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Assimilation, Hydrothermal Alteration and Graphite Mineralization in the Borrowdale Deposit (UK)

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Ortega Menor, Lorena and Luque del Villar, Francisco Javier and Barrenechea, Edurne and Rodas, Magdalena and Millward, David and Beyssac, Olivier (2009) Assimilation, Hydrothermal Alteration and Graphite Mineralization in the Borrowdale Deposit (UK). In Smart science for exploration and mining : proceedings of the 10th Biennial SGA Meeting, Townsville, Australia 17th-20th August 2009. James Cook University School of Earth & Environmental Studies. Economic Geology Research Unit, Townsville, Qld, pp. 257-259. ISBN 9780980558685.

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Abstract

The volcanic-hosted graphite deposit at Borrowdale was formed through precipitation from C-O-H fluids. The G13C 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.


Item Type:Book Section
Uncontrolled Keywords:Graphite, Borrowdale, fluid inclusions, carbon isotopes
Subjects:Sciences > Geology > Mineralogy
ID Code:44786
Deposited On:26 Sep 2017 08:33
Last Modified:11 Dec 2018 08:46

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