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The graphite deposit at Borrowdale (UK): A catastrophic mineralizing event associated with Ordovician magmatism

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Ortega Menor, Lorena y Millward, David y Luque del Villar, Francisco Javier y Fernández Barrenechea, José María y Beyssac, Olivier y Huizenga, Jan-Marten y Rodas, Magdalena y Clarke, S.M. (2010) The graphite deposit at Borrowdale (UK): A catastrophic mineralizing event associated with Ordovician magmatism. Geochimica et cosmochimica acta, 74 . pp. 2429-2449. ISSN 0016-7037

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Resumen

The volcanic-hosted graphite deposit at Borrowdale in Cumbria, UK, was formed through precipitation from C–O–H fluids.
The δ13C data indicate that carbon was incorporated into the mineralizing fluids by assimilation of carbonaceous metapelites
of the Skiddaw Group by andesite magmas of the Borrowdale Volcanic Group. The graphite mineralization occurred
as the fluids migrated upwards through normal conjugate fractures forming the main subvertical pipe-like bodies. The mineralizing
fluids evolved from CO2–CH4–H2O mixtures (XCO2 = 0.6–0.8) to CH4–H2O mixtures. Coevally with graphite deposition,
the andesite and dioritic wall rocks adjacent to the veins were intensely hydrothermally altered to a propylitic
assemblage. The initial graphite precipitation was probably triggered by the earliest hydration reactions in the volcanic host
rocks. During the main mineralization stage, graphite precipitated along the pipe-like bodies due to CO2 → C + O2. This
agrees with the isotopic data which indicate that the first graphite morphologies crystallizing from the fluid (cryptocrystalline
aggregates) are isotopically lighter than those crystallizing later (flakes). Late chlorite–graphite veins were formed from CH4-
enriched fluids following the reaction CH4+O2 → C + 2H2O, producing the successive precipitation of isotopically lighter
graphite morphologies. Thus, as mineralization proceeded, water-generating reactions were involved in graphite precipitation,
further favouring the propylitic alteration. The structural features of the pipe-like mineralized bodies as well as the isotopic
homogeneity of graphite suggest that the mineralization occurred in a very short period of time.


Tipo de documento:Artículo
Palabras clave:Graphite deposits; Ordovician magmatism; Borrowdale (UK)
Materias:Ciencias > Geología > Petrología
Código ID:17518
Depositado:20 Dic 2012 11:41
Última Modificación:20 Dic 2012 11:41

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