Evolution Of Hydrothermal Fluids Research Articles

Geochemistry of Hydrothermal Fluids From the E2-Segment of the East Scotia Ridge: Magmatic Input, Reaction Zone Processes, Fluid Mixing Regimes and Bioenergetic Landscapes

The compositions of hydrothermal fluids in back-arc basins (BABs) can be affected by the influx of magmatic fluids into systems that are dominated by reactions between basement rocks and seawater-derived fluids. The East Scotia Ridge (ESR) in the Scotia Sea hosts such hydrothermal systems where the role of magmatic fluid influx has not yet been addressed. During expedition PS119 in 2019, three chimneys were sampled from the E2 segment. These samples were analysed for their chemical and isotopic composition along with fluid inclusions in corresponding precipitates. Our data provide evidence for the temporal evolution of hydrothermal fluids in this remote back-arc system. Salinity variations in anhydrite-hosted fluid inclusions indicate that phase separation takes place in the subseafloor. Moderate-temperature (<53°C) fluids from the newly discovered E2-West hydrothermal vent field and high-temperature (>320°C) fluids from the E2-South area were sampled. Depletions in fluid-mobile elements, ΣREE and low δ18OH2O show that the basement in this root zone has been leached since the previous sampling in 2010. The results indicate that high-temperature fluid-rock interactions are key in setting the composition of the fluids with cation-to-chloride ratios suggesting a common root zone for both vent sites. The concentrations of dissolved gases provide new insights in the connection between magmatic degassing and its influence on endmember vent fluid composition. Specifically, stable isotope (O, H) data and elevated CO2 concentrations point to a minor influx of magmatic vapour. Stable sulphur isotopes provide no evidence for SO2 disproportionation suggesting a H2O-CO2 dominated nature of these vapours. The concentrations of conservative elements in the E2-W fluid reflects subseafloor mixing between E2-S endmember fluid and seawater. In contrast, non-conservative behaviour, and depletion of Fe, H2, and H2S point to a combination of sub-surface abiotic and biotic reactions affecting these fluids. Similarly, E2-W fluids show evidence for H2S and CH4 being metabolized in the subseafloor. Thermodynamic computations confirm that the E2 system is dominated by sulphide oxidation as a major catabolic pathway. Our results indicate that the conditions at E2 are favourable to hosting a robust subseafloor biosphere.

Geochemical Characteristics of Chlorite in Xiangshan Uranium Ore Field, South China and Its Exploration Implication

Chlorite is one of the most important hydrothermal minerals in many hydrothermal uranium deposits worldwide and is commonly closely associated with the uranium mineralization. Trace elements in chlorite have been extensively applied to fingerprinting the hydrothermal fluid evolution and indicating the concealed ore bodies in porphyry Cu (-Au) deposits and skarn-related Pb-Zn deposits. However, this approach was rarely attempted on hydrothermal uranium deposits to date. Xiangshan uranium ore field, located in the southeast part of Gan-Hang Metallogenic (or Volcanic) Belt (GHMB), is the largest volcanic-related ore field in the whole country. In this study, the focus was placed on the petrographic characteristics and trace elements in hydrothermal chlorite from two typical deposits (Zoujiashan and Yunji) at Xiangshan. Four types of chlorites were identified, i.e., Chl1-Y and Chl2 from Yunji deposit, and Chl1-Z and Chl3 from Zoujiashan deposit. The pre-ore Chl1-Y and Chl1-Z are formed through replacing the original magmatic biotite. Chl2 and Chl3 occur as veinlets or disseminated, and are closely associated with early-ore U mineralization and main-ore U mineralization, respectively. All the four types of chlorites are typically trioctahedral chlorite. Vein-type/disseminated Chl2 and Chl3 in ore veins were precipitated directly from the hydrothermal fluids through dissolution-migration-precipitation mechanism, whereas the replacement-type chlorite was formed by the dissolution–crystallization mechanism. Empirical geothermometry indicates that the chlorite from Yunji and Zoujiashan were crystallized at 179~277 °C, indicating a mesothermal-epithermal precipitation environment. EPMA and LA-ICP-MS results show that the replacement-type chlorite has relatively consistent compositions at Yunji and Zoujiashan. Both Chl2 and Chl3 are enriched in U, Th but depleted in Mn and Ti. Compared with the Chl2 related to early-ore U mineralization, Chl3 that formed at main-ore stage has higher concentrations of Fe, U, Th, REEs, Mn and Ti, as well as higer Fe/(Fe + Mg) ratios. Such compositional differences between Chl2 and Chl3 are mainly attributed to the formation temperatures and fluid compositions/natures. Combined with petrology and chemical compositions of different types of chlorite, we propose that the presence of vein-type/disseminated chlorite with high U and Fe/(Fe + Mg) ratio but relatively low Mn, Ti and Pb contents can be regarded as an effective vector toward the most economic (high U grade) mineralized zone, whereas the occurrence of Chl2 is likely to indicate the subeconomic U mineralization and less potential exploration for uranium at depth.

Evolution and genesis of hydrothermal fluids for the Cretaceous Dongnan Cu deposit, Zijinshan ore district (SE China)

The Dongnan Cu(-Mo) deposit is a newly discovered porphyry-epithermal system located between the Zijinshan high-sulfidation (HS) epithermal deposit and Luoboling porphyry deposit, within the Zijinshan ore district, a large porphyry-epithermal Cu-Au-Mo-Ag ore system in southeast China. The deposit offers a window to investigate the origins of hydrothermal fluids in a coupled porphyry-HS epithermal magmatic-hydrothermal system.Drill hole logging and alteration mineral mapping has identified four hydrothermal alteration and mineralization stages at the Dongnan deposit, namely: stage I porphyry-type alteration, which can be subdivided into stage IA potassic alteration and stage IB phyllic alteration, stage II HS mineralization and associated alteration which can be subdivided into stage IIA-1 pyrophyllite-dominated, stage IIA-2 illite and illite-montmorillonite and stage IIB dickite-dominated alteration, stage III late veins (i.e., calcite and fluorite) and stage IV supergene alteration. Chalcopyrite and molybdenite mineralization is associated with Stage I, whereas covellite and digenite mineralization formed during Stage II. Stage IB minerals (i.e., muscovite) were largely superimposed by minerals from stage II (i.e., dickite, kaolinite, pyrophyllite and alunite). Stage II epithermal alteration is located mainly in the western part of Zijinshan ore district, whereas the eastern part is dominated by porphyry-type mineralization and alteration. Microthermometry results show the average Th and salinity of each sample suite from stage IB ranging from 341 ± 22 to 442 ± 2 °C and 1.6 ± 1.6 to 19.2 ± 2.6 wt% NaCl equiv. By contrast, stage III fluid inclusions are liquid-dominated, with low temperature of homogenization and low salinity (156 ± 9 to 165 ± 4 °C and < 0.5 wt% NaCl equiv.). Thermodynamic modelling of stage II suggest that the fluid responsible for epithermal mineralization was characterized by high temperature (270 to 320 °C), high water to rock (w/r > 1) and high SO2 to H2S ratios (SO2/H2S > 1). Because stage IIA pyrophyllite alteration is overprinted by late stage IIB dickite alteration and relatively different forming condition for pyrophyllite (>270 °C) and kaolinite (<270 °C and w/r > 1), stage IIA and stage IIB may have resulted from two different fluids. The Dongnan porphyry-type mineralization formed mainly in response to the cooling and dilution, led by the mixing of acidic magmatic water with meteoric water, whereas stage II alteration was more likely related to volatile gas escaping, resulting in pH decrease and contributing to the formation of pyrophyllite, dickite and alunite.The distribution of alteration minerals and evolution of hydrothermal fluids all indicate that the hydrothermal fluids for the porphyry-type and HS epithermal-type alteration at Dongnan came separately from nearby Luoboling granodiorite porphyry and the Zijinshan HS epithermal Cu-Au deposit, which implies that Luoboling porphyry deposit and Zijinshan HS deposit formed from separate magmatic-hydrothermal events. It suggests that multiple magmatic centers exist in the Zijinshan ore district, a finding which will contribute to both understanding of ore genesis of this overprinted porphyry-epithermal deposit and further mineral exploration in the Zijinshan Ore district.

Fluid Inclusion Study of Epithermal Quartz Veins from the Kyaukmyet Prospect, Monywa Copper-Gold Ore Field, Central Myanmar

The Kyaukmyet prospect is located near the main ore bodies of the Kyisintaung and Sabetaung high-sulfidation Cu-Au deposits, Monywa copper-gold ore field, central Myanmar. Lithologic units in the research area are of mainly rhyolite lava, lapilli tuff and silicified sandstone, mudstone and siltstone units of Magyigon Formation which hosted to be polymetallic mineralization. Our field study recorded that epithermal quartz veins are hosted largely in rhyolite lava and lapilli tuff units. Those quartz veins show crustiform, banded (colloform), lattice bladed texture and comb quartz. The main objectives of the present research in which fluid inclusion studies were considered to conduct the nature, characteristics and hydrothermal fluids evolution from the epithermal quartz veins. In this research, there are three main types of fluid inclusions are classified according to their phase relationship (1) two-phase liquid-rich inclusions, (2) the coexisting liquid-rich and vapor-rich inclusions, and (3) only vapor-rich inclusions. Microthermometric measurements of fluid inclusions yielded homogenization temperatures (Th) of 148–282 °C and final ice-melting temperature (Tm) of -0.2°C to -1.4°C . The value of (Tm) are equal to the salinities reaching up 0.35 to 2.07 wt % NaCl equiv. respectively. Estimation formation temperature of the quartz veins provide 190°C and 210°C and paleo-depth of formation are estimated to be between 130m and 210m. Petrography of fluid inclusion and microthermometric data suggest that fluid boiling as well as mixing processes were likely to be happened during the hydrothermal fluid evolution at the Kyaukmyet prospect. According to the characteristics of many parameters including petrography of fluid inclusion, microthermometric data, paleo-depth, evidence of quartz vein textures and types of hydrothermal alteration from the Kyaukmyet prospect allows to interpret these data to be the low-sulfidation epithermal system.

Origin of metasomatic fluids in the Bayan Obo rare-earth-element deposit

The Bayan Obo REE–Fe–Nb deposit is the largest light rare earth element (REE) deposit in the world, and it has undergone serious hydrothermal reworking processes. However, the origin and nature of metasomatic fluids in this deposit remain controversial. To determine the origin of metasomatic fluids, we conducted a detailed mineralogical and geochemical investigation on the apatite from the ∼ 1.8-km drill core in the Bayan Obo deposit. The textural and geochemical variations of the apatite could trace the hydrothermal metasomatism during REE mineralization. Four types of apatites were identified according to their cathodoluminescence characteristics, including turbid euhedral apatite (Ap-0), rounded pinkish–violet apatite veinlets (Ap-1), yellowish–green apatite aggregates (Ap-2), and apatite with a pinkish–violet core and a yellowish–green rim (Ap-3). According to the in-situ Sr isotopic compositions, these apatites were classified into two groups. Ap-0, Ap-1, and Ap-2 display constant and low 87Sr/86Sr ratios (0.7030–0.7035), whereas Ap-3 displays various Sr isotopic compositions (0.7047–0.7105). The in-situ Th–Pb age of the bastnäsite associated with the Ap-0 ranges from 390 to 425 Ma. The Sr isotopic compositions and geochronological study indicate that metasomatic fluids in the Bayan Obo deposit were obtained from different sources, including the Silurian Bainaimiao arc plutons and sedimentary materials. The four apatite types display various REE distribution patterns, which could account for hydrothermal fluid properties. F-rich and Na-poor fluids metasomatized the precursor Ap-0, releasing REEs from apatites and resulting in low REE contents of Ap-0. Ap-1 and Ap-2 were derived from the same fluid but were characterized by different trace element compositions, which could be attributed to the evolution of hydrothermal fluids with changes in the Na content and F/Cl ratio. The primary Ap-3 was derived from a REE-rich source and was altered by Na-poor and Cl-rich fluids. Metasomatic fluids with different origins and variable chemical compositions result in complex mineral assemblages and geochemical features in the Bayan Obo deposit.

Mineralogy, Fluid Inclusions, and Isotopic Study of the Kargah Cu-Pb Polymetallic Vein-Type Deposit, Kohistan Island Arc, Northern Pakistan: Implication for Ore Genesis

The Kargah Cu-Pb polymetallic deposit is a newly discovered ore deposit from the Gilgit-Baltistan region, located in the Kohistan Island Arc, northern Pakistan. However, this area is poorly researched on the ore genesis, and its origin and the evolution of its magmatic-hydrothermal system remain unclear. Three stages of mineralization were identified, including quartz-pyrite, quartz-sulfide, and carbonate representing early, middle, and late stages, respectively. The major ore minerals are pyrite, chalcopyrite, galena, and zincian tetrahedrite with minor native silver, and native gold mainly distributed in pyrite. Here, we present a systematic study on ore geology, hydrothermal alterations, trace element composition of pyrite, fluid inclusions, and isotopes (S and Pb) characteristics to gain insights into the nature of the ore-forming fluids, types of unknown deposits, and hydrothermal fluid evolution. The high Co/Ni ratio (1.3–16.4) and Co content (average 1201 ppm), the low Mo/Ni ratio (0.43–0.94) and Mo contents (average 108 ppm) of both Py-I and Py-II suggest a mafic source for the mineralization. The Au-Ni plots, Co-As-Ni correlation, and the δ34S values range from −2.8 to 6.4‰ (average of 3.4‰) indicating the affiliation of the mineralization with a mantle-derived magmatic-hydrothermal provenance. The Pb isotope data showing the narrow variations in 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values suggest a single lead source from crustal-derived materials. The microthermometry data suggest that the dominant mechanisms are fluid boiling and mixing for mineral precipitation at temperatures ranging between 155 and 555 °C, and represent an intrusion-related magmatic-hydrothermal environment for the Kargah Cu-Pb polymetallic deposit.

A Distal, High-grade Irish-type Orebody: Petrographic, Sulfur Isotope, and Sulfide Chemistry of the Island Pod Zn-Pb Orebody, Lisheen, Ireland

AbstractIrish-type Zn-Pb deposits are important global sources of zinc, but despite a fundamental understanding of ore genesis within the Irish orefield, a detailed understanding of fluid migration and chemical evolution pathways related to sulfide and carbonate precipitation is lacking. We present the first petrographic, paragenetically constrained sulfur isotope and mineral chemistry study of mineralization at the Island Pod orebody, Lisheen deposit. The Island Pod orebody comprises high-grade mineralization that is less deformed than elsewhere in the Irish orefield. Consequently, studies of the Island Pod orebody and its mineralization provide information on the evolving nature of hydrothermal fluids involved in ore deposition.The Island Pod orebody consists almost exclusively of pyrite, sphalerite, and galena, with several stages of calcite and dolomite precipitation. Pre-ore, diagenetic pyrite is commonly overgrown by early main ore-stage pyrite, with both phases frequently replaced by main ore-stage sphalerite. In many cases, early main ore-stage pyrite is texturally zoned and exhibits chemical zoning patterns, reflecting that episodic influxes of hydrothermal fluids contained variable concentrations of As, Co, Ni, and Tl. The main ore stage was dominated by the formation of sphalerite and galena from mineralizing fluids that were depleted in these trace elements (e.g., As, Co, Tl) compared to the early main ore stage. Sulfur isotope analysis reveals four distinctive but slightly overlapping isotopic groupings, corresponding to different mineral and paragenetic stages: (1) δ34S values range from –47.7 to –30.7‰, associated with diagenetic pyrite; (2) δ34S values range from –34.3 to –14.7‰, related to early main ore-stage pyrite; (3) δ34S values range from –15.5 to +1.7‰, corresponding to main ore-stage sphalerite; and (4) δ34S values range from –11.1 to +17.4‰, associated with galena. Large variations in S isotope composition are common at intragrain and at other small spatial scales. The textures, paragenetic sequence, and ranges in δ34S values are consistent with hydrothermal sulfide deposition where the fluids containing bacteriogenic sulfide mixed with metal-bearing fluids. Replacement and remobilization from other Lisheen orebodies may have contributed to some of the higher sulfur isotope ratios observed in the Island Pod orebody.The excellent preservation of sulfide textures in the Island Pod orebody observed during this study demonstrates that it is an ideal location to study hydrothermal fluid evolution, including episodic fluid flow, mixing, precipitation, and compositional variations during the early main ore stage. In other Irish Zn-Pb orebodies, these early-ore textures are often obscured due to more complex dissolution and replacement processes, making interpretation of the early hydrothermal activity challenging. Consequently, the petrographic, mineral chemistry, and sulfur isotope studies of the Island Pod orebody presented here contribute to an enhanced understanding of ore-forming processes in similar deposits, where mineralization is often associated with more complex deformation or repeated pulses of hydrothermal activity.

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

Neoproterozoic tin mineralization in South China was closely related to the highly-evolved peraluminous granites in the southeastern and western margins of the Yangtze Block. The Baotan (23 Mt @ 0.43 % Sn) is a largest tin deposit in the Jiuwandashan–Yuanbaoshan tin district, northern Guangxi, and the orebodies are mainly hosted in the Lower Neoproterozoic Sibao Group metasedimentary rocks and Early Neoproterozoic mafic rocks intruding the Sibao Group as well as the apex of the Pinying granitic pluton. This deposit shows widely important tourmaline alteration. Based on petrographic observations, four generations and eight occurrences of tourmalines are identified in the Baotan tin deposit. The first generation is late-magmatic tourmalines and occurs as disseminated (Tur1a) and tourmaline–quartz nodule (Tur1b) in the Pingying biotite granite. The pre-ore hydrothermal tourmalines (Tur2) occur as tourmaline–quartz veinlet in the biotite granite and can be subdivided into both the early Tur2a (second generation) and the late Tur2b (third generation) according to microscopic observation and BSE image. The fourth generation is syn-ore hydrothermal tourmalines (Tur3), and they occur as disseminated in mafic- (Tur3a) and metasedimentary (Tur3c) host rocks, or as vein-type tourmaline–quartz–cassiterite–chlorite ores in mafic- (Tur3b) and metasedimentary (Tur3d) host rocks.The different generations of tourmalines have both similarities and distinct differences in their compositions. Both the late-magmatic Tur1a and Tur1b show similar chemical compositions, with high Fe/(Fe + Mg) (0.73 ∼ 0.96, avg. 0.89) and Na/(Na + Ca) (0.87 ∼ 0.98, avg. 0.95) ratios, and Li, Zn, Ga, Nb and Ta contents, and belong to alkali group and schorl tourmaline. Based on mineral textural and compositional features and Mössbauer spectroscopic analyses, it can be speculated that the Tur1b in the tourmaline–quartz nodules and Tur1a are mainly sourced from a late-magmatic immiscible B–Na–Fe-rich hydrous melt in a reduced environment. The pre-ore Tur2a has similar SiO2, Al2O3, TiO2, Na2O, Li, Be, V, Sn and Zr contents, Fe/(Fe + Mg) and Na/(Na + Ca) ratios and total Al (apfu) with the late-magmatic Tur1a and Tur1b, indicating that it also originates from the Pingying granitic magma. The lower compatible elements (e.g., Fe, Mn, Co, Ni and Zn) and higher LILEs (e.g., Sr, Rb, Ba and Cs) in the pre-ore Tur2a and Tur2b suggest that these tourmalines precipitate from the early evolved magmatic hydrothermal fluids. The wide variations of Fe/(Fe + Mg) and Na/(Na + Ca) ranges, elevated Sr and V contents for the syn-ore Tu3a to Tur3d are attributed to the compositional differences of the host rocks and different degrees of interaction between the ore-forming fluids and the host rocks. The increasing Sr content from the late-magmatic Tur1 and pre-ore Tur2 to the syn-ore Tur3 suggests an involvement of the meteoric water. The Mössbauer spectral features of tourmalines from the late-magmatic Tur1 and syn-ore Tur3 confirm that they form under the different redox environments. The early fluids are relatively reduced magma-derived fluids. The addition of recycled meteoric water causes the fluids at the Tur3 stage to become relatively oxidized and also to be Sr-rich. The significant increase of Sn content from the late-magmatic Tur1 and pre-ore Tur2 to the syn-ore Tur3 also reflect a change from the early reduced fluid to the late oxidized fluid. The textural and compositional changes of Tur1 to Tur3 reflect the evolution of the ore-forming fluids. Fluid mixing between an acidic, reduced, Sn-rich magmatic hydrothermal fluid and cooler, oxidizing meteoric water and as well intensive water–rock reaction are considered as the main processes that cause extensive cassiterite precipitation.

Explorations on footprints of salt-rich fluid and salt-depleted fluid immiscibility in hydrothermal systems: Insights from divergent partitioning of sulfate and perchlorate in the ZnSO4–Zn(ClO4)2–H2O system

A variety of salt fluids (e.g., aqueous Li2SO4, ZnSO4 and UO2SO4 fluid) were observed to separate into a heavy salt-rich fluid (F1) and a light salt-depleted fluid (F2) when heated to appropriate temperatures. The F1–F2 immiscibility (FFI) likely provides an efficient pathway for hydrothermal transport and accumulation of certain ore elements and influences the compositional evolution of hydrothermal fluids, as it considerably concentrates these elements into the F1 phase. However, geological studies to date have paid little attention to the occurrence and influence of this kind of fluid immiscibility. The discrepancy may be explained from two perspectives. First, the complex pressure–temperature–composition (P–T–x) conditions of geological fluids may destabilize the FFI or suppress its occurrence. Alternatively, the FFI in geological fluids may be neglected during geologic investigations due to a lack of knowledge. To bridge the gap between experimental research and geological studies, this study aims to reveal more properties of the FFI and establish some geological constraints for discerning the FFI in geological systems. The partitioning behaviors of sulfate (FFI-relevant component) and perchlorate (FFI-irrelevant component) during the FFI and boiling process in the ZnSO4–Zn(ClO4)2–H2O system were quantified using Raman spectroscopic analyses integrated with the mass conservation law. Results show inconsistent partitioning behaviors of SO42− and ClO4− during the FFI; SO42− is highly concentrated into the F1 phase relative to the light F2 phase, while ClO4− partitions almost equally between the two phases. On the contrary, SO42− and ClO4− are concentrated proportionally in the residual liquid with boiling. Accordingly, some possible geological constraints, in addition to microthermometry, are postulated for the identification of the FFI in hydrothermal fluids, including (a) the P–T–x trajectory of the fluid intersecting with the FFI's stability region; (b) fluid inclusions showing divergent contents of FFI-relevant components but consistent concentrations of FFI-irrelevant components. This study sheds new light on the research of the salt-rich fluid and salt-depleted fluid immiscibility in geological systems.

Using garnet geochemistry discriminating different skarn mineralization systems: Perspective from Huangshaping W-Mo-Sn-Cu polymetallic deposit, South China

Important Mid-Late Jurassic W-Mo-Sn and Cu polymetallic skarn deposits are widely distributed in South China, yet differences in ore-forming processes and physiochemical conditions between these two skarn deposit types are poorly constrained. The large Huangshaping deposit in South China developed both W-Mo-Sn skarn (named 301 system) and Cu skarn (named 304 system) mineralization systems, which are genetically linked to granite porphyry and quartz porphyry, respectively. Both skarn systems are characterized by abundant garnet. Three types of garnet were distinguished in the 301 system, i.e., the texturally homogeneous Grt301a and veined Grt301b from deep-level (−176 m), and oscillatory-zoned Grt301c from shallow-level (−96 m). Moreover, Grt301a is commonly cut by Grt301b vein. Meanwhile, two types of garnet were identified in the 304 system, i.e., the texturally anisotropic Grt304a and oscillatory zoned Grt304b, with the former being commonly replaced by the latter.Trace element geochemistry shows that REE3+ entered garnet mainly by YAG-type substitution ([Ca2+]VIII −1[REE3+]VIII + 1[Si4+]IV −1[Z3+]IV + 1) in the 301 system and menzerite substitution ([Ca2+]VIII −1[REE3+]VIII + 1[Al3+]VI −1[Y2+]VI + 1) in the 304 system. Grt301a, Grt301b and Grt304a are HREE-enriched, suggesting that they were formed under nearly-neutral pH and low water/rock (W/R) ratios in closed and stable systems via diffusive metasomatism. Meanwhile, Grt301c and Grt304b are LREE-enriched, suggesting that they were formed under mildly-acidic pH and high W/R ratios in open and fluctuating systems via infiltration metasomatism. The elevated Sn content in Grt301c (median 5039 ppm) indicates its higher oxygen fugacity (fO2) than Grt301a (median 1861 ppm) and Grt301b (median 1776 ppm). Similarly, the high Sn content in Grt304b (median 1444 ppm) also suggests higher fO2 than that of Grt304a (median 338 ppm). The different formation conditions between deep-level garnet (Grt301a and Grt301b) and shallow-level garnet (Grt301c) in the 301 system are not consistent with garnet in the 304 system that shows similar texture, trace element geochemistry and formation conditions at different depths. This reveals differences of formation environments between two skarn systems at Huangshaping. Partial least squares-discriminant analysis (PLS-DA) suggests that W in garnet can discriminate the 301 and 304 skarn systems, which may be related to the lower fO2 and higher fractionation of the ore-causative granite porphyry in the former, which favors W enrichment in corresponding magmatic-hydrothermal fluids in 301 system. Moreover, Mo can discriminate garnet from the deep- and shallow-levels of the 301 system, which may be related to the higher fO2 in the latter. We highlight the importance of garnet trace element geochemistry on tracing hydrothermal fluid evolution and discriminating between different skarn systems.

Genesis and hydrothermal evolution of the Zhazigou skarn W (Mo) deposit, East Qinling, China: Constraints from fluid inclusions and H–O–S–Pb isotopes

The Zhazigou is a typical redox-intermediate skarn W (Mo) deposit, located in the Luanchuan ore district within the East Qinling orogenic belt, central China. Skarn and W (Mo) orebodies are hosted mainly in the contact zones between the Yuku porphyritic granite and carbonaceous sedimentary rocks of the Luanchuan and Guandaokou groups. This paper presents detailed fluid inclusion (FI) and isotope data that are used to reconstruct the hydrothermal processes and explore the role of CO2 during scheelite precipitation at the Zhazigou. Four mineralization and alteration stages are recognized: anhydrous skarn (I), hydrous skarn (II), quartz–sulfide (III), and carbonate (IV). Systematic analysis of the FIs elucidates the evolution of hydrothermal fluids through these stages. Four primary types of FIs are recognized: two-phase liquid-rich inclusions (L-type), two-phase vapor-rich inclusions (V-type), halite-bearing (hypersaline) inclusions (H-type), and two- or three-phase CO2-bearing inclusions (C-type). In stage I, coexisting H-type (36.19–46.37 wt% NaCl equiv.) and low salinity V-type FIs have similar homogenization temperatures, indicating that fluid boiling during the formation of anhydrous skarn minerals occurred at 500–550 °C and 550–700 bar (2.0–2.5 km lithostatic depth). In stage II, coexisting V- and L-type FIs in quartz and scheelite within quartz–scheelite ± actinolite ± tremolite veins have different salinities but similar homogenization temperatures, indicating they were also trapped in the boiling process at 350–380 °C and ~240 bar (1.8–2.8 km hydrostatic depth). In stage III, L-, V-, and minor H-type FIs have homogenization temperatures indicating continued fluid boiling at 300–340 °C and <100–200 bar (1.1–1.8 km hydrostatic depth). In stage IV, only minor primary L-type FIs were observed in quartz and calcite, which have lower salinities (0.88–11.34 wt% NaCl equiv.) and homogenization temperatures of 150–200 °C, reflecting typical post-ore conditions. The O and H isotope data from garnet, scheelite, and quartz indicate that the ore-forming fluids at the Zhazigou deposit have a dominantly magmatic water signature, with δ18Ofluid values from 7.9 to 1.8 ‰ and δ18Dfluid values from −99.1 to −84.4 ‰. The S isotopic compositions, which range from 1.8 to 5.6 ‰, and the relatively homogeneous Pb isotopic compositions, demonstrate that the ore-forming materials were dominantly derived from magmatic sources with only a minor contribution from sedimentary sources. Scheelite was precipitated as a result of fluid–rock interactions and/or fluid boiling, and the former released abundant Ca2+ by altering calcium-rich minerals (e.g., andradite, diopside, hedenbergite, and plagioclase) into the mineralizing fluids. Abundant C-type FIs occur in quartz within the hydrous skarn stage, providing evidence of CO2-rich fluids that would have decrease the pH and generated relatively acidic conditions to help precipitate scheelite.

Hydrothermal fluid evolution in the Escondida porphyry copper deposit, northern Chile: evidence from SEM-CL imaging of quartz veins and LA-ICP-MS of fluid inclusions

The origin of hypogene alteration and mineralization features in the Escondida porphyry Cu deposit resulted from intense overprinting related to three main hydrothermal stages. The beginning of each stage is recorded by deposition of bright quartz crystals on vein walls that precipitated from high-temperature fluids. In the deepest zones of the deposit, the earliest stage started with exsolution of intermediate-density fluids, which transported high concentrations of Cu, whereas Mo was not detected. However, in the shallow zones, the early stage began with depressurization and unmixing of intermediate-density fluids, which generated both a hypersaline and vapour-rich fluid phase, producing an important geochemical segregation between both phases. The transitional stage also started with circulation of intermediate-density fluids but never experienced unmixing. These fluids transported the highest Mo concentrations in the deposit; however, Cu displays lower concentrations relative to intermediate-density fluids from the early stage. The beginning of the late stage was also associated with intermediate-density fluids; however, Cu and Mo were below the detection limits in most of the analysed fluids. During the evolution of the three stages, the fluids experienced gradual cooling, which promoted the precipitation of euhedral and zoned quartz crystals that overgrew the early high-luminescence quartz generations. Sometimes, sulfide minerals display euhedral crystal boundaries with zoned quartz, suggesting that mineralization started during these phases. The latest events detected in each main stage are linked to cooler and low-salinity fluids, from which dark quartz and hypogene sulfides precipitated along microfractures and interstitial spaces developed in the earlier quartz generations.

Fluid Inclusion Study of Epithermal Gold-Base Metal Mineralization System in the Shwebontha Prospect, Monywa Mining District, Central Myanmar Monywa

The Shwebontha Prospect area is one of prominent epithermal Au-Ag prospects in Monywa mining district, central Myanmar, characterized by the appearance of gold-bearing and base metal quartz veins with gold grade is around 3g/t -10.4g/t. The geology of the area consists of the volcanic and volcaniclastic rocks of Upper Oligocene-Middle Miocene Magyigon Formation that served as the host rock of the ore mineralization. This research focused on fluid inclusion study is aimed to know the characteristics of hydrothermal fluids during ore mineralization as well as the possible paleo- depth and temperature of formation of gold-bearing and base metal quartz veins. The mineralization styles are gold-bearing brecciated quartz veins and chalcedonic quartz veins where sulfides are clustered as well as disseminated both in quartz gangue and volcanic host rocks. Those quartz veins include pyrite, sphalerite, galena, chalcopyrite and gold (electrum). Fluid inclusion microthermometry indicates that the ore mineralization is characterized by the values of homogenization temperature range from 158°C to 310°C and salinities range from 0.35 to 2.41wt.% NaCl equiv. This temperature is consistent with the formation temperature of 250°C to 270 °C and also their estimate paleo-depth of formation is between 440m and 640m respectively. Microthermometric data indicates that fluid mixing and dilution were significant processes during ore mineralization and evolution of hydrothermal fluids. Based on the petrography of fluid inclusion, microthermometric measurements and ore minerals assemblage as well as estimation of paleo-depth from the Shwebontha Prospect imply that forming in under shallow level epithermal environment

Major and trace elements analyses of garnet from the Haobugao Zn-Fe-Sn polymetallic deposit, northeast China: Implications for skarn formation and hydrothermal fluid evolution

The Haobugao Zn-Fe-Sn skarn deposit in the Southern Great Xing’an Range of NE China contains three generations of garnets that were distinguished using petrography. This paper discusses skarn formation and hydrothermal fluid evolution in the Zn-Fe-Sn skarn system based on the interpretation of diverse textures and compositions of garnets. Crystals of first-generation garnet (Grt 1) from the proximal exoskarn are red-brown, coarse-grained garnets that are subhedral and optically isotropic. They are dominantly grossular (Adr49-18Grs51-82) and have the highest REE contents of the garnets at Haobugao. They are LREE-depleted, HREE-enriched, with negative Eu anomalies, and they formed under low water/rock (W/R) ratios and neutral pH conditions. Crystals of second-generation garnet (Grt 2) from the central exoskarn are yellow-green and andradite dominated (Adr63-97Grs2-36). They have relatively isotropic cores with chaotic anisotropism (Grt 2a) or oscillatory zoned rims (Grt 2b). The Grt 2 cores have low REE contents, are LREE-enriched, HREE-depleted, with positive Eu anomalies, and formed under high W/R ratios and acidic conditions. The Grt 2 rims have higher REE contents, are LREE-depleted, HREE-enriched, with weak negative Eu anomalies, and formed under low W/R ratios and acidic condition. Crystals of third-generation garnet (Grt 3) coexist with pyroxene and are light yellow–brown, zoned or optically homogeneous. They are relatively Fe-rich grossular (Adr49-51Grs48-50), compared with Grt 1. They have the lowest REE contents of the garnets at Haobugao, and a REE pattern similar to that of the Grt 1, which indicates they formed under low W/R ratios, reduced to oxidized and neutral pH conditions. The REE content and fractionation of garnets indicate periodic fluctuations in fluid composition occurred in the Haobugao Zn-Fe-Sn skarn system, likely caused by the joint effects of magmatic and meteoric water. Diverse mechanisms contributed to the ΣREE variation in garnets.

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.

Texture, trace elements, sulfur and He-Ar isotopes in pyrite: Implication for ore-forming processes and fluid source of the Guoluolongwa gold deposit, East Kunlun metallogenic belt

The Gouli goldfield is the most important gold producers of the East Kunlun metallogenic belt. Gold mineralization in this region is enigmatic, primarily because of uncertainties as to the mineralization processes, and the nature and sources of ore-forming fluids. Here, we investigate the ore-forming fluids and processes through a comprehensive study of the texture and geochemistry (LA-(MC)-ICPMS trace element analysis, mapping, S isotopes, and He-Ar isotopes) of pyrite from the Guoluolongwa gold deposit, the largest deposit in the Gouli goldfield. Four types of pyrite with distinct textures, trace element contents and sulfur isotopes were identified, and provide evidence for a multi-stage hydrothermal fluid evolution. The sedimentary-diagenetic Py0, representing the pre-ore stage, is characterized by high contents of Co and Ni, and negative δ34S values (−11.6‰ to −4.0‰), which are notably different from the subsequent hydrothermal Py1 to Py3. Hydrothermal Py1, comprising subtypes Py1a and Py1b, was deposited in pyrite-quartz veins during stage I. The Py1a subtype shows characteristic Co-Ni-As oscillatory zoning that replaced Py0 via dissolution-reprecipitation, and hence inherited high concentrations of Co and Ni from Py0. In contrast, the Py1b subtype was devoid of Py0 influence and represented the initial ore-forming fluid with low concentrations of Co and Ni, moderate Cu, Pb, Zn, Sb, and Bi, but high As, Au, and Ag. The next stage, Py2, formed in the quartz-pyrite veins during stage II and showed clear textural and compositional contrasts to Py1. Specifically, Py2 has elevated Cu, Pb, Zn, Ag, and Bi contents and somewhat lower δ34S values (+1.5‰ to +3.2‰) compared to Py1 (+1.6‰ to +5.6‰). In addition, Py2 shows irregular compositional zones, which we attribute to fluid phase separation caused by pressure fluctuation. Abundant Py2 precipitation triggered native gold deposition due to desulfidation of the ore-forming fluid. The next stage, Py3 in polymetallic sulfides-calcite-quartz veins at stage III, implies a new pulse of Cu-Pb-Zn-As-Au-Ag-rich and Bi-poor ore-forming fluid. This latter pyrite is characterized by inclusion-rich cores (Py3c), which contain abundant inclusions of chalcopyrite, galena, sphalerite, and native gold, and inclusion-free rims (Py3r). The cores, Py3c, likely represent abrupt destabilization of metal complexes and rapid precipitation during vigorous phase separation, whereas the euhedral Py3r rims formed under steadier physico-chemical conditions, contributing to lower δ34S values of Py3c (+0.5‰ to +3.0‰) than Py3r (+1.9‰ to +3.6‰). Noble gas isotopes of Py1-Py3 have crust-dominated 3He/4He (0.04–0.4 Ra) and 40Ar/36Ar (623–2081), mixed with minor mantle components. Sulfur isotopes and low Co and Ni contents of hydrothermal pyrite, and He-Ar isotopes of fluid inclusions in pyrite, are consistent with a model in which multiple pulses of ore-forming fluids were exsolved during sequential episodes of primarily crustal-derived felsic magmatism with lesser mantle influences. This model is also supported by previous H-O isotope analyses, as well as widespread granitoid magmatism in the eastern Central East Kunlun Belt.

Apatite Texture, Composition, and O-Sr-Nd Isotope Signatures Record Magmatic and Hydrothermal Fluid Characteristics at the Black Mountain Porphyry Deposit, Philippines

AbstractThe trace elemental and isotopic signatures in apatite can be modified during hydrothermal alteration. This study investigates the suitability of apatite as an indicator of the source, chemistry, and evolution of magma and hydrothermal fluids. In situ textural, elemental, and O-Sr-Nd isotope analyses were performed on apatite in thin sections, from fresh and propylitically altered pre- and synmineralized dioritic porphyries from the Black Mountain porphyry Cu deposit in the Philippines.All studied apatite crystals have similar subhedral to euhedral shapes and are homogeneous in the grayscale in backscattered electron images. In cathodoluminescence images, the apatite in fresh and altered rocks displays yellow to yellow-green and green to brown luminescence, respectively. Apatite in fresh rocks has a higher Cl and Mn content, and lower Fe, Mg, Sr, Pb, and calculated XOH-apatite, compared to apatite in altered rocks. The content of F, rare earth elements (REEs), Y, U, Th, and Zr, and the Sr-Nd isotope signatures of apatite from fresh and altered rocks are similar in all apatite grains (87Sr/86Sr = 0.7034–0.7042 vs. 0.7032–0.7043, εNd(t) = 5.3–8.0 vs. 5.1–8.4). The X-ray maps and elemental and oxygen isotope signatures across individual apatite crystals are typically homogeneous in apatite from both fresh and altered rocks. The distinct luminescence colors, coupled with distinct mobile element compositions (Cl, OH, Mn, Mg, Fe, Sr, Pb), indicate modification of primary magmatic apatite during interaction with hydrothermal fluids. The similarities in Sr isotope ratios (87Sr/86Sr = 0.7032–0.7043) but slight differences in O isotope signatures (δ18O = 6.0 ± 0.3‰ vs. 6.6 ± 0.3‰) in apatite from fresh and altered rocks are consistent with the magma and hydrothermal fluids having the same source and suggest significant phase separation in the hydrothermal fluids given that 18O preferentially fractionates into the residual liquid relative to 16O during phase separation. The similarity of immobile element (REE, Y, U, Th, and Zr) contents in both populations of apatite, consistency of textures and Nd isotope compositions, and absence of obvious dissolution-reprecipitation features all suggest that altered apatite retains some magmatic characteristics. The apatite in fresh rocks has oxygen isotope compositions similar to that of zircons from the same sample (δ18O = 5.9 ± 0.3‰), indicating little to no oxygen isotope fractionation between zircon and apatite and that apatite can be a good proxy for the oxygen isotope composition of the magma. Based on the Cl contents of the magmatic and replacement apatite, and assuming their equilibrium with high-temperature magma fluid and replacement hydrothermal fluid, respectively, the calculated Cl content of the early magmatic fluid and the later replacement fluid can be estimated to be 6.4 to 15.1 wt % and ~0.25 ± 0.03 wt %, respectively. This indicates a depletion of Cl from the early high-temperature fluid to the replacement fluid, consistent with phase separation.This study demonstrates that cathodoluminescence, elemental compositions (such as Cl, Mn, Mg, Fe, Sr, Pb) and Sr-O isotope signatures in apatite can be modified during hydrothermal alteration, whereas other components (REE, Y, U, Th, and Zr) and the Nd isotope composition are preserved. These features can be used to constrain the origin, chemistry, and evolution of the primary magma and ore-forming hydrothermal fluids.

Integration of stable isotopes, radiometric dating and microthermometry of saddle dolomite and host dolostones (Cretaceous carbonates, Kurdistan, Iraq): New insights into hydrothermal dolomitization

This paper integrates Mg-, C-, O-, Sr-isotopes, fluid-inclusions microthermometry, and radiometric dating of dolostones and vug-filling saddle dolomite and calcite of the Upper Cretaceous Bekhme Formation, the Kurdistan Region of Iraq, provide new insights into the origin, timing and geochemical evolution of hydrothermal dolomitizing fluids. Similar isotopic compositions and fluid-inclusion microthermometry of the dolostones and saddle dolomite suggest formation by similar fluids. The U–Pb ages of dolostones and saddle dolomite suggest that dolomitization (74.8 Ma) was accomplished in 300 Ka at near Earth's surface by flow of hydrothermal fluids followed by dolostone dissolution and precipitation of vug-filling saddle dolomite (73.8 Ma). Mg isotopic compositions of the dolomites (26MgDSM3=−0.81‰ to −0.76‰) are interpreted to indicate derivation of Mg from marine connate waters entrapped in overpressured basinal mudstones. Relatively high Sr-isotope ratios of the dolomites precludes the role of ophiolites as an important source of Mg in the basin.

Chemical and boron isotopic compositions of tourmaline at the Dachang Sn-polymetallic ore district in South China: Constraints on the origin and evolution of hydrothermal fluids

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.

Petrography, geochemistry and tectonic setting of adakitic bodies in the Tighanab area and their relationship with iron skarn mineralization (southeast of Sarbisheh-east of Iran)

Petrography, geochemistry and tectonic setting of adakitic bodies in the Tighanab area and their relationship with iron skarn mineralization (southeast of Sarbisheh-east of Iran)