Fluid-rock Interaction Processes Research Articles

Omphacitite formation and fluid-rock interaction processes in an intra-slab eclogite-facies shear zone

We document the formation of omphacitites in the Monviso Lago Superiore Unit (W. Alps) where eclogite-facies, serpentinite-bearing shear zones host ample evidence for intense intra-slab fluid flow. In the first locality studied, a serpentinite-hosted jadeitite block is rimmed by strained omphacitite (with lawsonite pseudomorphs and minor phengite). U-Th-Pb zircon dating yields a protolith age of c.150 Ma, thus pointing to a burial-related replacement of a former Tethyan seafloor material, and revealed a lack of alpine rims. Multi-mineral Rb-Sr dating of the minerals forming this omphacitite rind yields an isochron age of 41.6 ± 0.7 Ma, corresponding to the early exhumation path still within the eclogite-facies. In this case, omphacitization led to the nearly complete replacement of the former prograde jadeitite through solution-precipitation and was followed by crystal-plastic deformation processes.In various Monviso localities, Fe-Ti metagabbro blocks (commonly exhibiting brecciated garnetite fragments) display evidence for intense fluid-rock interactions leading to the formation of decimeter-thick, weakly strained omphacite-dominated rinds (with minor amounts of lawsonite, clinochlore and talc) around the blocks at the contact with the surrounding serpentinite. Partly dissolved zircon crystals separated from omphacitite rimming an eclogitic Fe-Ti metagabbro block only yielded middle-Jurassic protolith ages. This indicates that omphacitite rinds did not overgrow the eclogite blocks but rather replaced metasomatically the garnet-rich eclogitic assemblage. The omphacitization overprint occurred independently of the nature of the protolith (namely, forming after a jadeitite or after an Fe-Ti eclogite) as a consequence of infiltration of externally-derived fluids and subsequent element sequestration from the incoming fluid phase. These observations highlight the extraordinary ability of eclogite-facies metasomatic fluids to nearly totally overprint rocks as resistant and impermeable as Monviso eclogite breccias or jadeitites.

Hydrothermal alteration and its geochemistry of the Xiadian gold deposit, Jiaodong Peninsula, China: Implications for fluid-rock interaction processes and mineral exploration

The interaction between hydrothermal fluids and host rocks, along with associated alteration geochemistry within the Mesozoic Jiaodong gold system, presents significant gaps in our understanding. This study systematically examines the petrographic, mineralogical, and geochemical characteristics of alteration at the Xiadian gold deposit, including comprehensive whole-rock geochemistry analyses, mass balance calculations, and phase equilibria modeling. The alteration history of Xiadian could be divided into four stages. Amid Stage 1, the predominant process involves the replacement of plagioclase by K-feldspar-albite, accompanied by the simultaneous bleaching of amphibolite and associated significant depletion of MgO (ΔCi/CiO = −21.2 %) and Fe2O3 (−27.2 %) owing to the decomposition of mafic minerals. Stage 2a involves sericitization of albite and continued mafic mineral decomposition, resulting in the lowest recorded MgO (avg. 0.23 wt%) and Fe2O3 (avg. 0.92 wt%) contents in all alteration lithologies. Stage 2b is characterized by abundant chlorite vein formation, corresponding to the proximal enrichment of MgO (158.1 %) and Fe2O3 (108.2 %), accompanied by aMg2+/(aH+)2 decreased in the fluid. Stage 3, the main stage of gold mineralization, involves chlorite substitution with sericite during quartz-sericite-pyrite (QSP) alteration, suggesting mild acidification of the ore-forming fluid. Insights from phase equilibrium modeling suggest that sulfidation has contributed to gold precipitation, leading to increased fO2 and reduced fH2S (extending into quartz-carbonate alteration of stage 4). Mass balance calculations indicate gold is enriched compared to protolith at all stages, with the most intense enrichment in stage 3. This suggests that hydrothermal fluids at Xiadian were auriferous throughout their evolution. The process may induce fluid modifications, particularly through the dissolution of mafic minerals, more so in mafic mineral-rich rocks (e.g., amphibolite) than in granite, promoting the incorporation of hydrothermal chlorite into the Au system to contribute to sulfidation. Our results provide insights into lithology controls on gold mineralization at Xiadian, indicating that felsic rocks (Linglong granite and quartz porphyry) with lower initial CaO contents often develop K-metasomatism and Au-bearing QSP alteration, while mafic rocks (amphibolite and lamprophyre) with higher CaO contents develop carbonization. This is likely due to the influence of lower CaO contents on alteration minerals, which promote rock porosity, affecting rock deformation, pore-fluid flow, and reacting with wall rocks. Principal component analysis (PCA) results indicate that elements including Au, Ag, As, Bi, W, and Cu are proxies for Au mineralization, suggesting their mobilization and deposition together during the gold mineralization process. This highlights their significance as a proxy for gold exploration.

Mineral chemistry evolution of magmatic Li-bearing micas and Li-loss during postmagmatic phengitization in an A-type sieno-monzogranite: A mass balance approach at La Chinchilla pluton, Sierra de Velasco, La Rioja, Argentina

La Chinchilla granite (∼3.75 km2) is an epizonal pluton intruded during the Lower Carboniferous in Sierra de Velasco, Sierras Pampeanas, northwestern Argentina. Three facies have been distinguished in the pluton (equigranular, porphyritic and fine-grained border zone facies), being equigranular and porphyritic the main two granite types, which host abundant millimeter to < 2 m sized miarolitic pegmatites and pockets of simple granitic mineralogy (quartz + albite + microcline + micas) ± beryl. Micrometer-sized accessory magmatic species are monazite-(Ce), several high field strength element oxide species, ilmenite, cassiterite (1), fluorapatite and fluorite. Primary Li-bearing micas with variable degrees of late to postmagmatic replacement occur in abundances that range from ∼2 to 7 %. Textural evidence and mineral chemistry allow to distinguish between early granitic and late bladed miarolitic micas. The mineral chemistry composition of the primary micas shows members of the siderophyllite-polylithionite series. Replacing phases are Li-muscovite (phengite). Electron probe microanalyses of primary micas of both main granite types suggest that these are compositionally distinct intrusives. According to mica chemistry, the porphyritic unit is more enriched in Li than the equigranular granite. The equigranular unit with interconnected miarolitic texture was more intensely affected by fluid-rock interaction processes and is significantly richer in U and Be. An F–Na rich fluid phase caused strong albitization during late miarolitic stages, along with crystallization of polylithionite, fluorite, pyrochlore group species and cassiterite (2). In terms of mass balance, the phengite replacement process is characterized by the conservation of Si, Al and K, a slight gain of Na and Ba, and a systematic loss of orderly decreasing Fe > F > Li > Mn > Ti > Mg > Zn. Likely, the muscovite-forming fluid was the same simultaneously involved in the hydrothermal alteration of high field strength element accessory species, largely included in primary micas, which generated secondary U- and Nb-rich species such as carlosbarbosaite. The hydrothermal late miarolitic to postmagmatic fluid-mineral replacement of primary magmatic micas released significant amounts of Li, of which about 59 wt % of the Li2O content of protholithic micas (i.e., 1 to 2.4 wt % Li2O) was inherited by replacing Li-bearing muscovite and the remaining 41 wt % was released from the system. It is calculated that ∼1.5 km3 of La Chinchilla body would have expelled ∼0.38 million tons of Li2O, just considering than only 30 % of primary siderophyllite-polylithionite was muscovitized. Such a Li2O tonnage was likely transferred to the paleohydrological cycle during Carboniferous times, when the La Chinchilla stock was still undergoing its cooling down path. Currently, evidence of the final fate of mobilized Li is lacking.

Gold source and deposition in the Sanakham gold deposit, SW Laos: Constrains from textures, trace element geochemistry and in-situ sulfur isotopes of pyrite

The Luang Prabang (Laos) – Loei (Thailand) metallogenetic belt is endowed with major porphyry-skarn, and epithermal gold deposits. The Sanakham gold deposit is a newly discovered hydrothermal vein-type orogenic gold deposits (10.6 t Au @ 3.05 g/t) in the belt, and consist of a series of quartz-sulfide veins that are controlled by secondary structures of the regional NNE-trending brittle-ductile shear zone. Its mineralization can be divided into four stages: (I) quartz–pyrite–arsenopyrite, (II) quartz–pyrite–pyrrhotite–chalcopyrite, (III) quartz–pyrite–base-metal sulfide, and (IV) quartz–carbonate. Three pyrite generations were identified: py1 (stage I) as disseminated euhedral to subhedral coarse grains; py2 (stage II) as subhedral to anhedral fine grains occur around the cataclastic and corrosion textures in py1; py3 (stage III) as subhedral to anhedral fine grains occurred in sulfide veins or veinlets that overprint the existing quartz-sulfides. All three generations of pyrite have low trace-element contents including very low lattice-bound gold (py1: 0.040 ppm; py2: 0.098 ppm; py3: 0.028 ppm) and arsenic (py1: 227 ppm; py2: 133 ppm; py3: 13.3 ppm) contents. Visible gold grains are sited in textures of pyrite grains that relate to brittle cataclastic deformation. Considering the relatively low contents of pyrite in the ores of Sanakham, it is impossible to derive the high-gold grade orebodies through local remobilization of originally lattice-bound gold in pyrite. The py2 might be formed during hydrothermal overprinting and dissolution of py1 led by continuous supply of ore fluids with relatively higher contents of Au, Ag, Cu, Sb, Te, Pb, and Bi. A reduction reaction related to reduced carbon species in wallrock is an assignable cause for gold deposition, even though the fluid-rock interaction and fluid immiscibility processes might be important. The Au- and As-poor pyrites have similar δ34S values (py1: –2.58 to –0.83 ‰; py2: –2.17 ‰ to + 0.25 ‰; py3: –1.24 to –0.60 ‰ ppm) that close to 0 ‰, indicating the sulfur may have been originated from magmatic or mantle sources. Therefore, we propose that the Sankham gold deposit is a member of the orogenic deposit class, but notably characterized by the low concentrations of invisible gold in the relatively As-poor pyrite lattice.

The state of Zr and Hf in chloride hydrothermal fluids from in situ X-ray absorption spectroscopy

In order to quantify the hydrothermal mobility of Zr and Hf in chloride fluids we performed an investigation of the speciation of these metals by means of in situ X-ray absorption spectroscopy (XAS). The experiments included registration of Zr K-edge X-ray absorption near edge structure/extended X-ray absorption fine structure (XANES/EXAFS), and Hf L1-edge (XANES) and Hf L3-edge (EXAFS) spectra. The capillary method, when the experimental solution together with baddeleyite ZrO2(cr) or HfO2(cr) is sealed inside a silica glass capillary, was used. The spectra were recorded between 320 °C/600 bar and 480 °C/2200 bar (Zr) and at 370 °C/2100 bar (Hf) after the stationary solubility of the solid phase was attained. The aqueous fluid composition varied from 3.8 m HCl to 3.5 m HCl/3.2 m CsCl and 3.3 m HCl/2.2 m NaCl (Zr), and was set to 7.7 m HCl in the case of Hf. The composition of the dominant complexes was determined by means of EXAFS fitting and theoretical XANES modeling as ZrCl3(OH)2−/ ZrCl5(H2O)− and HfCl4OH−/ HfCl5(H2O)−. In Zr complexes the metal-to-ligand distances varied between 2.10 ± 0.06 Å - 2.16 ± 0.06 Å (OH−), 2.22 ± 0.03 Å - 2.32 ± 0.07 Å (OH2), and 2.46 ± 0.02 Å - 2.49 ± 0.02 Å (Cl−) depending on the fluid PT-compositional parameters. In Hf-bearing species the average HfO distance is 2.07 ± 0.12 Å, and the HfCl distance is 2.43 ± 0.03 Å. Both the distances are slightly shorter compared to the interatomic distances in Zr complexes. Higher number of Cl ligands and shorter metal-to-ligand distance determined for Hf complexes mean higher affinity of Hf to oxygen-bearing ligands and chloride ion, which can result in separation of Zr and Hf in hydrothermal processes. Reactions of the dissolution of Zr- and Hf-bearing minerals imply that both ionic associate HCl(aq) and ion Cl− are necessary for the formation of the negatively charged complexes. The region, that best satisfy these conditions, is located between ca. 300 and 450 °C. At higher temperature the neutral Zr/Hf-OH-Cl complexes can support the hydrothermal mobility of these metals. The complexation mechanisms determined for Zr and Hf demonstrate that deep chloride fluids can play an important role in mobilization and separation of these metals in subduction zones and in the fluid-rock interaction processes.

Lithium enrichment processes in sedimentary formation waters

Reservoirs exploited for oil, gas, or geothermal energy in sedimentary basins may represent an alternative lithium resource to salars and hard rocks. Similarities in the geochemical characteristics of the basins were searched using a database of 2400 water analyses available from more than 40 sedimentary basins worldwide, notably a selection of representative geochemical trends on 13 basins. The objective was to understand better the factors controlling Li concentrations and the potentiality of such basins for the Li-recovery. Considering LiCl and MgCl relationships, major cation (Na/K, Na/Ca) and halogen (Cl/Br) ratios, Li concentrations are monitored in several basins by mixing processes between low salinity recharge waters, seawater and brines. Lithium concentrations span three orders of magnitude in brines between evaporated seawater that has passed saturation with halite and a particularly Li-enriched pole. In the latter, significant Li-enrichment is obtained when brines are primary, in equilibrium with halite, and then enriched in Li through fluid-rock interaction processes. Li-rich lithologies such as volcanic rocks, Li-enriched clay formations, or Li-phyllosilicates dispersed in siliciclastic rocks or the underlying crystalline basement probably act as the primary Li sources. Finally, Li extraction is promoted by the rise in temperature, which explains the high Li concentrations in the waters of geothermal systems. Therefore, favourable reservoirs for Li-rich brines are characterised by a brine collection from evaporites during tectonic events, such as compressive events or salt tectonics, at a relatively short distance from the present reservoir. They must be preserved from convection and surface water ingress via recharge zones to prevent dilution and Li depletion.

Multistage fluorite mineralization in the southern Black Forest, Germany: evidence from rare earth element (REE) geochemistry

Abstract. The Black Forest hosts a wide range of hydrothermal mineralization, including fluorite–barite vein deposits. In a detailed investigation of the Finstergrund and Tannenboden deposits in the Wieden mining district (southern Black Forest), the diversity, geochemical evolution and relative chronology of multistage fluorite precipitation is tracked on the basis of rare earth element (REE) geochemistry, geologic field relationships and crystal zoning. Geochemical discrimination and mathematical λ coefficients suggest a total of seven fluorite REE groups, at least three distinguishable post-Variscan fluid mobilization events and independent formation histories for the deposits despite their spatial proximity. Fluorite vein mineralization at the Finstergrund deposit evolved over three fluid generations, was derived from gneissic source aquifers and comprises five distinct fluorite REE groups: the first fluid generation is characterized by fluorite precipitation above 200 ∘C (“group III”), below 200 ∘C (“group I”) and after fractional crystallization (“group IV”); the second generation comprises remobilized fluorite (“group II”); and the third generation revealed fluorite precipitation by meteoric water mixing (“group V”). Fluorite vein formation at the Tannenboden deposit is associated with two distinct fluorite REE patterns derived from the same fluid generation: fluorite precipitation above 200 ∘C (“group VII”) and after cooling below 200 ∘C (“group VI”). Its fluid source aquifer lithology best matches migmatites contrary to previous models that suggest either gneissic or granitic aquifer rocks for fluorite vein precipitation in the Black Forest. The decoupled formation history between the deposits is tectonically controlled as suggested by a new genetic model for the Wieden mining district. The model argues for a change in the local fluid percolation network and the termination of hydrothermal activity at the Tannenboden deposit after the first fluid mobilization event. The geochemical evolution of multistage fluorite mineralization, as exemplified by the Tannenboden and Finstergrund deposits in combination with other fluorite mineralizations in the Black Forest, provides unique insights into the lithospheric origin and precipitation behaviour of fluorite by various fluid–rock interaction processes occurring in large hydrothermal systems. The local diversity of REE patterns emphasizes the need for detailed investigations of individual hydrothermal vein deposits.

Hydrothermal Alteration in the Nevados de Chillán Geothermal System, Southern Andes: Multidisciplinary Analysis of a Fractured Reservoir

The interplay between a heat source, primary plus secondary permeability, and hydrothermal fluids makes geothermal systems a highly dynamic environment where evolving physico-chemical conditions are recorded in alteration mineralogy. A comprehensive characterization of hydrothermal alteration is therefore essential to decipher the major processes associated with geothermal system development. In this study, we defined the hydrothermal mineralogical evolution of the Nevados de Chillán Geothermal System (NChGS), located in the Southern Volcanic Zone (SVZ) of the central Andes, where the regional framework of the system is formed by a direct association with a currently active volcanic complex, a favorable structural control, and vertically inhibited fluid circulation. To characterize the secondary mineralogy present in the NChGS, we integrated optical petrography, Scanning Electron Microscopy (SEM) observations, X-ray Diffraction (XRD) analysis, and microthermometric measurements along a drill core with a depth of 1000 m at the Nieblas-1 well. These mineralogical approaches were combined with a structural field analysis to highlight the relevance of multidisciplinary study in understanding active geothermal systems. The results indicated that the evolution of the system involved four paragenetic stages, with the main processes in each phase being the heating, boiling, and mixing of fluids and re-equilibration to new physico-chemical conditions. Additionally, three hydrothermal zones were recognized: an upper argillic section, an intermediate sub-propylitic zone, and a deep propylitic domain. Sampled thermal springs are characterized by pH values of 2.4–5.9 and high SO4= concentrations (&gt;290 ppm). These acid-sulfate steam-heated waters suggest the contribution of primary magmatic volatiles to the hydrothermal system. Alunite recorded in the alteration halos of veinlets presents at depths of 170–230 m denote the circulation of acidic fluids at these levels which were favored by reverse faults. These findings indicate that, at this depth range, the condensation of magmatic volatiles into shallow aquifers controls the recharge area of the superficial thermal manifestations. Conversely, deep-seated hydrothermal fluids correspond to near-neutral chloride fluids, with salinities ranging from 0.1 to 6.9 wt.% NaCl eq. The distribution of illite/smectite and chlorite/smectite mixed-layered minerals outline the presence of a significant clay cap, which, in this system, separates the steam-heated domain from the deep hydrothermal realm and restricts fluid circulation to existing permeable channels. Our mineralogical and structural study provides critical data for the interpretation of heat–fluid–rock interaction processes in the NChGS. The interplay between hydrothermal fluids and active faults is also discussed in the context of the complex of geological processes in active geothermal systems along the Chilean Southern Volcanic Zone.

Spectral characterisation of hydrothermal alteration associated with sediment-hosted Cu–Ag mineralisation in the central European Kupferschiefer

Abstract. The analysis of hydrothermal alteration in exploration drill cores allows for fluid–rock interaction processes to be traced, for fluid flow paths to be identified, and thus for vectors in mineral systems to be determined. Hyperspectral imaging techniques are increasingly being employed to fill the scale gap between lab-based petrographic or geochemical analyses and the typical size of exploration targets. Hyperspectral imaging permits the rapid, cost-efficient, and continuous characterisation of alteration mineralogy and texture along entire drill cores, with a spatial sampling of a few millimetres. In this contribution, we present the results of an exploratory study on three mineralised drill cores from the Spremberg–Graustein Kupferschiefer-type Cu–Ag deposit in the Lusatia region of Germany. We demonstrate that hyperspectral imaging is well-suited to recognising and tracking the effects of hydrothermal alteration associated with strata-bound hydrothermal mineralisation. Micro X-ray fluorescence spectrometry was used to corroborate the alteration mineral assemblages identified in hyperspectral data acquired in the visible, near- (400 to 970 nm), shortwave (970 to 2500 nm), mid-wave (2700 to 5300 nm), and longwave infrared (7700 to 12 300 nm). We identified two main shortcomings of the technique, namely the overlapping of some mineral features (e.g. carbonate and illite absorption in the shortwave infrared) and the darkness of the organic-matter-rich dolostones and shales that results in low reflectance. Nevertheless, spectral features associated with iron oxide, kaolinite, sulfate, and carbonates were successfully identified and mapped. We identified different markers of hydrothermal alteration spatially associated with or stratigraphically adjacent to Cu–Ag mineralisation. Importantly, we can clearly distinguish two mineralogically distinct styles of alteration (hematite and ferroan carbonate) that bracket high-grade Cu–Ag mineralisation. Intensive hydrothermal alteration is characterised by the occurrence of well-crystallised kaolinite in the sandstone units immediately below the Kupferschiefer horizon sensu stricto. Proximal Fe-carbonate and kaolinite alteration have not previously been documented for the high-grade Cu–Ag deposits of the central European Kupferschiefer, whereas hematite alteration is well-known in Kupferschiefer-type ore deposits. The latter marks the flow path of oxidising, metal-bearing hydrothermal fluids towards the site of hydrothermal sulfide mineralisation. In contrast, ferroan carbonate alteration in carbonate rocks located above the main mineralised zone is interpreted as a mark of hydrothermal fluid discharge from the mineralising system. Although this study is limited to a small number of drill cores, our results suggest that hyperspectral imaging techniques may be used to identify vectors towards high-grade Cu–Ag mineralisation in Kupferschiefer-type mineral systems.

Linking top and subsoil types, alteration and degassing processes at Rotokawa geothermal field, New Zealand

Surface geothermal expressions such as mud pools, fumaroles, mineral deposits, collapse pits, and hydrothermal eruption craters vary in scale and type over space and time. The evolution in space and time of these surficial thermal features strongly relates to alteration processes caused by hydrothermal fluids. The changes in the physical and mechanical properties of top and subsoils that can control fluid flow, degassing patterns, and occurrence of geothermal hazards remain understudied. The thermal area located south of the Rotokawa geothermal field (New Zealand) includes a variety of major natural surface thermal features and widespread sulphur deposits precipitated by acid–sulphate fluids in steam-heated zones. Decades of sulphur mining induced the formation of new thermal features. Such a setting represents an exemplary case study for investigating old and recently formed thermal features, soil characteristics, types and patterns of superposed hydrothermal alteration, and degassing processes. We combined field and laboratory methods to define groups of thermal features, soil types, and associated alteration. Their spatial distribution indicates that fluid circulation, alteration intensity, and degassing are strongly influenced by the local and regional geological and structural settings, as well as by mining activity. We found that at the water table level and within excavated areas, acidic fluids led to the deposition of mud deposits, sinters, and stromatolites around warm springs and mud pools. In the vadose, steam-heated portions, fluids generally leach and degrade the Taupo Pumice, resulting in broad unstable grounds and collapse structures, while extensive sulphur-encrusted grounds and sulphur-rich soils formed in excavated sites. In this framework, the degassing and fluid circulation within the top and subsoils are strongly affected by the dominant soil layer type, and in turn by its granulometry, texture, and alteration state. Our study of top and subsoils yields precious insights into surface expression variability, fluid–rock interaction processes, and sulphur deposition patterns within steam-heated zones. Processes at such a scale may strongly influence the migration of thermal manifestations, gas outputs, and ground subsidence within geothermal environments. From a broader perspective, our results will help assess the evolution of geothermal activity and related hazards in similar areas worldwide.

Tracing the Source of Hydrothermal Fluid in Ophiolite-Related Volcanogenic Massive Sulfide Deposits: A Case Study from the Italian Northern Apennines

The Italian Northern Apennines contain several Fe-Cu-Zn-bearing, Cyprus-type volcanogenic massive sulfide (VMS) deposits, which significantly contribute to the Cu resources of Italy. The massive sulfide lenses and related stockwork mineralizations are hosted by several levels (from basalt to serpentinite) of the unmetamorphosed ophiolitic series; therefore, this region offers perfect locations to study the ore-forming hydrothermal system in detail. A combination of fluid inclusion microthermometry, Raman spectroscopy, electron probe analyses (chlorite thermometry) and stable and noble gas isotope geochemistry was used to determine the fluid source of the VMS system at Bargone, Boccassuolo, Campegli, Casali–Monte Loreto, Corchia, Reppia and Vigonzano. This question of the fluid source is the focus of modern VMS research worldwide, as it has a direct influence on the metal content of the deposit. The obtained temperature and compositional data are both in the typical range of VMS systems and basically suggest evolved seawater origin for the mineralizing fluid. Modification of seawater was most commonly due to fluid–rock interaction processes, which happened during long-lasting circulation in the crust. The role of a small amount of magmatic fluid input was traced only at the lower block of Boccassuolo, which may be responsible for its higher ore grade. This fluid origin model is evidenced by O, H and C stable isotopic as well as He, Ne and Ar noble gas isotopic values.

Halogen ratios in crustal fluids through time—Proxies for the emergence of aerobic life?

Abstract Halogens (Cl, Br, I) are exceptional provenance tracers in crustal fluids because their ratios are not strongly altered during most fluid-rock interaction processes. The halogen systematics of metamorphic fluids are of particular interest because such fluids are key drivers of crustal-scale element fluxes and ore formation in orogenic belts, but they remain poorly studied due to analytical challenges. We present novel triple-halogen laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) fluid-inclusion data from metamorphic systems ranging in age from Archean to Phanerozoic. Our results show that the halogen signatures in Phanerozoic metamorphic fluids are controlled by variable degrees of organic-matter interaction in their source rocks, leading to increased I/Cl and decreased Br/I ratios relative to seawater. By contrast, Archean metamorphic fluids from organic matter–rich source rocks have low I/Cl and very high Br/I ratios, distinctly different from any known fluid source signature. We propose that these signatures nevertheless are consistent with organic-matter interaction because dominantly prokaryotic Archean lifeforms did not yet produce iodine-bearing metabolites. This prevented biosequestration and accumulation of iodine-rich organic matter in sediments and imposed halogen signatures onto Archean metamorphic fluids entirely unlike those in younger fluids.

Deep hydrochemical section through the Central Alps: evolution of deep water in the continental upper crust and solute acquisition during water\u2013rock-interaction along the Sedrun section of the Gotthard Base Tunnel

Drilling of the Gotthard Rail Base Tunnel through the Central Alps from 2005 to 2010 opened up fractured basement units and frequent water inflows provided access to the major fluid-rock interactions processes in orogenic crystalline upper crust. Construction of the 57 km long tunnel was divided into five different sections. Here we report data and observations from the 10 km long central Sedrun section 211 water samples were collected at inflow points 900 to 2350 m below the surface. The exceptional samples and data provide a comprehension of the hydrochemical evolution and solute acquisition of deep groundwater in basement units. The investigated tunnel section drilled through steeply dipping rock units and vertical fracture systems at high angle. It cuts across granite, gneiss and schist of the pre-Alpine basement and across two narrow zones of meta-sediments. Rock temperature along the Sedrun section varies from 30 to 45 °C depending on the thickness of the overburden. The fracture water is of meteoric origin and acquires its composition exclusively by chemical interaction with the surrounding rocks along the flow path. Water from inflow points in the basement of the Gotthard Massif has typically a high pH of about 10 and total dissolved solids in the range of 100 to 300 mg L−1. Sodium is the prime cation of most waters. Although plentiful in the rocks, calcium, potassium and magnesium are low to very low in water. The anions associated with Na are carbonate/bicarbonate, sulfate, fluoride and chloride in widely varying proportions. High fluoride concentrations of up to 15.4 mg L−1 are characteristic for most waters. As a result of the high pH dissolved silica (SiO2) reached concentrations of up to 58 mg L−1 and represents 25—30 wt% of the solutes. The meteoric recharge provides dissolved O2 and CO2 to the fluid-rock interaction processes. The solutes derive from the dissolution of feldspar (Na+, SiO2aq), oxidation of sulfides to sulfate (SO42−), alteration of biotite (F−), and fluid inclusions opened by brittle deformation (Cl−). The solids formed during fluid-rock interaction, mainly zeolites, chlorite (and other clay minerals) and secondary Fe-minerals, remove Mg, Fe, and K almost quantitatively from the water. The high pH results from hydrolysis of silicates. The data distinctly show that within the depth interval of 1.0–2.5 km below surface deep water in continental basement evolves to a low TDS, high pH, sodium carbonate and silica solution by interaction of gneiss and granite with infiltrating pristine meteoric water, snow and rain.

Lithium-rich geothermal brines in Europe: An up-date about geochemical characteristics and implications for potential Li resources

Lithium (Li) is a strategic metal - especially for batteries in electric vehicles - for which worldwide demand is constantly increasing. Presently, several investigations are examining if a part of lithium could be extracted from deep European geothermal fluids. Among the data from geothermal and hydrocarbon wells found in the literature review carried out by BRGM and EIFER, only six areas stand out with deep fluids containing high Li concentrations from 125 to 480 mg/l in Italy, Germany, France and the United-Kingdom. Except the UK fluid, which has a relatively low salinity (TDS = 19 g/l) and reservoir temperature (around 52 °C), these deep Li-rich fluids are Na-Cl brines (TDS ≥ 56 g/l) with high concentrations of Na (> 18 g/l) and Cl (> 25 g/l) and high temperatures (≥ 120 °C and up to 380 °C in Italy). If high TDS and temperature values seem to be key factors for triggering high Li concentrations in such fluids, these two factors alone cannot be sufficient. Indeed, our study confirms that Li concentrations not only depend on temperature and fluid salinity, but also on the type of reservoir rock and its mineralogical constituents, as demonstrated by the good results obtained using two different Na-Li thermometric relationships existing in the literature. One fits well with the brines from low to high temperature (120–250 °C) reservoirs in deep tectonic sedimentary basins over a crystalline basement (France, Germany) and the other with the fluids from ultra-high temperature (≥ 300 °C) reservoirs in volcano-sedimentary environment (Italy). If the high chloride concentrations values in these brines mainly depend on the fluid origin (evaporated seawater or freshwater, halite dissolution, primary neutralization fluids or parent-geothermal fluids in high-temperature and -pressure volcanic environments, water mixing, etc.), the other aqueous major species are mostly controlled by hydrothermal water-rock interaction processes. At these temperatures (≥ 120 °C), the fluid-rock interaction processes are generally dominated by plagioclase and K-feldspar dissolution, followed by albitization of these minerals, dissolution of white micas and biotite, precipitation of illite, and chloritization. According to the two Na-Li thermometric relationships, existing mineralogical and isotopic data, this study suggests that the main sources of Li are white micas and biotite dissolution. Among the European areas, it shows that the Upper Rhine Graben (URG) along the French/German border is probably the most promising zone. For the URG geothermal brines, the main source and control of lithium, at about 225 °C, at reservoir depth, is probably the up to 450-m-thick micaceous continental sandstone of Triassic Buntsandstein. A minor contribution from the granite basement can also not be excluded. Though the Li concentration values of these brines (≥ 150 mg/l) seem to be favourable for geothermal Li exploitation, it is essential to make an as accurate as possible estimate of the Li resource of these brines, design the Li extraction process, and examine the economic conditions of its exploitation.

An Integrated Study of Water Weakening and Fluid Rock Interaction Processes in Porous Rocks: Linking Mechanical Behavior to Surface Properties

We investigated the impact of water weakening on the mechanical behavior of Obourg Chalk and Ciply Chalk (Mons Basin, Belgium). Different mechanical tests were conducted to estimate the unconfined compressive strength (UCS), tensile strength, Young’s modulus, mechanical strength under triaxial loading, critical pressure, fracture toughness, cohesion, and internal friction coefficient on samples either dry or saturated with water or brine. This extensive dataset allowed us to calculate wet-to-dry ratios (WDR), i.e., the ratio between any property for a dry sample to that for the water-saturated sample. For both chalks, we found that water has a strong weakening effect with WDR ranging from 0.4 to 0.75. Ciply Chalk exhibits more water weakening than Obourg Chalk. The highest water weakening effect was obtained for UCS, critical pressure, and Young’s modulus. Weakening effects are still present in brine-saturated samples but their magnitude depends on the fluid composition. The mechanical data were correlated to variations in surface energy derived from three different methods: fracture mechanics, contact angle goniometry, and atomic force microscopy. Water weakening in the tested chalks can be explained by a clear reduction in surface energy and by the existence of repulsive forces which lower the cohesion.

Fluid-Rock Interactions in a Paleo-Geothermal Reservoir (Noble Hills Granite, California, USA). Part 2: The Influence of Fracturing on Granite Alteration Processes and Fluid Circulation at Low to Moderate Regional Strain

Fracture connectivity within fractured granitic basement geothermal reservoirs is an important factor controlling their permeability. This study aims to improve the understanding of fluid–rock interaction processes at low to moderate regional strain. The Noble Hills range (Death Valley, CA, USA) was chosen as a naturally exhumed paleo geothermal reservoir. A series of petrographic, petrophysical, and geochemical investigations, combined with a fracture distribution analysis, were carried out on samples collected across fracture zones. Our results indicate that several generations of fluids have percolated through the reservoir. An increase of (1) the alteration degree; (2) the porosity values; and (3) the calcite content was observed when approaching fracture zones. No correlation was identified among the alteration degree, the porosity, or the calcite content. At a local scale, samples showed that the degree of alteration does not necessarily depend on the fracture density or on the amount of the strain. It is concluded that the combined influence of strain and coeval fluid–rock interaction processes drastically influence the petrophysical properties of fracture zones, which in turn impact geothermal production potential.

The role of competitive fluid-rock interaction processes in the formation of high-grade gold deposits

Although forming one unified class, orogenic gold deposits may be associated with a range of ore mineral assemblages controlled by a variety of gold deposition processes that have a direct impact on gold grade. Whereas, a better understanding of the deposition process may be critical for effective targeting of highly-endowed gold deposits, it is challenging to establish a direct comparison between mineral systems formed under different conditions. Accordingly our ability to predict whether contrasted mineral associations may be more or less prospective for high grade mineralization remains poor. In this paper, we compare fluid-rock interaction processes that led to the coeval formation of two contrasting gold mineral assemblages within the world-class Callie orogenic gold deposit. Stratabound and vein-hosted lodes formed within the same structural framework but with contrasting grades (∼3 g/t Au and ∼8 g/t Au, respectively). The stratabound mineralization consists of bedding-parallel sulfides in which gold is preserved in arsenopyrite micro-fractures. The vein-hosted mineralization consists of locally very high-grade (up to 10,000 ppm Au) visible gold concentrated in quartz veins. Both mineralization styles are hosted within different metasedimentary stratigraphic horizons with contrasting compositions. The stratabound mineralization occurs in iron-oxide rich siltstones whereas the vein-hosted mineralization occurs in carbonaceous siltstones. We investigate the fluid-rock interaction mechanisms of the two styles of mineralization at the Callie deposit by combining petrographic observation with whole-rock geochemistry, quantitative chlorite and gold analyses and thermodynamic modeling. Our results show that the stratabound mineralization occurred in response to sulfidation of the host-rock whereas decarbonation alteration, associated with the vein-hosted mineralization, records oxidation of the host-rock. Our investigation shows that gold deposition induced by oxidation of the host-rock produces the valuable, locally very-high grade visible gold mineralization whereas the sulfidation process lead to the formation of lower grade gold mineralization disseminated within the sulphide ore assemblage. We conclude that the nature of the host-rock applies a first-order control on the gold deposition efficiency in metasedimentary-hosted gold deposits.

Fe and O isotopes in coesite-bearing jadeite quartzite from the Western Alps record multistage fluid-rock interactions in a continental subduction zone

Fluid is a key agent for the mass transfer between the subducting slab and the mantle wedge, which greatly affects the evolution of the crust-mantle system at convergent plate boundaries. A geochemical study was carried out for coesite-bearing jadeite quartzite and its country rock granitic gneiss from the Dora Maira Massif in the Western Alps. The results provide new insights into the composition of metamorphic fluids and fluid-rock interaction processes in the continental subduction zone. Coesite inclusions are found for the first time in metamorphic zircons of the granitic gneiss. The ultrahigh-pressure (UHP) metamorphism was dated to occur at 34.7 ± 0.3 Ma, confirming that the country rock experienced the UHP metamorphism synchronously with the whiteschist and jadeite quartzite. The jadeite quartzite occurs as layers and boudins within the coesite-bearing whiteschist, and shows similar whole-rock REE distribution patterns to both granitic gneiss and whiteschist. In addition, relict domains of magmatic zircon in the three types of UHP metamorphic rocks exhibit similar U-Pb ages and δ18O values, indicating that they have the same protolith of granites. However, metamorphic zircons in the jadeite quartzite show significantly lower δ18O values of 6.1–7.3‰ than the relict magmatic domains of 9.4–10.6‰. Furthermore, the jadeite quartzite has whole-rock δ18O values of 7.9–8.7‰, significantly lower than those of 9.5–10.9‰ for the granitic gneiss but higher than 6.5–7.9‰ for the whiteschist. These differences suggest that the jadeite quartzite was probably formed through metasomatism of metagranite by external fluids with relatively low δ18O values. The jadeite quartzite shows high δ56Fe values of 0.69–0.87‰, considerably higher than those of 0.18–0.38‰ for the granitic gneiss, but falling within the range of 0.32–1.03‰ for the whiteschist from the same outcrop. The Fe isotope modeling results suggest that the metasomatic fluids responsible for the formation of jadeite quartzite were possibly derived from the host whiteschist under UHP metamorphic conditions. Such fluids are enriched in Si, Al and Mg, and have high δ56Fe but low δ18O values. Therefore, the jadeite quartzite records the action of metamorphic supercritical fluids at the subarc depth. If the metasomatized rocks in the continental subduction zone would dehydrate to generate fluids with variable geochemical compositions due to multistage fluid-rock interactions, the fluids could further infiltrate the surrounding rock and possibly the mantle wedge to result in further geochemical transfer from the subducting slab to the mantle wedge.

Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

Differentiating between hydrothermal and diagenetic carbonate using rare earth element and yttrium (REE+Y) geochemistry: a case study from the Paleoproterozoic George Fisher massive sulfide Zn deposit, Mount Isa, Australia

Carbonate minerals are ubiquitous in most sediment-hosted mineral deposits. These deposits can contain a variety of carbonate types with complex paragenetic relationships. When normalized to chondritic values (CN), rare-earth elements and yttrium (REE+YCN) can be used to constrain fluid chemistry and fluid-rock interaction processes in both low- and high-temperature settings. Unlike other phases (e.g., pyrite), the application of in situ laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS) data to the differentiation of pre-ore and hydrothermal carbonates remains relatively untested. To assess the potential applicability of carbonate in situ REE+Y data, we combined transmitted light and cathodoluminescence (CL) petrography with LA-ICP-MS analysis of carbonate mineral phases from (1) the Proterozoic George Fisher clastic dominated (CD-type) massive sulfide deposit and from (2) correlative, barren host rock lithologies (Urquhart Shale Formation). The REE+YCN composition of pre-ore calcite suggests it formed during diagenesis from diagenetic pore fluids derived from ferruginous, anoxic seawater. Hydrothermal and hydrothermally altered calcite and dolomite from George Fisher is generally more LREE depleted than the pre-ore calcite, whole-rock REE concentrations, and shale reference values. We suggest this is the result of hydrothermal alteration by saline Cl--rich mineralizing fluids. Furthermore, the presence of both positive and negative Eu/Eu* values in calcite and dolomite indicates that the mineralizing fluids were relatively hot (>250°C) and cooled below 200–250°C during ore formation. This study confirms the hypothesis that in situ REE+Y data can be used to differentiate between pre-ore and hydrothermal carbonate and provide important constraints on the conditions of ore formation.