%0 Journal Article
%A Fernández Barrenechea, José María
%A Luque Del Villar, Francisco Javier
%A Millward, David
%A Ortega Menor, Lorena
%A Beyssac, Olivier
%A Rodas González, Magdalena
%T Graphite morphologies from the Borrowdale deposit(NW England, UK): Raman and SIMS data
%D 2009
%@ 0010-7999
%U https://hdl.handle.net/20.500.14352/50259
%X Graphite in the Borrowdale (Cumbria, UK)deposit occurs as large masses within mineralized pipe-likebodies, in late graphite–chlorite veins, and disseminatedthrough the volcanic host rocks. This occurrence shows thegreatest variety of crystalline graphite morphologies recognizedto date from a single deposit. These morphologiesdescribed herein include flakes, cryptocrystalline andspherulitic aggregates, and dish-like forms. Colloformtextures, displayed by many of the cryptocrystallineaggregates, are reported here for the first time from anygraphite deposit worldwide. Textural relationships indicatethat spherulitic aggregates and colloform graphite formedearlier than flaky crystals. This sequence of crystallizationis in agreement with the precipitation of graphite fromfluids with progressively decreasing supersaturation. Thestructural characterization carried out by means of Ramanspectroscopy shows that, with the exception of colloformgraphite around silicate grains and pyrite within the hostrocks, all graphite morphologies display very high crystallinity.The microscale SIMS study reveals light stablecarbon isotope ratios for graphite (δ13C = -34.5 to-30.2%), which are compatible with the assimilation ofcarbon-bearing metapelites in the Borrowdale VolcanicGroup magmas. Within the main mineralized breccia pipelikebodies, the isotopic signatures (with cryptocrystallinegraphite being lighter than flaky graphite) are consistentwith the composition and evolution of the mineralizingfluids inferred from fluid inclusion data which indicate aprogressive loss of CO2. Late graphite–chlorite veinscontain isotopically heavier spherulitic graphite than flakygraphite. This agrees with CH4-enriched fluids at this stageof the mineralizing event, resulting in the successive precipitationof isotopically heavier graphite morphologies.The isotopic variations of the different graphite morphologiescan be attributed therefore, to changes in thespeciation of carbon in the fluids coupled with concomitantchanges in the XH2O during precipitation of graphite andassociated hydrous minerals (mainly epidote and chlorite).
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