macla nº 9. septiembre ‘08 revista de la sociedad española de mineralogía 91

Graphite Morphologies in the Volcanic- Hosted Deposit at Borrowdale (NW England, UK): Preliminary Raman and SIMS data / JOSÉ FERNÁNDEZ BARRENECHEA (1, *), JAVIER LUQUE (1), LORENA ORTEGA (1), MAGDALENA RODAS (1), DAVID MILLWARD (2), OLIVIER BEYSSAC (3)

(1) Dpto. Cristalografía y Mineralogía, Facultad de Geología, Universidad Complutense de Madrid, 28040 Madrid ( Spain) (2) British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA (UK) (3) Laboratoire de Geologie, CNRS-UMR 8538, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 5 (France)

INTRODUCTION. over 100 m in length. The pipe-like RESULTS.

bodies contain nodular masses and Graphite structure consists of a patches of graphite, typically 1-2 cm Raman spectra were collected with a continuous bidimensional array of six- across, but ranging from a few Renishaw INVIA spectrometer at the fold rings of carbon atoms stacked along millimetres to 1 m or more. The mineral Ecole Normale Superieure (Paris, the c-axis. This determines that crystals association of the deposit includes France) on the polished thin sections commonly show laminar (platy) habits. graphite, chlorite, epidote, and quartz. used for the petrographic study. This In spite of this, other morphologies have method allows for measurements in situ been reported, but the widest range of The study of fluid inclusions within and recognition of the textural morphologies known within a single quartz associated with graphite relationships between graphite and the paragenesis is reported here from the indicates that the mineralizing fluids rest of the minerals in the assemblage. Borrowdale graphite deposit in Cumbria, evolved from H2O-CO2-CH4 mixtures All the measurements were done UK. There, graphite mineralization is (X H2O = 0.646; X CO2 = 0.244; X CH4 = focusing the laser beam beneath the hosted within volcanic rocks, one of only 0.114; X NaCl = 0.006) to H2O-CH4 (X H2O surface of the transparent minerals two examples of such an association = 0.93; X CH4 = 0.02; X NaCl = 0.05) fluids. associated with graphite to avoid the known worldwide. The crystal mechanical disruption of the graphite morphology of any mineral is controlled The wide diversity of graphite structure at the surface of the thin by both its structure and the physico- morphologies recognized within the section due to polishing (Pasteris, 1989; chemical conditions prevailing during deposit can be grouped into three Beyssac et al., 2003). The 514.5 nm nucleation and growth. On the other categories: laminar, cryptocrystalline, wavelength of a 20 mW spectra Argon hand, some geochemical features are and spherulitic. Laminar graphite laser focused through a 100x objective also dependent upon the conditions of crystals (up to 300 µm long and 50 µm was used for the analyses. Under these crystal growth. Thus, the structural and wide) are by far the most abundant (≈ conditions the spatial resolution is ~1 isotopic characterization of the graphite 90%). Along fault zones, “graphic-like” µm and the spectral resolution is close morphologies in the volcanic-hosted intergrowth textures, consisting of thin to 1 cm-1. The Raman parameters (peak deposit at Borrowdale could help unravel curved and tapering graphite flakes or position, band area, and band intensity) the evolution of the mineralizing process “vermiform” graphite crystals within were determined with the computer in this particular environment. chlorite have been observed. program PeakFit 3.0 using a Voigt Cryptocrystalline graphite may form: 1) function. GEOLOGICAL SETTING AND “composite nodules” consisting of both MINERALOGICAL FEATURES. flaky and cryptocrystalline graphite, 2) According to the Raman data all rounded patches within flaky graphite, graphite morphologies from the The Borrowdale graphite deposit and 3) “colloform” bands and globules Borrowdale deposit correspond to consists of mineralized faults hosted by dispersed within the host rock. structurally well ordered graphite. The andesite lavas and sills belonging to the Spherulites occur in four different intensity and area ratios for the D and G upper Ordovician (Caradoc) Borrowdale settings: 1) as individual forms, 5-40 µm bands in the first-order spectra (R1 and Volcanic Group, the geochemical in diameter, within flaky graphite, 2) as R2) are consistent with highly crystalline characteristics of which show evidence individual forms, 1-3 µm in diameter, graphite (R2 averages 0.06 for laminar of assimilation of pelitic material from included in quartz fragments, 3) as and spherulitic graphite and 0.05 for the underlying Skiddaw Group individual forms, 1-5 µm in diameter, or cryptocrystalline graphite within (McConnell et al., 2002). The richest aggregates disseminated within the composite nodules). In addition, no graphite deposits are developed at the volcanic rock, and 4) as aggregates, 5- significant changes were observed in the intersections of the faults within steeply 10 µm in diameter, associated with second order Raman region, all the inclined pipe-like bodies up to 1 x 3 m in chlorite along fault zones. spectra showing well-defined shoulders

palabras clave: Grafito, Raman, SIMS, Borrowdale key words: Graphite, Raman, SIMS, Borrowdale

resumen SEM/SEA 2008 * corresponding author: [email protected] 92

at ≈2685 cm-1 on the S1-peak, which is nodules has significantly lighter isotopic In the fractionation that occurs between indicative of attaining tri-periodic order signatures (average δ13C = -33.70 ‰) two carbon-bearing phases, the more in the structure (Lespade et al., 1982). with no apparent zoning across the oxidized species of the pair becomes Cryptocrystalline graphite forming banded texture (Fig. 1). The lightest relatively enriched in the heavier colloform textures within the host rock, carbon isotope ratios correspond to isotope, i.e. 13C. Thus, graphite however, shows a significantly lower vermicular graphite within chlorite precipitating from CO2-rich fluids would crystallinity (R2=0.25). (average δ13C = -34.49 ‰). be lighter than CO2 in equilibrium. This agrees with the fact that the earlier Secondary ion mass spectrometry DISCUSSION. morphologies (colloform) crystallizing in (SIMS) analyses were performed at the the pipe-like bodies display lighter Department of Geology and Geophysics, According to classical theories of signatures than flaky graphite that University of Edinburgh, using a Cameca nucleation, spherulites and colloform formed somewhat later. However, graphite within chlorite from fault-fills points to an inverse relationship: the

isotopic ratio of spherulitic graphite within chlorite is heavier than in the -34.73 vermicular intergrowths. This suggests, though does not prove, that graphite- chlorite veins represent a slightly later mineralizing event related to deposition from lower temperature, CH4-rich fluids. Since graphite is the oxidized phase with respect to methane, the earlier crystallizing morphologies (i.e. spherulites), retained heavier isotopic signatures than those precipitating later (vermicular graphite). -34.29 -35.12 -32.87 Finally, the small differences in the δ13C values between the different graphite -34.01 -34.01 morphologies suggest that graphite 140 µm precipitated over a narrow temperature interval.

fig. 1. SIMS data (δ13C relative to the PDB standard) of colloform graphite around quartz. The quartz grain REFERENCES. contains small graphite spherulites. Note the narrow range of δ13C values. ims 1270 ion microprobe. Gold-coated textures represent heterogeneous Beyssac, O., Goffé, B., Petitet, J.P., Froigneux, polished thin sections were analysed nucleation on pre-existing substrates E., Moreau, M., Rouzaud, J.-N. (2003): On under high supersaturation conditions. the characterization of disordered and using negative secondary ions sputtered heterogeneous carbonaceous materials + with a positively charged Cs beam. The Laminar graphite should nucleate later, by Raman spectroscopy. Spectrochim. spot size was 20 µm, and hence stable as a result of precipitation from fluids Acta, Part A, 59, 2267-2276. carbon isotope data were obtained only with significantly lower supersaturation. Cooper, D. C., Lee, M. K., Fortey, N. J., Cooper, from those morphologies displaying A. H., Rundle, C. C., Webb, B. C., Allen, P. sizes larger than this. The results were In spite of this sequence of M. (1988): The Crummock Water aureole: checked using international standards, crystallization of the different graphite a zone of metasomatism and source of including graphite USGS24 (δ13C = morphologies, Raman data indicate that ore metals in the English Lake District. J. there is not a significant change in the Geol. Soc., London, 145, 523-540. -16.05 ‰). Five measurements were Lespade, P., Al-Jishi, R., Dresselhaus, M.S. made on the standard at the start and structural order of graphite. (1982): Model for Raman scattering from end of each 20 analyses – thus enabling incompletely graphitized carbons. calibration of the sample measurements The light isotopic signatures of the Carbon, 20, 427-431. against 10 standard measurements. different morphologies of graphite from McConnell, B.J., Menuge, J.F., Hertogen, J. Under these conditions, the precision of the Borrowdale deposit suggest that the (2002): Andesite petrogenesis in the the point analysis is close to 0.2 ‰. carbon was derived from a biogenic Ordovician Borrowdale Volcanic Group of source. This is in good agreement with the English Lake District by fractionation, the evidence of assimilation of Skiddaw assimilation and mixing. J. Geol. Soc., It is worth noting that the analyses of London, 159, 417-424. each morphological type show very Group metapelites by the volcanic host Pasteris, J.D. (1989): In situ analysis in homogeneous values, thus falling within rocks, and with the presence of zones geological thin-sections by Laser Raman narrow ranges. Graphite flakes have one within the metapelites that have been Microprobe Spectroscopy: a cautionary of the heaviest isotopic signatures depleted in carbon and other elements note. Appl. Spectroscopy, 43, 567-570. (average δ13C = -30.26 ‰). Such values during hydrothermal alteration (Cooper are close to those found in graphite et al., 1988). Independent bulk carbon spherulites within chlorite (average δ13C isotopic analyses of Skiddaw = -30.15 ‰). On the other hand, metapelites yielded values close to -28 cryptocrystalline graphite in composite ‰.