Exploration for volcanogenic mineralisation in Devonian rocks north of Wadebridge, Cornwall

The exploration for mineralisation associated with Middle Devonian volcanic rocks of the area north of Wadebridge in north Cornwall is described. There are no significant aeromagnetic anomalies associated with the volcanic rocks though some shallow density variations can be inferred from the Bouger gravity anomaly map. Reconnaissance overburden sampling was carried out along a series of traverses roughly at right angles to the mapped main outcrops of the volcanic rocks. The position of contacts between volcanic and sedimentary rocks was clearly discernable from the overburden geochemical data either as sharp increases in the concentration of elements like titanium or in the value of principal component 1 derived from a principal component analysis of the raw geochemical data. Follow-up overburden sampling with further traverse and grid surveys in two localities were carried out. This work delineated a number of varieties of overburden anomaly, some of which were investigated further with ground geophysical surveys. Finally a series of eight diamond drill holes were collared to test the source of five overburden anomalies. A total of 40 horizons of basic igneous rock were intersected in the drill holes, some clearly volcanic and others clearly intrusive, which varied in inclined thickness from a few centimetres to over 50 metres. Four compositional groups of basic igneous rock were recognised on the basis of relative concentrations of the "immobile elements" Ti, Y, Zr and Nb. Two varieties of quartz vein were found as loose blocks during the overburden sampling, containing boulangerite + galena and arsenopyrite + pyrite respectively. A significant amount of gold (up to 1.0 ppm) was subsequently found by analysis to be associated with the arsenopyrite-bearing veins. No veins corresponding exactly to these two varieties were intersected in the drill holes though quartz veins and veinlets with either manganoan siderite or ankerite are common. Associated with some of these veins and with chloritic veins are pyrite, arsenopyrite, chalcopyrite, sphalerite and galena in varying proportions and minor amounts of tetrahedrite, some of which is richly argentiferous. A second variety of mineralisation consisting of minor amounts of bournonite, jamesonite and stibnite is closely associated with intrusive greenstone bodies and their immediate aureoles. Stibnite and secondary products of its alteration in association with siderite is a third type of mineralisation.