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

Abstract

Massive graphite deposition resulting in volumetrically large occurrences in volcanic environments
is usually hindered by the low carbon contents of magmas and by the degassing processes occurring
during and after magma emplacement. In spite of this, two graphite deposits are known worldwide associated
with volcanic settings, at Borrowdale, UK, and Huelma, Spain. As inferred from the Borrowdale deposit,
graphite mineralization resulted from the complex interaction of several factors, so it can be considered as
an example of self-organized critical systems. These factors, in turn, could be used as potential guides for
exploration. The key factors influencing graphite mineralization in volcanic settings are as follows: (1) an
unusually high carbon content of the magmas, as a result of the assimilation of carbonaceous metasedimentary
rocks; (2) the absence of significant degassing, related to the presence of sub-volcanic rocks or hypabyssal
intrusions, acting as barriers to flow; (3) the exsolution of a carbon-bearing aqueous fluid phase; (4) the
local structural heterogeneity (represented at Borrowdale by the deep-seated Burtness Comb Fault); (5) the
structural control on the deposits, implying an overpressured, fluid-rich regime favouring a focused fluid
flow; (6) the temperature changes associated with fluid flow and hydration reactions, resulting in carbon
supersaturation in the fluid, and leading to disequilibrium in the system. This disequilibrium is regarded as the
driving force for massive graphite precipitation through irreversible mass-transfer reactions. Therefore, the
formation of volcanic-hosted graphite deposits can be explained in terms of a self-organized critical system.