Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Barrenechea, Edurne

Beyssac, Olivier

Huizenga, Jan-Marten

Luque del Villar, Francisco Javier

Millward, D.

Ortega Menor, Lorena

Rodas, Magdalena

Luque Del Villar, Francisco Javier

Rodas González, Magdalena

Docta Complutense

European Current Research On Fluids Inclusions                                                 ECROFI 2009  1 CONDITIONS OF GRAPHITE PRECIPITATION IN THE VOLCANIC-HOSTED DEPOSIT AT BORROWDALE (CUMBRIA, UK)  Ortega, L.a; Luque, F.J.a; Barrenechea, J.F.a; Beyssac, O. b; Huizenga, J-M. c; Millward, D. d and Rodas, M.a a Departamento de Cristalografía y Mineralogía, Facultad de Geología, Universidad Complutense de Madrid, Spain (lortega@geo.ucm.es; jluque@geo.ucm.es; barrene@geo.ucm.es; rodas@geo.ucm.es). b Laboratoire de Géologie, CNRS, Ecole Normale Supérieure, 24 rue Lhomond, 75005 Paris, France (beyssac@mailhost.geologie.ens.fr). c School of Environmental Science and Development, North-West University, Potchefstroom, South Africa (21907609@nwu.ac.za) d British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK (dmill@bgs.ac.uk).   Introduction  Most of the world’s graphite deposits from which the highest quality graphite is mined has been formed by precipitation from C-bearing fluids along fracture systems. In spite of the economic significance of this type of graphite deposit, little is known about the composition and characteristics of the fluids responsible for its formation. Thus, although fluid inclusions provide unique and direct information about the fluids involved in mineralization processes, there is only one reference on fluid inclusion studies in graphite deposits dealing with quartz–graphite veins hosted by metasedimentary rocks (Duke et al., 1990). More recently, Satish-Kumar (2005) has confirmed the usefulness of fluid inclusion studies to infer the mechanism of graphite precipitation and to model the carbon isotope evolution of the fluid–graphite system in granulites.   Here we present a study of fluid inclusions from the graphite deposit at Seathwaite in Borrowdale, Cumbria (UK), where graphite was discovered and mined from the 16th century until the 19th century. In addition to the historical and economic interest of this deposit, it is also one of the only two large graphite occurrences known worldwide to be hosted by volcanic rocks, the other being at Huelma in southern Spain. The characterization of the carbon-bearing fluids has provided a further insight into the progress of the mineralizing event in this deposit and therefore a tool for understanding the mineralization processes in other fluid-deposited graphite occurrences.  Geology and mineralogy of the deposit    The Borrowdale graphite deposit consists of mineralized faults hosted by andesite lavas and sills belonging to the upper Ordovician (Caradoc) Borrowdale Volcanic Group. The graphite mineralization occupies about a 400 m length of a conjugate set of normal faults hading up to 45°. The richest deposits are developed at the intersections of the faults where there are steeply inclined pipe-like bodies up to 1 x 3 m in cross-section and from a few metres to over 100 m in length. The pipe-like bodies contain nodular masses and patches of graphite, typically 1-2 cm across, and brecciated quartz within a yellow-brown matrix of intensely altered wall-rock.   Graphite nodules and patches frequently include radiating aggregates of elongated epidote crystals, chlorite, quartz, pyrite, chalcopyrite and minor sericite. The andesite and dioritic wall rocks adjacent to the veins have been intensely hydrothermally altered to hematite and an assemblage containing quartz, chlorite, and albite. These features are indicative of an intense propylitic alteration, and evidence that graphite precipitated during this hydrothermal event.  Fluid inclusion study   Microthermometric and Raman data allowed the definition of four types of inclusions: V, VS, L1 and L2. Type V inclusions are two-phase vapour-rich (Vv/Vt = 50-95%), made up by H2O-CO2-CH4 (XCO2=0.6-0.8). Total homogenization of the inclusions ranges from 295 to 340 ºC (into vapour) and from 328 to 350 ºC (critical behaviour) indicating that the fluid was a vapour-like supercritical phase at the trapping conditions. Type VS inclusions consist of a large bubble containing pure CH4 vapour plus highly crystalline graphite and a liquid H2O phase (up to 15 % vol). Type L1 are two-phase liquid-rich H2O-CO2-CH4-bearing inclusions (Vv/Vt = 25-40%) and occurs spatially associated with type V. Raman European Current Research On Fluids Inclusions                                                 ECROFI 2009  2analyses indicate XCO2 between 0.03-0.28 in most L1 inclusions.  Total homogenization occurs between 276 and 372 ºC (into liquid). Type L2 are two-phase liquid-rich inclusions (Vv/Vt < 10%), with CH4 as the only carbon species. Total homogenization temperatures are between 123 and 204 ºC (into liquid).  The petrographic analysis of the fluid inclusions, along with their composition, allows establish the timing of fluid circulation as follows: V → L1→ L2. Thus, the overall fluid evolution is characterized by: 1) depletion in volatiles, i.e. the carbonic species are transferred to the solid state as graphite, and 2) a progressive decrease in the #XCO2 (#XCO2=XCO2/(XCO2+CH4)) ratio.   Conditions of graphite deposition  According to the proposed chronology for the fluid inclusions, the type V, vapour-rich fluid would be the fluid circulating at the earliest stages of the process. Thermodynamic calculations for this fluid, characterized by #XCO2 of 0.69  and a fO2 in equilibrium with the FMQ buffer, (as indicated by a epidote composition of Ps25; Liou, 1993), has allow to estimate the PT conditions for the beginning of the mineralization process at 500 ºC and 2-3 kb (Luque et al., 2009). This pressure interval was considered the most likely, as it is compatible with an emplacement of the graphite deposit in a subvolcanic setting (Millward, 2004).   Two chemical reactions may account for the graphite precipitation at Borrowdale: 1) CO2 → C + O2, and 2) CH4 + O2 → C + 2H2O. The conspicuous relationship between epidote and graphite in the breccia pipe bodies strongly suggests that the graphite precipitation mainly occurred following the reaction CO2 → C + O2. The type V fluid contain XCO2=0.24 whereas epidote is not stable for XCO2>0.2 (Liou, 1993). Thus, epidote crystallization was likely triggered by the consumption of CO2 in the reaction of graphite precipitation. VS inclusions containing graphite along with CH4+H2O may be recording this reaction. The mechanism inducing initial nucleation and subsequent growth of graphite previous to epidote crystallization could be the early hydrothermal alteration of the host rocks, notably to form chlorite. The significant decrease in #CO2 is recorded in L1 inclusions which would have been formed while the reduction of CO2 proceeded.   Therefore, the dominant reaction of graphite crystallization within the pipes was CO2 → C + O2. However, this reaction needs to be coupled with an oxygen-consuming reaction during graphite deposition to buffer the fO2, because the composition of the epidote, notably the content in Fe3+ expressed as the pistacite content, is extremely sensitive to fO2 conditions (Liou, 1993). Since fluid inclusion data reveal a progressive enrichment in water as graphite precipitation proceeded, the reaction CH4 + O2 → C + 2H2O is considered as the most likely to have occurred. Such a reaction would have consumed the O2 produced by the main reaction, thus maintaining the overall fO2 conditions close to the FMQ buffer. During this process, the buffering capacity of the rock would have contributed to keeping fO2 conditions fairly constant, as supported by the widespread hematization of the wall rock. The absence of important changes in the fO2 conditions is also supported by the lack of significant isotopic variations at the microscale in the precipitated graphite (Barrenechea et al., 2009).  References Barrenechea, J.F.; Luque F. J.; Millward, D.; Ortega, L.; Beyssac, O.; Rodas, M; Contrib. Mineral. Petrol. 158, 37-51. (2009). Duke, E.F.; Galbreath, K.C.; Trusty, K.J. Geochim. Cosmochim. Acta, 54, 683-698 (1990). Liou, J.G. in: V. Hock and F. Koller, editors, Proceedings of the 125 Jahre Kappenwand Symposium, 7-16 (1993). Luque, F.J.; Ortega, L.; Barrenechea, J,F.; Millward, D.; Beyssac, O.; Huizenga, J-M.  Geology, 37, 275-278 (2009). Millward, D. Proc. Yorkshire Geol. Soc. 55, 73-105. (2004). Satish-Kumar M. Geochim. Cosmochim. Acta 69, 3841-3856. (2005). 

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Conditions of graphite precipitation in the volcanic-hosted deposits at Borrowdale (Cumbria, UK)

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Conditions of graphite precipitation in the volcanic-hosted deposits at Borrowdale (Cumbria, UK)

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