Bernard Poty

Fluid regimes during late stages of a continental collision: Physical, chemical, and stable isotope measurements of fluid inclusions in fissure quartz from a geotraverse through the Central Alps, Switzerland☆

Fluid evolution during neo-alpine metamorphism during late stages of the continental collision between Europe and Africa was studied by analyzing fluid inclusions in alpine fissure quartz collected in forty-nine localities along a geotraverse through the Central Alps, Switzerland. The methods employed include microthermometry, micro-Raman spectroscopy, K/Na thermometry, and stable isotope analysis. Early fluid inclusions provide evidence of close to peak metamorphic temperatures of the late Tertiary or neo-alpine metamorphic event. Fluid composition evolved along the geotraverse from north to south as follows: higher hydrocarbons were dominant in the low- and medium-grade diagenetic zones, methane was the main volatile in the high-grade diagenetic and low-grade anchizone, water dominated in the highgrade anchizone and low-grade epizone, with CO2 > 10 mol% in the high-grade epizone and in the mesozone. Higher hydrocarbons and CH4 were the products of kerogen maturation and cracking of preexisting petroleum. Large water supplies originated from the dehydration of cooler metasedimentary rocks that were overthrust by crystalline basements of the Lepontines, Aar, and Gotthard massifs. Carbon isotope analyses suggest that the CO2 component was derived from oxidation of graphitic matter, especially in the vicinity of sulfate-bearing metasediments and from decarbonation reactions. In the Aar and Gotthard massifs as well as in the Helvetic Axen nappe and its underlying North Helvetic flysch, high fluid pressures prevailed and favored nappe transport. By contrast, in the southern Lepontine area, very low early fluid pressures were probably related to dry rocks and scarce metasediments, and to high geothermal gradients that resulted from intense uplift and erosion between 26 and 18 Ma. Retrograde fluid evolution was recorded by a succession of fluid inclusion populations in each alpine fissure. It was controlled by uplift and cooling and characterized by decreasing contents of volatiles and an increase in δ18O of host quartz. Tectonic activity led to episodic pressure drops of at least 0.5 to 2 kbar and promoted fluid unmixing, channelized flow, and rapid growth of skeletal quartz. Channelized rather than pervasive fluid migration at temperatures < 450°C and under conditions of brittle deformation is documented by episodic increases in salinity and by fluid flushing through the massifs. There is stable isotope evidence for involvement of meteoric water only in late-crystallizing quartz. Formation of Alpine fissures and fissure minerals was the result of a unique coincidence of late continental collision (< 450°C), fluid expulsion from overthrust metasediments, uplift, and erosion.

Radiolysis evidenced by H2-O2 and H2-bearing fluid inclusions in three uranium deposits

H2 with or without O2 has been identified by Raman spectroscopy in fluid inclusions in quartz from three Precambrian uranium deposits: Oklo (Gabon), Rabbit Lake and Cluff Lake D (Saskatchewan, Canada). In the Oklo uranium deposit, heterogeneous trapping of fluids in the system H2O-H2-CH4-salts is spatially associated with the natural nuclear reaction zones. At Rabbit Lake and Cluff Lake D, fluid inclusions show heterogeneous trapping of an aqueous liquid phase and a vapour phase with variable X(O2)X(H2) ratios always greater than one. H2 and O2 formation is discussed in terms of chemical reactions, chemical reactions indirectly induced by nuclear reactions and radiolytic processes. Numerical calculations of water radiolysis were carried out for two models: 1) water at the contact with an uranium oxide and 2) water inside a fluid inclusion containing either dissolved uranium or a small UO2 crystal. The concentrations of O2 and H2 inside fluid inclusions from Rabbit Lake and Cluff Lake D deposits are consistent with the first model. In the Oklo deposit, the H2-bearing fluid inclusions found in the quartz and the anomalous high OC atomic ratio of organic matter from nuclear reactor zones could result from radiolysis of water and organic matter due to the fission products of the chain nuclear reactions, but α radiolysis is not definitely ruled out.

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.

Pyrolysis of organic matter in cold-seal pressure autoclaves. Experimental approach and applications

Abstract Comparison of natural series of type III coals and type II kerogens with experimental results have shown that the cold-seal autoclave system offers the possibility to simulate the maturation of organic matter. The experimental vessel as well as the temperature regulation and monitoring systems are described in some detail. Limitations of the technique for mass balance calculations and simulations on whole rocks are discussed and indicate necessary improvements of this system for organic geochemistry purposes.

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.

Geochemistry of the rhyolitic magmas from the Streltsovka caldera (Transbaikalia, Russia): a melt inclusion study

The Streltsovka caldera located in Transbaikalia near the Chinese–Mongolian border represents the largest uranium field associated with volcanics in the world. All the rocks, volcanics and underlying granites, are deeply altered by hydrothermal fluids and their original major and trace element chemistry was strongly modified, particularly their primary mobile element concentrations such as fluorine, alkalis, uranium. To determine the primary geochemical characteristics of the magma associated with this giant ore field, a study of melt inclusions has been undertaken on the rhyolites. Many of the melt inclusions are well preserved in quartz phenocrysts from strongly altered rhyolites of the caldera. Si, Al, Fe, Na, K, Ca, F, Cl, Mn, Ti, P, S and Mg were analyzed by electron microprobe, U by fission tracks and H, U, Th, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Yb, Lu, Sc, Ti, V, Sr, Y, Nb, Ba, Hf by ion microprobe. The presence in melt inclusions, of daughter minerals such as fluorite, F-rich mica and REE fluocarbonate, and the composition of the silicate melt after homogenization, indicate that the Streltsovka rhyolites correspond to highly fractionated mildly peralkaline melts (1.04<Na+K/Al<1.10) whereas whole rock analyses indicate a peraluminous composition. The Streltsovka rhyolitic melts are rich in F (from 1.4 to 2.7 wt.%), and in U (14 to 23 ppm). They are relatively dry with a water content inferior to 2.4% H2O. The low Cl contents (0.2% Cl) in melt inclusions of the present study may be related to the mildly peralkaline character of the Streltsovka melts (NK/Al: 1.04 to 1.10). These melts are rich in REE and present moderate LREE fractionation, low HREE fractionation and a strongly negative Eu anomaly. The similar REE contents of Tulukuevskoe melt inclusions and whole rock indicate that there was no significant fractionation after trapping of the magma. On the other hand, the glasses from Krasny Kamen and Streltsovskoe rhyolites are richer in HREE than the whole rock. This agrees with an REE saturation of the magma at the trapping time that is confirmed by the crystallization of parisite as a daughter mineral in melt inclusions. Melt temperatures obtained from homogenization of the inclusions are relatively high for such magma (1015±30°C) and may be overestimated because of a possible volatile loss since the rhyolite formation. The lower Th (1000±20°C) obtained in Tulukuevskoe inclusions agrees with their higher F content (2.2% to 3.2%). The higher homogenization temperature of Krasny Kamen inclusions also agrees with the lower F content (0.6% to 0.8%).

Three examples illustrating the analysis of organic matter associated with uranium ores

Abstract Sedimentary uranium ore deposits provide interesting case histories for studies in organic geochemistry. Conversely, organic geochemical data often contribute to the understanding of uranium ore forming processes. Recent improvements in analytical techniques have also facilitated sophisticated investigations of organic matter related to the deposits. Three examples of organic geochemical studies are discussed. 1. 1. Barren and highly mineralized bitumens from Saskatchewan deposits (Canada) display very different isotopic composition but quite similar gross geochemical characteristics. NMR and Py-GC analyses—distinguish between these two types of bitumens on the basis of the amount and length of alkyl substituents on aromatic rings. 2. 2. Organic matter from the Precambrian Witwatersrand deposits (South Africa) display micro-textured variations which have been investigated by micro-Raman spectroscopy. Spectral characteristics vary with respect to the uranium content and petrographic characteristics. 3. 3. The maximum age of uranium mineralization in the breccia pipes in Arizona (U.S.A.) has been determined by modelling the maturation of the source rocks responsible for the genesis of hydrocarbons closely associated with the uranium mineralization. These examples illustrate how sophisticated studies are useful and necessary to acknowledge the complexity of organics associated to uranium ore deposits.

Simulation of natural coalification by high-pressure pyrolysis

Abstract Simulating natural organic matter maturation (coalification) is an important topic in organic geochemistry since experiments dating from the beginning of the century. However, because of the lack of an accurate natural reference series of coals, comparative efficiency of different simulation methods has not been tested. In this work, artificial maturation experiments were performed on raw lignites and coal extracts from a homogeneous sample set of increasing maturity (Mahakam Delta, Indonesia). In order to verify the validity of the experimental technique, the agreement between the chemical behaviours of the natural and artificial series was checked by different analytical techniques (C,H,O analysis; reflectance measurements; infrared and NMR spectroscopies; gas chromatography and mass spectrometry of hydrocarbons). (Monthioux et al., 1985, 1986; Landais et al., 1988). Thermal experiments were performed as follows: (1) in a open system, swept by an inert gas where the confinement (inverse of the dead-volume) is nil; (2) in a closed system, in sealed glass tubes where the confinement is intermediate; and (3) in a confined system, in cold-seal pressure autoclaves where the dead volume is minimal. For this last technique which was derived from hydrothermal experiments, 150 to 200 mg of coal were placed in an argon atmosphere inside a thin-walled gold tube (Landais et al., 1989). The gold tube was welded and then introduced in a cold-seal autoclave equipped to exert pressures ranging between 500 and 4000 bars (Fig. 1). Heat treatment temperatures range between 150 and 500°C for 24 hours. The H/C vs. O/C diagram of Figure 2 illustrates from a chemical standpoint the improving effect obtained by using increasing confinement conditions in the natural maturation of a lignite. Only a confined system is able to accurately duplicate the natural model, whereas other techniques quickly fall out of the range of natural coals. Numerous other parameters (evolution of infrared bands, aromaticity factor, reflectance, proportions of different classes of hydrocarbons, etc.) indicate the same conclusion. Thus, the confinement notion is an important parameter in the evolution of organic matter. In our experiments, confinement increase was due to the elevation of the effluents partial pressure. As far as a pyrolysis experiment involving the formation of free radicals, a pressure increase favours bimolecular reactions such as recombination or hydrogenation, whereas monomolecular reactions (unsaturation) are enhanced in open-system pyrolysis. Careful examination of the distribution of hydrocarbons shows that in confined pyrolysis, as well as in natural coalification, hydrogenation reactions are enhanced (Monthioux, 1988). Confined-system pyrolysis gives results comparable to those of natural maturation. Thus they can be applied to investigate the behaviour of different types of organic matter such as oxidized coals during maturation or coalification. It is shown that the behaviour of the end-member of an oxidation series of coals during maturation is roughly comparable to that of an unoxidized reference coal. Nevertheless, it was demonstrated that the hydrocarbon potential that has been lost during oxidation cannot be recovered during subsequent maturation. Artificial coalification of individual coal macerals extracted from the same parent coal also yields interesting results on their respective behaviours during coalification (Landais et al., in press).