Of cannonballs and pencils: Borrowdale graphite.
A packed Tullie House lecture theatre got the full story of the Seathwaite graphite mine, from David Millward of the British Geological Survey, in a very clear and well structured account. Many images and facts from the presentation will linger long in the mind including [1] the impressive cavity left by the main pipe-shaped orebody, [2] the size of the graphite nodule in the Tullie House collection, [3] that the moderate temperature volcanic hydrothermal system was capable of producing graphite as highly crystalline and of the same high quality as found in ultra-metamorphic regions, [4] the wide array of modern high tech kit that was used in the study, [5] the intriguing history of exploitation and ancillary socio-economic developments, [6] the range of ancient and modern uses of graphite, [7] the mine’s special characteristics in that it is one of only two volcanic-hosted graphite deposits and it was the first place where colloform graphite was identified, [8] the challenge of keeping CO2 in the magma to form such a deposit, [9] the clear biogenic source of the carbon very likely derived through assimilation of Skiddaw Group material into the magma, a process also responsible for the globally-rare garnet phenocrysts in the BVG, [10] the fundamental nature of the Burtness Comb Fault that probably played a role in channelling magmas to the site of mineralisation, [11] the bureaucratic complexities involved in working on an Scheduled Ancient Monument and the five year delay to the project when the 2001 foot-and-mouth outbreak coincided with the planned start, and [12] the desirability of having caving skills to do geology underground in old workings.
Perhaps more for the ore deposit aficionados were aspects like [1] the Raman spectrometry to determine the degree of crystallinity of the graphite, [2] the Secondary Ion Mass Spectrometry to study isotopes in micrometre-sized areas of the separate graphite forms, [3] the array of microscopy techniques employed down to Scanning Electron Microscope resolutions, [4] the extensive use of fluid inclusion data to characterise the nature of the fluid and how it evolved during the mineralisation process, [5] the structural control at fault/fault intersections and the catastrophic disturbance of fluid conditions at fault rupture events, [6] the extent of fluid-rock interaction and the crucial role this process played in forming the deposit as the hydration of country-rock minerals led to carbon supersaturation in the fluid driving massive graphite precipitation, [7] by combining all the data, the deposit clearly is shown to be epigenetic and must have formed after the Birker Fell Formation but whilst the Borrowdale volcanics were still evolving, and [8] the remarkable sequence of events in the depositional model and the number of critical steps required to create such a special deposit. All of these results come from a highly fruitful collaboration with a group of Spanish researchers.
The final thought put to the group was that if, industrially, we could replicate the natural process at Seathwaite we could produce synthetic graphite with much greater energy efficiency than happens at the moment for this critical raw material. Audrey Brown proposed the vote of thanks supported by great acclaim from the audience.
CAB
