Metamorphic Fluids Research Articles

Genesis of the Bailugou Vein-Type Zinc-Lead-Silver Deposit, Eastern Qinling Orogen, China: Constraints from Ore Geology and Fluid Inclusions

The Bailugou vein-type zinc-lead-silver deposit is located in the Eastern Qinling Orogen, China. There has been a long-standing debate about whether its formation is related to magmatism or metamorphism. To determine the origin of ore-forming materials and fluids, we conducted a geological and fluid inclusion investigation of the Bailugou. Field surveys show that the vein-type orebodies are controlled by faults in the dolomitic marbles of the Mesoproterozoic Guandaokou Group, and they are distal to the regional Yanshanian intrusions. Four ore stages, i.e., quartz–pyrite ± sphalerite (Stage 1), quartz–polymetallic sulfides (Stage 2), dolomite–polymetallic sulfides (Stage 3), and calcite (Stage 4), are identified through microscopic observation. The homogenization temperatures of measured fluid inclusions vary in the range of 100 °C to 400 °C, with the dominating concentration at 350 °C to 400 °C, displaying a descending trend from early to late stages. The estimated formation depth of the Bailugou deposit varies from 2 km to 12 km, which is deeper than the metallogenic limit of the epithermal hydrothermal deposit but conforms to the typical characteristics of a fault-controlled deposit. The ore-forming fluid in Stage 1 originates from a fluid mixture and experiences a phase separation (or fluid immiscibility) between the metamorphic-sourced fluid and the fluids associated with ore-bearing carbonate-shale-chert association (CSC) strata. This process results in the transition to metamorphic hydrothermal fluid due to water–rock interactions in Stage 2, culminating in gradual weakening and potential fluid boiling during the mineralization of Stage 3. Collectively, the Bailugou lead-zinc-silver mineralization resembles an orogenic-type deposit formed by metamorphic fluids in the Qinling Yanshanian intracontinental orogeny.

Gold Mineralization at the Syenite-Hosted Anwangshan Gold Deposit, Western Qinling Orogen, Central China

The Anwangshan gold deposit is located in the northwestern part of the Fengtai Basin, Western Qinling Orogen (WQO). The gold ore is hosted within quartz syenite and its contact zone. The U–Pb weighted mean age of the quartz syenite is 231 ± 1.8 Ma. It is characterized by high potassium (K2O = 10.13%, K2O/Na2O > 1) and high magnesium (Mg# = 55.31 to 72.78) content, enriched in large ion lithophile elements (Th, U, and Ba) and light rare earth elements (LREE), with a typical “TNT” (Ti, Nb, and Ta) deficiency. The geochemical features and Hf isotope compositions (εHf(t) = −6.68 to +2.25) suggest that the quartz syenite would form from partial melting of an enriched lithospheric mantle under an extensional setting. Three generations of gold mineralization have been identified, including the quartz–sericite–pyrite (Py1) stage I, the quartz–pyrite (Py2)–polymetallic sulfide–early calcite stage II, and the epidote–late calcite stage III. In situ sulfur isotope analysis of pyrite shows that Py1 (δ34S = −1.1 to +3.8‰) possesses mantle sulfur characteristics. However, Py2 has totally different δ34S (+5.1 to +6.7‰), which lies between the typical orogenic gold deposits in the WQO (δ34S = +8 to +12‰) and mantle sulfur. This suggests a mixed source of metamorphosed sediments and magmatic sulfur during stage II gold mineralization. The fluid inclusions in auriferous quartz have three different types, including the liquid-rich phase type, pure (gas or liquid)-phase type, and daughter-minerals-bearing phase type. Multiple-stage fluid inclusions indicate that the ore fluids are medium-temperature (concentrated at 220 to 270 °C), medium-salinity (7.85 to 13.80% NaCleq) CO2–H2O–NaCl systems. The salinity is quite different from typical orogenic gold deposits in WQO and worldwide, and this is more likely to be a mixture of magmatic and metamorphic fluids as well. In summary, the quartz syenite should have not only a spatio-temporal but also a genetical relationship with the Anwangshan gold deposit. It could provide most of the gold and ore fluids at the first stage, with metamorphic fluids and/or gold joining in during the later stages.

Cold-subduction biogeodynamics boosts deep energy delivery to the forearc

Metamorphic fluids in subduction zones carry C–H–N–O–P–S species, which are crucial for sustaining subsurface microbial life at shallower crustal depths in the forearc region. Upwards migration of deeply released fluids to shallower levels, where temperatures permit the persistence of microbial life, is recorded by metasomatic rocks formed along the plate interface. Variations in the redox state and component speciation of metamorphic fluids – from local to secular, and highly dependent on thermal gradients and redox state of subduction inputs – may strongly control microbial pathways or even the possibility for metamorphic fluids to sustain microbial communities in the subsurface biosphere at convergent plate margins. We show that metamorphic fluids containing reduced energy sources for microbial life – e.g., CH4, H2 – are common in Phanerozoic, high-pressure/low-temperature plate-interface metasomatic rocks such as jadeitites and albitites worldwide. Based on the stability fields of minerals hosting CH4, H2 and graphite inclusions, we pinpoint the protracted, probably episodic migration of energy sources in the mantle wedge via fluid circulation being mediated by jadeitites from > ca. 35 km depth, and by their retrogressed counterparts forming from between 35–15 km depth. These fluids can cross the so-called biotic fringe – whose limit is the depth corresponding to ca. 122–135 °C (as deep as ca. 13 km depth depending on geothermal gradients) – as suggested by previous documentation of slab-derived fluids reaching subsurface microbial communities. Thermodynamic modeling indicates that cool thermal gradients, possibly combined with increased inputs of organic matter-rich sediments into subduction, favor the abundance of reduced energy sources relative to more oxidized species (e.g., CO2), thus promoting the proliferation of subsurface microbial life at convergent margins.

Genesis and fluid evolution of the Hatu orogenic gold deposit in the West Junggar, Western China

The West Junggar in Xinjiang, western China, represents a significant gold mineralization belt hosting over 200 gold deposits, with the Hatu gold deposit being the largest among them. In this study, ore geology and fluid inclusion assemblages of quartz samples from the Hatu gold deposit were investigated in an effort to clarify the mineralization process and its relationship with the geodynamic settings. The mineralization process is delineated into early (pyrite-albite-quartz veins), middle-a (quartz-ankerite veins), middle-b (quartz-ankerite-sulfide-native gold veins), and late (quartz-calcite veins) stages. The early stage veins suggest a compressional setting while the middle-a and middle-b stage veins suggest a tensional shear setting. The late stage veins are typically filled fissures cutting through earlier veins. Scanning electron microscope-cathodoluminescence (SEM-CL) reveals that early stage quartz (Q1) exhibits concentric CL-oscillatory growth zoning, while middle-a stage quartz (Q2a) displays unidirectional zoning and ranges from CL-bright to CL-dark, middle-b stage quartz (Q2b) is CL-bright and weakly zoned, and late-stage euhedral quartz (Q3) shows sector zoning transitioning from CL-gray to CL-dark. Quartz that formed in these stages developed three types of fluid inclusions: pure CO2 (PC-type), CO2–H2O (C-type), and H2O–NaCl (W-type). The early stage quartz contains C- and W-type fluid inclusions homogenizing at 309–345 °C, while the late stage quartz contains only W-type fluid inclusions with homogenization temperatures of 164–229 °C. Microthermometry of fluid inclusion indicated the evolutionary of the metallogenic fluid from a high-temperature, CO2-rich, and minor CH4 metamorphic fluid to a low-temperature, CO2-poor meteoric fluid. From the early to middle-a stages, fluid boiling caused the unloading of ore-forming elements whereas fluid mixing led to the precipitation of polymetallic sulfides and gold in the middle-b stage. Trapping pressures in the middle-b stage were estimated using C-type inclusions, illustrating that gold mineralization took placed at depth of 8.0 km. We propose classifying the Hatu gold deposit as an orogenic deposit, originating from the collisional orogenesis between the Yili-Kazakhstan and Siberian continents in the Late Carboniferous.

The Carboniferous Wuzunbulake orogenic gold deposit in South Tianshan Orogen (NW China): Sericite Rb[sbnd]Sr geochronology, pyrite geochemistry, and metallogeny

The Kumishi area is located in the eastern part of the South Tianshan Orogen, which hosts several gold deposits and has substantial gold discovery potential. The timing of gold mineralization at Kumishi, however, has been poorly constrained owing to the absence of suitable dating minerals. Hydrothermal activity at Wuzunbulake is divided into the pre-ore stage 1 pyrite-quartz, syn-ore stage 2 quartz(-sulfide) and post-ore stage 3 quartz-calcite alteration/mineralization. Three types of pyrite have been recognized, i.e., Py1 (stage 1), Py2 (stage 2), and PyWR (from wallrock). Our in-situ RbSr dating on stage 2 sericite yielded an isochron age of 351.0 ± 17.4 Ma, indicating Early Carboniferous gold mineralization. Py1 and Py2 have δ34SΣS = 8.28–15.97 ‰ (avg. 12.88 ‰) and 6.92–8.70 ‰ (avg. 7.67 ‰), respectively, indicating that the sulfur in Py1 was metamorphic fluid sourced, while that of Py2 may have a mixed metamorphic fluid and wallrock source (0.84–3.27 ‰; avg. 2.31 ‰). For Py1, its contents of Au, As, Ag, Bi, Co, Cu, Mn, Ni, Pb, Sb, Tl are the lowest. Py2 has significantly higher Au-As-Ag, slightly higher Co-Cu-Ni-Sb-Tl, but lower Bi-Mn-Pb contents than those in PyWR. Considering also the sulfur isotope features, we considered that Py1 was primarily originated from the initial ore-forming fluid, and Py2 was derived from both the ore fluid and PyWR, with the former being more important and represents the source of gold. Based on that Py2 was formed by metasomatism on the PyWR margin and the element spatial coupling characteristics shown in EPMA geochemical maps, we inferred that the Au enrichment and precipitation are associated with fluid-rock reactions. The initial ore fluid is likely featured by the enrichments in Au, As, Ag, Co, Cu, Ni, Sb, Tl, and depletions in Bi, Mn, and Pb. The Wuzunbulake is best classified as an orogenic gold deposit based on its tectonic background, wallrock alteration style, and the ore-fluid source and characteristics.

The Permian Watershed tungsten deposit (northeast Queensland, Australia): fluid inclusion and stable isotope constraints

The Watershed scheelite deposit is located in an extinct fore-arc basin in the Mossman Orogen of North Queensland. This fore-arc region comprises multiply deformed, Ordovician–Silurian metasedimentary rocks of the Hodgkinson Formation, and it was intruded by Carboniferous–Permian granites of the Kennedy Igneous Association. At Watershed, the Hodgkinson Formation includes strongly deformed skarn-altered conglomerate, psammite and slate units, which record four deformation events that evolved from ductile (D1–3) to brittle–ductile (D4). Early, D1–2 scheelite mineralisation in Carboniferous monzonite and skarn-altered conglomerate formed during regional prograde metamorphism, which reached upper greenschist to lower amphibolite facies conditions. Permian, D4 scheelite mineralisation was deposited in transtensional, shear-related veins, vein haloes and skarn-altered conglomerate during retrograde, lower greenschist facies metamorphism. During D4, four stages of retrograde alteration (retrograde stages 1–4) affected the rocks. Fluid inclusion assemblages in retrograde stage 2, vein scheelite and quartz are characterised by a low-salinity H2O–NaCl ± CH4 fluid (XCH4 <0.01, 1.4–8.0 wt% NaCleq). The fluid inclusions show evidence for fluid mixing between a low (∼0 wt% NaCleq) and a medium (<8 wt% NaCleq) saline fluid. Scheelite mineralisation P–T conditions were determined at ∼300 °C and 1–1.8 kbar (i.e. a depth of 3.7–6.7 km), indicative of a high geothermal gradient (35–75 °C/km), which was likely caused by the heat from the Permian granites. The presence of pyrrhotite and arsenopyrite in D4 veins (retrograde stage 4), plus graphite and methane in the fluid inclusions in scheelite, indicates reduced mineralisation conditions. Oxygen isotope compositions (δ18OVSMOW) of retrograde stage 2 scheelite (+3.8 to +7.3‰), plagioclase (+7.0 to +11.8‰) and quartz (+12.6 to +15.5‰) indicate a fluid temperature of 306 ± 56 °C with δ18OVSMOW values between +4.7 and +8.3‰. Retrograde stage 3 muscovite δDVSMOW (−73.4 to −62.7‰) and δ18OVSMOW (+11.5 to +13.2‰) values were used to calculate the O–H isotopic compositions of the fluids in equilibrium with the minerals at various possible temperatures (250–300 °C). The results are consistent with a metamorphic origin for the mineralising fluid. Sulfur isotope compositions (δ34SCDT between −2.5 and +2.8‰) for vein-hosted, retrograde stage 4 sulfides indicate that sulfur could have come from seawater or seawater sulfate, which is consistent with the local geology, even though this range overlaps with magmatic sulfur isotope compositions. Metamorphic fluids probably originated from devolatilisation reactions in the Hodgkinson Formation during prograde metamorphism. Permian intrusions acted as heat source enhancing metamorphic fluid flow and metal transport.

Experimental metasomatic alteration of titanite in a series of metamorphic fluids at 700 °C and 200 MPa

Seven experiments exploring the reaction of titanite with various hydrothermal solutions have been carried out at 700 °C and 200 MPa for a run duration of 16 days. In experiments involving fluids consisting of NaCl+H2O, KCl+H2O, CaCl2+H2O, 2M NaOH, or 2M KOH, no reaction of the titanite with the fluid was observed other than a slight dissolution of the titanite. Experiments involving NaF+H2O and Ca(OH)2+H2O resulted in visible alteration of the titanite in texture and composition, coupled with the formation of perovskite. In the NaF+H2O experiment, perovskite, enriched with rare earth elements (REE), formed as euhedral to subhedral crystals on the surface of the recrystallized titanite. In the Ca(OH)2+H2O experiment perovskite took in minor amounts of REE, and formed as a reaction rim partially replacing the titanite via a coupled dissolution-reprecipitation reaction. Wollastonite, along with minor calcite, and grossular garnet, formed as an outer rim on the perovskite. In the NaF+H2O experiment major and trace elements were leached from the titanite, whereas in the Ca(OH)2+H2O experiment no leaching of major or trace elements was observed. Nb/Ta, Th/U, and Y/Ho were investigated as potential indicators of hydrothermal processes. While the Nb/Ta ratio was altered in the experimentally metasomatised titanite, the degree of alteration was the same for both fluids. In contrast, only small changes in the Th/U and Y/Ho ratios between the altered and original titanite were seen for either experiment. The formation of perovskite at the expense of titanite in NaF+H2O or Ca(OH)2+H2O fluids demonstrates how titanite reacts with these fluids in simple, low silica activity systems under mid to upper crustal P-T conditions.

Investigation of the mineralogical composition and origin analysis of black jadeite

Abstract Analyzing black jadeite variety is helpful not only to distinguish black jadeite from the common omphacite jadeite, but also to learn the origin of jadeite by studying these special specimens. The basic gemological properties, mineral composition, structural characteristics, spectroscopic features, and color-causing mechanisms of black jadeite were studied through testing methods such as Polarizing microscope, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy. Recent studies have confirmed the distinct crystalloblastic textures of black jadeite—namely columnar, granular, and fibrous—alongside its characteristic fracture structures. This research further identifies the presence of secondary minerals, primarily opaque black impurities consisting of a graphite and disordered graphite mixture, as revealed by Raman spectroscopy and X-ray diffraction analyses. These analyses have elucidated that the variable distribution of graphite mixtures contributes to differing degrees of light absorption, thereby forming various shades of black, a critical factor in the gemstone’s coloration. This study suggests that these carbonaceous materials are a byproduct of the carbonization of organic matter within metamorphic fluids, subsequently forming dispersed graphite structures within the mineral matrix and its fractures, indicative of multi-stage metamorphic processes. Additionally, the identification of graphite supports the theory that jadeite’s protolith originated from the subduction processes of the Neotethys Ocean, accompanied by metasomatism and high-pressure metamorphism of olivinite.

Fluid-mediated Cu and Zn isotope fractionation in subduction zones and implications for arc volcanism: Constraints from high pressure veins within eclogites in the Dabie Orogen

High-pressure (HP) veins in eclogites are the products of fluid-rock interaction and provide insight into the composition and evolution of fluids in subduction zones. We here present the Cu and Zn isotope data for different types of HP veins and their host eclogites from Ganghe and Hualiangting in the Dabie Orogen to reveal the behavior of Cu and Zn isotopes during fluid-rock interaction and fluid evolution. The HP veins include omphacite-epidote (Omp-Ep), epidote-quartz (Ep-Qtz), and kyanite-epidote-quartz (Ky-Ep-Qtz) veins. The Omp-Ep veins first crystallized from eclogite-derived, solute-rich fluids with the Ep-Qtz and Ky-Ep-Qtz veins successively crystallizing from the residual fluids after the Omp-Ep vein formation. The early Omp-Ep veins have variably lower δ65Cu (−1.32 to −1.12‰ at Ganghe, −0.70 to −0.66‰ at Hualiangting) but higher δ66Zn (0.36 to 0.38‰ at Ganghe, 0.40 to 0.41‰ at Hualiangting) relative to the host eclogites (δ65Cu: −0.71 vs. 1.84‰, δ66Zn: 0.22 vs. 0.33‰), indicating CuZn isotope fractionation during slab dehydration in subduction zones with the lighter Cu and heavier Zn isotopes preferentially entering the vein-forming fluids from the eclogites. Systematic increase of δ65Cu from the Omp-Ep (−0.70 to −0.66‰) through Ep-Qtz (0.01‰) to Ky-Ep-Qtz veins (0.46 to 0.95‰) can be attributed to isotope fractionation induced by redox changes during the evolution of metamorphic fluids, as revealed by the negative correlations of δ65Cu with redox-sensitive ratios of Fe3+/ΣFe and (Eu/Eu*)N in those veins. Additionally, the higher δ66Zn of the Omp-Ep veins relative to the Ky-Ep-Qtz veins can be explained by equilibrium isotope fractionation between crystallized minerals and evolved metamorphic fluids. Our results thus demonstrate that CuZn isotope fractionation occurred during the evolution of slab-derived metamorphic fluids in subduction zones. Binary mixing calculations show that fluids derived from dehydration of mafic rocks in subducted slabs, represented by the multistage HP veins in the present study, cannot account for the heavier Cu and lighter Zn isotope compositions in most arc magmas than in mid-ocean ridge basalts. This offset can be resolved by addition of forearc serpentinite-derived fluids enriched in heavy Cu and light Zn isotopes into the mantle source of arc magmas.

A supergene-hydrothermal origin of the itabirite-hosted high-grade iron ores in the Mbarga prospect, Mbalam iron ore district, southern Cameroon, Congo Craton

The Mbarga itabirite deposit in the Mbalam iron district on the northwest edge of the Congo Craton (CC) hosts two main types of iron ore enrichments: supergene and specularite ores. This study presents mineralogical, geochemical, and isotopic datasets on these ores to determine their genesis.Ore microscopic studies indicate that the itabirites are of the oxide facies type, with magnetite showing partial to extensive alteration to hematite-martite. The supergene ores consist of hematite + martite + goethite ± gibbsite ± magnetite ± quartz, while the specularite ores are mainly composed of hematite + martite ± quartz. Magnetite microchemistry suggests formation under low-T hydrothermal conditions (~200–300 °C) with high fO2. Geochemical analyses show that the supergene and specularite ores have higher Fe2O3 (88.27 to ~100 wt%) and lower SiO2 (<0.01 to 0.18 wt%) contents than the itabirites (31.95 wt% Fe2O3, 67.16 wt% SiO2). The enrichment of Fe in the supergene ores is attributed to the depletion of major oxides and trace elements due to weathering and supergene enrichment, while the high Fe content in the specularite ores stems from the precipitation of iron-rich, but trace- and rare earth elements (REE)-deficient hydrothermal fluids. The slightly higher Al2O3 content and positive Ce anomalies in the supergene ores suggest the retention of Al-bearing minerals (gibbsite) and reveal highly oxidative conditions during martitization. Stable isotope analyses reveal that the supergene and specularite ores have δ18O values of −2.5 to −0.3 ‰ and − 2.0 to −3.4 ‰, and δ2H values of −75 to −123 ‰ and − 70 to −119 ‰, respectively, suggesting the involvement of isotopically light-evolved meteoric water in their formation. In contrast, the itabirites exhibit heavier δ18O (8.5 to 10.2 ‰) and δ2H (−85 to −91 ‰) values, suggesting formation from mixed magmatic and metamorphic fluid sources. A “polygenic-supergene-hydrothermal” model is suggested for the formation of the Mbarga itabirite-hosted iron ores.

The chromitites of the Herbeira massif (Cabo Ortegal Complex, Spain) revisited

The ultramafic rocks of the Herbeira Massif in the Cabo Ortegal Complex (NW Iberia) host chromitite bodies. The textural and compositional study of the host rocks and the chromitites classified them into: (1) Type-I chromitites, forming massive pods of intermediate-Cr chromite (Cr# = 0.60–0.66) within dunites; and (2) Type-II chromitites forming semi-massive horizons of high-Cr chromite (Cr# = 0.75–0.82) interlayered with dunites and pyroxenites. Minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V and Sc) contents in the unaltered chromite cores from both types show patterns very similar to fore-arc chromitites, mimicked by the host dunites and pyroxenites. Calculated parental melt compositions suggest that Type-I chromitites crystallized from a melt akin to fore-arc basalt (FAB), while Type-II chromitites originated from a boninite-like parental melt. Both melts are characteristic of a fore-arc setting affected by extension during rollback subduction and have been related to the development of a Cambrian-Ordovician arc. These chromitites are extremely enriched in platinum-group elements (PGE), with bulk-rock PGE contents between 2,460 and 3,600 ppb. Also, the host dunites and pyroxenites exhibit high PGE contents (167 and 324 ppb, respectively), which are higher than those from the primitive mantle and global ophiolitic mantle peridotites. The PGE enrichment is expressed in positively-sloped chondrite-normalized PGE patterns, characterized by an enrichment in Pd-group PGE (PPGE: Rh, Pt and Pd) over the Ir-group PGE (IPGE: Os, Ir and Ru) and abundant platinum-group minerals (PGM) dominated by Rh-Pt-Pd phases (i.e. Rh-Ir-Pt-bearing arsenides and sulfarsenides, Pt-Ir-Pd-base-metal-bearing alloys, and Pt-Pd-bearing sulfides). The PGM assemblage is associated with base-metal sulfides (mostly pentlandite and chalcopyrite) and occurs at the edges of chromite or embedded within the interstitial (serpentinized) silicate groundmass. Their origin has been linked to direct crystallization from a S-As-rich melt(s), segregated by immiscibility from evolved volatile-rich small volume melts during subduction. At c. 380 Ma, retrograde amphibolite-facies metamorphism occurred during the exhumation of the HP-HT rocks of the Capelada Unit, which affected chromitites and their host rocks but preserved the primary composition of chromite cores of the chromitites. This event contributed to local remobilization of PGE as suggested by the negative slope between Pt and Pd and high Pt/Pd ratios in the studied chromitites, and host dunites and pyroxenites. In addition, it promoted the alteration of primary PGM assemblage and the formation of secondary PGM. Nanoscale observations made by focused ion beam high-resolution transmission electron microscopy (FIB/HRTEM) analysis of a composite grain of Rh-bearing arsenide with PGE-base-metal bearing alloys suggest the mobilization and accumulation of small nanoparticles of PGE and base-metals that precipitated from metamorphic fluids forming PGE-alloys. Finally, we offer a comparison of the Cabo Ortegal chromitites with other ophiolitic chromitites involved in the Variscan orogeny, from the Iberian Peninsula to the Polish Sudetes. The studied Cabo Ortegal chromitites are similar to the Variscan chromitites documented in the Bragança (northern Portugal) and Kraubath (Styria, Austria) ophiolitic massifs.

Chromian minerals and talc veins in the metasandstones of the Ochi nappe, southern Evia, Greece: The role of fluids in retrograde metamorphism during exhumation

Unusual minerals were formed during the retrograde metamorphism of high-pressure metamorphic rocks in the Ochi nappe, southern Evia, Greece, specifically widespread sub-mm-scale talc veins and chromian varieties of chlorite, phengite, biotite and allanite. Albite-quartz-calcite and calcite-quartz schists are cut by common 10–200 μm wide veins of talc, rimmed by Cr-chlorite with <0.6 apfu Cr. Hydraulic fracturing by fluids derived from dehydration of subducted chromite-rich serpentinite in the Ochi nappe created fractures in the schists where talc precipitated. Mineral chemistry and textures are related to regional metamorphic and tectonic phases, which included Eocene blueschist metamorphism followed by Oligocene syn-orogenic wedge extrusion. The overprint resulted in retrograde growth of albite porphyroblasts, emplacement of talc veins, and formation of phengite with <0.08 apfu Cr and later biotite with <0.3 apfu Cr. Late Oligocene shearing flattened many pre-existing minerals. Neogene exhumation within the North Cycladic detachment system produced decompression fractures that nucleated precipitation of allanite (some with <0.4 apfu Cr and > 0.8 apfu REE), apatite, titanite and zircon, reflecting the availability of halogen-rich fluids. The evolution of the retrograde metamorphic minerals assemblages can thus be tracked, with pervasive Cr-Mg infiltration in the Oligocene, and more focussed Miocene metasomatism by halogen-rich fluids.

Fluid source and ore-forming process of the Lishupo Au deposit, NE Hunan: Constraints from fluid inclusions, SIMS oxygen isotope, LA-ICP-MS pyrite trace elements and fsLA-ICP-MS quartz trace elements

The Lishupo Au deposit (4.3 t @ 4.76 g/t) is located in the central Jiangnan Orogen. The processes and origins of regional gold mineralization, as well as the characteristics of the ore-forming fluids, continue to be poorly understood. This study integrates field geology, fluid inclusions, secondary ion mass spectrometry (SIMS) O isotope, femtosecond laser ablation inductively coupled plasma mass spectrometry (fsLA-ICP-MS) trace elements analysis for quartz, and LA-ICP-MS for trace elements study of pyrite to constrain the nature and source of ore-forming fluids, mechanism of gold precipitation, and ore genesis. Field investigations and microscopic observations reveal three mineralization stages at Lishupo: (1) the quartz-pyrite (Qtz1-Py1) stage; (2) the quartz-dolomite-polymetallic sulfides-gold (Qtz2-Py2) stage; and (3) the quartz-carbonate-pyrite (Qtz3-Py3) stage.Cathodoluminescence (CL) observations reveal that Qtz1 and Qtz2 have a homogeneous texture, while Qtz3 exhibits oscillatory zoning. Fluid inclusions in quartz suggest a CO2-H2O-NaCl ± N2 ± CH4 fluid system, which comprise H2O-CO2-NaCl, CO2-rich, and aqueous types. Microthermometry of fluid inclusions indicates low salinity (stage 1: 3.7–7.9 wt% NaCl equiv.; stage 2: 2.2–7.5 wt% NaCl equiv.; stage 3: 1.4–3.9 wt% NaCl equiv.) and medium to low temperature (stage 1: 313–341 °C; stage 2: 209–286 °C; stage 3: 139–198 °C) for ore-forming fluids, consistent with the low Ti content (<10 ppm) found in the quartz. The low Al content (7.99–20.7 ppm) of Qtz1 suggests that the ore-forming fluids were saturated with CO2 and had a near-neutral pH. The Al contents are higher and more variable in Qtz2 (10.7–1959 ppm) and Qtz3 (21.6–713 ppm), which may imply fluctuations in fluid pH. δ18OFluid values for various types of quartz (Qtz1: 9.02–10.95 ‰, Qtz2: 6.30–7.68 ‰, Qtz3: 3.45–3.94 ‰) indicate a metamorphic fluid origin for stages 1 and 2 fluids, which were mixed with the meteoric water during stage 3. The highest concentrations of Au and As in Py2 suggest that the Qtz2-Py2 stage is the primary ore stage. The Co/Ni ratios of all types of pyrite are less than 1, indicating that the ore-forming materials are derived from metamorphic sedimentary rocks. Fluid inclusion microthermometry suggests that fluid immiscibility was responsible for the gold deposition during the Qtz2-Py2 stage. Fluid immiscibility destabilizes Au(HS)2-, leading to the precipitation of native gold. These data strongly suggest that the Lishupo gold deposit is classified as an orogenic-type deposit. Integrating the existing geochronology data and geological context, we emphasize the presence of the Late Triassic orogenic gold deposit in the Jiangnan orogen. This highlights a significant exploration target area within this zone for future gold prospecting.

MINÉRALISATION POLYMÉTALLIQUE Zn-Pb-Cu (Au, Ag, V) DU GISEMENT DE BAMBA-KILENDA, DANS L’AVANT-PAYS DE LA CHAÎNE WEST CONGO (WCB) : PHASES DE MINÉRALISATION ET NATURES DES FLUIDES »

Bamba-Kilenda deposit is located 70 km (as the crow flies) south of Kinshasa. The studies of Verhaegen (2005) based on fluid inclusions made it possible to highlight two spatially and temporally distinct episodes of mineralization. The decrepitation technology used by Verhaegen (2005) remains imprecise. For this, Fe-Ti, Ca-Ti and Fe-Ca geothermometry and the alkalinity index evaluation (K+Na/Ca) proposed Intiomale (2013 and 2014) uses those as part of this study. The staging of mineral concentrations reflects the installation in the FK2/4a drilling of the Bamba-Kilenda deposit, from bottom to top: 1) Phase of sedimentary ores (Phase 1), the pyrite sheath spared by oxidation and impurities provides information on an alkanity index of 0.21 typical of fluids of sedimentary origin; 2) Installation phase by orogenic fluids (Phase 2) of the Mississippi Valley (MVT) type, with alkalinity varying between 3.5 – 6.5 and temperatures 56 – 70°C; 3) Installation phase by metamorphic fluids (Phase 3), rich in copper and zinc, at a temperature of 400°C. The Bamba-Kilenda FK2/4a drilling did not intersect veins of Qtz-Cu-Fe coming from thermal reflux fluids (THERMOREF: Phase 4), the ores formed between 455 - 475°C ores and no longer the ores linked to N-S rift faults (POST-RIFT: Phase 5) observed at Nsienene Wogila and Mbusu, observed in the foreland of the West Congo Belt. Finally, meteoric fluids (Phase 6) infiltrated into the fault zone reworked and oxidized the ores observed in the Bamba-Kilenda FK2/4a borehole.

Spatial variation of body wave attenuation in Garhwal-Kumaun Himalaya region, India

We have investigated the spatial variation of body wave attenuation in Garhwal-Kumaun Himalaya region from the datasets of 465 well located earthquakes, recorded at 52 broadband stations between 2017 and 2020. The body wave attenuation parameters QP and QS were estimated at each station by applying the extended coda normalization method for five different central frequencies ranging from 1.5 to 18 Hz. Strong frequency-dependent body wave attenuation was observed at each station over the whole region. Also, we found a significant variation of QP and QS from north to south which is consistent with geotectonic diversity beneath the study region. We have also made separate estimations of frequency-dependent relations for Garhwal and Kumaun Himalaya region in order to investigate the lateral variation in attenuation characteristics, and obtained the frequency-dependent relations as follows: QP=30±3f1.05±0.05, QS=143±20f0.88±0.07 for the Garhwal Himalaya region and QP=31±1f1.09±0.02, QS=121±11f1.00±0.04 for the Kuamun Himalaya region. Obtained Q values indicate strong body wave attenuation for both the region with no significant lateral variation. It may suggest the presence of crustal level folding, faulting, aqueous fluids, and metamorphic fluids below both the regions. Also, the ratios of QS/QP are high (>1) for the entire analyzed frequency range, suggesting a significant level of heterogeneity and tectonic complexities in the crust of the study region.

Fluid–Melt–Rock Interaction during the Transition from Magmatism to HT-UHT-Granulite- and Amphibolite-Facies Metamorphism in the Ediacaran Adrar–Suttuf Metamafic Complex, NW Margin of the West African Craton (Southern Morocco)

Abstract Underplated mafic intrusions ponded at the base of the lower continental crust in extensional settings can experience ultra-high-temperature (UHT) granulite-facies metamorphism during tens of My due to slow cooling rates. These intrusions are also the source of heat and carbonic fluids for regional high-temperature (HT) granulite-facies metamorphism in the continental crust. This work analyses the fluid–melt–rock interaction processes that occurred during the magmatic to HT-UHT-granulite- and amphibolite-facies metamorphic evolution of high-grade mafic rocks from the Eastern Ediacaran Adrar–Suttuf Metamafic Complex (EASMC) of the Oulad Dlim Massif (West African Craton Margin, Southern Morocco). P–T conditions were determined using Ti-in-amphibole thermometry, two-pyroxene and amphibole–plagioclase thermobarometry, and phase diagram calculations. The thermobarometric study reveals the presence of tectonically juxtaposed lower- and mid-crustal blocks in EASMC that experienced decompression-cooling paths from, respectively, UHT and HT granulite-facies conditions at ca. 1.2 ± 0.28 GPa and 975 ± 50°C, and ca. 0.82 ± 0.15 GPa and 894 ± 50°C, to amphibole-facies conditions at ca. 0.28 ± 0.28 GPa and 787 ± 45°C (precision reported for the calibrations at 1 s level). An age for the magmatic to UHT granulite-facies metamorphic transition of 604 Ma was constrained from published SHRIMP Th–U–Pb zircon ages of the igneous protoliths. An amphibole 40Ar–39Ar cooling age of 499 ± 8 Ma (precision at 2 s level) was obtained for the lower-crustal blocks. Amphibole 40Ar–39Ar closure temperatures of 520–555°C were obtained for an age range of 604–499 Ma and an average constant cooling rate of 4.2°C/My, suggesting that the lower-crustal blocks cooled down to the greenschist–amphibolite facies transition in ca. 100 My. During the high-temperature stage, interstitial hydrous melts assisted textural maturation of the rock matrix and caused incongruent dissolution melting of olivine and pyroxenes, and, probably, development of An-rich spikes at the grain rims of plagioclase, and local segregation of pargasite into veins. Subsequent infiltration of reactive hydrous metamorphic fluids along mineral grain boundaries during cooling down to amphibolite-facies conditions promoted mineral replacements by coupled dissolution-precipitation mechanisms and metasomatism. Ubiquitous dolomite grains, with, in some cases, evidence for significant textural maturation, appear in the granoblastic aggregates of the high-grade mafic rocks. However, calculated phase relationships reveal that dolomite could not coexist with H2O–CO2 fluids at HT-UHT granulite- and low-medium P amphibolite-facies conditions. Therefore, it is proposed that it may have been generated from another CO2-bearing phase, such as an immiscible carbonatitic melt exsolved from the parental mafic magma, and preserved during cooling due to the prevalence of fluid-absent conditions in the granoblastic matrix containing dolomite. The lower-crustal mafic intrusions from EASMC can represent an example of a source of heat for granulitisation of the mid crust, but a sink for carbon due to the apparent stability of dolomite under fluid-absent conditions.

Unveiling the connection between lode gold mineralization and the metamorphic core complex evolution from the large Yangzhaiyu gold deposit, North China Craton

The Xiaoqinling gold province, located in the Neoarchean−Paleoproterozoic uplifted footwalls of the Xiaoqinling metamorphic core complex (XMCC), is one of China’s largest gold producers; however, achieving a consensus regarding their metallogenic model remains elusive. Scheelite is an indicator mineral that commonly occurs in lode gold deposits worldwide used to recognize deposit types and understand hydrothermal evolution and the origin of features. Xenotime, monazite, and rutile are common hydrothermal minerals in association with lode gold deposits worldwide. Here, we provide textual, in situ U-Pb geochronology of xenotime, monazite, and rutile, and in situ elemental and Sr-Nd isotopic compositions of scheelite within different stages from the large Yangzhaiyu lode gold deposit, aiming to elucidate its genesis and, for the first time, establish a holistic correlation between the lode gold mineralization and the evolution of the XMCC. Notably low εNd(t) values (−30.7 to −23.7), high 87Sr/86Sr ratios (0.72659−0.75914), and distinct rare earth elements, Sr, Mo, and As contents of scheelite confirm a metamorphic crustal source. Xenotime U-Pb dating and pre-ore (Stage I) scheelite reveal that ore-barren metamorphic fluids at ca. 140 Ma were oxidized with low Bi contents and buffered by greenschist facies metamorphism when the XMCC initiated. Monazite and rutile U-Pb dating combined with ore-stage scheelite geochemistry indicate a compositional shift in the more reduced auriferous metamorphic fluids, which dominated during major gold deposition periods (stages II and III) from 130 Ma to 120 Ma, characterized by significantly depleted Na and increased Bi contents. This resulted from the prograde greenschist-to-amphibolite metamorphism at mid-lower crustal depths as the result of the XMCC isostatic doming and the lithospheric mantle thinning after 130 Ma. This study highlights the crucial role of metamorphic core complexes in governing the timing, locations, and resources of the lode gold metallogenic system.

Genesis of a sediment-hosted vein-type Cu deposit in the Lanping Basin, SW China: New insights from geology and LA–ICP–MS analysis of fluid inclusions

Sediment-hosted Cu deposits, including stratiform and epigenetic vein-type Cu deposits, are the second largest source of Cu worldwide. However, many facets of these vein-type Cu deposits, such as the nature and origin of ore fluids and geodynamic mechanisms that produced these fluids, remain controversial, limiting our interpretations of the ore genesis and hampering the selection of exploration strategies. Numerous sediment-hosted vein-type Cu deposits are present in the Lanping Basin, SW China, of which the Jinman deposit is the largest. Copper orebodies at Jinman consist of Cu-sulfide–quartz–carbonate veins and altered wall-rocks hosted mainly in faults and fracture zones in metamorphosed and deformed clastic rocks. The deposit was formed in three stages, pre-, syn- and post-ore, with the first two stages producing hydrothermal veins. The veins in pre-ore stage consist of quartz + ankerite ± sericite ± pyrite, whereas those in syn-ore stage consist of Cu-sulfides + quartz + carbonate ± sericite. During the syn-ore stage, tennantite, quartz, and ankerite crystallized early, then bornite was precipitated, followed by chalcopyrite and chalcocite and finally calcite. Only minor sericite was formed during the post-ore stage. Two major types of fluid inclusions—CO2-rich and aqueous inclusions—are present in both pre- and syn-ore quartz, whereas only aqueous inclusions are present in syn-ore calcite. In pre- and syn-ore quartz and calcite, CO2-rich inclusions have higher homogenization temperatures (275–323 °C) and pressures (307–3495 bar) and lower salinities (0.22–2.7 wt% NaCl eqv.) than aqueous inclusions (164–299 °C, 6.5–80.4 bar and 3.7–20.9 wt% NaCl eqv.). However, the small variations in Cs/Na and Cs/(Na + K) ratios [log Cs/Na = –3.3 ∼ –1.9, log Cs/(Na + K) = –3.3 ∼ –2.1] of the fluid inclusions suggest a same source for the ore fluids. We propose that during both stages, CO2-rich inclusions resulted from unmixing of initial deep-seated, single-phase CO2-rich fluids, whereas aqueous inclusions likely represent the fluids that evolved from these unmixed fluids by extensive CO2 degassing. This interpretation, combined with microthermometric results of CO2-rich inclusions, suggests a mesothermal, low-salinity H2O–NaCl–CO2 ore fluid system. The fluid inclusions from Jinman have similar compositions to those of metamorphic fluids, suggesting a metamorphic origin for the ore-forming fluids. The evolution of fluid compositions suggests that fluid degassing likely acted as a major mechanism for Cu precipitation. We propose that the ore fluids were generated through metamorphic devolatilization of basement of the Lanping Basin in the Biluoxueshan–Chongshan metamorphic zone under a compressional or transpressional regime caused by Cenozoic India–Asia collision. The Jinman deposit has many similarities to orogenic gold deposits worldwide and thus can be thought of as an orogenic deposit. Based on a compilation of the geological characteristics of global sediment-hosted epigenetic vein-type Cu deposits, we suggest that other sediment-hosted epigenetic vein-type Cu deposits with similar genesis to the Jinman deposit are likely present in other regions worldwide. This highlights the exploration potential of this type of deposit in these regions and the need for further research.

Mineral precipitation sequence from multi-stage fluids released by eclogite during high-pressure metamorphism

Abstract Arc magmas above subduction zones hold abundant fluid-mobile elements attributed to fluids released from the dehydrating subducted oceanic crust. However, the quantity of trace elements in the fluids and their evolution with the metamorphic processes during subduction and exhumation are still unclear. The precipitation sequence of vein minerals preserves the nature of multi-stage high-pressure (HP) metamorphic fluids and the fingerprint of mass exchange in deep subduction zones. In this contribution, we conducted detailed petrological studies and phase equilibria modeling on a unique HP omphacite-rich vein and its host eclogite from the Chinese southwestern Tianshan. The host eclogite consists mainly of garnet, omphacite, epidote, glaucophane, phengite, quartz, and rutile. Garnet in the eclogite records prograde subduction and early exhumation characterized by decompression heating at P-T conditions of ~2.4–2.6 GPa and 460–540 °C. The embedded omphacite-rich vein has similar mineral assemblage to the host eclogite. Garnet grains in this vein are predominantly distributed along or intersect the vein wall, which record similar eclogite-facies metamorphic conditions to that of the host eclogite. Omphacite is dominant in the vein, while epidote and glaucophane occur interstitially. Phase equilibria modeling reveals a sequential growth of garnet-dominated, omphacite-dominated, and epidote-dominated assemblages from fluids originating from the breakdown of different hydrous minerals. These lines of evidence suggest that the formation of multi-stage HP fluids are a continuous long-term process with a spontaneously short-distance transport and sequential mineral precipitation. Calculated fluid compositions demonstrate that the fluids released by lawsonite breakdown during exhumation have great potential to modify the trace element systematics of arc magmas. Our findings reveal the nature and evolution of multi-stage HP metamorphic fluids from internal sources during subduction and exhumation of oceanic crust, providing valuable insights into the chemical compositions of arc magmas.

Discriminating Superimposed Alteration Associated with Epigenetic Base and Precious Metal Vein Systems in the Rouyn-Noranda Mining District, Quebec; Implications for Exploration in Ancient Volcanic Districts

Abstract The Rouyn-Noranda mining district of Quebec contains 20 Cu-Zn (±Au ±Ag) volcanogenic massive sulfide (VMS) deposits, including the giant and gold-rich Quemont and Horne deposits. Mineralized epigenetic veins are also present, but their origin and relative timing remain enigmatic. The nature and extent of their alteration signatures and the effect of their superposition on district-scale alteration patterns is unknown. The VMS-related quartz-sulfide Cu-Zn-Ag veins have δ18Oquartz values of 8.5 ± 0.8‰, reflecting δ18Ofluid compositions of –0.4 to 3.1‰ (250°–350°C) that are typical of Archean seawater. They are associated with a proximal Fe-rich chlorite alteration and marginal spotted sericite-chlorite alteration with whole-rock δ18O values of 2.9 to 5.9‰ and are interpreted to have formed within the structurally controlled discordant upflow zones of a VMS hydrothermal system. Younger gold-bearing quartz-carbonate veins were emplaced along mechanical anisotropies created by mafic dikes during north-south compression and the formation of regional E-trending faults, folds, and cleavage. They are characterized by δ18Oquartz values of 11.3 ± 0.8‰, reflecting δ18Ofluid compositions of 2.4 to 5.9‰ (250°–350°C), typical of a metamorphic fluid, possibly mixed with a lower δ18O upper crustal fluid. They are associated with ankerite, calcite, muscovite, chlorite, albite, and quartz ± hematite alteration with whole-rock δ18O values of 5.8 to 10.3‰. Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon ages for two tonalite intrusions constrain the maximum age of the Cu-Zn-Ag veins to 2697.6 ± 0.7 Ma and the minimum age to 2695.3 ± 1.0 Ma, which is also the maximum age of the gold quartz-carbonate veins. Superposition of alteration related to the gold quartz-carbonate veins on previously chlorite- and sericite-altered rocks has resulted in mixed alteration signals with whole-rock δ18O values of ~6 to 8‰ that have perturbed and masked regional alteration patterns related to older VMS mineralization, such as those found in the Quemont and Horne deposits. These results indicate that defining alteration vectors in camps that have superimposed hydrothermal systems requires full consideration of the hydrothermal history of the camp, and if such constraints are lacking, whole-rock δ18O values should not be used as a stand-alone exploration method.