A lamprophyre province has been identified in the Truro area comprising numerous dykes and at least one diatreme. The regional extent of the province has not been determined. The lamprophyre dykes, of the calc-alkaline suite and classed as minette, generally intrude joint planes of Cambrian metasediments of the Karinya Syncline. An age of about 480 Ma has been determined by K-Ar geochronology indicating a genetic association with the Ordovician granite and norite intrusions to the southeast. A possible diatreme with multi-phase dolerite-lamprophyre core, fluidised pebble breccia margin and the talc-asbestos-tourmaline alteration of carbonate host rock has been located near Dutton. Deep-seated crustal weakness is indicated by the association of many of the dykes with Landsat lineaments. Consequently the region is considered prospective for diamonds, gold and base metal mineralisation.