Exploration for porphyry-style copper mineralisation near Llandeloy

Magnetic surveys
Numerous magnetic anomalies were recorded with
maximum amplitudes up to 500 nT on the ground and
200 nT from the detailed airborne survey (Fig. 7).
Measurements on borehole cores show that
susceptibilities of the magnetic rocks lie generally
around 0.6 x 10-3 SI units. The magnetic data available
for the Llandeloy area provide a picture of considerable
complexity, but several useful conclusions can still be
drawn. Strong short-wavelength anomalies due to near
surface magnetic rocks are superimposed on a broad high
indicating an underlying magnetic body of some depth
extent. The pat tern of aeromagne tic anomalies in the
wider area and their interpretation has been discussed in
the regional geophysics section.
In the Llandeloy area two groups of short wavelength
anomalies occur separated by the line of the Tref fynnon
Fault (Figs. 44 and 45). To the north-west of the fault
lies a roughly triangular group of anomalies with a
250 nT high along the northern edge. There is no
geological indication of the source of these anomalies
which are not accompanied by high chargeabilities but do
coincide in part with weak copper-insoil anomalies to
the north of Ty Llwyd.
To the east of the Treffynnon Fault a roughly linear
magnetic high up to 300 nT on the ground runs NE-SW
along the line of the major chargeability and
geochemical anomalies. The anomaly is not highly
peaked, and on one traverse (3700E) becomes noticeably
broader. Drilling showed that this effect was due to the
lacustrine deposits. Several smaller anomalies occur
north of the main high, but to the south the magnetic
field falls away quite rapidly. To the west the anomalies
of this group end abruptly alonjg a line sub-parallel to the
Treffynnon Fault but 200 to 300 metres to its east. A
second fault along this line is implied, which could also
explain differences in the degree of alteration between
boreholes 7 and 8, and the 2 to 5 group. Double or
multiple faults with this orientation occur elsewhere in
the St David's area, notably south west of the city.
Table 16 shows the results of the susceptibility
measurements and profiles are plotted on the graphic
logs (Figs. 20-28). High suscep tibillties occur almost
exclusively in altered intrusive rocks, but not all
members of the intrusion complex are magnetic, and
petrographically similar rocks from different boreholes
have widely different susceptibilities. Magnetic
susceptibility cannot, therefore, be used to distinguish
different types of igneous rocks in the complex,
different petrographic types having overlaming ranges.
This is partly at least because of two secondary factors
superimposed on the primary magnetic susceptibility
variation. Firstly, there is a general relationship between
susceptibility and degree of alteration, the most highly
altered rocks of the area intersected by boreholes two to
five showing average susceptibility eight times that of
the other cores. Secondly, rocks from the weathered
zone give lower results. Except for some thin magnetic
horizons, low results were also obtained from the shales,
sandstones and volcanic rocks. The lacustrine sediments
were mostly weakly magnetis but contained some
material of moderate susceptibility. If this magnetism
arises from detrital magnetite a local source for the
sediments is implied, as there is no other magnetic rock
within a considerable distance.
There is good evidence, reviewed by Parker (1983) that
magnetic susceptibility is distributed log-normally
through a rock. Good descriptors of this distribution are
the mean and standard deviation of the log-transformed
data. Figure 46 shows these quantities calculated for the
various lithologies at Llandeloy as well as the frequency
histogram of susceptibility on a logarithmic axis. The
histogram, constructed from all the data, is clearly
poly modal and was split into component populations
using cumulative frequency curve analysis (Sinclair,
Figure 44 Aeromagnetic anomaly map of the area around the boreholes with contours at 4 nT intervals
45