Michel Cathelineau

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.

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.

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.

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.

Fluid percolations in a fault zone: a study of fluid inclusion planes in the St Sylvestre granite, northwest Massif Central, France

Abstract A method for relating fluid flow evolution (direction of fluid percolation) to the bulk brittle network (microfracture system) at each state in a sequence of deformational events is presented. Fluids percolating through cracks are generally trapped as fluid inclusion planes (alignment of fluid inclusions). The method involves the use of fluid inclusions not only as a tool for the establishment of the physico-chemical conditions under which fluids were entrapped, but also as a microstructural marker of the geometry of permeability. The study shows that through the determination of the paleopressures for each stage of fluid migration, the chronology of the cracks and P-T conditions prevailing during each crack formation can be established. This methodology has focussed on a granitic example from the French Massif Central, but may be applied to other geological environments.

Fluid transfers at a basement/cover interface Part II. Large-scale introduction of chlorine into the basement by Mesozoic basinal brines

Significant fracture and porosity sealing characterizes the sedimentary cover-basement interface in the northwestern margin of the Aquitaine Basin (France). Dolomite and calcite (and sometimes fluorite, barite and quartz) constitute most of the fracture fillings. They contain primary inclusions of brines having chlorinities in the range of 3.3 to 5.5 mol Cl/kg solution, with total homogenization temperatures in between 65 and 130 °C for quartz and slightly lower for dolomite, barite, fluorite and calcite. Crush-leach analyses indicate that brines are characterized by Na/K ratios of 5 to 40, Na/Li ratios of 20 to 530, and Cl/Br ratios of 200 to 1000, which are rather typical of deep basinal brines. The fluid δ18O signature is estimated to be ≈6.6±1.8‰ SMOW for a crystallization temperature of 100±20 °C between 80 and 120 °C and the δD value is −30±10‰ SMOW. The fluid source for the fracture filling mineral is interpreted as a deep sedimentary brine expelled during a period of maximum subsidence in the Aquitaine Basin, which migrated along the sediment cover/basement interface which is characterized by high permeabilities below the Toarcian shales. The sealing is likely to be linked to the mixing of the brines with dilute ascending, hot waters. These dilute waters infiltrated from emerged zones, convected and heated at depth, reaching temperatures of 100 °C (up to 150 °C on the basis of cation geothermometry). Extensional activity, of probable Cretaceous age, related to the Gascogne Gulf rifting could be considered as the most likely cause of a significant fluid migration event at the basement/cover interface all along the margins of the French Massif Central. These processes are large scale as shown by the similarities of mineral sequences, fluid types and general features of most of the F–Ba–Pb–Zn deposits located at the basement–sedimentary cover interface. The mass transfer of Cl linked to this stage is significant and accounts for the early introduction of large amounts of chlorine in the granitoid microporosity. This probably explains the significant chlorine concentrations of the present-day fluids recovered in the granitoid aquifer.

A three stage fluid flow model for Variscan gold metallogenesis in northern Portugal

Mineralogical, fluid inclusion and geochemical studies were made on two intra-granitic gold deposits (Grovelas and Pene- dono), together with a deposit linked to sub-vertical structures in silicified metasediments at Tres-Minas, and several intra- metamorphic occurrences at Vila Pouca de Aguiar. They all possess similar mineral assemblages, deformational state, fluid flow characteristics, ore fluid composition and have comparable P-T conditions. Three successive crystallisation stages are recorded during the formation of gold-bearing structures independent of their location or host rocks (granites or metasedi- ments). They are: Stage 1 — the development of milky quartz veins that formed primarily after the emplacement of peraluminous two-mica granites (315-310 Ma) at P-T conditions reflecting high temperature and low pressure. They are similar to those from pluton induced metamorphismOPa 300-350 MPa and Ta 500-5508CU: No clear evidence was found for gold deposition during this stage. Stage 2 — during orogenic uplift and repeated tectonic reactivation a clear quartz was deposited in the early milky quartz veins (Stage 1) at P-T conditions between 100 and 300 MPa and 300 and 4508C. Local sulphide deposition (arsenopyrite II and pyrite II) occurred in clear quartz, but was never massive. The fluids percolating within the granite were mainly aqueous- carbonic and reflect equilibrium with the metamorphic host rocks. They are very similar to those found in metamorphic environments. No evidence for the involvement of magmatic fluids was found. Stage 3 — intense microfissuring of the earlier vein infillings occurred, associated with the main episode of gold deposition. The P-T conditions were ,100 MPa and ,3008C based on aqueous fluid inclusions. Native gold and electrum crystallised together with sulphides (galena, chalcopyrite and bismuthinite), native Bi and sulphosalts (Pb-Bi-Ag dominated). The fractures frequently contain chlorite (^ sericite) especially where they crosscut earlier sulphides (arsenopyrite). These processes and fluid types are similar in both the granites and metamorphic host rocks. Therefore, the gold ores appear to be the result of successive periods of fluid circulation, in this case related to the uplift of the Variscan basement in response to high heat flow and the intrusion of granites. Without exception, these fluids have been re-equilibrated with the metamorphic rocks. However magmatic fluids are absent; the granites thus act passively as heat engines for fluid circulation. q 2000 Elsevier Science B.V. All rights reserved.

EFFECTS OF ETHYLENE GLYCOL SATURATION PROTOCOLS ON XRD PATTERNS: A CRITICAL REVIEW AND DISCUSSION

Study of the transformation of smectite to illite, chlorite or vermiculite via interstratified clay minerals needs precise qualitative and quantitative determinations of the different layers in the mixed-layer clays and is generally based on X-ray diffraction (XRD) patterns after specific treatments of the clay samples. Saturation with K or Mg followed by ethylene glycol (EG) solvation are classical methods used to identify high-charge smectite and vermiculite. These procedures have been applied to two experimental clays, one composed of smectite layers and the second, a mixture of vermiculite and smectite layers. Different methods of glycolation (EG vapor or liquid EG) produce significant differences in the XRD patterns. Comparison with literature data indicates that K-saturated, high-charge smectite (≈0.8 < total charge <1/unit-cell) and Mg-vermiculite (whatever its charge) do not expand in ethylene glycol vapor (d values ≈14–15 Å). Expansion to 17 Å in liquid ethylene glycol occurs for Mg-vermiculite with a total charge of <~1.2/unit-cell and for K-saturated, high-charge smectite, when the tetrahedral charge is <≈0.7/unit-cell. This study shows that: (1) glycolation procedures need to be standardized; (2) the use of saturation protocols using both liquid ethylene glycol and ethylene glycol vapor yields useful additional information about the distribution of charges in clay minerals.

Fluid inclusion evidence of the differential migration of H2 and O2in the McArthur River unconformity-type uranium deposit (Saskatchewan, Canada). Possible role on post-ore modifications of the host rocks

A fluid inclusion study was performed in the vicinity of the McArthur River unconformity-type uranium deposit,Saskatchewan, Canada. Euhedral quartz veins and breccias were sampled from the contact of the uranium orebody to a distance of 50 m from it. Raman analyses of the fluid inclusions vapor phase indicated the presence of H2 and O2 with various proportions. Fluids located at the immediate vicinity of the uranium ore contain nearly exclusively O2. At 1 m from the ore, the oxygen proportion relative to other gases rapidly decreases, whereas H2 increases and occasional traces of CH4 occur. Additional C2H6 as well as CO2 appear at about 10 m from mineralization, in the basement. Finally, only occasional traces of H2 were detected in fluid inclusions located at 50 m from the mineralization, in the sandstones. Thus, fluids that circulated at the contact of the orebody became oxidizing, whereas fluids circulating at distance from it were more reduced, due to the combination of water radiolysis and differential migration of H2 and O2. This repartition of gases probably induced indirectly significant modifications in the host rock after the ore formation, such as hematization or graphite alteration and subsequent gas production. In addition, the migration of H2 at long distance from the orebody should represent an potential prospecting tool.