Individual Fluid Inclusions Research Articles

Tracing metal source of copper-rich tungsten mineralization: Evidence from individual fluid inclusion analysis of the Huangsha Cu-rich W deposit in South China

The Huangsha Cu-rich W deposit, located in the northeast part of the Nanling region, is a typical large-size wolframite-quartz vein-type W deposit with considerable reserves of copper. Wolframite and chalcopyrite usually distribute in the same quartz veins, while field and petrographic observation show that wolframite commonly precipitates earlier than coexisting quartz and sulfides. Veined quartz can be divided into three successive generations (Q1, Q2, and Q3) based on SEM-CL imaging, and only Q2 is strictly coeval with chalcopyrite and other sulfides. Primary two-phase aqueous (type Lw) fluid inclusions are identified from wolframite with homogenization temperatures of 292 to 327 ℃ and salinities of 6.9 to 9.9 wt% NaCl equiv. Pseudosecondary two-phase aqueous (type Lq) and vapor-rich (type Vq) fluid inclusions are recognized from Q2, supporting the occurrence of fluid immiscibility. Homogenization temperatures of type Lq inclusions in Q2 range from 200 to 258 ℃ with salinities of 4.3 to 7.7 wt% NaCl equiv. LA-ICP-MS analysis shows that all inclusions contain Na and K as the major cations. For all the detected elements, type Lw inclusions in wolframite have higher concentration ratios X/Na than type Lq inclusions to varying degrees. The ore-forming fluid in both W and Cu mineralization stages share the same characteristics of high B concentrations, consistent Rb/Cs ratios and evolving nature of ore metals, indicating that W and Cu were sourced from the same magmatic fluid but precipitated in different mineralization stages. The variation of fluid Sr concentration likely indicates fluid-rock interaction during precipitation of wolframite, whereas the deposition of chalcopyrite is possibly influenced by fluid cooling and immiscibility. Compared with typical porphyry Cu systems and W/Sn mineralized systems worldwide, the ore-forming fluid in the W mineralization stage at Huangsha shows considerably higher mineralization potential of Cu, which is consistent with the geological fact. The Cu contents in initial W ore-forming fluids may serve as an indicator in identifying Cu-rich W deposits.

Spatially resolved CO2 carbon stable isotope analyses at the microscale using Raman spectroscopy

Measuring the carbon stable isotope ratio (13C/12C, expressed as δ13CCO2) in geogenic CO2 fluids is a crucial geochemical tool for studying Earth's degassing. Carbon stable isotope analysis is traditionally performed by bulk mass spectrometry. Although Raman spectroscopy distinguishes 12CO2 and 13CO2 isotopologue bands in spectra, using this technique to determine CO2 isotopic signature has been challenging. Here, we report on in-situ non-destructive analyses of the C stable isotopic composition of CO2, applying a novel high-resolution Raman configuration on 42 high-density CO2 fluid inclusions in mantle rocks from the Lake Tana region (Ethiopia) and El Hierro (Canary Islands). We collected two sets of three spectra with different acquisition times at high spectral resolution in each fluid inclusion. Among the 84 sets of spectra, 58 were characterised by integrated 13CO2/12CO2 band area ratios with reproducibility better than 4‰. Our results demonstrate the determination of δ13CCO2 by Raman spectroscopy in individual fluid inclusions with an error better than 2.5 ‰, which satisfactorily matches bulk mass spectrometry analyses in the same rock samples, supporting the accuracy of the measurements. We thus show that Raman Spectroscopy can provide a fundamental methodology for non-destructive, site-specific, and spatially resolved carbon isotope labelling at the microscale.

The magmatic-hydrothermal transition in P-rich pegmatitic melts: Crystal-melt-fluid interactions recorded by phosphate minerals

Rare-element pegmatites are the crystallization product of melts strongly enriched in incompatible elements. Some of these elements, e.g., Li, B, P, F, and H2O, function as melt structure modifiers, allowing such melts to reach unusually low crystallization temperatures and regulating the saturation of aqueous-carbonic fluids from the melt. However, the timing of fluid saturation is highly debated, and the processes that mark the magmatic-hydrothermal transition in rare-element pegmatites are still elusive. The crystallization sequence of phosphate minerals can be used to assess the timing and nature of fluid-related processes in pegmatitic systems. In the Buranga pegmatite, western Rwanda, magmatic phosphates are replaced by (hydrous) alkali-rich phosphates and later by strongly hydrated phosphates as the dike cools. This reaction series indicates a disequilibrium in the crystal-melt-fluid system and records complex fluid-rock interactions during dike consolidation. This contribution compares these interactions with the fluid record preserved in fluid inclusions in the main phosphates of the crystallization sequence. Three major fluid inclusion types are observed, aqueous-carbonic (mostly-two-phase, L-V), carbonic-aqueous (two-phase, V-L, or three-phase, L-L-V), and crystal-rich inclusions (multiphase, L-V-Solids). Crystal-rich fluid inclusions in trolleite [Al4(PO4)3(OH)3], the last primary phosphate to precipitate from the melt, show a paragenesis of solid phases similar to the secondary phosphates observed in the dike. LA-ICPMS analyses of individual fluid inclusions show high concentrations of Na and K, with some content of Li, Rb, Cs, B, Fe, and Mn in the fluid. A similar composition is present in all inclusion types within the same mineral. Crystal-rich fluid inclusions demonstrate that during the magmatic-hydrothermal transition, primary phosphates react with the fluid in the presence of melt to generate secondary phosphates. No evidence of external fluid sources has been observed during the magmatic-hydrothermal transition. The fluid inclusion record shows that the fluid salinity is kept constant due to fluid-mineral interactions and corroborates the hypothesis that fluid-soluble elements are internally remobilized in the melt-crystal-fluid system during the magmatic-hydrothermal stage. These interpretations have implications for the general evolution of P-rich pegmatitic melts and the mineralization of Nb and Ta in these systems.

The genetic association between vein and skarn type tungsten mineralization in the Yaogangxian tungsten deposit, South China: Constraints from LA-ICP-MS analysis of individual fluid inclusion

The large-scale Yaogangxian W deposit, located in central of the Nanling metallogenic belt, South China, comprises independent quartz vein-type and skarn-type W mineralization. Wolframite-bearing quartz veins occur invariably in metasandstone and slate to the northwest of the Yaogangxian granitic intrusion, whereas scheelite-skarn are mainly hosted by the limestone to the east. The vertical extension of single wolframite-bearing quartz vein can reach up to 1000 m and we focused on the vein samples collected from the deepest level (26 level) which likely recorded early magmatic fluids. For skarn mineralization, two scheelite precipitation stages were recognized, namely scheelite-1 in retrograde stage and scheelite-2 from sulfide-stage. To reveal the fluid sources and characteristics of two mineralizing systems, detailed petrography and microthermometry analyses were carried out on fluid inclusion assemblages (FIA) in scheelite, wolframite and their coexisting quartz. Fluid compositions were obtained from individual fluid inclusion (FI) in ore and gangue minerals via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. These data aid elucidation on the source and nature of fluids in the two distinct W mineralization styles. The chemical compositions of FIs hosted in wolframite and scheelite points to a common magmatic source. The fluid in the scheelite ore section exhibits lower As and B than the wolframite ore section, indicating that boiling may have occurred at early skarn stage. Fluid evolution in the two mineralizing systems suggest that formation of diverse W mineralization styles is mainly controlled by different host lithology and fluid processes. In the wolframite ore section, ore-forming fluid recorded in the deepest level of vein show elevated As, B, K and Rb contents than previously reported ore-forming fluid obtained from the tip of the vein. This likely indicate absence of intensive fluid immiscibility in the deep during wolframite formation, which may rather be accompanied by fluid-rock interaction. In the scheelite ore section, constant injection of meteoric water is revealed. FI hosted in scheelite-1 and 2 exhibit similar compositions, and the lower K content in scheelite-2 fluid is thought to be caused by sericitization in the retrograde stage skarn. The Mo content of scheelite decreased from the retrograde stage to sulfide-stage, indicating a decrease in oxygen fugacity. Based on these conclusion and previous geological and geochronological studies, we propose a metallogenic model of the coupled wolframite-quartz vein and scheelite-skarn mineralization.

Insights into the formation of Shihuiyao Ta-Nb deposit in southern Great Xing'an Range, NE China: Evidence from chronology and fluid inclusion

Shihuiyao is a typical granite-type Ta-Nb deposit in the southern Great Xing'an Range (SGXR), Northeast China. The ore field is comprised of several granitic intrusions that were emplaced into the Early Permian Linxi Formation during the Late Jurassic (ca. 145 ∼ 150 Ma). The main Ta-Nb mineralization (stage 1–2) is found within the leucogranite, with minor identified in the porphyritic granite in the deposit. Four distinct stages in the metallogenic process can be identified: weakly mineralized magmatic stage (stage 1), strongly mineralized magmatic stage (stage 2), post-magmatic hydrothermal stage (stage 3) and low-temperature hydrothermal stage (stage 4). Cassiterite samples collected from stage 2 were dated to be 147 ± 3.5 Ma and 147.7 ± 1.9 Ma, providing evidence for the latest Jurassic Ta-Nb metallogenic event at Shihuiyao. To acquire a more profound comprehension of the properties and behaviors of the fluids, fluid inclusions within quartz, albite, amazonite and fluorite were analyzed among all stages. The results showed that the Shihuiyao fluid inclusions were relatively homogeneous and predominantly comprised liquid-rich inclusions, with occasional occurrences of CO2-type, vapor-rich type, and solid-type inclusions. From stage 1 to stage 4, the temperature of the ore-forming fluids decreased gradually, while the salinity showed increasing during stage 4, which indicates a possible contribution from surrounding strata sourced fluids. Raman analysis of the inclusions at each stage revealed that the vapor components were primarily H2O, CO2, and CH4, with additional N2 and CH4 appearing in the late stage, pointing to mixing between the metallogenic fluids and the surrounding strata source materials. Notably, the simultaneous occurrence of liquid-rich and solid-type inclusions under low pressure conditions (0.6 ∼ 1.5 km) in the early ore-forming stage (stages 1–2) suggests a fluid boiling process. We argue that the fluid boiling and rapid chemical quenching of the granitic melt play a crucial role in changing the physicochemical conditions of the ore-forming fluids, ultimately resulting in the enrichment and precipitation of niobium and tantalum. The fluid mixing and intense water–rock interaction in the late stages (stages 3–4) may have also contributed to the minor mineralization.Drawing upon our investigations utilizing petrographic features and composition analysis of individual fluid inclusions, we have prognosticated on the Sn mineralization potential within the Shihuiyao deposit. Our results indicate the Shihuiyao granites are comparable to the tin granites worldwide. Genesis of cassiterites is linked to both magmatic and hydrothermal processes, exsolution from tin-rich fluids (up to 233 ppm) and interaction with country rocks attribute to the probable economic tin mineralization. Consequently, the Shihuiyao deposit has been noteworthy prospectively for further tin mineralization.

A tale of three fluids: Fluid-inclusion and carbonate clumped-isotope paleothermometry reveals complex dolomitization and dedolomitization history of the Latemar platform

Abstract This work focuses on an exceptionally complex natural laboratory, the Triassic Latemar isolated platform in the Dolomite Mountains of northern Italy. It explores spatial and temporal gradients in processes and products related to contact metamorphism, dolomitization, and the dedolomitization of marine limestones. Rock samples were studied using dual fluid-inclusion thermometry and clumped-isotope thermometry. Independent of the spatial position at Latemar, Δ47 clumped-isotope and fluid-inclusion data provide contrasting paleotemperature estimates. An apparent lack of systematic patterns in fluid-inclusion data (homogenization temperature, salinity, density) results from analyses of micrometer-sized growth zones within a single crystal. The composition of the individual fluid inclusions represents a “snapshot” of fluid mixing with variable endmember elemental ratios. The bulk crush-leach data and slopes in Caexcessversus Nadeficit diagrams indicate different water–rock interactions and fluid signatures with evaporation sequences and crystalline rocks. The presence of three fluid types (crystalline basement brine, halite-dissolution brine, seawater) in all carbonates suggests that all fluids coexisted during contact metamorphism and dolomitization of Latemar carbonates. Non-equilibrium processes overruled thermodynamic controls on the precipitation of diagenetic phases. Fluid mixing resulted in the precipitation of two complex carbonate successions. The Δ47 data represent bulk temperatures, averaging the mixing ratio of fluids with different temperatures and their respective volume. Fluid-inclusions record patterns of remarkable complexity and shed light on the complexity of a multi-fluid system. Data shown here provide answers to the controversial interpretation of dolomitizing fluid temperature in the Latemar and exemplify the strengths of multi-proxy paleotemperature studies.

Ore genesis of the Narenwula quartz-vein type W polymetallic deposit in the southern Great Xing’an Range W belt, NE China: Constraints from wolframite geochronology and individual fluid inclusion analysis

The Narenwula deposit, situated in the southern Great Xing’an Range W belt, is a large-scale quartz-vein type W polymetallic deposit in NE China. The mineralization is spatially associated with Late Jurassic monzogranite and granite porphyry. In this study, we present detailed description of the ore geology, wolframite in-situ U-Pb dating, and fluid inclusion analyses including microthermometry, laser Raman spectra, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) microanalyses to precisely constrain the timing of ore formation, origin and evolution of ore-forming fluids as well as metal precipitation mechanisms. In-situ U-Pb dating of hydrothermal wolframite yields a W mineralization age of 136.4 ± 2.0 Ma (1σ, MSWD = 1.3), which is ∼14 Ma younger than the zircon U-Pb ages of W-bearing granitoids, indicating the W-bearing granitoids are not genetically related to the W mineralization at Narenwula. The hydrothermal process can be divided into four stages: stage I is characterized by abundant wolframite (I), which is the dominant stage of W mineralization; stage II is featured with the development of wolframite (II) together with minor amounts of pyrite and chalcopyrite; stage III is the predominant stage of sulfide mineralization; and stage Ⅳ is characterized by lack of sulfide or wolframite and occurrence of quartz, carbonate, and fluorite. Cathodoluminescence analysis of quartz further revealed four generations of quartz, corresponding to the four hydrothermal stages. Four types of fluid inclusions were distinguished for quartz, including CO2-rich (C1-type), CO2-bearing (C2-type), liquid-rich (l-type), and brine (B-type) fluid inclusions. Fluid inclusion microthermometry shows a decrease in homogenization temperatures and salinities from the early to late stages. The ranges of the homogenization temperature for stages I to Ⅳ are 440–280, 320–240, 260–180, and 200–160 ℃, respectively, and the ranges of salinities for stages II to Ⅳ are 2.9–17.8, 1.4–10.2, and 0.4–6.7 wt% NaCl equiv., respectively. The salinities of fluid inclusions from stage I have a bimodal distribution in the ranges of 9.6–17.4 and 31.1–35.6 wt% NaCl equiv. Individual fluid inclusion LA-ICP-MS analyses suggest a magmatic source for the fluids of all stages, evidenced by the high Rb and Mn concentrations, high Rb/Na, K/Na, Cs/Na, Li/Na, and Zn/Na ratios, which are distinctly different from those of basinal brines. The Rb/Sr and K/Rb ratios of fluid inclusions are similar to those of Late Mesozoic highly fractionated W-mineralized granitoids in NE China, indicating that the metallogenic parent rocks at Narenwula are highly evolved. In combination with evidence of consistent Cs/Rb and Cs/(Na + K) ratios of fluid inclusions from all stages, we suggest that the mineralizing fluids originated from an underlying, geochemically uniform, and highly fractionated granitic magma. Fluid immiscibility and fluid-rock interaction (especially greisenization) are the main mechanisms for wolframite precipitation, whereas natural cooling of fluids may be subordinate. Fluid inclusion data provide evidence that the addition of meteoric water to the magmatic-hydrothermal fluid initiated at the sulfide mineralization stage, and the fluid dilution and cooling due to fluid mixing played an important role in the process of sulfide precipitation. This study highlights that W and base metals (e.g., Cu, Pb, Zn) in ore-forming fluids display different migration and precipitation progresses, the knowledge of which is important for elucidating multi-stage ore-forming processes of quartz-vein type W polymetallic deposits in NE China.

Fluid origin and critical ore-forming processes for the giant gold mineralization in the Jiaodong Peninsula, China: Constraints from in situ elemental and oxygen isotopic compositions of quartz and LA–ICP–MS analysis of fluid inclusions

The Jiaodong Peninsula, China, is one of the largest gold provinces in the world. During a short time interval in the Early Cretaceous, over 5000 t of gold were accumulated there. The mechanisms controlling the focused Au deposition and the origins of massive amounts of auriferous fluids are still controversial. Quartz–sulfide lode mineralization (Linglong-type) and disseminated/stockwork mineralization (Jiaojia-type) are the two major mineralization styles in the Jiaodong Peninsula. Herein, we conducted in situ textural, elemental and oxygen isotopic analyses using scanning electron microscopy (SEM), laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–MS) and secondary ion mass spectrometry (SIMS) on quartz from different hydrothermal stages and depths for three representative Linglong-type (Linglong deposit) and Jiaojia-type (Xiadian and Jiangjiayao deposits) gold deposits in the northwestern Jiaodong Peninsula, with the aim to constrain the ore-forming processes. The LA–ICP–MS analysis of individual fluid inclusions was also conducted to unravel the origin of the auriferous fluids. The results show that from the pre-mineralization to the ore-forming stages in the Linglong-type mineralization, the homogenization temperatures and salinities of fluid inclusions and the δ18Oquartz values change slightly, but the Al concentrations of quartz decrease continuously. Combined with the coexistence of CO2- and H2O-rich fluid inclusion endmembers, these trends indicate the increase of pH induced by CO2 degassing (fluid–fluid unmixing), which led to focused Au deposition. The quartz grains from the Jiaojia-type mineralization typically show a homogeneous texture with well-correlated lithophile elements (e.g., K, Al and Rb), which are distinct from those of the Linglong-type that are characterized by oscillatory zoning with much higher Al concentrations and δ18Oquartz values. These phenomena indicate that enhanced fluid–rock interaction occurred in the Jiaojia-type mineralization. In addition, the δ18Oquartz values show an increase from deep to shallow depths. In combination with thermodynamic modelling, it suggests decreasing fluid–rock interaction with decreasing depth. The above results corroborate that multiple mechanisms led to the efficient Au deposition in the Jiaodong Peninsula. Fluid–fluid unmixing typically occurred in the Linglong-type mineralization, while fluid–rock interaction was dominant in the Jiaojia-type mineralization. The different mechanisms might render mining challenging due to the changeable locations of favorable Au unloading.The LA–ICP–MS results of fluid inclusions suggest that the ore-forming fluids are metamorphic fluids rather than magmatic–hydrothermal fluids. In combination with the isotopic compositions of ores showing considerable mantle affinities and large-scale mafic underplating occurring beneath the study region, we propose a new genetic model that accounts for the origin of the auriferous fluids. The new model invokes the devolatilization of previously emplaced H2O- and Au-rich amphibole cumulates through amphibolite- to granulite-facies metamorphism in the middle–lower crust. It suggests an alternative way to generate the ‘orogenic-like’ gold deposits.

Formation of the Banxi Sb deposit in Eastern Yangtze Block: Evidence from individual fluid inclusion analyses, trace element chemistry, and He-Ar-S isotopes

The Banxi Sb deposit (>100,000 t Sb metal with a grade of 0.02%–64.5% Sb (typically 15.3%–25.9% Sb) in the Eastern Yangtze Block is hosted in Neoproterozoic low-grade metamorphic clastic rocks of the Banxi Group. The deposit is characterized by vein-type Sb-only mineralization. Analyses of individual quartz-hosted fluid inclusions obtained by LA-ICP-MS identified Na as the most abundant cation (thousands to tens of thousands of ppm) in the ore-forming fluids of the Banxi Sb deposit, together with variable contents of K, Ca, Al, As, Fe and Sb (tens to thousands of ppm). Multiple chemical criteria, including high Na+/K+ (typically 3–15) and low Mn2+/Na+ (0.003–0.2) and Sr2+/Na+ (0.001–0.01) ratios of individual fluid inclusions, suggest that the fluids responsible for quartz and stibnite precipitation in the Banxi Sb deposit were likely dominated by basinal brines within low-grade metasedimentary rocks. Moreover, the extremely low 3He/4He ratios (0.001–0.048Ra) but relatively high 3He/36Ar (0.5 × 10−5–5.8 × 10−4) and 40Ar/36Ar ratios (323–1111 with an average of 737) of fluid inclusions trapped in ore-stage quartz further support a sedimentary origin of ore-forming fluids in this deposit. The positive and homogeneous sulfur isotopic values (δ34S = 8.0–9.5‰) of stibnite and arsenopyrite are comparable to those of sulfides in Proterozoic metasedimentary rocks (δ34S = 5.6–11.5‰), indicating these metasedimentary rocks may have served as the primary source of S for the Banxi Sb deposit. Combined with geological features and new trace element chemistry data for quartz and stibnite, we envisaged a new formation model for the Banxi Sb deposit that assumes that circulation of weakly alkaline basinal brines through faults under an extensional tectonic setting during the Early Cretaceous (∼130 Ma), mobilization of Sb and S from the underlying and host metasedimentary rocks and ascent of ore- forming fluids along faults, and ultimately precipitation of Sb minerals in or around fault/fracture zones as a result of the decreasing pH of the ore-forming fluids as well as sulfidation during fluid-rock reaction.

Co-genetic formation of scheelite- and wolframite-bearing quartz veins in the Chuankou W deposit, South China: Evidence from individual fluid inclusion and wall-rock alteration analysis

The large-scale Chuankou W deposit is located in the north part of the Nanling metallogenic belt, South China. It comprises independent quartz vein-type scheelite and wolframite mineralization, both associated with the underlying granitic intrusion spatially. The scheelite-bearing quartz veins occur invariably in sandstone and slate to the west, whereas the wolframite-bearing quartz veins are mainly hosted by the granite to the east. To reveal the fluid characteristics of the two mineralizing vein systems, detailed petrography, microthermometry, and Raman spectroscopic analyses were carried out on fluid inclusions in scheelite, wolframite and their coexisting quartz. Major and trace element concentrations of fluids were obtained from individual fluid inclusions in quartz via LA-ICP-MS analysis. Wall-rock alteration of both vein systems were evaluated by TESCAN Integrated Mineral Analyser (TIMA) analysis, and based on which the bulk geochemical data of wall-rock as a function of distance from the vein were obtained for scheelite-bearing quartz vein. These data aid elucidation on the nature of fluids in the two types of W mineralization. Fluid inclusions in quartz and its coexisting tungsten minerals share similar homogenization temperature and salinity, suggesting near concurrent formation of quartz and ore minerals. LA-ICP-MS analyses on pseudosecondary fluid inclusions in quartz associated with ore minerals show considerable concentrations of W in both mineralizing fluids, whereas Fe, Mn and Ca are generally absent irrespective of their higher detection limits. This may imply that Ca, Fe, and Mn were not initially derived from the granitic magma. The fluid compositions of both types of W mineralization display similar Rb/Na, K/Na, and Rb/Cs ratios, indicating a similar initial fluid source. The TIMA image shows that the host rock (sandstone) of scheelite-bearing quartz vein contains abundant Ca-rich minerals, such as apatite and calcium feldspar, indicating wall-rock as potential source providing Ca for the precipitation of scheelite. This assumption is strongly supported by bulk geochemical data of wall rock as a function of distance from hydrothermal quartz vein. In contrast, the tourmaline-rich granite hosting wolframite quartz veins may be a potential source of Fe for the precipitation of wolframite. Study on fluid composition combined with wall rock alteration suggest that the magmatic-sourced fluids and fluid–wall-rock interactions exerted primary control on the large-scale variations in mineralization types in the deposit, with scheelite- and wolframite-bearing quartz veins being both genetically and spatially related. Results of this study provide new guidelines for mineral exploration, suggesting a homologous scheelite-dominated deposit being likely to exist in association with a wolframite-dominated deposit, where the lithology of Ca-bearing formations may indicate potential prospecting areas.

Characterization of hydrothermal fluids that alter the upper oceanic crust to spilite and epidosite: Fluid inclusion evidence from the Semail (Oman) and Troodos (Cyprus) ophiolites

Pervasive alteration of basaltic oceanic crust by heated seawater at greenschist facies conditions produces two contrasting hydrothermal rocks. “Spilites”, consisting of chlorite + albite + quartz ± actinolite ± epidote, occur typically with regional extents. Locally spilites are metasomatically transformed to “epidosites” consisting of epidote + quartz + titanite + hematite or magnetite. Both alteration types have been proposed as markers of deep hydrothermal upflow in sub-seafloor convection cells, and as sources of the ore metals in basalt-hosted seafloor massive sulfide deposits. Little direct evidence is available for the chemical compositions of these fluids in their states deep in the upflow zones prior to their discharge at the seafloor. To better characterize them we have conducted a field, petrographic and fluid inclusion study of the lavas, sheeted dikes and plagiogranites in the Semail ophiolite, with supporting samples from the Troodos ophiolite. Our results show that both the spilite- and epidosite-forming fluids were single-phase aqueous liquids during the hydrothermal alteration. At some sites their salinity is 3.1–3.2 wt.% NaCleq, which we take to represent the chlorinity of Cenomanian seawater in the Semail realm. At other sites salinities are as low as 2.4 wt.% NaCleq or as high as 5.7 wt.% NaCleq, attributable to liquid–vapor separation and partial remixing deep in the crust along the dew curve of seawater, prior to ascent of the fluids to the sites of fluid inclusion trapping. Hypersaline brines, often accompanied by vapor, are restricted to plagiogranites in both the Semail and Troodos ophiolites and they represent magmatic–hydrothermal fluids that pre-date and are genetically unrelated to the spilite and epidosite alteration. The volcanostratigraphic locations of the samples constrain their maximum depths to 1470–3600 m below seafloor during alteration. The range of possible fluid trapping pressures for all samples is 31–68 MPa. Trapping temperatures vary between sites from 145 to 440 °C for spilite fluids and 255 to 435 °C for epidosite fluids. Quantitative analyses of 12 elements in individual fluid inclusions by LA-ICP-MS define the chemical characters of the two alteration fluids. The Br/Cl ratio in the spilite fluid is the same as in modern seawater and the other elements fit expectations from seawater–basalt experiments at elevated temperature. Accordingly, concentrations of Li, B, Na, Cl, K, Br and Sr in the spilite fluid match those in modern black-smoker vent fluids in basaltic crust. Exceptions are Ca and Fe, which are enriched in the spilite fluid. As these elements may precipitate below or at the seafloor prior to vent sampling, we conclude that the spilite fluids are plausible feeders of basalt-hosted black-smoker vents. The epidosite fluid has broadly similar elemental concentrations to the spilite fluid, but vastly lower Fe, reflecting the highly oxidized state of epidosites. This suggests that epidosite fluids are incapabable of forming basalt-hosted seafloor massive sulfide deposits.

Constraining the genesis of tungsten mineralization in the Jiaoxi deposit, Tibet: A fluid inclusion and H, O, S and Pb isotope investigation

The Jiaoxi deposit, located in the western part of the Lhasa terrane, is the first discovered Miocene (ca. 13 Ma) quartz vein-type tungsten deposit in this belt. Ore bodies mainly occur as wolframite-bearing veins hosted in the shales and granite. Three stages were identified for the formation of the Jiaoxi deposit, including the oxide stage (dominated by wolframite and quartz), the sulfide stage (quartz-sulfides veins) and the fluorite stage (late veins comprising fluorite, calcite, and quartz). Here, we investigate the genesis of this deposit based on fluid inclusion and H-O-S-Pb isotope studies. Fluid inclusion microthermometry and oxygen isotope data reveal that the wolframite precipitated from a moderate temperature (340–380 °C), low salinity (2.1–7.5 wt% NaCl equivalent) hydrothermal fluid at a depth of ∼ 6 km. The sulfides from the oxide and sulfide stage have uniform δ34SCDT values (+2.5 to + 3.7‰), and consistent Pb isotope compositions with the coeval granites (206Pb/204Pb: 17.866–18.789; 207Pb/204Pb: 15.555–15.743; 208Pb/204Pb: 37.157–39.335). The δ18OVSMOW value of the hydrothermal fluid decreased from the oxide (δ18OVSMOW = +7.2 to +14.2‰) to the fluorite stage (δ18OVSMOW = +2.1 to +5.7‰), indicating that the exsolved magmatic hydrothermal fluid mixed with meteoric water. This is also suggested by the positive correlation between the salinity and homogenization temperature of the aqueous fluid inclusions. LA-ICP-MS analysis of individual fluid inclusion reveals that the W concentrations of ore-forming fluid in the oxide stage (1.2–70.2 ppm) are much higher than those in the sulfide and fluorite stage (1.1–2.2 ppm) and show positive correlations with the homogenization temperatures and the Cl concentrations. We propose that fluid dilution and cooling due to fluid mixing played an important role during the wolframite precipitation in the Jiaoxi deposit.

Fluid evolution along the Patterson Lake corridor in the southwestern Athabasca Basin: constraints from fluid inclusions and implications for unconformity-related uranium mineralization

The Patterson Lake corridor (PLC) in the southwestern margin of the Athabasca Basin hosts several high-grade uranium deposits. These deposits are located in the basement up to 900 m below the unconformity surface, raising questions about their affiliation with typical unconformity-related uranium (URU) deposits elsewhere in the basin. Based on cross-cutting relationships four pre- and three syn- to post-mineralization quartz generations were identified. Fluid inclusion analyses indicate that pre-mineralization fluids have salinities ranging from 0.2 to 27.2 wt% NaCl equiv. (avg. 9.0 wt%), whereas syn-mineralization fluids have salinities ranging from 8.8 to 33.8 wt% NaCl + CaCl 2 (avg. 25.4 wt%), with NaCl- and CaCl 2 -rich varieties. The homogenization temperatures ( T h ) of fluid inclusions from pre-mineralization quartz range from 80 to 244°C (avg. 147°C), and from syn-mineralization quartz range from 64 to 248°C (avg. 128°C). Fluid boiling is indicated by the co-development of liquid-dominated and vapour-dominated fluid inclusions within individual fluid inclusion assemblages from the syn-mineralization quartz and is related to episodic fluid pressure drops caused by reactivation of basement faults. Our results indicate that composition and P – T conditions of the ore fluids in the PLC are comparable to those of typical URU deposits in the Athabasca Basin, indicating that the uranium deposits in the PLC formed under similar hydrothermal conditions. Episodic reactivation of basement faults was an important driving force to draw uraniferous fluids from the basin and reducing fluids from the basement to the mineralization sites, forming deep basement-hosted deposits. Supplementary material: Table 1, microthermometric results of type-2 and -5 fluid inclusion assembladges and isolated inclusions from the Patterson Lake corridor, and table 2, microthermometric results of type-6 fluid inclusions from the Patterson Lake corridor are available at https://doi.org/10.6084/m9.figshare.c.5510179 Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Characteristics and evolution of ore-forming fluids in the Diyanqinamu Mo deposit, Inner Mongolia: Evidence from LA-ICP-MS analysis of individual fluid inclusion

The Diyanqinamu Mo deposit in Dong Ujimqin Banner of Inner Mongolia is a large deposit. In this study, hydrothermal activity is divided into five stages: quartz - potassium feldspar - fluorite, quartz - muscovite, epidote - magnetite, chlorite - carbonate – pyritization, and argillization. The results show that early exsolved fluid has the characteristic of high temperature (up to 470°C), high oxygen fugacity, and Fe- and sulfide-rich in daughter minerals. LA-ICP-MS analysis of individual fluid inclusion exhibit early fluid is rich in W, Mo, Pb, Zn, Fe, Mn, As, Ag, Sn, and rare earth elements of La, Ce. The ore-forming fluids are NaCl-H2O system and contain gases CO2 and a small amount of H2 and N2. In general, the ore-forming fluids evolved from high temperature (340 °C - 470°C) with high salinity (up to 63.9 wt%NaCl equiv) to low temperature (189 °C - 202°C) with low salinity (as low as 0.18 wt% NaCl equiv). The ore-bearing fluid comes from the exsolution of the volatile-rich magma, and hydrothermal fluid boiling result from the drop of the pressure may have led to Mo precipitation. Based on the alteration, mineralization, and fluid characteristics, the Diyanqinamu Mo deposit is a porphyry deposit related to magmatic activity in Yanshanian.

Chlorine isotope fractionation during serpentinization and hydrothermal mineralization: A density functional theory study

Because of the large-scale recycling of volatile chlorine from the interior to the surface of the Earth, it is possible to use the isotopic composition of this element (δ37Cl) in serpentinite to study mantle-crust interactions in subduction zones. It is also possible to use chlorine isotopes to track the evolution of fluids (through fluid inclusions/minerals) in hydrothermal ore-forming systems. Here, we report the results of a study of equilibrium chlorine isotope fractionation during serpentinization and hydrothermal mineralization based on density functional theory (DFT) and ab initio molecular dynamics simulations (AIMD). The chlorine isotope fractionation between lizardite and brine under variable thermodynamic conditions is described by the relationship 1000lnαlizardite-fluid = 0.4170 × (1000/T)2–0.0281 × (1000/T) + 0.0582, which yielded Δ37Cllizardite-fluid values of +0.49 to +4.46‰, +0.41 to +0.59‰ and + 0.49 to +3.57‰ for conditions at the seafloor, in the mantle wedge and in subduction zones. As a proxy for the diversity of metal-chloride complexes in hydrothermal fluids, the stable configurations of ferrous chloride complexes were acquired from long trajectories of AIMD simulation. Using this information, the chlorine isotope fractionation between minerals (i.e., apatite-group minerals, muscovite, phlogopite, tremolite, lizardite, marialite and metal halides) and ore-forming fluid was estimated. The relatively large chlorine isotope fractionation (Δ37Cl minerals-hydrothermal fluid values from −1.99 to +2.18‰) might partly explain the large variation of δ37Cl in individual fluid inclusions observed in hydrothermal ore deposits. Because of the limited chlorine isotope fractionation between apatite and hydrothermal fluid (i.e., Δ37Cl apatite-ore-forming fluid of 0.06‰–0.69‰), apatite-group minerals might be an alternative to fluid inclusions for constraining the origin and evolution of hydrothermal fluids using δ37Cl values. The theoretical constraints provided in this paper will facilitate the use of chlorine isotopes in tracking the sources of chloride in subduction zones and the origin of mineralizing fluids in ore deposits.

Basement aquifer evolution and the formation of unconformity-related hydrothermal vein deposits: LA-ICP-MS analyses of single fluid inclusions in fluorite from SW Germany

The main ore stage of three similar unconformity-related vein systems in the Schwarzwald, SW Germany spanning a period of activity of ~150 Ma, was investigated to understand the details of fluid penetration from overlying sediments/marine environment into the basement, their evolution as well as the processes involved in vein formation. To investigate temporal and spatial variations of the hydrothermal fluids responsible for mineralization, over 1650 fluid inclusions were analyzed by microthermometry. Of these, a total of 108 fluid inclusions (mainly in fluorite) were successfully analyzed by LA-ICP-MS. The fluid inclusions reveal a binary mixing trend between a CaCl2- and a NaCl-rich endmember. Independent of major element composition, the fluids are metal-bearing (e.g., up to ~100 mg/kg Ba, Pb, Zn, Ni and up to 10 mg/kg Ag), show high As (up to 1000 mg/kg) and low S (below the detection limit in most analyses). Over time, the mixed fluid shows a gradual decrease in CaCl2 and increase in NaCl with slightly decreasing total salinity.Based on earlier studies and geochemical arguments, the veins formed by anisothermal binary fluid mixing of two fluids, which both were originally derived from seawater and chemically modified through interaction with the basement and sedimentary rocks in different ways. This produced a gradual stratified basement fluid reservoir comprising a modified bittern/halite dissolution brine. The fluids involved in the vein formation are sourced from different depths of this modified bittern/halite dissolution basement brine reservoir: fluid A, a CaCl2-dominated, KCl-poor, deeper seated brine with a salinity of ~25 wt% CaCl2 + NaCl, and fluid B, an NaCl-dominated and KCl-richer brine situated at shallower depths in the crystalline basement with salinities of ~22 wt% NaCl+CaCl2. Based on the Na-Ca-K and NaK thermometers and on Rb/Cs systematics, fluid A records alteration of the Na-, K- and Ca-bearing feldspars of the host rocks; progressive alteration led to consumption of mainly Ca-rich plagioclase in contact with these basement brines. Accordingly, fluid B that subsequently entered the basement was only in equilibrium with alkali feldspars and clay minerals. This scenario produced a gradual change of fluid composition with depth that was pushed to greater depth over time.The source temperatures are estimated to ~250 °C while vein formation occurred at 100–170 °C, based on fluid inclusion homogenization temperatures. Thus, significant fluid cooling without abundant fluid mixing (and without major mineralization) must have occurred during fluid ascent (3–7 km, depending on the assumed geothermal gradient). Fluid mixing then resulted in the formation of the major gangue minerals fluorite, quartz, barite and calcite, while the locally confined sulfides must have formed due to an influx of sulfide into the binary mixed fluid. The process of fluid mixing is a rapid and turbulent process. This is recorded by a great diversity of fluid compositions (including ones close to the mixing endmembers) trapped within the same crystal. Compositional variations are even visible within individual fluid inclusion trails.

New Insights into the Origin of the World-Class Jinding Sediment-Hosted Zn-Pb Deposit, Southwestern China: Evidence from LA-ICP-MS Analysis of Individual Fluid Inclusions

Abstract The Jinding deposit in the Lanping basin, southwest China, is the largest sandstone-hosted Zn deposit in the world and the second largest Zn-Pb deposit in China. However, questions related to the metal compositions and origin of the ore fluids remain. In this study, microthermometry and laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS) were employed to determine the properties and compositions of individual fluid inclusions trapped in sphalerite and calcite. The results show that the fluid inclusions trapped in sphalerite and calcite have similar homogenization temperatures (79°–173°C with the majority 100°–130°C), salinities (10.3–29.1 wt % NaCl + CaCl2 equiv with the majority 24.5–27.4 wt % NaCl + CaCl2 equiv), and concentrations of alkali and alkali earth elements (e.g., Na, Ca, Mg, K, Sr, Ba, Li, Rb, and Cs). However, the concentrations of ore and associated metals (e.g., Pb, Sb, Ag, and Tl) in the fluid inclusions hosted by sphalerite are significantly higher than those hosted by calcite. Based on these observations, we propose that the sulfides including sphalerite were precipitated from a low-temperature, high-salinity, Ca-rich, metal-rich fluid, while the gangue minerals such as calcite crystallized subsequently from fluids depleted in metals due to prior precipitation of sulfides, and that the high salinities of the fluid inclusions are likely due to a combination of seawater evaporation and subsequent dissolution of evaporitic sequences during fluid percolation. The LA-ICP-MS analyses reveal that the fluid inclusions have K/Na, Rb/Na, and Cs/Na ratios within the range of modern basinal brines, and Li/Na, Ba/Na, and Ca/Na ratios share similar compositions with the ore fluids of basement interacted deposits in the world. The Jinding ore fluids contain ~200 to 650 ppm Pb, based on the data of fluid inclusions trapped in sphalerite. The estimated concentrations of Zn in the ore fluids are also very high at ~200 to 6,500 ppm. Our results reveal that anomalously metal rich fluids played a critical role in the formation of the giant Jinding sediment-hosted Pb-Zn deposit. We concur with the previous suggestion that sulfide precipitation at Jinding occurred when ascending metal-rich brines encountered an H2S-rich, Ca-rich fluid, which was produced by interaction of hydrocarbons with evaporites, in the cap of the Jinding dome.

Quantitative calculation of a confocal synchrotron radiation micro-X-ray fluorescence imaging technique and application on individual fluid inclusion

Elemental distribution representations at varying depths by the confocal μ-SRXRF imaging technique are effectively compensated for after quantitative calculation.

Mineral composition and formation conditions of the Inkur tungsten deposit ores (Dzhidinsky ore field, South-Western Transbaikalia)

The aim of the study is to clarify the mineral composition and determine the conditions of the formation of the quartz-hubnerite veins of the Inkur stockwork tungsten deposit (the Dzhidinsky ore field, South-Western Transbaikalia). The research methods include a mineralogical and petrographic description of the ore quartz-hubnerite veins; an electron microprobe analysis of the mineral associations; thermometry, cryometry, and Raman spectroscopy of the individual fluid inclusions in quartz, fluorite, hubnerite, and muscovite. The mineralogical and petrographic studies has made it possible to clarify the mineral composition of the Inkur deposit ores and determine the mineral paragenesis formation sequence. The fluid inclusion studies have established that the ore deposition was occurring in the relatively low-salinity (~5.7–14.6 wt. % eq. NaCl) homogeneous solutions due to a decrease of the temperature. The study of the salt composition of the solutions has identified Ca chloride as a prevailing component, with NaCl, KCl, and MgCl as admixtures. CO2 and N2 have been identified in the gas phase of inclusions. Two stages of mineral formation have been defined: high-temperature (≥300 °С) and low-temperature (≥2.00–300 °С). The conducted studies allow qualitative estimation of the chemical composition of the ore-forming solutions. It has been established that one of the main factors of the hubnerite deposition is a temperature factor.

Fluid composition and evolution of the Langxi Ba-F deposit, Yangtze Block, China: New insight from LA-ICP-MS study of individual fluid inclusion

The Langxi vein-type Ba-F deposit, located in the southeast of the Sichuan Basin in the central part of Yangtze Block, South China, is hosted by Ordovician limestone. The paragenesis of the deposit’s veins is subdivided into pre-, syn- and post-ore stages. The syn-ore stage includes two fluorite generations with the first (Fl-1) deposited at ~140° to 200° C and the second (Fl-2) at ~140° to 180 °C. The syn-ore barite mineralisation was deposited with the temperature range of 110°–160 °C, and the syn-ore calcite records a temperature range of 90° to 140 °C. These temperature ranges show that the mineralising fluids were low- to moderate in temperature and decreased with the initial deposition of fluorite followed by barite and then calcite. The types of fluid inclusions in the veins are classified as monophase liquid (L-type, type I), biphase liquid + vapour (L + V type, type II) and polyphase (L + V + S type, type III) brine inclusions. The salinity of the mineralising fluid for the three types of inclusions during the deposition of F-Ba-Ca is 0.35–21.7% NaCl eqv., indicating that the ore-forming fluid varied greatly during the mineralising process. There are no obvious differences in the fluid inclusion density in the different minerals with the average densities being 0.98 g/cm3 for Fl-1, 0.99 g/cm3 for Fl-2, 1.00 g/cm3 for barite, and 1.01 g/cm3 for calcite. This shows that the density of the F–Ba–Ca mineralising fluid belong to a moderate- low density. The deposition of the minerals took place with a change from a reducing to an oxidising environment. The liquid phase initially included Na+ and Cl−, and evolved containing Ca2+, SO42−, K+, and Mg2+. LA-ICP-MS analyses reveal that the L-V type fluid inclusions associated with Fl-1 assays 46,600 ppm Na, 3470 ppm K (with a mean Na/K ratio of ~24), ~28,200 ppm for Cl, and 1060 ppm Br (with a mean Cl/Br mass ratio of ~27). The second-generation fluorite L-V type fluid inclusions average around 47,000 ppm Na and 12,000 ppm K, corresponding to and a Na/K mass ratio of 7. The halogen content of Fl-2 averages 26,200 ppm Cl and 950 ppm Br, corresponding to a Cl/Br mass ratio of 28. These analyses indicate that the mineralising fluid was rich in NaCl, assuming the fluid inclusions represent samples of the mineralising fluid. It is proposed that Ba-F concentrated in the mineralising fluid were derived from basinal fluids mixed with meteoric water circulating through faults at increased temperature gradients. The driving force was Late Cretaceous tectonic activity related to the subduction of the Paleo-Pacific Oceanic Plate. This model is consistent with the geodynamic setting of other barite-fluorite-rich districts in the Yangtze Block of South China.