Marie-Christine Boiron

Identification of fluid inclusions in relation to their host microstructural domains in quartz by cathodoluminescence

Abstract The geometry of fluid trapping in relation to microfracture healing or filling was investigated using massive quartz vein samples characterized by multistage deformation and fluid trapping. The multidisciplinary approach used in this study includes • 1) mapping of oriented thin sections of quartz using the cathodoluminescence (CL) mode of a scanning electron microscope (SEM). SEM CL images display areas of different intensities from dark to white, which correspond to the intensity of the CL and are caused by trace elements present in the quartz. Microfracture healing by newly formed quartz crystallization may occur under physical-chemical conditions different from those which characterized the early quartz matrix formation. Thus, evidences of linear cathodoluminescent markers have been searched and identified by paralleling SEM CL images and transmitted light microphotographs. Data show that cathodoluminescence makes clearly visible the healed crack networks which are only detected in part by transmitted light microscopy. The technique is essential to establish the microcrack chronology and to estimate the width affected by healing along a fluid inclusion trail. Such data are useful for the estimation of the microfracture permeability changes throughout the history of fluid migration; • 2) systematic and statistical analysis of the microstructural markers in horizontal and oriented planes using an interactive videographic analyzer which gives the geometry of the paleofluid pathways (orientation, dip and width of the veinlets, fluid inclusion trails, and cathodoluminescent microdomains); • 3) microthermometric and Raman analyses of fluid inclusions in selected fluid inclusions, for the determination of the P-V-T-X conditions of fluid migration in a specific microcrack network. Studied quartz samples come from Au-mineralized quartz veins from the French Massif Central, and some quartz samples from the Cassiar Mountains in British Columbia, have been observed for comparison. In all samples, the quartz matrix exhibits intense microfracturing, suggesting strong brittle deformation which postdates the main stage of quartz lens formation. The quartz veins are characterized by multistage quartz crystallization. Healed microfractures, clear quartz bands, as well as quartz comb veinlets, were studied as functions of the physicochemical conditions in relation to stages of deformation and ore deposition. This study shows that systematic measurements of microstructural marker orientations together with detailed fluid inclusion and mineral petrography and microthermometry may lead to a precise determination of ore fluid pathways and chronology.

Determination of Chlorinity in Aqueous Fluids Using Raman Spectroscopy of the Stretching Band of Water at Room Temperature: Application to Fluid Inclusions

A new analytical method, based on the Raman spectroscopy of the ν(OH) stretching vibration of water, has been developed for the determination of the concentration of chloride in aqueous solutions with the goal of reconstructing the bulk ion content of fluid inclusions that are relics of paleo-fluid circulation in rocks. The method involves calibrating the area of one band of the spectrum difference between pure water and solutions of appropriate composition with respect to the chloride concentration. Calibration curves were constructed for the major geological chemical salts LiCl, NaCl, KCl, CaCl2, and MgCl2, and NaCl–CaCl2 systems. The application to fluid inclusions has been confirmed using synthetic fluid inclusions. For cubic minerals such as fluorite, the calibration curve for the NaCl system correctly estimates the chlorinity. For birefringent minerals, such as quartz, the Raman spectrum of the aqueous solution depends on the orientation of the host crystal. The crystal must be oriented in such a way that one axis of the ellipse of the indicatrix projects parallel to the spectrometer slit. This method complements micro-thermometric data and allows the determination of chlorinity when ice-melting temperature cannot be used.

Mixing of metamorphic and surficial fluids during the uplift of the Hercynian upper crust: consequences for gold deposition

A detailed geochemical study of fluids from representative quartz-sealed faults hosting late Hercynian gold concentrations shows that fluids percolating the mineralised faults had two main distinct reservoirs: one was a quite shallow and the other rather deep-seated. Both fluids have lost a great part of their original geochemical signature through interactions with host metamorphic formations. Early fluids, present during the primary sealing of the faults by quartz, are considered to have effectively equilibrated with the metamorphic pile and then predominantly flowed upwards along the faults. They are characterised by CH4/CO2/H2O ratios rather typical of fluids equilibrated with graphite, and moderate to medium chlorinities with a high Br/Cl ratio. The striking feature of the gold-bearing quartz is that gold is not synchronous within any quartz deposition, and appears located in late microfractures and associated with Pb–Bi–Sb sulphosalts and sulphides. These late stages are characterised by fluids whose salinities decrease to very low values indicating their progressive dilution by waters of more surficial origin in the fault system. The long-lived activity of the fault favoured the connection between two distinct fluid reservoirs at a critical time during the basement uplift. The fluids evolved through two main driving mechanisms which were responsible for the Au deposition: (i) decrease in temperature accompanying decompression from supra-lithostatic to hydrostatic conditions which yielded, in some instances, volatile unmixing in the faulted systems, (ii) mixing of the resulting fluids with waters entering the hydrological systems from shallower reservoirs. In addition to dilution and fluid mixing which are favourable factors for decreasing the gold solubility, the presence of microfractured sulphides could have enhanced gold precipitation through electrochemical processes. D 2002 Elsevier Science B.V. All rights reserved.

Evidence for Li-rich brines and early magmatic fluid-rock interactionin the Larderello geothermal system☆

Abstract The geochemical features of fluids accompanying the first stages of geothermal activity linked to magmatic intrusions have been documented for the Larderello geothermal system (Italy). Deep drilling has provided samples which preserve evidence of this early geothermal activity. Four wells (San Pompeo 2, Monteverdi 7, Sasso 22, and Serrazzano, VC 11) penetrated the deeper parts of the Larderello system, located in a metamorphic basement underlying the Tertiary nappe complex which constitutes the main aquifer at Larderello. The drill holes terminated close to the inferred roof of a granitic complex thought to be responsible for geothermal activity. Fluid inclusion data were obtained from recrystallized quartz lenses and quartz veins in samples displaying high temperature assemblages (plagioclase-actinolite-biotite-tourmaline; clinopyroxene ± andradite-wollastonite) and also from magmatic quartz in a leucogranite dike. The inclusions are mainly secondary in origin, oriented in fluid inclusion planes (FIP) related to hydrothermal circulation in the Larderello system. Several generations of high temperature fluids were trapped and include: 1. (1) H2OCO2 dominated vapors displaying variable but significant contents of CH4 and N2; 2. (2) aqueous vapors containing LiCl, with variable salinity; 3. (3) aqueous LiCl brine, often oversaturated with respect to halite at room temperature; 4. (4) complex brine, always oversaturated at room temperature with respect to two (halite and sylvite) or more (n ≤ 4) salts. The presence of LiCl was confirmed by identification of the salt hydrate (LiCl5H2O) at very low temperature using Raman spectroscopy. Bulk salinities could be roughly estimated at around 30 wt% eq. LiCl for the LiCl brine. Geometric and chronologic relationships between FIP reveal close relationships and mutual contamination between the H2OCO2 vapors and LiCl brine, indicating synchronism in their trapping. These fluids were generated and trapped at pressures of 100–130 MPa, nearly 23 MPa above the estimated present-day lithostatic pressure. This implies a denudation rate between 0.2 and 0.5 mrn· a−1 since the onset of hydrothermal activity, compatible with the setting of Larderello in a young (Tortonian) collision belt. Fluid inclusion trapping temperatures (425–650°C) show a monotonous increase towards the inferred granite, and are around 100–200°C higher than the highest present-day temperatures. The results are interpreted as recording the interaction between magmatic and contact metamorphic fluids in the early Larderello system. The H2OCO2 vapors resulted from the reheating of the basement metamorphic series (often C-rich) under relatively high temperatures during contact metamorphism. Lirich fluids expelled from an underlying Li-rich leucogranite migrated through the metamorphic series and the already cooled granite dikes and experienced local boiling. The fluid inclusion data demonstrate the involvement of magmatic fluids during the initial development of this high energy geothermal field.

Advances in lithium analysis in solids by means of laser-induced breakdown spectroscopy: An exploratory study

Lithium is an important geochemical tracer for fluids or solids. However, because the electron microprobe cannot detect Li, variations of Li abundance at the micrometric scale are most often estimated from bulk analyses. In this study, the Li intense emission line at 670.706 nm in optical emission spectroscopy was used to perfect the analysis of Li at the micrometric scale by means of laser-induced breakdown spectroscopy (LIBS). To estimate lithium content for different geological materials, LIBS calibration of the emission line at 670.706 nm was achieved by use of synthetic glasses and natural minerals. The detection limit for this method is ∼5 ppm Li. Three applications to geological materials show the potential of LIBS for lithium determination, namely for Li-bearing minerals, melt inclusions, quartz, and associated fluid inclusions. For spodumene and petalite from granite pegmatite dikes (Portugal), the Li2O concentrations are 7.6 ± 1.6 wt% and 6.3 ± 1.3 wt%, respectively, by use of LIBS. These values agree with ion microprobe analyses, bulk analyses, or both. For eucryptite crystals, the Li concentrations are scattered because grain size is smaller than the LIBS spatial resolution (6 to 8 μm). Lithium concentrations of melt inclusions from the Streltsovka U deposit (Siberia) are in the range of 2 to 6.2 wt% (Li2O) for Li-rich daughter minerals. Lithium estimations on silicate glasses display values between 90 and 400 ppm. Lithium was also analyzed as a trace element in quartz. Transverse profiles were performed in hydrothermal barren quartz veins from the Spanish Central System (Sierra de Guadarrama). The highest Li concentrations (250 to 370 ppm) were found in specific growth bands in conjunction with the observed variation in optical cathodoluminescence intensity. Considering the fluid inclusion analysis, the source of fluid responsible to the Li enrichment in quartz is probably high-salinity fluids derived from sedimentary basins.

Metallogenesis in the French part of the Variscan orogen. Part I: U preconcentrations in pre-Variscan and Variscan formations — a comparison with Sn, W and Au

Abstract In the French part of the Variscan orogen, the pre-Variscan metamorphic rocks and the Variscan granites resulting from pure crustal recycling are enriched in uranium and tin. Data on tungsten and gold are too scarce for a reasonable estimation of the average concentrations of these elements to be obtained. The spatial distribution of uranium preconcentrations is transverse to the main Variscan structural domains, and the distribution of the Sn-W mineralizations prompts the same conclusions. U, Sn, W and Au metal enrichment in crustal domains in Europe seems to result from late Proterozoic to early Palaeozoic basic and acid magmatism. For uranium, at least two main stages of partial melting, early Palaeozoic and Late Carboniferous, lead to enrichment of the metal in peraluminous leucogranites; these leucogranites represent the main source of late Variscan uranium and tin. Within the peraluminous granitic complexes, the main metal source is related to the late intrusion of small granitic cupolas strongly enriched in metals. In the case of uranium the efficiency of the hydrothermal remobilization depends on the proportion of metal located in uraninite, which is easily leachable by hydrothermal solutions. The emplacement of these cupolas is structurally controlled in well-defined areas by large shear zones. These shear zones are continuously active during the successive emplacement of the granitic magmas. Most of the hydrothermal uranium mineralization is related to the Permian reactivation of the shear zones in a brittle stage. These zones then channel the hydrothermal fluids involved in the last metal concentration step, leading to the formation of the orebodies. The efficiency of metal remobilization is dependent on a close spatial relationship between magmatic structures enriched in uranium and brittle structures channeling the hydrothermal fluids.

Metallogenesis of the French part of the Variscan orogen. Part II: Time-space relationships between U, Au and SnW ore deposition and geodynamic events — mineralogical and UPb data

Abstract Mineralogical, geochemical, structural and isotopic (UPb) studies were carried out on representative uranium and gold deposits from the French part of the Variscan orogen. Results are compared with complementary data from the literature in order to propose a reconstruction of the metallogenesis in the Variscan terranes, with a particular emphasis on U deposits. The data show that the early metal concentrations or pre-enrichments in the terranes are affected by successive metallogenic stages which caused significant metal transport and deposition. Thus, the significant geodynamic events which affected the Variscan terranes were responsible for multistage metal deposition or alteration of the early concentrations and for the superimposed features of the metal deposits. UPb geochronological results complemented by a compilation of geochronological data on U and PbZnBaF deposits show that three major stages of U deposition took place in Western Europe in the periods 325–300 Ma, 290–260 Ma and 190–170 Ma. In addition, small uranium mineralization events have been dated at 425 Ma, 90–110 Ma and as Tertiary (about 40 Ma). The Permo-Stephanian stage is associated with late brittle deformation associated with major shear zone movements and constitutes the major stage of U concentration. The Jurassic phase has a prominent part in the hydrothermal remobilization of uranium and in FBa (PbZn) deposition throughout Western Europe, especially in the eastern part of the Massif Central. Other stages correspond to subordinate uranium migration within Variscan terranes during earlier or later geodynamic phases: the Silurian metamorphism in Vendee and the Mesozoic to Tertiary tectonic movements related to the Pyrenean or Alpine orogenies in the south eastern part of the Massif Central and in the Alps. Data available on Au and SnW (As, Sb) deposits have been considered for comparison with the U metallogenesis. Stages related to the deposition of such metals display significant similarities with those related to uranium deposition. They occurred mostly during Visean to Permian events related to the intrusion of the late Visean (SnW) and Late Carboniferous (SnW, LiF) granitoids and to the late brittle deformation stages which affected the Hercynian basement in the vicinity of major shear zones (Au, AsSb). However, the significant discrepancies between the physical-chemical conditions of solubilization, transport and deposition of the metal led to distinct deposit location independently of the metal source location within the terranes.

Palaeofluid chemistry of a single fluid event: a bulk and in-situ multi-technique analysis (LIBS, Raman Spectroscopy) of an Alpine fluid (Mont-Blanc)

Abstract The only direct evidence of palaeofluids that circulated in the crust is contained within fluid inclusions. To properly understand such processes requires the complete anion and cation analysis of the fluid inclusions, which are examples of single fluid events. This analysis is a major analytical challenge when an average inclusion ( −9 g of material. An integrated multi-technique approach has been applied to the analysis of single inclusions from the water–chloride (+CO 2 ) system. It includes, after a careful selection of the inclusion and its microthermometric study, the use of non-destructive techniques (Raman spectroscopy for Cl − content and gas ratios) followed by destructive techniques such as laser-induced breakdown spectroscopy for major element ratios, in parallel to a bulk determination using a miniaturised crush leach technique on the whole fluid population. The laser-induced breakdown spectroscopy technique was used in this study for the first time in a routine way for fluid inclusion analysis. It provided Na/K, Na/Ca and Na/Li ratios. Detection limits are low and adequate for the determination of ion concentrations in individual inclusions (10 ppm for Na and Li, 20 ppm for Ca and 750 ppm for K). The application was carried out on a single fluid event typical of Alpine retrograde metamorphism that affected the Mont-Blanc granite (Glacier du Geant, France). The fluids are mostly aqueous (H 2 O is dominant, XCl around 1 mol% with X=Na (930±90 mmol/kg H 2 O), K (135±35 mmol/kg H 2 O) Li (30±8 mmol/kg H 2 O), Ca (105±25 mmol/kg H 2 O), and small amounts of CO 2 +N 2 mol%). The reconstruction of the fluid composition, using major element ratios obtained by the laser-induced breakdown spectroscopy technique, is in good agreement with the estimate obtained by the crush leach method on the same sample that is dominated by a single fluid population. The low Na/Li and Na/K are indicative of interaction with the host rocks at high temperature, the latter being indicative of equilibrium with two alkali feldspars at the temperatures estimated for the fluid event.

Boiling and fluid mixing in the chlorite zone of the Larderello geothermal system

Abstract The geochemical features of the geothermal fluids produced within the boiling zone in the relatively shallow parts of the Larderello geothermal system (Italy) have been documented as a result of deep drilling which provided samples from 1480 to 2500 m depth. Four wells (Monteverdi 1, Monteverdi 2A, Sasso 22 and Capannoli 2B) have been sampled in the intermediate parts of the Larderello aquifer located in a metamorphic basement underlying the Tertiary nappe complex which constitutes the shallow aquifer at Larderello. Fluid inclusions from recrystallized quartz lenses and quartz veins in samples displaying a predominant quartz–chlorite–(epidote–adularia) paragenesis have been studied by microthermometry and Raman spectroscopy. The inclusions are primary and pseudo-secondary in origin when formed in authigenic quartz, or of secondary origin, when located in fluid inclusion planes related to microfracturing of metamorphic quartz lenses. Several generations of fluids are present and include: H 2 O–(CO 2 )-dominated vapours and liquids, and a series of aqueous liquids, for the most part of relatively low salinity. The T m -ice of both early and late inclusions are mostly between 0.0 and −4.5°C, indicating that the salinities of the hydrothermal fluid were very low to moderate. However, rare fluid inclusions with lower T m -ice (from −4.9 to −25.0°C) were also observed. These inclusions may record the occasional input of saline fluids, which may be derived from the interaction of the hydrothermal waters with the evaporites present in the shallow part of the Larderello field. At Capannoli 2B, earliest H 2 O–(CO 2 ) liquids were trapped under minimal pressures of 610–645 bars, which bracket the estimated present-day lithostatic pressure (640 bars). In all other samples, the main stage of quartz–chlorite crystallization occurs under boiling conditions attested by the presence of liquid and vapour-rich inclusions, that, in some instances, can be texturally interpreted to be coeval. Their trapping conditions (350–375°C, 160–215 bars) are higher than the present day temperatures at the same depth. Later fluid inclusions attest to a significant cooling of the fluids down to temperatures similar to the present-day temperatures. During this time, pressures were close to hydrostatic conditions. Most fluid inclusions were trapped within the liquid field, this indicating that a significant pressure drop has since affected the main aquifers or fractured zones which are, at present, under vaporstatic conditions.

P-T-X conditions of late Hercynian fluid penetration and the origin of granite-hosted gold quartz veins in northwestern Iberia: A multidisciplinary study of fluid inclusions and their chemistry

Quartz veins hosted by late Hercynian granites and their host rocks occur across the Variscides of the northern Iberian peninsula, and locally display Au-As mineralization. Four separate occurrences at Corcoesto, Tomino, Penedono, and Pino have been investigated to determine P-T-X conditions of formation and likely fluid sources. Special attention has been paid to fluid chemistry using a multidisciplinary investigation of fluid inclusion gases by Raman spectroscopy (individual fluid inclusion analysis) and mass spectrometry (bulk analysis), and ion chemistry using microthermometry and bulk leachate chemical analysis. Two major changes in the chemical and physical environment have been identified: (1) a progressive change in the bulk chemical composition from early CO2-rich, C-H-O-(N) fluids, equilibrated with graphitic metamorphic host rocks, to late H2O-dominated fluids, inferred from their halogen signature to result from an influx of meteoric or upper crustal fluids affecting the basement at the end of Variscan orogenesis, and (2) changes in the P-T conditions from early stage sulphide deposition in quartz veins, at ca. 450 °C and 150-300 MPa, towards epithermal conditions, ca. 260–310 °C and <75 MPa, at the stage of gold mineralization. Several chemical trends are shown by the fluid inclusions: (1) dilution of the early volatile-rich fluids, (2) a break of graphite buffering activity demonstrated by the CH4 content increase in the volatile fraction of the latest As mineralizing fluids, and (3) increasing contribution of a relatively oxidizing fluid enriched in sulphate and bromide during the latest stages of fluid percolation (Au stage). These latest fluid stages are interpreted as indicative of extended fluid penetration downward in the crust enhanced by late brittle deformation and decompression, and played an important role in mass transfer at the end of the Hercynian orogeny, especially in transport of metals. Gold ores have formed mostly in granites because main fluid pathways developed in the main structurally active zones which favoured the emplacement of the granites. However, there is no evidence of genetic link between gold ores and their enclosing granites.