This report describes stream sediment and gravity
surveys carried out across the Berwyn Dome and adjacent
areas. The gravity survey confirmed the presence of a
broad regional Bouguer anomaly low in the central part of
the Dome, on which is superimposed several smaller irregular
highs and lows. Some of these local anomalies
possibly reflect small igneous bodies but more detailed
gravity surveys would be needed to determine their form.
Near Corwen the Bryneglwys Fault coincides with a 4.5
mGa1 anomaly but southwards the two features diverge,
suggesting that the density interface is related either to a
splay fault or to the eastern margin of the Lower
Palaeozoic Montgomery trough. Some other structural
trends are weakly reflected on the Bouguer anomaly and
aeromagnetic maps, but there is no clear correlation with
known base metal mineralisation. The Bouguer
anomalies cannot be attributed to particular structures
with any certainty but are probably due to a number of
factors, including variation in the Precambrian basement
and changes in the lithology and thickness of Lower
Palaeozoic sedimentary rocks. There is no evidence for a
large granitic body in Lower Palaeozoic rocks underlying
the mineralisation at Llangynog. The aeromagnetic map
suggests the presence of a magnetic basement at a depth
of 3-4 km centred beneath the northwestern margin of
the Dome.
The stream sediment survey involved the collection of a
- 100 mesh stream sediment, panned concentrate and
water sample from each of the 399 sites sampled. The
sample density was 1 site per 1.5 km*. Cu, Pb, Zn, Ba,
Fe, Mn, Co, V, Cr, Ni, Zr, MO and Sn were determined
in the stream sediments, Cu, Pb, Zn, Ba, Fe, Mn, Ce,
Sn, Sb, Ti, Ni and As in the panned concentrates and Cu,
Pb and Zn in stream waters. Major variations in the
results are related to (i) hydrous oxide precipitation processes,
(ii) contamination from human activities, (iii) base
metal and baryte mineralisation, (iv) monazite concentrations
in panned concentrates, (v) hitherto unrecorded
gold mineralisation and (vi) lithological variations. The
latter were related principally to shale-sandstone variation,
but groups of elements attributable to the presence
of basic intrusions, phosphatic rocks, coal measures,
sandstones, limestones and volcanics were also discerned.
Threshold levels were established from cumulative frequency
curve analysis, and some anomalous sites were examined
in the field. Anomalies did not form prominent
coherent groups and were generally weak and scattered,
with a wide variety of element groupings reflecting a
range of causes. Many anomalous panned concentrates
were examined mineralogically to try to . determine
whether anomalies were related to chemically extreme
background lithologies, contamination, or mineralisation.
All the anomalies were related to one or more of the
major causes of variation, although because of the very
limited amount of follow-up work carried out the precise
cause of many anomalies remains uncertain. No anomaly
is considered to represent a strong prospect but several
deserve further limited investigation, notably those
associated with (i) gold mineralisation in the northwest of
the area, (ii) baryte, perhaps accompanied by base metal
’ mineralisation, associated with Caradocian volcanics and
phosphatic rocks at several localities, (iii) mineralisation
associated with Llandeilian limestones and volcanic rocks
north of Llanrhaeadr, and (iv) copper mineralisation
associated with intrusives near the eastern margin of the
Dome, where survey data is most incomplete