The metalliferous mineral potential of the basic rocks of the Penmynydd Zone, south-east Anglesey

Geophysical and geochemical investigations have been carried out to assess the mineral potential of an area in south-east Anglesey containing positive gravity anomalies and coincident aeromagnetic anomalies. The anomalous area contains outcrops of dense, basic hornblende schist of the Precambrian Penmynydd Zone, which is a complex, fault-bounded zone of tectonised metasediients and basic schists. The existing regional gravity survey coverage has been augmented by over 150 additional gravity stations and three detailed traverses made on limes normal to the regional strike. Over 250 soil samples have been collected on 7 traverse lines which, together with over 30 rock samples, have been analysed for a suite of up to 13 elements (so&) and up to 28 elements (rocks). Some of the rocks have been analysed also for gold and platinum. The geophysical data suggest that the gravity and aeromagnetic anomalies over the Penmynydd Zone can best be explained by the presence of a near-surface, relatively dense body of low magnetic susceptibility underlain at around 3 km depth by a body with a much higher magnetic susceptibility. The anomalies could therefore be due to a layered basic intrusion, fault-bounded on both its northwest and south-east sides. Along-strike modelling of this body indicates that it is disrupted by block faulting along a north-west trend, with segments becoming progressively deeper towards the northeast. The geochemistry of the basic hornblende schists indicates that they have an oceanic tholeitic basalt parentage. The soil sampling results show several barium anomalies, probably associated with thin baryte veins, and one Pb/Zn anomaly over Carboniferous Limestone. There are no immediate geological or geochemical indications of potentially economic near-surface mineralisation in the area.