Publication: Graphite morphologies from the Borrowdale deposit
(NW England, UK): Raman and SIMS data
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Publication Date
2009
Authors
Luque del Villar, Francisco Javier
Millward, David
Beyssac, Olivier
Rodas, Magdalena
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Springer Science Business Media
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Abstract
Graphite in the Borrowdale (Cumbria, UK)
deposit occurs as large masses within mineralized pipe-like
bodies, in late graphite–chlorite veins, and disseminated
through the volcanic host rocks. This occurrence shows the
greatest variety of crystalline graphite morphologies recognized
to date from a single deposit. These morphologies
described herein include flakes, cryptocrystalline and
spherulitic aggregates, and dish-like forms. Colloform
textures, displayed by many of the cryptocrystalline
aggregates, are reported here for the first time from any
graphite deposit worldwide. Textural relationships indicate
that spherulitic aggregates and colloform graphite formed
earlier than flaky crystals. This sequence of crystallization
is in agreement with the precipitation of graphite from
fluids with progressively decreasing supersaturation. The
structural characterization carried out by means of Raman
spectroscopy shows that, with the exception of colloform
graphite around silicate grains and pyrite within the host
rocks, all graphite morphologies display very high crystallinity.
The microscale SIMS study reveals light stable
carbon isotope ratios for graphite (δ13C = -34.5 to
-30.2%), which are compatible with the assimilation of
carbon-bearing metapelites in the Borrowdale Volcanic
Group magmas. Within the main mineralized breccia pipelike
bodies, the isotopic signatures (with cryptocrystalline
graphite being lighter than flaky graphite) are consistent
with the composition and evolution of the mineralizing
fluids inferred from fluid inclusion data which indicate a
progressive loss of CO2. Late graphite–chlorite veins
contain isotopically heavier spherulitic graphite than flaky
graphite. This agrees with CH4-enriched fluids at this stage
of the mineralizing event, resulting in the successive precipitation
of isotopically heavier graphite morphologies.
The isotopic variations of the different graphite morphologies
can be attributed therefore, to changes in the
speciation of carbon in the fluids coupled with concomitant
changes in the XH2O during precipitation of graphite and
associated hydrous minerals (mainly epidote and chlorite).