Constraints on graphite crystallinity in some Spanish fluid-deposited occurrences from different geologic settings

Epigenetic, vein-type graphite mineralization originates by deposition from C-O-H fluids in high-temperature environments. Consequently, fluid-deposit ed graphite is uniformly highly crystalline in volumetrically large occurre nces. This work examines the factors controlling graphite crystallinity in fluid-deposited occurrences with reference to some case studies from southe rn Spain where vein-type graphite is associated with a variety of host rock s. Possible causes influencing high crystallinity of graphite in these occu rrences include: (1) large graphite occurrences are generated from large vo lumes of fluids that maintain their temperatures for long periods of time, which is easier at higher temperatures; (2) high temperature conditions are required for a fluid to precipitate a major part of its dissolved carbon d uring a small temperature decrease, (3) carbon is incorporated into C-O-H f luids mainly through devolatilization reactions which also require high tem peratures; (4) highly crystalline graphite generated at high-T/high-P condi tions is less susceptible to resorption as P decreases or by subsequent flu id flow; (5) graphite precipitation involves high activation energy that ca n be overcome only if the temperature is high enough. These causes can be e xtrapolated to most vein-type graphite deposits worldwide.