Sr isotopes, we suggest that hydrothermal fluids responsible for the precipitation of type-I and type-II calcite samples were derived from a mixed metamorphic fluid and meteoric water source. In contrast, the type-III calcite samples, associated with realgar and orpiment, have distinct Mn, Sr, and As contents, REE patterns, and C-O-Sr isotopic composition from the type-I and II calcites, suggestive of different fluid sources. Based on our and previously published data, we propose that the fluid evolution, gold mineralization, and hydrocarbon accumulation in the Nanpanjiang basin are closely related to the Indosinian and Yanshanian orogenies in South China.

Origins and chemical evolution of hydrothermal fluids in the Shazhou volcanic-related type U deposit at the Xiangshan district, South China: New insights from LA-ICP-MS analysis of fluid inclusions

Experimental data on the mobility of uranium in hydrothermal fluids are required to accurately model the formation of uranium ore deposits. This study aims to conduct experiments on the solubility and speciation of U(IV) and U(VI) in fluoride- and phosphate-bearing solutions, as well as flow-through, fluid-rock interaction experiments designed to assess the optimal P-T-X conditions for U precipitation, which will be integrated into a comprehensive genetic model. Early work has focused on method development for the solubility and flow-through experiments. Preliminary experiments on UO2 solubility in 0.01 M F- solutions at 200°C produced concentrations of 1.76 to 3.92 ppm U, demonstrating the surprisingly high mobility of U(IV) and the importance of U fluoride complexes for ore-forming systems.

Pyrite textures, trace element and sulfur isotopes of Yanlinsi slate-hosted deposit in the Jiangnan Orogen, South China: Implications for gold mineralization processes

The volcanic‐hosted Xiangshan uranium ore field is the largest uranium deposit in Jiangxi Province, South China. In this ore field, Pb–Zn mineralization has been discovered beneath the uranium orebodies at Niutoushan‐Heyuanbei area. Apatite is a ubiquitous accessory mineral in the volcanic rocks, as well as the U and Pb–Zn alteration–mineralization zones. Six main types of apatite, which show the evolution of volcanic magma and hydrothermal fluids, have been observed on the basis of textures, associated mineral assemblages, chemical compositions (halogen, MnO, FeO, and SO3, etc.), and mineral inclusions. For magmatic primary apatite, porphyritic lava‐hosted P‐Apa and P‐Apb have higher F/Cl ratios and lower contents of MnO and FeO than rhyodacite‐hosted R‐Ap. For hydrothermal apatite, U‐Ap1‐a, U‐Ap1‐b, and S‐Ap1 are recognized as hydrothermal‐affected apatite, while U‐Ap2 and S‐Ap2 as apatite precipitating from hydrothermal fluid. U‐Ap1‐a and U‐Ap1‐b, hosted in the green‐stained alteration zone, are characterized by highly heterogeneous BSE imaging and contain monazite and ThSiO4 inclusions. Such apatites record the remobilization and reprecipitation of rare earth elements (REEs), Th and U by a relatively high‐temperature, F‐ and CO2‐enriched but Na‐depleted fluid at the early REE‐phase stage. U‐Ap2 occurring in pitchblende–fluorite dominated ore sample, is featured with negligible Cl, variable F and relatively higher Ti contents. It records a new hydrothermal pulse of relatively low temperature, oxidized, highly Ca‐, Ti‐, and F‐enriched but Cl‐depleted which probably conduces to leaching and transporting significant amounts of uranium. S‐Ap1 and S‐Ap2 hosted within the sericite–quartz–carbonate alteration halo on both sides of Pb–Zn–Fe sulphide veinlets, have relatively high Cl and variable F content. This apatite records reduced, Cl‐, Ca‐enriched, magmatic‐derived fluid events. Combined with previous researches, the apatite data suggest that U and Pb–Zn mineralization may form from two spatially and temporally distinct hydrothermal systems. Thus, apatite can fingerprint multi‐stage hydrothermal fluids and be treated as a useful tool for mineral exploration.

The Middle–Late Permian witnessed an unusual chert accumulation event along the margin of the Pangea and Paleo-Tethys realms, known as the “Permian Chert Event (PCE).” The PCE is well recognized in the Permian limestone from South China, in the forms of nodular and bedded cherts. Previous studies suggested that PCE was caused by hydrothermal fluids related to the Emeishan large igneous province (ELIP). Meanwhile, another hypothesis supported the biogenic origin of PCE, i.e., the Permian chert derived from biosilicification of abundant sponges and radiolarian. Thus, sources of silica from the Permian chert remain uncertain. To understand linkages among PCE, biosilicification mechanism, and the ELIP event, this study focused on chert nodules collected from the Permian Maokou and Wujiaping formations in the Lianziya and Maoertang sections, South China. We measured germanium/silicon ratios (Ge/Si) and rare earth element (REE) compositions of chert nodules on the basis of petrographic analysis. Ge/Si ratios range from 0.14 to 0.63 μmol/mol with an average of 0.33 μmol/mol (n=18) in the Lianziya section and from 0.02 to 0.75 μmol/mol with an average of 0.18 μmol/mol (n=45) in the Maoertang section, both of which are close to the seawater value. The REE pattern is characterized by LREE depleted with a positive Eu anomaly ranging from 0.66 to 2.16 in the Lianziya section and from 1.05 to 9.57 in the Maoertang section. Our results indicate that the silica of the Permian chert predominantly originated from seawater with limited contributions from hydrothermal fluids. To further quantify the contributions of hydrothermal fluids, we applied a binary (seawater and hydrothermal fluid) mixing model based on two geochemical proxies, i.e., the Ge/Si ratio and Eu anomaly. The modeling results suggest a mixing of 0.5 vol% to 1 vol% hydrothermal fluids with contemporaneous seawater, verifying the dominant seawater source of silica in the PCE. Although it has been widely accepted that positive Eu anomaly points to the hydrothermal fluid origin of silica, our study demonstrates that positive Eu anomaly could also be present in cherts that was predominantly derived from normal seawater. Therefore, the analysis of the Ge/Si ratio or REE compositions is highly recommended when determining the Si source of cherts.

Submarine hydrothermal contribution for the extreme element accumulation during the early Cambrian, South China

Tourmaline as an indicator for late-magmatic to hydrothermal fluid evolution of the Neoproterozoic Baotan tin deposit, South China

Analyses of six drill cores from and surrounding the North Canning uranium deposit in the Copper Mountain uranium district, Wyoming, suggest that a low-temperature hydrothermal system may have transported and precipitated uranium. Uranium is found in highly altered breccia zones and fractures in Archean granite and in overlying Tertiary sediment. Na montmorillonite invariably forms the matrix around stable and secondary K-feldspar and quartz in the breccias. Uraninite and amorphous pitchblende occur as disseminated fracture fillings and as encrustations on lithic fragments in the breccias. Hematite staining commonly occurs around uranium mineralization, and pyrite is disseminated in fractures and is intimately associated with uraninite. In the granite host rocks, sericite replaces plagioclase, epidote replaces plagioclase and biotite, and chlorite sporadically replaces biotite. Homogenization temperatures of fluid inclusions in quartz microveinlets average 136 degrees C.Anomalous uranium concentrations occur in the Copper Mountain granite (5 ppm regionally, but more than 20 ppm in a central core). Isotope systematics show 30 to 50 percent uranium losses in Tertiary time, from which preloss values greater than 35 ppm are calculated. The heat-generating capacity of this granite, using the back-calculated U value and averages of 28 ppm Th and 4.5 percent K 2 O is 11.2 mu W/m 3 . Steady state conductive temperatures greater than 150 degrees C would have been generated under 4.5 km of Cambrian to Cretaceous sediments. Laramide faulting opened the heated rock to convective water circulation, allowing silicate alteration, mobilization of uranium within the granite, and then deposition of uranium in the breccia zones.Activity diagrams show solution reaction paths associated with alteration and stability relationships for uraninite. Unaltered K-feldspar and plagioclase altered to sericite and Na montmorillonite suggest constant a (sub K (super +) ) /a (sub H (super +) ) and increasing a (sub Na (super +) ) /a (sub H (super +) ) ratios. The assemblage uraninite-pyrite-hematite is stable within f (sub o 2 ) values of 10 (super -44) to 10 (super -35) and f (sub s 2 ) values of 10 (super -20) to 10 (super -6 5) atm at an assumed P (sub CO 2 ) = 10 (super -2) atm and Sigma U = 10 ppb in the hydrothermal fluid. Oxidation of magnitite and sulfide solution species to hematite and hydrothermal pyrite caused reduction and precipitation of uranium from solution.

The influence of syngenetic hydrothermal silica fluid on organic matter preservation in lower Cambrian Niutitang Formation, South China

Fluid evolution and mineralizing process of the Bawang Fe-Zn-Sn deposit, Danchi Fold-and-Thrust belt, South China

In-situ sulfur isotope and trace element analysis of pyrite from the Xiwang uranium ore deposit in South China: Implication for ore genesis