Fluid-rock Interaction Research Articles

Fluid–rock interaction controlled by integrated hydrothermal fluid and fault: Implications for reservoir development

Hydrothermal activity is prevalent in petroliferous basins, particularly in fractured reservoirs. Hydrothermal fluids enter the reservoirs and cause significant fluid–rock interaction, which ultimately controls reservoir development. The Mahu Sag in western China is a hydrocarbon-rich region where hydrothermal fluids and faults control the reservoirs, making it challenging to understand the development mechanism of the shale reservoirs. This study focused on the Fengcheng Formation in the Mahu Sag. Based on a range of previous lithological test results, numerical models of hydrothermal fluid charging were established to explore differences in fluid–rock interactions under varying temperature, fluid, and flow rates. Results revealed that within a specific range, increasing temperature and Mg2+ concentration promoted hydrothermal processes. The hydrothermal flow rate influenced ion accumulation on the fluid-rock contact surface, thus controlling mineral dissolution rates. In a certain range, when the flow rate is increased by 3 times, the growth rate of dolomite content is increased by more than 15 times. Hydrothermal fluid flowing near the fault underwent chemical action more readily than in other areas. The influence of hydrothermal fluid and fault on reservoir development was both constructive and destructive. The decrease and increase of porosity are 30 % and 25 %, respectively. This study could provide a theoretical basis for identifying factors influencing shale reservoir development.

Genesis of high-grade lode gold shoot dominated by ore fluid overprinting during a ductile-brittle shear event

Lode gold deposits hosted by ductile-brittle shear zones account for more than one-third of the world’s gold production. High-grade ore shoots from this type of deposit are the most critical exploration targets. The ore shoots can form through the post-depositional deformation of auriferous sulfides or overprinting of ore fluids accompanied by coupled dissolution-reprecipitation (CDR) reactions. However, the mechanism that dominates ore shoot genesis remains unknown, primarily due to the controversial single progressive or polyphase nature of ore-bearing shear zones. Here, we report on geological and geochemical analyses we conducted at the large Hetai goldfield, South China, to construct an accurate gold upgrading model for the formation of ore shoots. Stages 1−3 of mineralization at Hetai show features typical of ductile shearing, while Stage 4 is characterized by quartz-sulfide veinlets in brittle fractures. 40Ar/39Ar ages of ca. 184 Ma and 157 Ma for the mineralization of stages 1 and 4 overlap with the regionally dextral ductile-brittle shear that occurred during ca. 210−162 Ma. Thus, the gold event at Hetai should have been controlled by a single progressive ductile-brittle shear episode, rather than polyphase structural events. The auriferous fluids at Hetai precipitated minor invisible gold in pyrites (mean 0.173 ppm) produced during stages 1−4 through fluid-rock interaction. The systematic increase of elements Au, As, Sb, Bi, Ag, and Cu and δ34S values in ductile-deformed pyrites from stages 1−3 indicate that early invisible gold upgrading should be the result of the post-depositional remobilization of auriferous sulfides during the long-lived ductile-brittle transition. Cataclastic pyrites hosting invisible gold from Stage 4 have zoned and porous mantles with elevated invisible gold (mean 0.503 ppm) and Sb, Bi, Pb, Co, Ni, and Ti contents. These pyrites are further replaced by chalcopyrite and pyrrhotite with increasing invisible gold, Co, and Ni contents. In addition, numerous visible native gold grains in Stage 4 are included in sulfides formed by replacement and develop along the microfractures and grain boundaries of these sulfides. We suggest the late invisible and visible gold upgrading events in Stage 4 can be attributed to the auriferous fluid superposition and subsequent replacement of pyrite via CDR reactions in a brittle regime. Therefore, the gold upgrading process at Hetai is jointly caused by the early remobilization induced by ductile-brittle deformation and the late ore fluid superposition with accompanying CDR reactions within a brittle domain. As the ore fluid superposition and CDR reactions in Stage 4 produce a significant amount of visible gold, they exert a first-order control on the genesis of ore shoots at Hetai. The refined model may be widely applicable to lode gold deposits elsewhere and can be used to identify regions with promising exploration targets.

Solutions and case studies for thermally driven reactive transport and porosity evolution in geothermal systems (reactive Lauwerier problem)

Abstract. Subsurface non-isothermal fluid injection is a ubiquitous scenario in energy and water resource applications, which can lead to geochemical disequilibrium and thermally driven solubility changes and reactions. Depending on the nature of the solubility of a mineral, the thermal change can lead to either mineral dissolution or precipitation (due to undersaturation or supersaturation conditions). Here, by considering this thermo-hydro-chemical (THC) scenario and by calculating the temperature-dependent solubility using a non-isothermal solution (the so-called Lauwerier solution), thermally driven reactive transport solutions are derived for a confined aquifer. The coupled solutions, hereafter termed the “reactive Lauwerier problem”, are developed for axisymmetric and Cartesian symmetries and additionally provide the porosity evolution in the aquifer. The solutions are then used to study two common cases: (I) hot CO2-rich water injection into a carbonate aquifer and (II) hot silica-rich water injection into a sandstone aquifer, leading to mineral dissolution and precipitation, respectively. We discuss the timescales of such fluid–rock interactions and the changes in hydraulic system properties. The solutions and findings contribute to the understanding and management of subsurface energy and water resources, such as aquifer thermal energy storage, aquifer storage and recovery and reinjection of used geothermal water. The solutions are also useful for developing and benchmarking complex coupled numerical codes.

Geochemical modeling of U-Ni-Co-As transport and deposition in acidic basinal brines: Implications for unconformity-related U-(Ni-Co-As) mineralization in the Athabasca Basin (Canada)

The unconformity-related uranium (URU) deposits, which are best developed in the Proterozoic Athabasca Basin (Canada), can be divided into the monometallic (U) and polymetallic (U-Ni-Co-As) subtypes. Different models have been proposed to explain the formation of the two subtypes of URU deposits. The conventional “diagenetic-hydrothermal” model suggests that both subtypes formed from oxidizing, acidic basinal brines in a unified mineralization system, whereas a newly proposed model suggests that the polymetallic deposits formed from monometallic U deposits superimposed by Ni-Co-As mineralization. While the second model has been the subject of ongoing debates, there are also unanswered questions in the diagenetic-hydrothermal model; in particular, it remains unclear what geological factors controlled the formation of one subtype or another in a unified mineralization system. This paper aims to address this question with geochemical modeling of hydrothermal U-Ni-Co-As transport and deposition using the Geochemist's Workbench (GWB) software. The conditions for U-Ni-Co-As transport were examined with thermodynamic modeling. The results indicate that highly oxidizing (log fO2(g) > −25), acidic (pH = 3.5) brines can transport large amounts (>100 ppm) of U together with Ni-Co-As, whereas moderately oxidizing (−40 < log fO2(g) < −25), less acidic (3.5 < pH < 6) brines can transport only minor amounts (<1 ppm) of U while still capable of carrying large amounts (>100 ppm) of Ni-Co-As, and very reducing (log fO2(g) < −40) brines with pH from 3.5 to 6 can dissolve only minor amounts (<1 ppm) of U and Ni-Co-As. The mechanisms of U-Ni-Co-As deposition were examined with reaction path modeling involving fluid-rock interaction and fluid-fluid mixing. The results indicate that as a U-Ni-Co-As-bearing, acidic, oxidizing brine progressively reacts with basement rocks or mixes with a reducing fluid, U precipitates before Ni-Co-As in relation to pH increase and fO2(g) decrease. In the case of fluid mixing, while U precipitation can be caused by any of the reducing agents (CH4(aq), H2(aq), H2S(aq), and Fe2+(aq)) in the reducing fluids examined, Ni-Co-As deposition requires the participation of CH4(aq) or H2(aq). Based on the modeling results, it is inferred that U and Ni-Co-As were not transported by the same fluid, otherwise all the URU deposits would be polymetallic. The U was most likely derived from the basin rather than from the basement, because as oxidizing basinal brines infiltrate and interact with basement rocks, their ability to dissolve and transport U quickly diminishes due to fO2(g) decrease and pH increase. In contrast, as these basinal brines infiltrate the upper part of the basement, which is moderately oxidizing, they retain the ability to dissolve and transport significant amounts of Ni-Co-As, enabling the scavenging of these metals from the basement. While all URU mineralization requires U-bearing basinal brines and reducing agents, the formation of polymetallic (U-Ni-Co-As) deposits depends on the availability of Ni-Co-As-rich lithologies in the upper part of the basement, together with an ample supply of robust reducing agents, such as CH4 and H2 that were derived from the deeper part of the basement. Therefore, although the possibility that polymetallic U-Ni-Co-As deposits formed from Ni-Co-As mineralization superimposing on monometallic U deposits cannot be ruled out, the results of this study demonstrate that both polymetallic and monometallic URU deposits can be formed in a unified diagenetic-hydrothermal mineralization system in the Athabasca Basin.

Review on spontaneous imbibition mechanisms in gas-water systems: Impacts on unconventional gas production and CO2 geo-sequestration

Spontaneous imbibition, a fundamental process in porous media, involves the displacement of a non-wetting phase by a wetting phase driven by capillary pressure. It plays a key role in various applications, particularly in unconventional gas production and greenhouse gas geo-sequestration. Despite extensive research in this area, conflicting results and explanations persist regarding imbibition phenomena, particularly in gas-water systems. This paper aims to address this gap by providing a comprehensive review of basic concepts, mechanical analyses, pore-filling patterns, front evolutions, and influencing factors associated with spontaneous imbibition. Mechanical factors including capillary force, viscous force, gravitational force, hydrostatic force, inertial force, capillary back force, and dead end force, play crucial roles in water imbibition with different boundary types. The pore-filling pattern significantly affects microscopic fluid distribution and front evolution during the imbibition process. To resolve conflicting findings, we systematically analyze the influencing factors of spontaneous imbibition within gas-water systems, encompassing rock properties, fluid characteristics, rock-fluid interactions, and reservoir properties. Furthermore, we present an in-depth discussion on the imbibition and trapping mechanisms relevant to unconventional gas production and CO2 geo-sequestration, providing insights into force analyses and influencing factors. Gas production strategies and favorable conditions for CO2 capillary trapping are proposed. Finally, we outline existing knowledge gaps and suggest potential directions for future research. This review thus provides useful insights and suggestions for advancing our understanding of spontaneous imbibition within gas-water systems and optimizing unconventional gas production and CO2 geo-sequestration practices.

Modeling the impact of pH and reservoir temperature on the dissolution of quartz mineral due to oilfield chemical treatment

The petroleum industry frequently uses oilfield chemicals, such as corrosion inhibitors, scavengers, drilling fluid, and chemical enhanced oil recovery methods, to improve oil recovery. However, these chemicals can negatively impact the geochemical characteristics of the formation, potentially leading to facility failure and reservoir formation damage. A study using PHREEQC was conducted to predict the impact of reservoir temperature and pH on quartz rock dissolution under different temperatures and pH levels. The results showed consistent dissolving rates for quartz at different pH levels. The solubility of quartz minerals was slightly affected by the solution pH, suggesting that the process variable pH has little effect on the equilibrium rate. However, precipitation caused quartz to dissolve at a rate of 0.04 mmol.L−1.s−1 at 45 °C but increased to 0.07 mmol.L−1.s−1 at 65 °C in 0.8 s. As temperature increased, quartz and NaNO3 solution interaction reached equilibrium concentrations in 0.8 and 2.7 s. The study concluded that temperature significantly impacts quartz dissolution during interaction with sodium nitrate oilfield chemicals compared to pH. The PHREEQ software is a useful research tool for simulating rock-fluid interactions resulting in reservoir formation dissolution and precipitation, which can cause formation damage and sand production.

Anomalous tellurium enrichment associated with gold mineralization: A mineralogical and isotopic study of the Yongxin Te-Au deposit, northeast China

The Yongxin tellurium-gold (Te-Au) deposit, a large epithermal deposit in the Duobaoshan polymetallic metallogenic belt (DPMB) within eastern section of the Central Asian Orogenic Belt (CAOB), is mainly hosted by syenogranite and mylonite. However, the Te-Au occurrence, precipitation mechanism and genesis in this deposit remain elusive. In this study, pyrite, the primary host of Te-Au mineralization, was studied utilizing multiparametric techniques such as scanning electron microscope (SEM), electron probe microanalysis (EPMA), in-situ laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) and femtosecond laser ablation coupled multi-collector inductively coupled plasma mass spectrometry (fs LA-MC-ICP-MS). The results show that there are three generations of pyrite termed here as Py1, Py2 and Py3. The coarse euhedral Py1 and fine vein Py2 contain negligible to low contents of Te and Au, whereas the anhedral aggregated Py3 with porosity and grain boundary (GB) shows the highest concentrations of Te and Au. Representative LA-ICP-MS profiles show that Te-Au occurs either as solid solution in the Py1 and Py2 or submicroscopic Au-Ag-Te-Bi inclusions, electrum and native gold in Py3. Thermodynamic data of telluride and sulfide show that the Te-Au was deposited under relatively oxidizing conditions with values of log f Te2 ranging from −15.2 to −11.2 and log f S2 from −16.7 to −12.1. at 200 °C. We infer that fluid mixing and fluid-rock interaction were the dominant mechanisms that triggered the precipitation of Te-Au in the Yongxin Te-Au deposit. Geochemical and geochronological data indicate that the likely source of Te is Te-rich oceanic sediments originating from the Western Pacific Plate. Pyrite and telluride from the gold deposits can be potential targets for Te exploration in the DPMB.

The use of combined C[sbnd]Mg isotope compositions of carbonates from orogenic Sb[sbnd]Au deposits as a tracer of fluid interaction with sea-floor altered crust

The Mesoarchaean Murchison Greenstone Belt is composed of a strongly deformed volcano-sedimentary succession metamorphosed up to amphibolite facies and surrounded by metaluminous and peraluminous granitoids in the north of the Kaapvaal Craton of southern Africa. A circa 40 km-long shear-zone of strongly carbonatised rocks (the Antimony Line, or Sb-Line) oriented along the main trend of the greenstone belt (WSW–ENE) hosts important Sb deposits with accessory Au. These commodities mostly occur as stibnite and native gold hosted by quartz‑carbonate rocks at the centre of the Sb-Line, underlining the importance of CO2-rich fluid flow for mineralisation during deformation and metamorphism.In this study, we compare the elemental and stable isotope (C-O-Mg) composition of carbonates from the Sb-Line with regionally distributed carbonates hosted by the volcano-sedimentary succession distal to mineralisation. The carbonates of the Sb-Line and regional rocks have overlapping major element compositions. Both magnesite and dolomite in most Sb-Line samples have marked light-REE-depleted patterns with variable positive Eu anomalies. Carbon isotope ratios define two clusters, with marked δ13C peaks at ca. -5 ‰ for Sb-Line rocks and ca. -2 ‰ for regional rocks, implying separate C-sources. The peak at ca. -2 ‰ likely represents early carbonatisation through sea-floor alteration, whereas the first peak at ca. -5 ‰ is indicative of deep CO2 (mantle, or magma-derived) introduced during tectonic activity of the Sb-Line. Magnesium isotope ratios of regional rocks reveal limited fractionation (bulk δ26Mg = −0.27 ± 0.10 ‰) that overlap with mantle values, but some carbonate-bearing veins present 26Mg-depleted compositions (bulk δ26Mg = −1.91 to −0.40 ‰). Sb-Line carbonated rocks have more fractionated Mg isotope compositions (bulk δ26Mg = −0.8 to 0.0 ‰) and carbonates display marked negative 26Mg values (δ26Mg from −1.46 to −0.31 ‰). We interpret these results in terms of preferential remobilisation of isotopically light Mg during fluid-rock interaction and dissolution of carbonate in the host-rocks. The lack of correlation between δ13C and δ26Mg indicates decoupling of these isotopic systems, implying contribution from isotopically distinct sources of C and Mg to the mineralised zone.Similar to other structurally controlled AuSb mineralisation in Archaean greenstone belts, metal transport and ore deposition in the Murchison Greenstone Belt was closely related to the deep cycle of carbon, linking C-draw-down during sea-floor alteration, carbonate re-mobilisation during metamorphism and CO2 degassing from deep sources along major crustal discontinuities. In contrast to models for orogenic AuSb deposits invoking a purely intra-crustal, metamorphic origin of mineralising fluids, our results underline the importance of deep-sourced fluids originating from an external source from the volcano-sedimentary succession.

Fluid inclusion and pyrite geochemistry of the Jiapigou gold deposit, North China Craton: Implication for origin of orogenic gold deposit?

The Jiapigou auriferous belt in Jilin province has been recognized as substantial gold-producers in the North China Craton (NCC). Gold mineralization in this district mainly occurs in mineralized quartz veins and alteration zones, characterized by silicification, pyritization, and argillitization. The quartz veins contain three generations of mineralized quartz, namely, qtz1 (probably oldest), qtz2 (slightly younger), and qtz3 (probably youngest) were identified using SEM-CL. The fluid inclusions in all three generations of quartz are broadly of three types, viz., bi-phase Ia (H2O-CO2), tri-phase Ib (H2O-CO2-NaCl ± CH4), and II (H2O-NaCl).The micro-thermometric analysis of the type Ia and Ib fluid inclusion in qtz1 & qtz2 have aqueous-carbonic composition and exhibit a similar salinity of 1.1 to 7.8 wt% NaCl equivalent, whereas the homogenization temperature (Th) range varies from ∼237 to ∼350 °C. These ore-related fluid inclusions are of low to moderate salinity, and show mixture of CO2 and CH4. On the other hand, Type II fluid inclusions are dominant in qtz3. They have salinity of 3.2 to 12.7 wt% NaCl equivalent and homogenization temperature varying between 180 °C and 210 °C. These data indicate that the ore-forming fluid evolved from a CO2–H2O–NaCl ± CH4 system during the mineralization period.The X-ray elemental maps of pyrite acquired using electron microprobe analysis (EPMA) show irregular zones of Co and Ni indicating two generations of pyrite. The early-stage pyrite which occupies core portion shows high Co and Ni, whereas the later-stage pyrite occupying rim has lower Co and Ni. The laser ablation-inductively coupled plasma-mass spectrometer (LA-ICPMS) analyses of pyrite has indicated that pyrite-1 is rich in Au + Ag + Se + Cu + Co + Ni, whereas, pyrite-2 has lower concentration of these elements. A positive correlation between Fe and other chalcophile elements reported here (i.e. Au + Ag + Se + Cu + Co + Ni), might be due to fluid-rock interaction resulting into a saturated fluid that subsequently precipitated along the microfractures within earlier-formed pyrite and quartz. The in-situ δ34S values in pyrite from Jiapigou deposits overlap in the range of +4.5 to +9.6 ‰, which is consistent with the ore-forming fluids of the crustal origin input during fluid-rock interaction. The systematic pyrite compositional observations and fluid inclusions study documented here to provide new insight into the process of ore formation for the Au enrichment in the Jiapigou deposit

Characteristics of antimony mineralization in the Yangla polymetallic deposit, northwestern Yunnan, SW China: Insights from calcite Sm-Nd dating and C-O-Sr isotopes

Calcite is the main gangue mineral in antimony (Sb) deposits, and its compositions can reflect the physicochemical conditions of Sb mineralization. The Yangla is the largest Sb deposit (10 kt Sb @ 14.87 %) in the Jinshajiang suture zone (SW China), and the lode-type Sb orebodies are stratabound or developed along NE-trending fracture zones in marble. To constrain the time of Sb mineralization and establish any genetic link with the local magmatism and wallrocks, we performed calcite Sm-Nd dating and bulk C-O and in-situ Sr isotope analyses. The results show that the Sb mineralization (∼155 Ma) was considerably younger than the Cu-Pb-Zn mineralization (∼230 Ma), skarn alteration (∼234 Ma), and granitoid emplacement (∼230 Ma) at Yangla, but much older than the local W mineralization (∼30 Ma). The initial 87Sr/86Sr ratio of calcite (0.71972–0.72208) is much higher than that of the Triassic granodiorite (0.71149– 0.71990) and Carboniferous basalt (0.70562–0.70995), suggesting mixed source of calcite from the ore fluids and Devonian wallrocks. The ore-related calcite has δ13CPDB (−4.53 to − 2.33 ‰) and δ18OSMOW (+14.98 to + 16.30 ‰) values that fall between the granite and marine carbonate isotopic fields. This suggests that the ore-forming fluid may be related to the low-temperature alteration of granites and marine carbonate dissolution. Simulated precipitation temperature calculation for the ore-related calcite yielded 200–150 °C, and the calcite C-O isotopes suggest that fluid mixing, fluid-rock interactions, and CO2 degassing may have precipitated the stibnite in the fracture zones under low-temperature conditions. Our new geochemical results and published data suggest that the Yangla polymetallic mineralization was multiphase, comprising the Indosinian Cu-Pb-Zn (∼230 Ma), Yanshanian Sb (∼155 Ma), and Himalayan W-Sb (∼30 Ma) metallogenic events.

Pyrite in-situ S–Fe isotope constraints on the ore-forming sources and mineralization processes of gold and polymetallic deposits in the Liaodong Peninsula, North China Craton

Plenty of gold and polymetallic deposits are widespread in the eastern part of the North China Craton. They are associated with mafic to felsic dikes and hosted by Precambrian basement or Mesozoic granitoids. However, the sources of gold and other metals in the ore-forming fluids remain controversial. Here we present in-situ S and Fe isotopes and trace element contents of pyrites from various gold and Pb–Zn–(Ag) deposits in the Liaodong Peninsula, China. Pyrites from quartz vein-type gold deposit hosted by Mesozoic granites in the Wulong deposit have relatively homogeneous magmatic-like S isotopes (δ34S values of 0.9 ‰ to 2.5 ‰) and Co/Ni ratios, indicating derivation of sulfur and, by inference, of ore fluids/materials most likely from Mesozoic magmas. In contrast, pyrites from gold and Pb–Zn–(Ag) deposits in the Qingchengzi orefield hosted by Precambrian basement have high and variable δ34S values (9.7 ‰ to 12.7 ‰ for the Wandigou altered rock-type gold deposit and 4.7 ‰ to 8.5 ‰ for the Xiquegou Pb–Zn deposit and the Zhenzigou Pb–Zn–Ag deposit), identical to those of host rocks, indicating the important contributions of gold and other metals from wall rocks to the ore deposits. Pyrites from the various deposits have variable δ56Fe values of 0.08 ‰ to 0.63 ‰ for the Pb–Zn–Ag deposit, –0.58 ‰ to 1.23 ‰ for the altered rock-type gold deposit, and –0.68 ‰ to 0.77 ‰ for the quartz vein-type gold deposit, indicating distinct mineralization processes. Rapid precipitation of pyrites (with negative δ56Fe values) in the alteration rock and subsequent deposition of pyrites (with positive δ56Fe values) from the residual fluids in an Fe-open hydrothermal system during intensive ore-forming fluid-wall rock interaction account for the Pb–Zn–Ag mineralization, while weak fluid-rock interaction and pyrites precipitation from a Fe-closed hydrothermal system contribute to gold mineralization. Our observations provide a robust S–Fe isotope evidence for the contribution of various sources and metallogenic processes for distinct gold and polymetallic deposits.

Integrative approach for formation damage diagnosis in a Colombian brownfield: a comprehensive methodology

Formation damage is the reduction of a well's productivity due to the alteration of the permeability of the subsurface rock, leading to economically inefficient operations. This research established a methodology to diagnose such damage, which is divided into: 1) Identification of operational problems, 2) Field sampling and analysis of geological and engineering data, 3) Adaptation of API standards for the study of fluid-fluid and rock-fluid interactions, 4) Identification of damage mechanisms and recommendations. This methodology was applied to two depleted wells in a brown oilfield with reduced productivity to address scale deposition and casing corrosion issues. Finally, the application of control and stimulation fluids must satisfy technical and environmental requirements, with the objective of inducing destabilization of the identified formation damage mechanisms.

Ca–Zn isotope fractionation during fluid–rock interaction in subduction zones and its implication for deep carbon cycle

Subducting slabs transport Earth's surface carbon into the deep mantle, with a portion of this carbon potentially recycled through volcanic eruptions at mid-ocean ridges or ocean islands. Non-traditional stable isotopes such as calcium (Ca) and zinc (Zn) are increasingly used to trace this deep carbon cycle. However, the potential for Ca and Zn isotope fractionation in the subduction zone, critical for the successful application of these tracers, remains insufficiently explored. In this study, we investigate high-pressure carbonated metamafic rocks and enclosed vein systems from a Paleo-oceanic subduction complex (SW Tianshan) to explore the possible fractionation of Ca and Zn isotope during subduction-related metamorphism and fluid–rock interaction. The δ44/40Ca (0.77 ‰–0.90 ‰) and δ66Zn isotope compositions (0.20 ‰–0.39 ‰) of greenschist-, blueschist- and eclogite-facies metabasites are consistent with their protoliths (OIBs and MORBs). We observe no systematic fractionation of either Ca and Zn isotopes across prograde metamorphism and associated dehydration. However, the precipitation of carbonate in surrounding rock along fluid pathways leads to an increase in the δ44/40Ca of residual fluids (>0.90 ‰). More significantly, we find detectable Zn isotope fractionation (ΔA-H = ẟ66Znaltered - ẟ66Znhost of 0.06–0.10 ‰) during fluid-mediated metasomatism of some blueschists/eclogites. Light Zn isotopes are preferentially released into the infiltrating fluid, resulting in an elevated δ66Zn isotope composition in the immediate carbonated eclogites along the vein. The modified slab fluids during migration are likely charactered by heavy δ44/40Ca and light δ66Zn compositions, which may play a crucial role in determining the low δ66Zn values found in some arc lavas. However, the MORB-like δ44/40Ca compositions observed in global arc magmatic rocks suggest that the slab-released Ca (possibly along with some carbon) is probably sequestered and stored along the migration path of the fluid, thereby hindering the slab–arc carbon recycling. Our study emphasizes that intraslab fluid–rock interaction results in similar δ44/40Ca and δ66Zn signature in carbonate-bearing eclogites compared to sedimentary carbonates. Consequently, deeply subducted carbonate-bearing eclogites –not necessarily sedimentary carbonates– might carry light δ44/40Ca and heavy δ66Zn signatures into the deep mantle, potentially influencing the corresponding Ca-Zn isotope compositions of some mantle-derived basalts.

Origin of the Narenwula quartz vein-type W deposit, Inner Mongolia, China: Insights from geochemistry of hosting granites, fluid inclusions, H-O-S isotopes, and wolframite trace elements

Wolframite that coexists with quartz is common hydrothermal mineral in tungsten deposits, and its geochemical features can be used to constrain the tungsten mineralization. Here we show that the Narenwula deposit is a typical quartz vein-type W deposit on the northern margin of the North China Craton, Northeast China, and temporally and spatially associated with Early Cretaceous granitoids, provides an opportunity to decipher the W mineralization process. All the orebodies are composed of a series of northeast- trending wolframite-polymetallic sulfides bearing quartz veins accompanied by intense alterations, mainly hosted in the Early Cretaceous monzogranite and porphyraceous monzogranite. Three paragenetic stages were identified: (I) quartz-wolframite, (II) quartz-wolframite-sulfide, and (III) quartz-fluorite-calcite. LA−ICP−MS trace element analyses revealed that both wolframite (I) and (II) have similar trace elements, REEN patterns, and overall consistent Nb/Ta and Y/Ho ratios, indicate a single fluid source and evolution in the W mineralization of the Narenwula deposit. Fluid inclusions microthermometry of quartz and wolframite, coupled with H-O isotopes suggest that the early ore-forming fluids of the Narenwula deposit are mainly magmatic water with relatively moderate-to-high temperature and salinity, while the late ore-forming fluids are mixed with meteoric water, with medium-to low temperature and low salinity. The δ34S values of pyrite (4.63–6.65 ‰, average = 5.44 ‰) further support a magmatic origin for the sulfides. The metallogenic mechanisms at the Narenwula deposit include fluid boiling and fluid-rock interactions were the two major factors controlling the deposition of wolframite and sulfide, while simple cooling and fluid mixing may have also promoted wolframite precipitation.

Newly discovered Early Cretaceous Au mineralization overprinting Late Jurassic Pb-Zn-Ag mineralization in the Qin-Hang metallogenic belt (South China)

Magmatic and mineralizing events in South China were traditionally thought to be limited between 150 Ma and 130 Ma despite the numerous Mesozoic-aged magmatic-hydrothermal deposits in the region. This viewpoint is being reevaluated in light of new age data. Kangjiawan is a representative Pb-Zn-Ag-Au deposit in the Hengyang Basin in the Qin-Hang metallogenic belt of South China that contains 1.96 Mt of Pb-Zn resource with 4.70% Zn, 6.02% Pb; 2280.5 t of Ag resource with 108.23 g/t of Ag; and 58 t of Au resource with 3.25 g/t of Au. Its mineralization can be divided into four stages: (I) pre-ore quartz−pyrite, (II) quartz−pyrite−galena−sphalerite, (III) quartz−galena−sphalerite, and (IV) calcite−pyrite. Multiple generations of pyrite were identified at Kangjiawan. Generation I pyrite (Py1, formed in Stage I) is coarse-grained, texturally homogeneous, and contains abundant silicate inclusions (e.g., K-feldspar and titanite). Generation II pyrite (Py2, formed in Stage II) is generally fine-grained and exhibits a core-rim texture, with the porous, sphalerite−galena inclusion-bearing core (Py2a) having replaced Py1 and the rim (Py2b) being homogeneous. Generation III pyrite (Py3, formed in Stage IV) is subdivided into Py3a and Py3b, with the homogeneous Py3a often replaced by the porous Py3b. The low Co/Ni ratio of Py1, along with the abundant silicate inclusions, is suggestive of intense fluid-rock interaction during Stage I. Py2a is As-Cu-Pb-Ag-Au−rich, Co-depleted, and has elevated Ag/Co (average 3.24) and As/Co (average 10,217.14) ratios, which are suggestive of extensive fluid boiling during the formation of Py2a. This is also supported by the ubiquitous Py2-cemented hydrothermal breccia. In contrast, Py2b is depleted in As-Cu-Pb-Ag-Au but enriched in Co, which is indicative of non-boiling conditions. Subsequent sealing of fractures by veins may have caused the formation of coarse-grained sphalerite and galena in Stage III. The similar 207Pb/206Pb values among Py1 (average 0.849), Stage II galena (average 0.849), Stage III galena (average 0.849), and the nearby Shuikoushan granodiorite stock (corrected average 0.848) suggests that mineralization in stages I−III may represent distal skarn-type Pb-Zn-Ag mineralization genetically related to the Shuikoushan stock. This interpretation is supported by the similar ages of mineralization Stage I (apatite U-Pb: 159.4 ± 1.0 Ma) and Stage II (fuchsite 40Ar-39Ar: 158.1 ± 0.4 Ma) to the age of Shuikoushan stock (zircon U-Pb age: ca. 158 Ma). As the primary Au-hosting mineral at Kangjiawan, the As- and Au-rich Py3a occurs in sharp contact with porous Co-Ni-Se-Bi-Mo-As−poor Py3b, which commonly contains abundant fine-grained galena and sphalerite inclusions. This suggests that Py3b formed via coupled dissolution−re-precipitation of Py3a. This coupled dissolution−re-precipitation reaction favors the remobilization of Au from within the Py3a structure and its re-enrichment as Au inclusions in Py3b; this is supported by the anomalous Au peaks in time-resolved signal profiles from laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and the abundant outliers for Au concentrations in Py3b. The markedly distinct trace-element geochemistry of Py3 compared to those of Py1 and Py2, and the U-Pb age of Stage IV calcite (ca. 138 Ma), are indicative of an Early Cretaceous Au mineralization event. Published calcite C-O isotope data, coupled with the narrow range of δ34S values (0.6‰−2.2‰) of Py3, suggest that the Kangjiawan Au mineralization may be associated with a concealed Early Cretaceous pluton. We propose that the Kangjiawan deposit formed via the overprinting of Late Jurassic distal skarn Pb-Zn-Ag mineralization by Early Cretaceous magmatic-hydrothermal Au mineralization, which demonstrates that 150−135 Ma may also be an important period for the formation of the magmatic-hydrothermal mineralization belt.

Metasedimentary “carbon filter” and its implication for subduction zone carbon recycling

The movement of carbon in subduction zones plays a crucial role in regulating the global carbon cycle, controlling Earth's climate, and maintaining its habitability. Recent work suggests that only a fraction of the carbon released from subducting slabs at sub-arc depths is ultimately released from volcanic arcs, necessitating the existence of hidden carbon reservoirs within the slab-to-arc pathways. However, the precise location of these reservoirs remains enigmatic. Slab fluid serves as the primary medium for carbon transport in subduction zones; thus, a comprehensive understanding of fluid-rock interaction during slab fluid migration is essential for reconciling the carbon flux imbalance between the slab and the arc. In this study, we explore rock carbonation along a fluid conduit in the Southwestern Tianshan HP metamorphic belt in northwest China. Field evidence and petrologic observation reveal significant carbonation of a siliciclastic metasediment at its contact with a high-pressure garnet-bearing calcite (formerly aragonite) vein. We find that rock carbonation (by progressive Fe-bearing magnesite, dolomite, then aragonite precipitation) occurred when slab-derived carbonic fluids migrated through the metasedimentary sequence at approximately 80 km depth. Furthermore, modeling demonstrates that the metasedimentary layer atop the slab has the capacity to sequester 20%–50% of the fluid carbon from the ascending slab devolatilization flux. We propose that the metasedimentary veneer at the plate interface functions as a “carbon filter”, hindering the transfer of carbon from the slab to the arc and helping to reconcile the carbon flux imbalance between the amount released by the slab and that emitted by the arc. This study also provides insights into decarbonation efficiency and mechanisms, carbon-transfer pathways, and temporal aspects of the subduction zone carbon cycle.

Re-examination of vesbine in vanadate-rich sublimate-related associations of Vesuvius (Italy): Mineralogical features and origin

Abstract A set of 23 vesbine-bearing samples from Vesuvius volcano (Italy), preserved in the collection of the Royal Mineralogical Museum of Naples, University Federico II (RMMN), have been investigated to identify the mineral assemblages and their mode of formation. In the late 19th century, fumarole-related yellow patinas coating some historical lavas from Vesuvius were believed by Scacchi to contain a new element, vesbium, similar to vanadium in a mineral he called vesbine. Subsequent studies rejected vesbium and showed that vesbine was a poorly defined mixture of copper vanadates and halides. The vesbine samples studied here consist of yellowish to yellow-green-blue encrustations on Vesuvius lavas and have been analyzed by combined optical microscopy, SEM-EDS, XRPD, FTIR, and TEM-HRTEM-EDS. Results reveal complex mineral associations, including vanadates, halides, carbonates, oxides, silicates, tungstates/molybdates, and sulfates. The vanadates correspond to mottramite, volborthite, and vanadinite; subordinate amounts of descloizite were detected by XRPD and FTIR investigations. Several additional non-essential elements have been detected in the vanadates, including Mn, Zn, and As. The occurrence of wulfenite- and stolzite-rich phases indicates the presence of Mo and W, along with Pb, in the mineralizing fluids. Mn-rich phases, commonly in mixtures with silicates and vanadates, were also observed. These minerals are formed by a combination of different processes, including rock-fluid interactions, gas-water interactions, and alteration/oxidation of primary fumarolic minerals. Temperatures for the depositions of the vanadates-bearing assemblages are interpreted to be in the range of 100 to 400 °C.

Footwall refrigeration versus footwall hydration: Reassessing steep thermal gradients below detachment faults with in situ SIMS oxygen isotope analysis

The extensional detachment systems of metamorphic core complexes (MCC) have the potential to facilitate significant fluid movement between Earth's surface and the warm, ductile middle crust. One long-standing and influential detachment fault-fluid interaction model called the “footwall refrigeration hypothesis” (Morrison and Anderson, 1998) postulates that cool meteoric fluids circulating downward along a detachment can facilitate large-scale heat loss at depth before rocks are exhumed by faulting. Stable isotopes of oxygen and hydrogen have long been recognized as potentially useful tracers of surface-to-depth fluid flow in core complexes; however, typical sampling methods and the competing effects between temperature and fluid composition have made oxygen isotope signatures of meteoric fluid infiltration difficult to detect and/or interpret in the rock record.Here, we use in situ δ18O measurements by secondary ion mass spectrometry (SIMS) to address the rock record of meteoric fluid interactions and temperature variation within the Whipple Mountains MCC footwall beneath the Whipple detachment fault (WDF). The micro-scale sampling of SIMS (10 µm diameter x 1–2 µm deep pits) allows investigation of isotope variability as a function of mineral geochemistry and microstructure and enables more accurate interpretation of the causes of isotope variability. In the Whipple footwall mylonites, quartz and epidote show grain-to-grain oxygen-isotope variability within samples and as a function of distance from the main detachment. Approaching the WDF, both quartz and epidote show increasing spread toward low δ18O values. The lowest SIMS-measured δ18O epidote values require exchange with a low δ18O, meteoric fluid at high temperature. However, SIMS data also reveal that meteoric fluid-rock interactions were spatially heterogeneous at the scale of individual grains, resulting in local preservation of metamorphic quartz-epidote oxygen isotope equilibrium unaffected by infiltrating meteoric fluid. Within preserved metamorphic domains at all investigated structural levels of the WDF footwall, quartz and epidote δ18O values are tightly clustered and yield similar fractionations (∆18Oqz-ep). For samples from the longest footwall traverse, these ∆18Oqz-ep values are 4.0 ± 0.4‰, consistent with δ18O equilibrium at 521+43/-37 °C. We conclude that there is no evidence of a large paleo-thermal gradient associated with the Whipple Detachment Fault (i.e., no evidence for footwall refrigeration).

Scheelite as a microtextural and geochemical tracer of multistage ore-forming processes in skarn mineralization: A case study from the giant Xintianling W deposit, South China

The Xintianling deposit is one of the largest skarn-type scheelite deposit in China. Recent discoveries of quartz-vein-type scheelite mineralization within the deposit have raised questions about its origin and the evolution of ore-forming fluids, hindering a comprehensive understanding of the ore-forming process. We investigate the microtextures, trace elements, and oxygen isotope compositions of scheelite from different stages in both skarn-type and quartz-vein-type W mineralizations. Combined with apatite geochemistry and U–Pb dating, we determine the timing of quartz-vein-type mineralization and the evolution of magmatic-hydrothermal system. Based on detailed petrological observation, three types (seven subtypes) of scheelite and two types of apatite are identified. The Mo contents and Eu/Eu* ratios of scheelite, along with the Ce/Ce* ratios of apatite indicate the oxygen fugacity of fluids during the skarn metallogenic episode is generally higher than that during the quartz-vein metallogenic episode. Furthermore, the Y/Ho ratios and REE patterns of scheelite indicate the presence of at least three significant stages of fluid influx and fluid-rock interactions throughout entire ore-forming process. The O isotope compositions of scheelite in the quartz-vein-type metallogenic episode reveal that the ore-forming fluids are originated from a magmatic source, and meteoric water was mixed into the system, leading to the precipitation of the latest stage of scheelite. The apatites closely coexist with scheelite, from the early and the late stages of quartz-vein metallogenic episode, yield consistent U–Pb ages within error of 160.4 ± 2.4 Ma and 158.4 ± 1.3 Ma, respectively, indicating a close genetic link between quartz-vein-type W mineralization and the fine-grained porphyritic biotite granite. Our study highlights the significance of pulsed fluid exsolution and the combined effects of multiple mechanisms, including fluid-rock interaction, fluid mixing, and physicochemical condition changes, in the formation of large W deposits.

Fluid-rock interaction related to alpine subduction of Allalingabbro deduced from large-size μXRF element maps

Alpine subduction of a block of olivine gabbro produced a plethora of different assemblages and local structures typical of high-pressure and low-temperature metamorphism. However, gabbro with the locally well-preserved igneous assemblage olivine - augite - plagioclase occurs in some domains of a 2 km long outcrop (Allalinhorn, western Alps).A large variety of hydrate minerals including zoisite, glaucophane, talc, Mg-chloritoid, chlorite, paragonite and muscovite formed during Alpine subduction of the metagabbro. The three primary igneous minerals have been replaced by hydration reactions during Eocene subduction. Talc, omphacite, and zoisite + jadeite and are the main minerals in the local domains after olivine, augite and plagioclase respectively. The rocks preserved the coarse grained igneous texture.An excellent overview of the complex coarse textures of the eclogite facies rocks has been provided by element distribution maps produced from up to 25 cm large polished slabs of typical samples of Allalingabbro using a μXRF instrument. The RGB images suggest that the minerals formed at the high-pressure in the presence of an aqueous fluid. The pseudomorphing reactions progressed close to volume conservation. The hydration reactions required an H2O fluid containing dissolved solids. The mapped distribution of elements including trace elements provides important clues on the origin and mobility of these elements in the hydration fluid. The homogeneously distributed immobile Cr in omphacite portrays primary augite. The irregular patchy Sr distribution resulted from small-scale migration of components replacing plagioclase by Sr-bearing zoisite and jadeite. Coherent Ni signals are related to the presence of talc. The talc occurs as a late infill of structures produced by dissolving olivine. Mg-chloritoid, kyanite and mica coat the former grain-boundary of olivine and plagioclase suggesting that olivine dissolved at eclogite facies conditions. Talc formed at distinctly lower pressures, however, as implied by computed model reactions. The olivine replacement structures are often larger than the size thin section and can be excellently studied by the large size μXRF patterns used in this study.The fractured surroundings of the locations of olivine provided the necessary hydraulic conductivity for hydration reactions to progress. The fractures and small veinlets contain late Ni-bearing talc. Cl is only present in very late vein amphibole suggesting that hydration fluids in the subduction zone transported dissolved components as hydrous complexes (and their dissociated ions). There is no evidence for the presence of a Cl-brine during subduction. In addition to primary olivine as a local Ni source, the element may also have partly been imported with the hydration fluid and originate from dehydration of ophiolitic serpentinites in the slab.