Exploration for porphyry-style copper mineralisation near Llandeloy

Hercynian orogenies (M. 3. Arthur, private
com munication, 198 2). The related suggestion that the
south Wales magnetic anomalies have been displaced by
140 km dextral transcurrent motion along the precursor
to the Bala Fault, from an original position adjacent to
the Harlech Dome magnetic anomaly, is of particular
interest to this investigation. Dextral transcurrent
movement of the same order has also been proposed for
the Menai Fault (Nutt and Smith, 1981).
On the more detailed airborne magnetic survey map
(Fig. 7) the broad Llandeloy/St. David's high has
numerous short-wavelength features superimposed on it
indicating that the magnetic rocks reach the surface.
Near the coast, west of the present survey area, the
anomalies can be seen to be due to Precambrian rocks of
the Pebidian Volcanic Series and perhaps also to
intrusions within them. The pattern of small, short
features is ascribed to the disruption of magnetic
horizons by the numerous faults in the area. With the
exception of the Ramsey Sound Group, most of the
Pebidian rocks are more or less magnetic. The Pebidian
forms the core of a horst running the length of the St.
David's Peninsula, and the breadth of the main anomaly
shows that the sides of this block must extend to some
depth. Numerical modelling of the three profiles shown
on Fig. 7 indicates sides dipping away at 40-60° to a
depth of about 3 km (Figs. 8-10). Gradients indicate that
the dip of the southern margin lessens to the west. To
effect this interpretation some sweeping assumptions
had to be made, namely: a block uniformly magnetised
by induction with a value of susceptibility typical of acid
igneous rocks. Consequently the models should be
regarded only as order-of-magnitude indications of the
true shape of the block. (See Appendix 5 for details of
the interpretation.)
Two groups of ill-clef ined magne t ic anomalies indicate
that the porphyry intrusions around Middle Mill, and the
Ordovician Brunel Beds are also weakly magnetic.
O-
dtstance (km)
Figure 8 Interpretation of aeromagnetic profile 1
12
0
I 1
2
I
3
I
4
I
5
I
6
1
7
d~stance (km)
Figure 9 Interpretation of aeromagnetic profile 2
I I I I I I I I
0 1 2 3 4 5 6 7
d~stanca (km)
Figure 10 Interpretation of aeromagnetic profile 3
Gravity survex
The Middle Mill intrusions correspond to a slight low in
the Bouguer gravity anomaly field (Fig. 111, implying a
density slightly lower than the surrounding -1ocks. A
Bouguer anomaly gradient of about 1 mGal km crosses
the survey area, values increasing northwards from a low
associated with intrusions into the Hayscastle Anticline
to the south, towards a high which lies offshore along
the coast of Cardigan Bay. There is no gravity anomaly
associated with the magnetic feature interpreted above.