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.