Mechanical graphite transport in fault zones and the formation of graphite veins

Abstract

This paper describes a vein-shaped graphite occurrence in which, for the first time, the geological, mineralogical and isotopic evidence support its formation by physical remobilization of previously formed syngenetic graphite. The deposit studied is located in the Spanish Central System and it occurs along the contact between a hydrothermal Ag-bearing quartz vein and a graphite-bearing quartzite layer. The characteristics of this occurrence differ from those of fluid-deposited vein-type graphite mineralization in that: (1) graphite flakes are oriented parallel to the vein walls; (2) graphite crystallinity is slightly lower than in the syngenetic precursor (graphite disseminated in the quartzite); and (3) the isotopic signatures of both types of graphite are identical and correspond to biogenic carbon. In addition, the P-T conditions of the hydrothermal Ag-bearing quartz veins in the study area (P <1 kbar, and T up to 360ëC) contrast with the high degree of structural order of graphite in the vein. Therefore, physical remobilization of graphite can be regarded as a suitable alternative mechanism to account for some cases of vein-shaped graphite deposits. Such a mechanism would require a previous concentration of disseminated syngenetic graphite promoted, in this case, by the retrograde solubility of quartz. This process would generate monomineralic graphite aggregates enhancing its lubricant properties and permitting graphite to move in the solid state along distances in the range of up to several metres.