Exploration for porphyry-style copper mineralisation near Llandeloy

QnD (BH 7) 1
Log rwceptibitity (9 uniu x 1~3)
1 -0.5 0 0.5 +I +1.5
L I I 1 I f
T I n Y
V - t
mS t 8 Y
QnD (BH 2) 1
n -
w " I
QDI
- I
n Y
w - I
h u
V -
I
0.1 0.5 1.0 2 5 10 20
Figure 46 Magnetic susceptibilities of borehole samples
1976). Three co m~onent populations with geometric
means of 0.22, 0.6 and 17 x 10-3 SI units were defined
(Fig. 47). The largest population (mean 0.6), consisting of
about 75 percent of the sample, can be regarded as the
'primary' background susceptibility of the rocks and
probably contains unresolved populations related to
different lithologies. The lower population is related to
weathered rocks whilst the upper population can be
ascribed to rocks containing magnetite generated during
late stage propylitic alteration. Magnetic susceptibility
may, therefore, be useful in detecting such rocks in
boreholes. The magnetic complexity of the area as a
whole, however, means that this magnetic signature is of
little use on a wider scale because there are at least four
modes of occurrence of magnetite here: secondary
hydrothermal, primary magmatic, magnetite horizons
within sedimentary rocks and detrital magnetite in
lacustrine sediments.
The identification of localised highly altered rock as
the most magnetic is partly at odds with the
interpretation of anomalies in the wider area discussed
above. The model of an anticlinal core of magnetic
Susceptibility (SI units X ~ O - ~ J
Precambrian rock clearly cannot be adopted without
qualification in the Llandeloy area, but the broad
magnetic anomaly further west does extend unbroken to
Llandeloy, and it is likely that these rocks underlie the
area at no great depth. The forms of some of the ground
profiles (Appendix 2) certainly suggest some local, near
surface effects superimposed on a much deeper seated
anomaly. An alternative possibility is that the broader
anomaly at Llandeloy is due to a subvolcanic intrusion,
perhaps representing the remains of a magma chamber
underlying the hypothetical Treffynnon volcano.
However, the magnetic source indicated by numerical
modelling has sides sloping outwards and a base width of
5 km or more at a depth of about 3 km, not a likely form
for a subvolcanic intrusion. The susceptibility used for
the models, about 7.5 x 10-3 SI units, is much greater
than that of the unaltered intrusions at Llandeloy, but
more typical of the Precambrian rocks which cause the
western part of this anomaly. An anticlinal core
therefore provides a more likely explanation of the broad
anomaly than a subvolcanic intrusion.