THE high electrical conductivity of the lower continental crust, as inferred from geophysical measurements1, has defied a simple explanation. Both pore-saturating brines2 and interconnected carbon films3-5 have been proposed as conducting pathways, but their relative importance remains uncertain, and may vary from place to place6. So far, attempts to address this question in the laboratory have had to rely on samples of metamorphic rock collected from surface exposures7-10. Although these rocks almost certainly originated in the lower crust, chemical alteration during their transport to, and residence at, the surface is likely to have affected their conducting properties. Here we report conductivity data for pristine crustal rocks recovered from depths of up to 4.6 km in the KTB boreholes in southern Germany. Our results show that the electrical conductivity of accessory phases such as Fe-Ti oxides and sulphides can enhance, or even surpass, the high conductivity produced by saline fluids.