Exploration for porphyry-style copper mineralisation near Llandeloy

and geophysical anomalies were identified. Explanations
for them were not evident from the examination of
surface exposures and nine boreholes were drilled to find
their causes. In addition, a gravity survey was carried
out in an attempt to find another method for refining the
setting of the mineralised zone. Down-hole geophysical
measurements were taken in five of the boreholes.
All the boreholes were drilled using the B.G.S.JKS 300
rig. Eight vertical holes were drilled to between 21.45
and 45.72 m; the ninth, an inclined hole, was stopped at
124.12 m. BQ core was retrieved from all holes above
the depth of 72.26 m; and AQ below that in the ninth
hole.
In boreholes 1 and 8 no samples were collected until
the first solid core was retrieved. In all the other holes
samples of the superficial deposits were taken while the
casing was being driven.
While drilling it was discovered that much of the area
was underlain by a previously unrecorded, locally thick
succession of superficial deposits. The third borehole
(3A) was stopped at 26.83 m and re-sited in an attempt
to avoid this deposit, but on the second attempt this
borehole (3B) penetrated the same thick deposit and
collapsed before solid was reached.
Detailed logs of all nine holes have been lodged with
the Aberystwyth office of B.G.S. Abbreviated logs are
presented in Appendix 1 and graphic logs in Figs. 20 to
28.
Introduction
The map showing the solid geology for the area around
Tref fynnon (Fig. 18) was constructed using data from the
few outcrops still accessible, the map of H. H. Thomas,
the geophysical surveys, the boreholes and the
distribution of float, Measurements of the dip and strike
Figure 18 The solid geology of the area around the boreholes
of stratified rocks were possible at only two localities
and the exposures were such that modification of the
orientation by hillcreep or other agencies could not be
ruled out. Information on the superficial deposits was
gained mainly by hand augering during the soil survey,
and from the boreholes.
Table 7 The stratigraphic succession in the
Treffynnon area
Sedimentary
and
volcanic
rocks
F
Igneous
rmks
In this section account is taken only of the small area of
the detailed survey carried out around Treffynnon. Notes
on the whole of the reconnaissance area are in the
regional account.
The succession (Table 7) consists of sedimentary and
volcanic rooks of possible Cambrian and Ordovician age
into part of which is emplaced a complex of
intermediate and acid intrusive rocks. Lacustrine
sediments of probable Tertiary age occur in much of the
- Road
, , - , Geological boundary
, , Fault
19
Drift deposits
Lacustrine deposits
Tetragrap tus Shales
Brunel Beds
TreffynnonGroup
volcanic rocks
Solva Group quartz-
wacke wd siltstone
Intermediate and
acid intrusions
Pleistocene
?Ter tiary
Ordovician
?Cambro/(Xdovician
C.mbrian
Post-Cambrian
Tetngraptua Shales
Tmffynnon Group
7'