Elemental analyses of Palaeozoic mudrocks from south Cornwall indicate the presence of 0.04–0.74 wt% carbon primarily in an elemental and/or organic form, in addition to 0.04–0.10 wt% nitrogen as NH4+ in micas and in organic material. Both elements are depleted in samples from the lithologically homogeneous Mylor Slate Formation next to the Tregonning Granite, an offshoot of the Cornubian batholith. The depletions are due to an H2O-CO2-N2(-CH4) fluid formed during contact metamorphic devolatilization of the pelitic country rocks. Fluids of this type have been reported from a number of Hercynian Sn–W provinces, and may play an important role in mobilizing metals in pelitic rocks during granite-related contact metamorphism and hence in the metallogenic evolution of such regions.

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