Vein-type graphite in Jurassic volcanic rocks of the external zone of the Betic Cordillera, Southern Spain.

Abstract

We have investigated volcanic-rock-hosted graphitem in eralization in the Huelma area of the Betic Cordillera, in southern Spain, in order to establish the mineralogical characteristics of the graphite andthe mechanism of its formation. The graphite is highlycrystalline, asrevealed by XRDandRaman spectroscopic data, andrelatively pure chemically (C content >90wt.%). XRD data demonstrate that some of themineralized bodies contain both hexagonal and rhombohedral graphite. Carbon isotopic compositions (-23.0< δ13 C <-20.7 ‰ ) and other geological observations indicate that thecarbon in the graphite was derived from organic matter included in the metamorphic basement. Cooling of a C-O-H fluid appears to be the most likely mechanism for precipitation of the graphite, although hydration reactions also may have played a role. The presence of rhombohedral graphiteis discussed with reference to the kinetics of graphite formation.