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

Abstract

The volcanic-hosted graphite deposit at Borrowdale in Cumbria, UK, was formed through precipitation from C-O-H flu ids. The 0 13 C data indicate that carbon was incorporated into the mineralizing fluids by assimilation of carbonaceous met a pelites 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 min eralizing fluids evolved from COrCH4-H20 mixtures (XC02 = 0.6-0.8) to CHCH20 mixtures. Coevally with graphite depo sition, 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 + 2H20, 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.

Key factors controlling massive graphite deposition in volcanic settings: an example of a self-organized critical system

“…X-ray diffraction (XRD) and Raman data reveal that the graphite is highly crystalline as evi denced by the sharp and symmetrical peaks in the XRD patterns (Luque et al 2009a) as well as by the very low intensity and area ratios of the disorder peak (c. 1350 cm-1) to the order peak (c. 1580 cm-1) in the fIrst-order Raman spectra ). In the same way, the ClITVes of differential thennal analysis of graphite from the Borrowdale deposit are also in agreement with its high crystallinity, showing strong exothennic peaks at tempera tures in excess of740 °c (Ortega et al 2010). Stable carbon isotope signatures of single graphite morphologies and bulk concentrates from the Borrowdale deposit are highly negative (013C from -34.5 to -24.30/00).…”

“…The wall rocks show an intense hydrothermal alteration to an assemblage containing quartz, chlorite, and albite, along with some disseminated small aggregates of graphite and late calcite veinlets. These features are indicative of an intense propylitic alteration, and provide evidence that the main stage of graphite precipitation occurred during this hydrothennal event (Luque et al 2009a;Ortega et al 2009Ortega et al , 2010. In addition to the pipe-like bodies, graphite also occurs associated with chlorite along fault planes (Fig.…”

“…Stable carbon isotope signatures of single graphite morphologies and bulk concentrates from the Borrowdale deposit are highly negative (013C from -34.5 to -24.30/00). In particular, within the pipe-like bodies, the homoge neous carbon isotopic signature both at the microscale and at the scale of each single graphite nodule suggests deposition between 500 and 400cC at about 2kbar (Ortega et al 2010). The highly negative 013C values suggest that the carbon was derived from a biogenic source.…”

“…The Borrowdale Volcanic Group is llllderlain by anchizonal to epizonal metapelites and sandstones of the upper Cambrian to middle Ordovician Skiddaw Group. There is petrological and geochemical evidence for assimilation of Skiddaw Group lithologies by the Borrowdale Volcanic Group magmas (Fitton 1972;McCOIlllell et al 2002;Ortega et al 2010). …”

“…2c). Graphite--chlorite veins represent a late event of graphite mineralization with respect to that of the pipe-like bodies (Ortega et al 2010). …”