Exploration for porphyry-style copper mineralisation near Llandeloy

Figure 41 Interpreted induced polarisation sources in the area around the boreholes
material transported only a short distance, perhaps a few
hundred metres or less.
Weaker IP anomalies occur over the altered intrusive
rocks where the lacustrine sediments are thin or absent.
A linear anomaly of 25-35 ms follows the mapped
southern margin of the intrusion complex eastwards from
Pwll-ceffyl ruin, but it is coincident with a road and may
not be geological in origin. The volcanic Treffynnon
Group produces moderately high chargeability in parts
with values around 10 to 25 ms.
The IP results in the Llandeloy follow-up area,
therefore, provide a complicated picture which reflects
bedrock lithology and overburden composition. It does
not, perhaps as a result of these complications or
because only the root zone of the deposit has been
preserved, allow evaluation of the mineralised intrusions
in terms of a conventional porphyry-copper model.
Resistivity and VLF-EM
Measured apparent resistivities range from 100 to
5000 ohm-metres while VLF horizontal intensity,
recorded by the airborne survey and expressed as
percentages of the 'normal' intensity, varies from 90 to
115% with one or two larger anomalies due to
powerlines. The two quantities are inversely related in a
general way: low VLF intensities arise over resistive
ground.
The regional maps (Figs. 12 and 13) show that three
important linear conductive features enter the
Treffynnon area (Figs. 42 and 43). In the north, an east-
west fault brings the resistive Cambrian and intrusive
rocks which form the core of the horst against
conductive Tetragraptus Shales. Resistivities are around
200-300 ohm-metres north of this fault and mostly over
2000 ohm-metres to the south. A second conductive
zone, about 200-400 metres wide, passes NNE-SSW
through Treffynnon. The pseudosections suggest that the
source is a fairly narrow vertical structure near the
western side of the anomaly, with surficial low
resistivity material extending some way to the east.
Magnetic maps discussed below show a marked
discontinuity along this line which is therefore
confidently interpreted as a fault and named the
Treffynnon Fault. Geological indications place this fault
somewhat to the west of its geophysical position. The
third conductive feature entering the area is the NE-SW
trending lineation of Fig. 13 which divides a zone of E-\V
VLF-EM trends from one of NE-SW trends. This may be
the southern fault margin of the horst, but it is nowhere
well defined and in the Treffynnon area becomes
confused with the low resistivities associated with the
lacustrine deposits. These surficial low resistivities
occur around the line of boreholes 2 to 5 and to the south
and west of Treffynnon. They have been used to provide
the estimate of the extent of lacustrine deposits shown
on Fig. 19. Attempts to model the conductive overburden
mathematically from the dipole-dipole depth information
were largely frustrated by lateral variations in
resistivity (Appendix 5). One reasonably successful
example from the east of the area indicated a 2.3 m
layer of overburden of resistivity 1.15% that of the
bedrock. Typical values might be 60 ohm-metres for
overburden and 5000 ohm-metres for bedrock. The
valley of the Solfach, although low-lying and marshy, did
not noticeably depress apparent resistivities along the
south and west of the area. This may imply that the river
has eroded close to bedrock and that the valley is floored
by a thin veneer of clean alluvium.