Exploration for porphyry-style copper mineralisation near Llandeloy

APPENDIX 5
COMPUTER MODELLING OF AEROMAGNETiC AND
RESISTIVITY DATA
Aeromagne tic anomaly
Three profiles from the aeromagnetic map (Fig. 7) were
subjected to computer interpretation. The IGS Applied
Geophysics Unit Program GAM2D2 was used (Lee, 1979).
Profiles were taken from the contour map and sampled
at 250 m intervals. The Llandeloy magnetic anomaly lies
within the influence of two larger anomalies to the south:
that due to the rocks in the core of the Hayscastle
anticline, and that which extends along the south coast of
Wales. The mutual interference of these anomalies made
it impossible to define the regional field accurately. A
value of 2nT km-1 was used though there are indications
for a gradient of up to 5nT kmwl negative northward.
At Llandeloy, the width and gentle flanks of the
anomaly indicate a source extending to some depth. The
base of the model was therefore set at 3 km, and the
shape of the top surface and the susceptibility were
adjusted to give the best possible fit of calculated to
observed anomalies, while permitting the body to be
exposed at the surface where appropriate. The
susceptibility which gave the best fit within these
limitations was 7.5 x 10-3 cgs units, which is close to the
average value for acid to intermediate igneous rocks
often given in the literature.
The shape provided for the interpreted sources are all
generally similar. A steep southern margin dips
southwards at 60-70° from the surface just south of the
maximum anomaly, The northern margin, corresponding
at the surface with the steepest gradient, has dips as
little as 20" and is therefore somewhat concave on two
of the three profiles. The individual profiles and their
interpretations are presented below (Figs. A5.1 to A5.5).
For profile two, an iterative interpretation method was
also used, which adjusts the length of prisms (extending
to infinity at right angles to the paper) above a base,
which was again assumed to be 3 km. The two bodies
from the automatic iterative process and the manual
polygon process correspond closely to one another.
Thickness of conductive overburden
An attempt was made to model the resistivity vertical
profile at two points using data from the pseudosections.
The dipole-dipole array is not well suited to this use
however, because it is very sensitive to lateral
variations, and because the relatively large dipole length
makes for poor resolution of the shallower layers. The
two sites chosen were 3700E/500N where borehole 3
intersected over 20 m of lacustrine sediments, and
4100E/675NY where the pseudosection indicated freedom
from lateral effects. It was not possible to produce a
realistic model for the former site, suggesting that
lateral variations are predominant here. The model for
the more easterly site is shown on Fig. A5.6. Even with
this relatively thin overburden, the maximum measured
resistivity will be only half the true resistivity of the
bedrock,