Geophysical surveys in part of the Halkyn-Minera mining district, north-east Wales

Induced polarisation (IP) surveys were carried out at four localities in the Halkyn - Minera area, all sited mainly on rocks of the Cefn-y-Fedw Sandstone Group north of the Llanelidan Fault. The first survey covered the Llandegla Moor area but failed to produce any evidence for a north-westward continuation of the rich Minera lode system. Regional and detailed gravity data subsequently demonstrated the existence of a pronounced Bouguer anomaly low over the western margin of the Cefn-y-Fedw Sandstone Group, including the Llandegla Moor area, suggesting that these rocks thickened rapidly eastwards, perhaps along a concealed northerly-trending fault. In an attempt to locate this fault and to test its mineral potential, trial IP surveys were carried out at three sites covering the boundary between the Cefn-Fedw Sandstone Group and the underlying Vi&an limestones. No strong chargeability anomalies were discovered and the overlap and variability of the resistivities of the sandstones and the limestones prevented recognition of the boundary between these two rock types. The chargeability values recorded over sandstones seem to be largely dependent upon the resistivity values, suggesting that variations in the porosities of the rocks are mainly responsible. The Bouguer low is interpreted as probably being due to the presence of high-porosity and rapidly-thickening sandstones in the Cefn-y-Fedw Sandstone Group. For these reasons the area where the low was discovered is not strongly recommended for any future exploration for extensions of the mineral veins.