Fabric Development, Electrical Conductivity and Graphite Formation in graphite-bearing Marbles from the Central Damara Belt, Namibia

Abstract

Graphite-bearing marbles occur in crustal-scale dome structures of the central parts of the Damara Belt in north-western Namibia. They have been reported to show significant anomalies of high electrical conductivity in magnetotelluric profiles. This work presents conductivity measurements on a sample scale of different types of these graphite-bearing marbles. As the graphite-bearing marbles also form distinct shear zones along reactivated rims of the dome structures, these different types of graphite-bearing marbles were distinguished by their macro- and microscopic fabric characteristics. The investigation and classification of the different fabrics is the basis for understanding the conductivity potentials of the different types of graphite-bearing marbles. The classification was made using qualitative optical microscopy, cathodoluminescence microscopy (CL), scattered electron microscopy (SEM), bulk and local texture analysis and the quantification of the calcite-graphite ratios. Several studies were made to verify the tectono-metamorphic development of the different fabrics, and to characterise the modes of graphite formation within these marbles. These include Raman spectroscopic measurements, energy dispersive X-ray analysis (EDX) and investigations of the stable isotopes.Fabric investigations and field work show that the graphite-bearing marbles are abnormally coarse- grained. The large grain-size of the marble is according to these investigations related to the regional intrusion of granitic melts into high-grade metamorphic rocks. Calcite-graphite thermometry by carbon isotopes indicates regional peak temperatures of around 760° C. The coarse-grained marbles were subsequently deformed in brittle-ductile shear zones along the reactivated rims of the dome structures, producing complex fabrics. The studied shear zones are composed of a mylonitic core zone and a brittle-ductile deformed boundary zone. Part of the deformation in the shear zones was by pressure solution, which resulted in the formation of graphitic stylolites. The graphitic stylolites form network structures of varying degrees of intensity. On a sample scale, the graphite networks show resistivities of 400 to 540 Ω m. These networks are responsible for the anomalies of high electrical conductivity, measured in the magnetotelluric profiles. Many of the graphitic stylolites are cut by microveins. It is proposed that these veins were generated as tension fractures and hydrofractures during the Cretaceous break-up of Gondwana and the subsequent uplift to surface levels.Graphite was most probably formed during metamorphism, which also lead to the abnormal grain-coarsening of the marbles. The graphite is of uniform high crystallinity across all types of graphite-bearing marbles. Since carbon isotopes of graphite were equilibrated with the calcite marble host rock, no isotopic indications about the origin of the graphite are preserved in the marbles. Qualitative optical microscopy and EDX investigations show, that the graphite is commonly epitaxial-intergrown with mica minerals. The fine-grained graphitic stylolites formed as a residue of pressure solution of calcite. The calcite-graphite ratios indicate, that both calcite and graphite were subject to pressure solution deformation. The calcite textures correspond to so-called 'low-temperature' pure shear textures, both in the mylonitic core zones as well as in the brittle-ductile boundary zones. The intensity of lattice-preferred orientations is generally very high and varies strongly with the grain size of the investigated samples. The mylonitic core zones show a complex pattern of different domains of lattice preferred orientation within microscopic scales. Graphite also shows a strong lattice-preferred orientation in the mylonitic core zones with the basal plains oriented parallel to the foliation.The abnormal grain-coarsening resulted from a combination of regional and contact metamorphism. The complex brittle-ductile deformation fabrics were formed subsequently in a seismic-aseismic transition zone with isochronous brittle, ductile and pressure solution deformation. The graphitic stylolites developed during this deformation, are responsible for crustal anomalies of high electrical conductivity in the measured magnetotelluric profiles.