Isotopic Compositions Of Fluids Research Articles

Lithium isotope compositions of U.S. coals and source rocks: Potential tracer of hydrocarbons

Kerogen in organic-rich rocks contains trace amounts of lithium (Li) that has been overlooked as a contributor to the global Li geochemical cycle. This study examined a variety of coals where kerogen is concentrated (>50% organic carbon) and hydrocarbon source rocks of different ages, depositional environments and thermal maturity to determine their range of Li isotopic compositions (δ7Li‰) and factors that influence their compositions.Using Secondary Ion Mass Spectrometry (SIMS), we analyzed 22 coals and 4 hydrocarbon source rocks (Types I, II, III), to determine the δ7Li of kerogen in situ, without chemical isolation of phases that can alter their original isotopic compositions. The δ7Li values of the coals surveyed are distinctly isotopically light (<0‰) compared to most natural minerals and fluids. In immature coals, with a vitrinite reflectance in oil (VRo) of ≤0.5%, kerogen δ7Li values average –23.4 ± 1.1‰ and become heavier with increasing thermal grade to temperatures of gas generation (VRo ~1.3%). The linear correlation between δ7Li and VRo suggests that 6Li may be preferentially released to pore fluid from kerogen during thermal maturation. Notably, authigenic clays forming at diagenetic temperatures substitute Li from pore fluids into silicate layers, therefore, the Li isotopic composition of the clays may record fluid isotopic compositions influenced by organic-Li sources.NanoSIMS isotopic maps of the Lower Bakken Shale, and SIMS measurements of the Green River Shale show isotopically light Li associated with C-dominated areas, and heavier δ7Li with Si-dominated areas of the hydrocarbon source rocks. We conclude that kerogen is a source of isotopically light Li that contributes to fluids during thermal maturation and hydrocarbon generation. Kerogen may be a significant contributor of Li to pore fluids and its distinctly light Li isotopic composition relative to other terrestrial waters and minerals could provide a tracer of organic inputs to the global geochemical cycle.

Geology and genesis of the Early Paleozoic Zhengguang intermediate-sulfidation epithermal Au-Zn deposit, northeast China

Zhengguang is a partly eroded (about ≥500 m being eroded), structurally controlled intermediate sulfidation (IS) epithermal gold-zinc deposit, characterized by wide distribution of base metal sulfides, Fe-poor sphalerite and Mn-calcite, hosted by the Late Cambrian-Early Ordovician basaltic-andesitic volcanic and volcano-sedimentary rocks in northeast China. The deposit is located 4 and 8 km from the Early Ordovician Tongshan (including several IS veins about ~500 m away in the east called East Tongshan) and Duobaoshan porphyry copper deposits, respectively. A 484.8 ± 8.7 Ma (MSWD = 198) of Re-Os isochron age on pyrite indicates that it is the most ancient intermediate sulfidation deposit that has been dated.Four stages of veins have been discerned at Zhengguang. Early magnetite ± chalcopyrite ± epidote ± chlorite and K-feldspar ± pyrite ± chalcopyrite veins (stage I) were associated with restricted K-feldspar ± epidote (locally with chlorite) alteration and propylitic (epidote ± chlorite ± calcite) assemblages. Two primary epithermal gold stages developed at Zhengguang: pyrite-quartz ± Fe-poor sphalerite ± chalcopyrite ± galena ± electrum ± petzite ± hessite ± tetrahedrite/tennantite ± adularia veins (stage II) and (Mn-)calcite-quartz-pyrite ± Fe-poor sphalerite ± chalcopyrite ± galena ± hessite ± native gold (stage III), plus a late barren stage characterized by little sulfides (calcite ± chlorite ± hematite ± quartz, stage IV). Gold occurs chiefly as micro-inclusions of electrum and petzite in pyrite and sphalerite of stages II and III.Stable isotope analyses and fluid inclusion results indicate a magmatic-hydrothermal component in the ore fluids. Fluid inclusion data from stage I yielded the highest salinity (∼6.9 wt% NaCl) and homogenization temperature (average 264 °C). Calculated oxygen isotope composition of fluid (δ18Owater up to 4.9‰) equilibrated with quartz indicates a magmatic-hydrothermal source during stage I. A progressive addition of meteoric water component is suggested from stage I to IV of Zhengguang based on the average δ18Owater values equilibrated with quartz and homogenization temperatures (stage I: δ18Owater = 3.3‰, Th = 264 °C; stage II: δ18Owater = −1.4‰, Th = 182 °C; stage III: δ18Owater = −3.4‰, Th = 154 °C; stage IV: δ18Owater = −6.3‰, Th = 121 °C).Our new pyrite Re-Os dating result shows that Zhengguang formed at the same time with Tongshan and Duobaoshan within errors. Comparable sulfur isotopic compositions of pyrite from Zhengguang and Tongshan, analogous low-Fe content in sphalerite and similar mineral assemblage and alteration pattern between Zhengguang and East Tongshan IS veins collectively indicate a genetic connection between Zhengguang and Tongshan, which may derive from fluids exsolved from the same magma chamber. High-temperature assemblages (e.g., K-feldspr ± pyrite ± chalcopyrite) at Zhengguang indicate that there is potential for the discovery of porphyry copper–gold mineralization below the current level of diamond drilling. The confirmation of the Zhengguang as an Early Paleozoic epithermal deposit implies a good preservation and exploration potential of ancient epithermal deposits in eastern Central Asian Orogenic Belt.

Reconstruction of an Early Permian, Sublacustrine Magmatic-Hydrothermal System: Mount Carlton Epithermal Au-Ag-Cu Deposit, Northeastern Australia

Abstract The Mt. Carlton Au-Ag-Cu deposit, northern Bowen basin, northeastern Australia, is an uncommon example of a sublacustrine hydrothermal system containing economic high-sulfidation epithermal mineralization. The deposit formed in the early Permian and comprises vein- and hydrothermal breccia-hosted Au-Cu mineralization within a massive rhyodacite porphyry (V2 open pit) and stratabound Ag-barite mineralization within volcano-lacustrine sedimentary rocks (A39 open pit). These orebodies are all associated with extensive advanced argillic alteration of the volcanic host rocks. Stable isotope data for disseminated alunite (δ34S = 6.3–29.2‰; δ18OSO4 = –0.1 to 9.8‰; δ18OOH = –15.3 to –3.4‰; δD = –102 to –79‰) and pyrite (δ34S = –8.8 to –2.7‰), and void-filling anhydrite (δ34S = 17.2–19.2‰; δ18OSO4 = 1.8–5.7‰), suggest that early advanced argillic alteration formed within a magmatic-hydrothermal system. The ascending magmatic vapor (δ34SΣS ≈ –1.3‰) was absorbed by meteoric water (~50–60% meteoric component), producing an acidic (pH ≈ 1) condensate that formed a silicic → quartz-alunite → quartz-dickite-kaolinite zoned alteration halo with increasing distance from feeder structures. The oxygen and hydrogen isotope compositions of alunite-forming fluids at Mt. Carlton are lighter than those documented at similar deposits elsewhere, probably due to the high paleolatitude (~S60°) of northeastern Australia in the early Permian. Veins of coarse-grained, banded plumose alunite (δ34S = 0.4– 7.0‰; δ18OSO4 = 2.3–6.0‰; δ18OOH = –10.3 to –2.9‰; δD = –106 to –93‰) formed within feeder structures during the final stages of advanced argillic alteration. Epithermal mineralization was deposited subsequently, initially as fracture- and fissure-filling, Au-Cu–rich assemblages within feeder structures at depth. As the mineralizing fluids discharged into lakes, they produced syngenetic Ag-barite ore. Isotope data for ore-related sulfides and sulfosalts (δ34S = –15.0 to –3.0‰) and barite (δ34S = 22.3–23.8‰; δ18OSO4 = –0.2 to 1.3‰), and microthermometric data for primary fluid inclusions in barite (Th = 116°– 233°C; 0.0–1.7 wt % NaCl), are consistent with metal deposition at temperatures of ~200 ± 40°C (for Au-Cu mineralization in V2 pit) and ~150 ± 30°C (Ag mineralization in A39 pit) from a low-salinity, sulfur- and metal-rich magmatic-hydrothermal liquid that mixed with vapor-heated meteoric water. The mineralizing fluids initially had a high-sulfidation state, producing enargite-dominated ore with associated silicification of the early-altered wall rock. With time, the fluids evolved to an intermediate-sulfidation state, depositing sphalerite- and tennantite-dominated ore mineral assemblages. Void-filling massive dickite (δ18O = –1.1 to 2.1‰; δD = –121 to –103‰) with pyrite was deposited from an increasingly diluted magmatic-hydrothermal liquid (≥70% meteoric component) exsolved from a progressively degassed magma. Gypsum (δ34S = 11.4–19.2‰; δ18OSO4 = 0.5–3.4‰) occurs in veins within postmineralization faults and fracture networks, likely derived from early anhydrite that was dissolved by circulating meteoric water during extensional deformation. This process may explain the apparent scarcity of hypogene anhydrite in lithocaps elsewhere. While the Mt. Carlton system is similar to those that form subaerial high-sulfidation epithermal deposits, it also shares several key characteristics with magmatic-hydrothermal systems that form base and precious metal mineralization in shallow-submarine volcanic arc and back-arc settings. The lacustrine paleosurface features documented at Mt. Carlton may be useful as exploration indicators for concealed epithermal mineralization in similar extensional terranes elsewhere.

Silicon isotope composition of subduction zone fluids as recorded by jadeitites from Myanmar

Silicon is one of the major elements in the solute of subduction zone fluids. The source information of Si in the fluids is useful for constraining the material exchange between the oceanic crust and mantle. Myanmar jadeitites result from crystallization of subduction zone fluids at high pressures, which can be used as a proxy for the chemical composition of the fluids. This paper reports high-precision Si isotope data of jadeitites, and associated amphibole-rich blackwall, serpentinites, and mica quartz schists to constrain the Si isotope composition of subduction zone fluids. The white jadeitites have higher δ30Si (− 0.04‰ to 0.23‰) than any igneous rocks reported in literature. The δ30Si of green jadeitites (− 0.35‰ to 0.03‰) and amphibole-rich blackwall (− 0.33‰ to 0.05‰) are generally lower than the values of white jadeitites. These data indicate that the jadeitite-forming fluids in the source subduction zone have heavy Si isotope compositions relative to the bulk silicate Earth. Based on the Si isotope fractionations among quartz, white jadeite, and the fluid, it is inferred that the subduction zone fluids have high δ30Si of about 0.7‰ to 1.2‰. The δ30Si of jadeitites and amphibole-rich blackwall show a positive correlation with SiO2 content with a slope of 0.0639, larger than that of the igneous rock array (0.0056). Such a correlation cannot be produced by magmatic differentiation or mineral fractional crystallization from fluids. Instead, it is more likely explained by a binary mixing model between fluids and mafic–ultramafic rocks. The model shows that about 0 to 25% of Si in green jadeitites and amphibole-rich blackwall is from the subduction zone fluids. In light of the relatively low δ30Si of abyssal clay sediment, altered oceanic crust, and mantle serpentinite, it is proposed that the deep-sea siliceous rocks with high δ30Si are likely the main source of Si in subduction zone fluids. In summary, this study shows that Si isotopes have useful application in tracing the source of solute in fluid-mediated processes.

The behavior of Fe and S isotopes in porphyry copper systems: Constraints from the Tongshankou Cu-Mo deposit, Eastern China

Iron isotopes have been widely applied to trace mineralization processes of magmatic-hydrothermal ore systems, yet the application of Fe isotopes in porphyry Cu deposits (PCDs) is still restricted by the lack of understanding of its behaviors during hydrothermal alteration and ore formation processes. In this study, Fe isotope systematics in whole-rocks and Fe-bearing minerals coupled with S isotope compositions of Fe-sulfides are investigated for the early Cretaceous Tongshankou porphyry Cu-Mo deposit from Eastern China. The results show that altered ore-hosting granodiorite porphyries display a small δ56Fe variation (0.04‰ to 0.17‰) with an average of 0.10 ± 0.08‰ (2SD, n = 10), indistinguishable from the values for global igneous rocks. The δ56Fe of magnetite (0.22‰) and biotite (-0.08‰ to 0.12‰) from the altered porphyries are also within the δ56Fe ranges of those from global igneous rocks. These results indicate that hydrothermal fluids have negligible effect on Fe isotope compositions of the porphyry host rocks, likely due to the insignificant contributions of fluid-derived iron relative to the total Fe budget of the host rocks.The Fe-sulfides in the porphyry system record geochemical and iron isotopic characteristics of the ore-forming fluids. Specifically, chalcopyrites show large Fe isotope variations with δ56Fe ranging from −0.60‰ to 0.61‰. The calculated Fe isotope compositions of ore-forming fluids based on the chalcopyrite δ56Fe are thus not uniform and fall in the range from −0.69 ± 0.18‰ to 0.52 ± 0.18‰ (2SD). On the other hand, pyrites exhibit a δ56Fe variation between −0.48‰ and 0.40‰ and show distinct Fe isotopic characteristics for disseminated and vein-type pyrite. The δ56Fe for disseminated pyrites are consistently higher than whole-rock values, ranging from 0.14‰ to 0.40‰, likely reflecting equilibrium Fe isotopic exchange between pyrite and dissolved Fe in fluid. By contrast, δ56Fe for pyrites in veins are highly variable (−0.48‰ to 0.34‰) and show an inverse correlation with the chalcopyrite δ56Fe. These observations, in concert with S isotope data, have been interpreted to reflect pyrite formation through the FeS pathway, whereas the wide range in δ56Fe for pyrite may record isotopic change from initial FeS-fluid equilibrium towards pyrite-fluid equilibrium due to different extent of reaction upon precipitation. Additionally, when compared to sulfides formed by non-magmatic, low-temperature processes, the sulfides in our study exhibit a relatively narrow range of δ56Fe and δ34S, implying a high-temperature magmatic origin for both elements. Overall, our study demonstrates that the coupling of Fe and S isotopes could be a useful tool for tracking the processes of sulfide mineralization in porphyry-related deposits.

Magmatic-Hydrothermal Gold Mineralization at the Lone Tree Mine, Battle Mountain District, Nevada

Abstract The Lone Tree deposit is located in the northern Battle Mountain mining district, Nevada. Prior to mine closure in 2006, Santa Fe Pacific Gold and Newmont produced 4.2 Moz of gold at an average grade of 2.06 g/t at Lone Tree, primarily from the N-S– to NNW-SSE–striking Wayne zone. The ore is located between the Roberts Mountain and Golconda thrusts in siliciclastic rocks of the Ordovician Valmy Formation and in the Pennsylvanian-Permian Battle Mountain and Edna Mountain Formations, and above the Golconda thrust in siliciclastic and carbonate rocks of the Mississippian to Permian Havallah sequence. Ore is also hosted by rhyolitic dikes that were emplaced at 40.95 ± 0.06 Ma based on zircon U-Pb chemical abrasion-thermal ionization mass spectrometry. The gold is associated with sericitic and argillic alteration of the siliciclastic rocks and dikes and with decarbonatization and Fe carbonate alteration of the carbonate-bearing units, as well as in Fe-As sulfide and finegrained quartz alteration of all rock types. Oxidation affects 30 to 45% of the deposit, penetrating into the stratigraphy along numerous steeply dipping north-south, east-west, and north-northeast–south-southwest structures. Gold is positively correlated with Ag, As, Hg, and Sb. The highest Au grades occur in quartz-sulfide ore hosted in siliciclastic and carbonate sedimentary rocks and rhyolitic intrusions. In this ore style, fine-grained quartz and sericite are intergrown with disseminated sulfide minerals (quartz-sericite-pyrite alteration), constituting cores of weakly mineralized pyrite or marcasite, which are surrounded by fuzzy arsenopyrite rims that contain up to ~2,000 ppm Au. Low gold grades occur in late-stage banded pyrite breccias consisting of a finely zoned Au-poor pyrite matrix surrounding jigsaw-fit clasts of quartz-, illite-, barite-, and adularia-altered siliciclastic rock. The timing of main-stage mineralization is bracketed between the emplacement of the dikes and an adularia 40Ar/39Ar age of 40.14 ± 0.74 Ma. Sericite intergrown with arsenopyrite-rimmed pyrite in phenocrysts of the rhyolite dikes gave δ18O values of 1.6 to 9.5‰ and δD values of –105 to –145‰. For temperatures of 300 ± 100°C, the calculated fluid isotopic compositions are consistent with felsic magmatic water and minor modifications by mixing with meteoric water and exchange with wall rocks. In the silica-sulfide ore, in situ isotopic laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) analyses of pyrite cores yielded δ34S values ranging from 3.4 to 7.7‰, with average values of 5.6‰ in the felsic dikes, 4.5‰ in the siliciclastic rocks, and 5.3‰ in the carbonate rocks. These values match conventional pyrite δ34S data reported for Eocene porphyry systems elsewhere in the district. Nanoscale secondary ion mass spectrometry analyses show that gold and associated trace elements occur in submicron-scale zones within arsenopyrite rims on pyrite. The average δ34S values of the arsenopyrite rims are 5.3 to 6.5‰ heavier than the pyrite cores, indicating cooling and an increasing H2S/SO2 ratio. The highest grades resulted from episodic pulses of a gold-rich fluid that was partly derived from, or exchanged with, the sedimentary host rocks. In situ LA-MC-ICP-MS δ34S values for the late-stage banded pyrite breccia become progressively lighter from veinlet margin to center, reaching a low of –32‰. These veinlets indicate a shift from main-stage quartz-sericite-pyrite and intermediate argillic alteration to more neutral pH and oxidizing conditions during late-stage mineralization, indicating either increasing interaction between the fluid and sedimentary sulfur sources in the host-rock package or bacterial sulfate reduction and supergene sulfide precipitation.

Badzhal tin magmatic-fluid system (Far east, Russia): the transition from the granite crystallization to the hydrothermal ore deposition

With a view to reveal special characteristics of the transition stage from granite crystallization to rare-metal ore deposition it is studied Badzhal tin-bearing magmatic-fluid system of eponymously-named volcano-plutonic zone of the Middle Priamyrie. For that end the detail research of melt, fluid-melt and fluid inclusions and oxygen isotopes from minerals of granitoids from Verkne-Urmi massif from Badzhal volcano-plutonic zone and also minerals of Sn-W deposits Pravo-Urmi and Blizhnee have been carried out. The formation of greisens and hydrothermal veins were caused by the development of the integrated system associating with establishing of Verkne-Urmi granite massif which is one of a dome fold of Badzhal cryptobatholith. For the first time for tin deposits it has been followed up the transition from the magmatic phase of granite crystallization to the hydrothermal ore formation stage and the evolution of magmatic fluid from its separation from magmatic melt to Sn-W ore deposition. The direct evidence of tin-bearing fluid separation under melt crystallization is combined fluid-melt inclusions. Glass composition in inclusions shows that granites and granite-porphyry were crystallizing from acid and from limited to high-aluminous melts, that is value ASI changes from 0.95 to 1.33 and a content of alkalies varies from 6.02 up to 9.02 mass.%. Cl and F concentrations in glasses are according 0.03–0.14 and 0.14–0.44 mass.% and turned out to be higher of same in the total composition of rocks (0.02 and 0.05–0.13 mass.% in accordance). These differences indicate that Cl and F could be separated from granite melt under its crystallization and degasation. H2O content made from total deficiency electron microprobe analysis is 8–11 mass.%. This evaluation was made inclusive of a probable effect of “Na loss” (Nielsen, Sigurdson, 1981) under aqueous glass crystallization. Considering a high error of a such estimation (Devine et al., 1995), it should take to obtained values as a very approximate evaluation and consider that examined melts contained about 9,5–10,0 mass.% of H2O.&#x0D; The results of melt inclusion examination show that at any rate a part of melt forming magmatic rocks of Badzhal Ore Magmatic System are crystallizing at about T = 650 °C. These melts were acid, limited fluoride and meta- and high aluminous. The reason of low temperatures of its crystallization are likely a high pressure of aqua and also a increased content of F. Most likely that examined inclusions characterize the final stage of establishing of the massif, herewith at the system crystals, residual liquor and magmatic fluid phase coexist.&#x0D; The fluid from which greisens of Pravo-Urmi deposit formed is similar in properties to the supercritical fluid absorbing by magmatic minerals. The salinity of this fluid varying from ~9 to 12 mass.% equiv. NaCl, maximal T = 550 °C (with consideration for the temperature correction of T gom on a pressure ~1 кbar) are similar to such of magmatic fluid, which permit to connect its origin with pluton cooling. The formation of greisens and quartz-topaz veins of Pravo-Urmi deposit is related to fall of temperature of magmatic fluid from 550–450 up to 480–380 °C. The evolution of fluid deposited quartz-cassiterite veins of Blizhnee deposit, which based upon oxygen isotope composition (d18ОН2О ≈ 8.5‰) also separated from magma, was going at more subsurface conditions under much lesser pressure. That led to the gas separation of a fluid with salinity ~13 mass.% equa. NaCl under T = 420–340 °C on thin low salinity vapour and brine with concentration 33.5–37.4 mass.% equiv. NaCl. The research of oxygen isotope system testifies that oxygen isotope composition of ore-forming fluid controlled by equilibrium with granites at wide interval tempera­tures (from ~700 °С up to the beginning of greisen crystallization). Correspondence of measured and calculation data of the offered model indicates that the considerable volume of external fluid with other isotope characteristics which did not reach the isotope equilibrium with Verkhne-Urmi massif did not come into the magmatic isotope system. The discovered differences of physico-chemical conditions for two studied deposits are not “critical” and support an idea about their formation as the single magmatic-fluid system.

Oxygen isotope disequilibrium during serpentinite dehydration

AbstractTalc + olivine in metaperidotites result from the serpentinite breakdown due to increasing temperature in the Bergell contact aureole. Jack‐straw olivine textures are present in close proximity to the serpentine breakdown reaction. As the intrusion is approached, the number of olivine crystals increases while the size of the crystals decreases; this feature documents increased overstepping with increased heating rates. Talc veinlets are observed in the outer parts of the talc–olivine zone and are interpreted to be pathways of fluid produced during devolatilization of serpentine. None of the talc–olivine oxygen isotope pairs analysed are in isotopic equilibrium with respect to the peak contact temperature. This implies that escaping fluids cannot be in equilibrium with both phases. Hence, the fluid produced by serpentine reaction does not directly reflect the protolith composition, and attention must be given to the reaction mechanism before interpreting fluid isotope composition.

Mantle-derived fluids in the Nankai Trough Kumano forearc basin

Noble gas abundance and isotope data from 12 drilling mud gas samples obtained during IODP Expeditions 338 and 348 provide new insights on fluid origin and fluid migration in the inner accretionary prism below the Kumano forearc basin offshore SE Japan. The samples originate from three adjoining boreholes (C0002F, C0002N and C0002P) spanning depths between 950 and 3050 meters below sea floor (mbsf) and are composed of air and variable contributions of mantle-derived and crustal fluids.Air-corrected 3He/4He ratios of samples from Exp. 338 fall between 0.44 ± 0.24 Ra and 3.26 ± 0.28 Ra. Samples #1400 and #1800 (sample number denotes depth in mbsf) are more influenced by radiogenic helium (1.33 ± 0.34 Ra resp. 0.44 ± 0.24 Ra), whereas the air-free helium isotopic composition of sample #950 is clearly higher (3.26 ± 0.28 Ra). Enhanced radon activity around 950 mbsf, 1100 mbsf, 1400 mbsf, and 1800 mbsf suggests active flow of fluids at these depths.The helium isotopic composition of two other samples (#1700 and #1850) collected during Exp. 338 and of all samples obtained during the 1-year-later Exp. 348 are less variable (1.74–2.46 Ra with a mean 3He/4He ratio of ~ 2.4 Ra) and no radon anomalies were observed during sampling. For these samples, we assume migration by diffusive flow of helium-bearing fluids through sediments of the accretionary prism. In contrast, elevated radon activities and the more variable helium isotopic composition of fluids from 950, 1400, and 1800 mbsf from Exp. 338 are better explained by channelized and active fluid flow through temporarily permeable strata, maybe as a short-term episodic flow event caused by reactivation of buried trust faults. The helium isotopic composition of #950 is in good agreement with estimated helium isotope values of the subducting Philippine Sea Plate (PSP) at present (~ 3.4 Ra), whereas fluids from 1400 and 1800 mbsf demonstrate the input of radiogenic helium from a crustal, probably terrestrial source.

Using stable Mg isotope signatures to assess the fate of magnesium during the in situ mineralisation of CO2 and H2S at the CarbFix site in SW-Iceland

The in-situ carbonation of basaltic rocks could provide a long-term carbon storage solution. To investigate the viability of this carbon storage solution, 175 tonnes of pure CO2 and 73 tonnes of a 75% CO2-24% H2S-1% H2-gas mixture were sequentially injected into basaltic rocks as a dissolved aqueous fluid at the CarbFix site at Hellisheidi, SW-Iceland. This paper reports the Mg stable isotope compositions of sub-surface fluids sampled prior to, during, and after the CO2 injections. These Mg isotopic compositions are used to trace the fate of this element during the subsurface carbonation of basalts. The measured Mg isotopic compositions of the monitoring well fluids are isotopically lighter than the dissolving basalts and continue to become increasingly lighter for at least two years after the gas-charged water injection was stopped. The results indicate that the formation of isotopically heavy Mg-clays rather than Mg-carbonates are the predominant Mg secondary phases precipitating from the sampled fluids. Isotope mass balance calculations suggest that more than 70% of the Mg liberated from the basalt by the injected gas charged water was precipitated as Mg-clays, with this percentage increasing with time after the injection, consistent with the continued precipitation of Mg clays over the whole of the study period. The formation of Mg clays in response to the injection of CO2 into basalts, as indicated in this study, could be detrimental to carbon storage efforts because the formation of these minerals consume divalent Mg that could otherwise be used for the formation of carbonate minerals and because such clays could decrease host rock permeability.

Shallow forearc mantle dynamics and geochemistry: New insights from IODP Expedition 366

The Mariana forearc is a unique setting on Earth where serpentinite mud volcanoes exhume clasts originating from depths of 15 km and more from the forearc mantle. These peridotite clasts are variably serpentinized by interaction with slab derived fluid, and provide a record of forearc mantle dynamics and changes in geochemistry with depth. During International Oceanic Discovery Program (IODP) Expedition 366, we recovered serpentinized ultramafic clasts contained within serpentinite muds of three different mud volcanoes located at increasing distance from the Mariana trench and at increasing depth to the slab/mantle interface: Yinazao (distance to the trench: 55 km / depth to the slab/mantle interface: 13 km), Fantangisña (62 km / 14 km) and Asùt Tesoru (72 km / 18 km). Four different types of ultramafic clasts were recovered: blue serpentinites, lizardite-serpentinites, antigorite/lizardite- and antigorite-serpentinites. Lizardite-serpentinites are primarily composed of orange serpentine, forming mesh and bastite textures. Raman and microprobe analyses revealed that these textures contain a mixture of Fe-rich brucite (XMg ~ 0.84) and lizardite/chrysotile. Antigorite/lizardite- and antigorite-serpentinites record the progressive recrystallization of mesh and bastite textures to antigorite, magnetite and pure Fe-poor brucite (XMg ~ 0.92). Oxygen isotope compositions of clasts and pore fluids showed that the transition from lizardite to antigorite is due to the increase in temperature from 200 °C to about 400 °C within the forearc area above the slab/mantle interface. Lizardite-, antigorite/lizardite- and antigorite-serpentinites displayed U-shaped chondrite normalized Rare Earth Element (REE) patterns and are characterized by high fluid mobile element concentrations (Cs, Li, Sr, As, Sb, B, Li) relative to abyssal peridotites and/or primitive mantle. The recrystallization of lizardite to antigorite is accompanied by a decrease in Cs, Li and Sr, and an increase in As and Sb concentrations in the bulk clasts, whereas B concentrations are relatively constant. Some clasts are overprinted by blue serpentine, often in association with sulfides. Most of these blue serpentinites were recovered at Yinazao and the uppermost units of Fantangisña and Asùt Tesoru suggesting alteration in the shallower portions of the forearc, possibly during exhumation of the clasts. This episode of alteration resulted in a flattening of REE spectra and an increase of Zn concentrations in serpentinites. Otherwise, no systematic changes of ultramafic clasts chemistry or mineralogy were observed with increasing depth to the slab. The samples document previously undescribed prograde metamorphic events in the shallow portions of the Mariana subduction zone, consistent with a continuous burial of the serpentinized forearc mantle during subduction. Similar processes, induced by the interaction with fluids released from the downgoing slab, likely occur in subduction zones worldwide. At greater depth, breakdown of brucite and antigorite will result in the massive transfer of fluids and fluid mobile elements, such as As, Sb and B, to the source of arc magmas.

An investigation of mid-ocean ridge degassing using He, CO2, and δ13C variations during the 2005–06 eruption at 9°50′N on the East Pacific Rise

We report 3He/4He, He and CO2 concentrations for the dissolved (glass) and vapor (vesicle) phase of basalts, for 23 lavas from the 2005–2006 eruption and 4 lavas from 1991–92 at 9°50′N on the East Pacific Rise. We also determined δ13C and δ18O of CO2 in vesicles in 18 of these lavas. These sample suites provide a rare opportunity to study volatile systematics related to magma recharge at a mid-ocean ridge volcano and to quantify degassing prior to and during the eruptions. Our study covers the spatial and temporal extent of the 2005–06 eruption and is complementary to previous studies of variations in basaltic volatiles with distance from the ridge axis during part of this multi-stage eruption (Soule et al., 2012; Gardner et al., 2016). 3He/4He shows minor variation with a mean value of 8.51 ± 0.03 RA. All but 1 sample lies in a narrow range of 280–420 ppm for total (vesicles + glass) CO2 indicating volatile saturation at 1.2–2.3 km depth in the crust, similar to depths of the seismically imaged magma lenses in the region. Vesicle He and CO2 concentrations vary throughout the lava flow field by factors of 17 and 250, respectively. The vapor phase CO2/He ratio co-varies positively with the fraction of CO2 contained in vesicles due to kinetic fractionation between He and CO2 during vesiculation. Vesicle δ13CPDB co-varies with the fraction of CO2 in vesicles (r2=0.83), ranging from −2.6 to −5.0‰, and straddles the isotope composition of EPR vent fluids. The vesicle 13C-enrichment (δvapor−δmelt) is +3.3‰ and comparable to the experimentally determined fractionation factor for basaltic systems. Collectively the observations indicate closed-system degassing during rapid ascent of melt from the axial magma lens through the conduit to its eruption and emplacement on the seafloor, with minimal bubble loss. In contrast, CO2/Ba systematics and a comparison of δ13C for lavas and vent fluids indicate an earlier period of open-system degassing during which 50% or more of the initial (parental magma) CO2 inventory was lost prior to magma storage in the shallow crust.

Genesis of the Jianzhupo Sb–Pb–Zn–Ag deposit and formation of an ore shoot in the Wuxu ore field, Guangxi, South China

The Jianzhupo Sb–Pb–Zn–Ag deposit in the southeastern Dachang Sn–polymetallic ore belt, Guangxi region, South China is the largest deposit in the Wuxu district and is hosted by Devonian limestone and mudstone. The deposit mainly comprises a stage I jamesonite–sphalerite ore shoot and stage II jamesonite–sphalerite–calcite–quartz (JSCQ) veins. We use homogenization temperatures, salinities, and the H–O isotopic compositions of fluid inclusions from the Jianzhupo deposit to constrain the genesis of the deposit and determine factors that controlled the formation of the ore shoot. Fluid inclusions from both stages of mineralization contain large volumes of CO2 and minor bitumen, C6H6, and CH4. Fluid inclusions from the ore shoot yield homogenization temperatures of 120 °C–290 °C (mainly 250 °C–270 °C) and salinities of 3.4–13.6 wt% NaCl equiv. Fluid inclusions from the stage II mineralization yield homogenization temperatures of 150 °C–350 °C (mainly 240 °C–310 °C) and salinities of 1.4–12.0 wt% NaCl equiv. Ore-forming fluids of the Jianzhupo deposit yield δ18OH2O and δDV-SMOW values of 4.9‰–9.9‰ and −78‰ to −111‰, respectively, and plot close to the composition of magmatic water on the δDV-SMOW vs. δ18OH2O diagram. CO2 in the ore-forming fluids was likely generated through deep skarnization beneath the Wuxu ore field rather than from metamorphism, thermal degradation, or the oxidization of organic matter. H–O isotopic compositions of ore-forming fluids that contain large volumes of CO2 (with minor bitumen and C6H6) indicate that they were derived from magmatic water that was mixed with basin brines. Ore-forming fluids of the stage I ore shoot yield lower δ18OH2O values (4.9‰–6.7‰) than those associated with the stage II mineralization (7.5‰–9.9‰). Moreover, ore-forming fluids of the ore shoot and the JSCQ veins yield strongly and weakly positive correlations between homogenization temperature and salinity, respectively. This difference suggests that rapid and large-scale mixing between magmatic water and basin brines was the key factor that controlled the formation of the ore shoot.

Magnesium Isotope Composition of Subduction Zone Fluids as Constrained by Jadeitites From Myanmar

AbstractSubduction zone fluids are critical for transporting materials from subducted slabs to the mantle wedge. Jadeitites from Myanmar record fluid compositions and reactions in the forearc subduction channel. Here we present high‐precision Mg isotope data of the Myanmar jadeitites and associated rocks to understand the Mg isotope composition of subduction zone fluids at forearc depths. Two types of jadeitites (white and green) exhibit distinct Mg isotope compositions. The white jadeitites precipitated from Na‐Al‐Si‐rich fluids and have low δ26Mg values, varying from −1.55‰ to −0.92‰, whereas the green jadeitites have higher δ26Mg values (−0.91‰ to −0.74‰) due to metasomatic reactions between fluids and Cr spinel. The amphibole‐rich blackwall in the contact boundaries between jadeitites and serpentinites also exhibits low δ26Mg values (−1.17‰ to −0.72‰). Therefore, the jadeite‐forming fluids have not only high concentrations of Na‐Al‐Si but also low δ26Mg values. The low δ26Mg signature of the fluids is explained by the dissolution of Ca‐rich carbonate in subducted sediments or altered oceanic crust, which is supported by the negative correlation of δ26Mg with CaO/TiO2, CaO/Al2O3, and Sr in the white jadeitites. Given the common occurrence of Ca‐rich carbonates in the subduction channel, the Mg isotope composition of low‐Mg aqueous fluids would be significantly modified by dissolved carbonates. Metasomatism by such fluids along conduits has the potential to generate centimeter‐scale Mg isotope heterogeneity in the forearc mantle wedge. Therefore, Mg isotopes could be a powerful tracer for recycled carbonates not only in the deep mantle but also in the shallow regions of subduction zones.

Geological and isotopic constraints on ore genesis, Huangjindong gold deposit, Jiangnan Orogen, southern China

The Huangjindong gold deposit, with gold resources of ∼80 t, is hosted in Neoproterozoic slates along the Chang-Ping Fault in the Jiangnan Orogen, southern China. The orebodies are dominated by auriferous arsenopyrite-pyrite-quartz-(scheelite) veins; quartz-rich breccias that are cemented by late arsenopyrite, pyrite, muscovite, and quartz; auriferous stibnite-quartz veins; and disseminated gold in arsenopyrite-pyrite-sericite-quartz altered rocks. Neither significant alteration zoning nor ore-element zoning are recognized within the orebodies, although the orebodies at shallower crustal levels are slightly more stibnite-rich. At the regional scale, the first-order control on the distribution of gold deposits is the NNE- to NE-trending crustal-scale, sinistral strike-slip Chang-Ping Fault. As in other gold provinces globally, this controlling fault is not gold mineralized. The gold occurrs in WNW to E-W-trending zones of thrusts and fracture arrays associated with tight ‘locked-up’ asymmetrical folds. The broadly E-W-trending orebodies formed independently within approximately NE-trending fault-bounded corridors defined by the first-order Chang-Ping, and second-order Ni-Wan and Keng-Shang/Chang-Long Faults. Hydrothermal activity was initiated at the ductile-brittle transition, but evolved to brittle-dominated deformation, all within a transpressional environment.Oxygen and hydrogen isotope compositions of fluids (δD = −44.4‰ to −34.2‰; δ18O = +8.6‰ to +9.5‰) calculated from gold-related hydrothermal quartz and hydrothermal sericite, respectively, are consistent with a metamorphic source. Sulfur isotope compositions of the gold-bearing arsenopyrite and pyrite (median values between −8.0‰ and −6.0‰) are generally distinct from δ34S of the regional Neoproterozoic slates in the Jiangnan Orogen, but overlap the compositions of pyrites in global sediment-hosted orogenic gold deposits. Lead isotope compositions of the gold-bearing arsenopyrite and pyrite (206Pb/204Pb: 17.049 to 18.417; 207Pb/204Pb: 15.263 to 15.712; 208Pb/204Pb: 37.597 to 39.220) suggest that the lead was sourced from metamorphic rocks involved in the orogenic events. Therefore, both the stable and radiogenic isotopic data indicate that ore-forming fluids and metals were derived from a metamorphic source, although the precise nature of that source is unclear.On the basis of these geological and isotopic data, Huangjindong is interpreted as an orogenic gold deposit. The gold ore bodies show classic structural controls, being adjacent at the province scale to the crustal-scale Chang-Ping Fault but located in thrusts and associated tight anticlinal fold hinges that lie in corridors between markedly oblique, relatively brittle fault arrays that parallel the Chang-Ping Fault.

Experimental partitioning of Ca isotopes and Sr into anhydrite: Consequences for the cycling of Ca and Sr in subseafloor mid-ocean ridge hydrothermal systems

The elemental and isotopic mass balance of Ca and Sr between seawater and the oceanic crust at mid-ocean ridge (MOR) hydrothermal systems integrates various physiochemical processes in the subseafloor, such as dissolution of primary silicate minerals, formation of secondary minerals, and phase separation in the subseafloor. In particular, the precipitation and recrystallization of anhydrite are recognized as important processes controlling the Ca and Sr elemental and isotope composition of high temperature vent fluids and coexisting ocean crust, and yet, little experimental data exist to constrain the mechanism and magnitude of these critical geochemical effects. Thus, this study experimentally examines Sr/Ca partitioning, Ca isotope fractionation, and the rate of exchange between anhydrite and dissolved constituents. These experimental constraints are then compared with Sr/Ca and Ca isotope compositions of anhydrite and vent fluids sampled from the TAG hydrothermal system. Accordingly, anhydrite precipitation and recrystallization experiments were performed at 175, 250, and 350 °C and 500 bar at chemical conditions characteristic of active MOR hydrothermal systems. Experimental data suggest that upon entrainment and recharge of seawater into MOR hydrothermal systems anhydrite will rapidly precipitate with a Ca isotopic composition that is depleted in the heavy isotope compared to the hydrothermal fluid. The magnitude of the Ca isotope fractionation, Δ44/40Ca(Anh-Fluid), is temperature dependent, −0.45, −0.22, and −0.02‰, for 175, 250, and 350 °C, respectively, but likely indicative of kinetic effects. Utilization of a 43Ca spike in solution was implemented to quantify the time-dependent extent of isotope exchange during anhydrite recrystallization at chemical equilibrium. These data indicate that the rate of exchange is a function of temperature, where 12, 46, and 45% exchange occurred within 1322, 867, 366 h at 175, 250, and 350 °C, respectively. The partitioning of Sr/Ca between anhydrite and constituent dissolved species during precipitation depends greatly on the saturation state of the hydrothermal fluid with respect to anhydrite at each experimental temperature, KD(Anh-Fluid) = 1.24–0.55 at 175–350 °C, broadly similar to results of earlier experimental observations by Shikazono and Holland (1983). Equilibrium KD(Anh-Fluid) values were estimated by taking explicit account of time dependent magnitude of exchange, yielding values of 0.43, 0.36, 0.29 at 175, 250, and 350 °C, respectively. Coupling these experimental constraints with the temperature gradient inferred for high temperature MOR hydrothermal systems suggests that the Ca isotope and Sr elemental composition of anhydrite formed near the seafloor will retain the composition derived upon initial formation conditions, which is indicative of disequilibrium. In contrast, at greater depths and at higher temperatures, anhydrite will reflect close to equilibrium Sr/Ca partitioning and Ca isotope fractionation conditions. The experimental and natural data presented in this study can be used to further understand the effect of anhydrite precipitation during hydrothermal circulation in the oceanic crust and on the chemical and isotopic composition of seawater on geologic timescales.

Fluid Inclusions and C, H, O, S, and Pb Isotopic Compositions of the Dabaoshan Cu Polymetallic Deposit, Northern Guangdong Province, South China

AbstractThe Dabaoshan deposit in Northern Guangdong Province, South China, is a Cu–Mo–W–Pb–Zn polymetallic deposit, located in the southern part of the Qin–Hang porphyry–skarn Cu–Mo ore belt. The deposit mainly comprises porphyry Mo and stratiform skarn Cu ore deposits. The genesis of the Cu ore deposit has been ascribed to a typical skarn ore deposit formed by the metasomatism of Devonian carbonate rock layers or to a volcanic rock‐hosted massive sulfide deposit formed by marine exhalation. In this paper, we report on the homogenization temperatures and salinities of fluid inclusions and C, H, O, S, and Pb isotopic compositions of fluids and minerals in this deposit. Homogenization temperatures and salinities of fluid inclusions in garnet, diopside, quartz, and calcite provide information on the skarnification, mineralization, and postmineralization stages. The data show that ore‐forming fluids experienced a continuous transition from high temperatures and salinities to low temperatures and salinities over the entire period of mineralization. C, H, and O isotopic compositions indicate that ore‐forming fluids were derived mainly from magmatic water. O isotopic compositions indicate that ore‐forming fluids mingled with atmospheric water during the last stage of mineralization. Sulfur in the ore came mainly from deep magmatic sources. Pb isotopic compositions in the orebody show that almost all the lead in the ore was derived from magma with a crustal source. Combined geological, geophysical, and geochemical data were achieved before we proposed that the Dabaoshan porphyry–skarn Cu–Mo–W–Pb–Zn deposit, as one member of the Qin–Hang porphyry–skarn Cu–Mo ore belt, formed during the Jurassic subduction of the paleo‐Pacific plate beneath the Eurasian continent at quite low angle. NE‐ and EW‐trending structures controlled the emplacement of magmatic rocks in the South China region. In the mining area, the Xiangguanping Fault and its branches were the main conduits for magmatic crystallization and mineralization. The many subfaults, folds, and interlayer fracture zones on both sides of the main fault provided the requisite space for the ore and, together, were the controlling structures of the orebody.

Origin of the high-grade Early Jurassic Brucejack epithermal Au-Ag deposits, Sulphurets Mining Camp, northwestern British Columbia

The high-grade (8.1 million ounces of gold; 15.6 Mt grading 16.1 g/t Au; Pretium, 2016) Brucejack epithermal Au-Ag deposits are located in the Canadian Cordillera of northwestern British Columbia, and formed in association with extensive early Mesozoic island arc magmatism. Porphyry-type Cu-Au-Mo mineralization occurs nearby at the Kerr-Sulphurets-Mitchell (196–190 Ma; Febbo et al., 2015), Bridge Zone (191.7 ± 0.8 Ma, 191.5 ± 0.8 Ma), and West Zone (188.9 ± 0.9 Ma) prospects. Gold-silver vein-type mineralization at Brucejack is hosted by variably altered and deformed Early Jurassic porphyritic latite lava flows, volcaniclastic rocks, and volcanic-derived sandstones, siltstones, and conglomerates. This study focuses on the Valley of the Kings Zone at Brucejack, where host rocks have been dated at 188–184 Ma (U-Pb, zircon). Post-mineralization (but not post-alteration) basaltic and trachybasaltic dikes that cut the veins are geochemically similar to the host volcanic sequence and follow the same structural trends of the mineralized vein corridors. We suggest these dikes are broadly coeval with mineralization, and that ore formation at the Valley of the Kings formed during the overall Jurassic tectonomagmatic event.Gold-silver mineralization is hosted by quartz-carbonate veins that cut sericitized and pyritized volcaniclastic rocks. Six stages of veining are defined, with Au-Ag mineralization (electrum) focused in stages III–V: stage I–III veins consist of quartz with minor carbonate, chlorite, sericite, and sulfide minerals; stage IV quartz veins contain more abundant base metal sulfides (pyrite, sphalerite, and galena with minor sulfosalts); stage V veins are dominantly calcitic; late post-mineralization stage VI veins are quartz-calcite and contain sparse pyrite and chlorite, but no electrum. The veins are interpreted to be syn- to late-tectonic, with deformation decreasing from locally penetrative in the host-rocks prior to veining (resulting in local foliation of sericite and pressure shadows around pyrite), to dismemberment and shearing of early stage I–II quartz veins, brittle disruption of stage III–IV quartz and stage V carbonate veins, and minimal deformation of post-mineralization stage VI quartz-carbonate veins. A much younger set of extensional muscovite veins locally cuts the deposit with Late Cretaceous apparent ages, and appears to be associated with a weak thermal overprint that has reset K-Ar and 40Ar/39Ar ages in sericite throughout the district.The deformation experienced by the vein system, particularly in the later stages (III–V), was not uniform, and original undeformed vein textures and fluid inclusion assemblages are locally preserved. We report fluid inclusion microthermometric data from carefully selected primary fluid inclusion assemblages from quartz, calcite, and sphalerite from vein stages III–IV, many of which show evidence for boiling (coexisting liquid- and vapor-rich primary fluid inclusions). Liquid-rich fluid inclusions from stage III and V veins have moderate homogenization temperatures (∼170 °C and ∼160 °C respectively) and salinities of 2–8 wt% NaCl equiv., whereas inclusions from base-metal-sulfide-bearing stage IV veins show evidence of mixing with a cooler, more saline brine (∼140 °C, ∼10–15 wt% NaCl equiv.). Carbon dioxide was observed as clathrate during cooling in some fluid inclusions, suggesting that minor amounts of CO2 were present in the fluids.Calculated oxygen isotopic compositions of fluids in equilibrium with quartz and calcite from vein stages III–V range from δ18Ofluid = −10.7 to +1.8‰, whereas δ13CCO2 ranges from −9.5 to −4.5‰, and δ34Spyrite ranges from −1.7 to +0.6‰. Taken together, these data suggest a magmatic source for S and some C, carried by a fluid of evolved or diluted magmatic origin, which variably mixed with meteoric-derived groundwater or seawater containing carbon of sedimentary (organic) origin. Mixing is supported by large salinity ranges of vein stage IV fluid inclusions, where individual assemblages range from ∼1 to 15 wt% NaCl equiv.These fluid temperatures, salinities, and isotopic compositions are typical of epithermal deposits where distal magmatic fluids mix with local heated groundwaters. The low fluid temperatures but evidence of boiling suggest formation at shallow crustal depths. This is despite the evidence for penetrative deformation in the sericitized volcanosedimetary host rocks, which we attribute to rapid uplift immediately prior to or during the earliest stages of mineralization. The vein system is therefore interpreted to have formed during the later stages of a deformational event related to arc accretion.

Multiple stable isotope fronts during non-isothermal fluid flow

Stable isotope signatures of oxygen, hydrogen and other elements in minerals from hydrothermal veins and metasomatized host rocks are widely used to investigate fluid sources and paths. Previous theoretical studies mostly focused on analyzing stable isotope fronts developing during single-phase, isothermal fluid flow. In this study, numerical simulations were performed to assess how temperature changes, transport phenomena, kinetic vs. equilibrium isotope exchange, and isotopic source signals determine mineral oxygen isotopic compositions during fluid-rock interaction. The simulations focus on one-dimensional scenarios, with non-isothermal single- and two-phase fluid flow, and include the effects of quartz precipitation and dissolution.If isotope exchange between fluid and mineral is fast, a previously unrecognized, significant enrichment in heavy oxygen isotopes of fluids and minerals occurs at the thermal front. The maximum enrichment depends on the initial isotopic composition of fluid and mineral, the fluid-rock ratio and the maximum change in temperature, but is independent of the isotopic composition of the incoming fluid. This thermally induced isotope front propagates faster than the signal related to the initial isotopic composition of the incoming fluid, which forms a trailing front behind the zone of transient heavy oxygen isotope enrichment. Temperature-dependent kinetic rates of isotope exchange between fluid and rock strongly influence the degree of enrichment at the thermal front. In systems where initial isotope values of fluids and rocks are far from equilibrium and isotope fractionation is controlled by kinetics, the temperature increase accelerates the approach of the fluid to equilibrium conditions with the host rock. Consequently, the increase at the thermal front can be less dominant and can even generate fluid values below the initial isotopic composition of the input fluid. As kinetics limit the degree of isotope exchange, a third front may develop in kinetically limited systems, which propagates with the advection speed of the incoming fluid and is, therefore, traveling fastest. The results show that oxygen isotope signatures at thermal fronts recorded in rocks and veins that experienced isotope exchange with fluids can easily be misinterpreted, namely if bulk analytical techniques are applied. However, stable isotope microanalysis on precipitated minerals may – if later isotope exchange is kinetically limited – provide a valuable archive of the transient thermal and hydrological evolution of a system.

The Arjuno-Welirang volcanic complex and the connected Lusi system: Geochemical evidences

For the first time we present the geochemical characterization of fluids emitted from the Arjuno-Welirang volcanic complex and compare the results with those obtained sampling the neighboring spectacular Lusi eruption site (Java Island, Indonesia).The isotopic composition of the hydrothermal and cold waters from the Welirang volcanic complex indicate a meteoric origin for these springs, with values ranging from −65 to −50 and −6 to −1‰ vs V-SMOW respectively for δD and δ18O. The water erupted from the Lusi site showed clustered higher δD and δ18O isotopic values, ranging around −6 and +10 vs V-SMOW respectively. We ascribe these results to mixing between hydrothermal fluids, meteoric water, saline formation fluids, and water released during clay mineral illitization ultimately altered by additional evaporation processes. The chemical and isotopic composition of fluids emitted from fumaroles and hydrothermal springs of the Welirang volcano showed a clear magmatic signature where a CO2-dominated gas reveals δ13CCO2 ranging between −5.9 and −2.4 and helium isotope with R/Ra = 7.3. These values are very close to those measured at Lusi site (R/Ra = 7) that also have high CO2/CH4 ratio (1.7–2.2) supporting the high contribution of magmatic gases. Moreover, a great contribution of andesitic water has been recognized in the water vapour emitted from the summit fumaroles.Converging geochemical data indicate that the plumbing system of the Lusi eruption site is connected at depth with the Arjuno-Welirang volcanic complex. These data support a scenario where hydrothermal fluids from the volcanic system migrated in the sedimentary basin triggering metamorphic reactions in the organic-rich sediments that ultimately resulted in a venting system at the surface. After eleven years of incessant activity this venting system remains constantly fed by the fluids from the volcanic complex and became world known as “Lusi”, the largest ongoing clastic geysering system on Earth.