Exploration for vanadiferous magnetite and ilmenite in the Lizard Complex, Cornwall

Exploration for enrichments of vanadiferous magnetite and ihnenite was carried out within the Trelan gabbro, a recently discovered component of the eastern part of the Lizard ophiolitic complex in Cornwall. Work comprised ground magnetic surveys, weathered bedrock and overburden sampling by means of a Minuteman power auger, and the drilling of four cored boreholes. In addition, limited power augering was used to assess the potential for placer concentrations of Fe-Ti oxides and other heavy minerals within the Crousa gravels, an unconsolidated deposit overlying the highest part of the Lizard. Drilling in the vicinity of high amplitude ground magnetic anomalies near Trelan proved the existence of extensive oxide-rich gabbro containing lo-15% combined ilmenite and magnetite. Ihnenite is the predominant oxide and only rarely is magnetite more than a minor constituent. The vanadium content of the magnetite is high, commonly between 2 and 3% and reaching up to 5% V2O3. Texturally the oxides occur in clusters and vein-like growths suggesting that some kind of filter pressing mechanism may have caused coalescence of oxide-rich liquid prior to crystallisation. The distribution of the oxide-rich gabbro in the boreholes suggests that this process occurred only on a local scale and that coalescence did not take place to such an extent as to produce a sufficient volume of Fe-rich liquid to produce economic concentrations of oxide. A provisional revised geological map of the eastern part of the Lizard based on geochemical mapping of the overburden samples and showing the extent of the Trelan gabbro is included in the report. Part of the Crousa gravel, here termed the Crousa gravel sensu sticto, is a channel deposit a few metres wide trending east-west and up to 9 m deep. At its base there is concentration of heavy minerals but these are of exotic origin and include cassiterite and tourmaline. The channel is cut through a mass of clay-rich material which caps the highest part of the Lizard. This is now interpreted as a glacial till containing large boulders of fresh gabbro derived from the coastal outcrop of the Crousa gabbro to the east. The till is thought to have been deposited as sea ice was pushed up against the coast during a relatively early glacial event which has left little trace of its presence in the rest of Britain, due to the effects of later glaciations. The chemistry of limited power auger sampling from the north-eastern part of the complex suggests potential for exhalative mineralisation. Siliceous sediments with anomalously high concentrations of Ba occur in close association with mafic rocks of probable volcanic origin. However, the area is structurally complex with no continuity of rock units along strike and may possibly be a melange.