Exploration for porphyry-style copper mineralisation near Llandeloy

Introduction
The main geophysical method employed along all
traverse lines was induced polar isation (IP), which is
particularly suited to the detection of low-grade
disseminated sulphides, but will also respond to more
massive deposits. The dipole-dipole electrode array was
used throughout with a dipole length (a) of 50 m. For the
600 m spaced reconnaissance traverses a constant dipole
separation of n = 2 was used, while along the 200 m .
spaced infill traverses at Treffy~on an expanding
separation from n = 2 to n = 6 was used to provide some
depth and dip information. Magnetic measurements were
taken on all traverse lines using proton magnetometers.
The very low frequency electromagnetic method (VLF-
EM) was used in the reconnaissance survey, but artificial
conductors are numerous and the method was not
helpful.
Cores from the boreholes were subjected to magnetic
susceptibility and density measurements and some of the
holes were geophysically logged, Aspects of these logs
are discussed below, and the detailed logs are held by the
Regional Geophysics Group of BGS.
The detailed geophysical results and descriptions are
contained in Appendix 2, with the main features
described and discussed below.
Induced polar isat ion
Over most of the area, IP chargeability values remained
in the range 5 to 15 milliseconds (ms). Values over 20 ms
are considered anomalous, the maximum measured (away
from artificial sources) was 50 ms. Two main areas of
anomalous chargeability occur (Fig. 40). The smaller lies
in the southwest, near the Cambrian-Ordovician
boundary mapped by Williams (1933). If the measured
maxima are connected between the traverse lines as
shown on Fig. 40, the strike of the chargeability anomaly
crosses the geological strike at about 40°. Continuity,
Figure 40 Chargeability anomalies in the Llandeloy area
however, has not been proved between these widely
spaced traverses, and the cause of the featurds) is
unknown.
The second area of high chargeability lies east of
Treffynnon, where it is accompanied by copper
anomalies in sail. Qualitative interpretation of the
chargeability pseudosections allows a map of IP sources
to be drawn (Fig. 41). The Tetragraptus Shales along the
north of the area produced chargeabilities up to 40 rns,
and similar values were recorded over the Brunel Beds
in the southeast. Both units include dark grey or black
mudstones which may contain carbonaceous matter in
sufficient quantity to produce these anomalies. In the
area of the geochemical anomalies near Treffynnon
there occurs a patchy linear chargeability high trending
about 065*, with values up to 35 ms. The maximum
values apparently lie where this lineat ion intersects
members of a set of weak east-west-trending anomalies
(Fig. 40). The boreholes over this feature (2, 3A, 3B, 4
and 5) intersected pyritous altered intrusive rocks under
locally thick lacustrine deposits. De trital magnetite and
hematite-coated pyrite occur in these deposits (Allen,
1981) and may be responsible for the fP anomaly.
However, under some circumstances clay-bearing rocks
can produce IP effects, sometimes equivalent to several
percent of sulphides, though by a different mechanism
(eg. Telford and others, 1976). The maximum effects
occur in sediments containing perhaps 10 percent clay
minerals, well distributed, and a low salinity pore fluid.
At Llandeloy the clay content of the lacustrine deposits
rises to perhaps SO percent, but much of this is
concentrated into thin clay beds, while the sandy beds
approach more closely the conditions described. The
downhole IP logs (available for boreholes 1 to 4, figs. 20-
24) show that the IP response of these surficial deposits
is locally strong, around 30 ms in borehole 3A and rising
to 70 ms in borehole 3B. The underlying intrusive rocks
produce chargeabilities around 20 ms with peaks up to
50 ms, and it seems likely that both contribute to the IP
anomaly measured at the surface. There is good evidence
from magnetic (see below) and other data that the
lacustrine sediments are composed of weathered bedrock