Causes of the uniformly high crystallinity of graphite in large epigenetic deposits

In low-pressure environments, precipitation of graphite is hindered at low to moderate temperatures by the high solubility of carbon in C-O-H fluids and by kinetic barriers to nucleation. Those low-temperature fluids that do attain saturation tend to precipitate graphite continuously during flow and cooling, thereby producing widely dispersed films of low-crystallinity graphite. In contrast, at high temperatures, particularly when combined with high pressures, the precipitation of graphite is enhanced by decreased solubility of carbon in C-O-H fluids and by improved nucleation under those conditions. The longevity of fluid systems in high-temperature, high-pressure terranes permits efficient, long-term scavenging of dispersed carbon from the crust. The latter may be redistributed in a much more concentrated form as fluids rise, cool and decompress, and as the carbon is finally precipitated as highly crystalline graphite in fracture systems. The combined effects of the thermochemical controls on carbon solubility and the geological controls on fluid generation, movement and P – T  pathways are the reason that large, epigenetic graphite deposits form dominantly at high temperatures and pressures. Those high-temperature, high-pressure conditions, in turn, account for the uniformly high crystallinity of the fluid-deposited graphite.