Exploration for porphyry-style copper mineralisation near Llandeloy

APPENDIX 3
GEOPHYSICAL RESULTS FROM THE MIDDLE MILL
AREA
The results are displayed on figures A3.1 to A3.6, line
numbers refer to Fig. 16. As in Appendix 2, a subjective
interpretation of the pseudosections has been made.
Line 150W
Severe artificial interference was encountered on this
line, and many readings were completely unobtainable.
There are two zones of high chargeability, both
apparently artificial is origin. At its northern end, the
line runs close to a road for 400 m, giving a complex
pattern of very high, low and even negative
chargeabilities. At the southern end of the line very high
chargeabilities arise from dipole positions near buildings
and a water pipe. The apparent resistivity also shows
interference. Values are generally 600-900 ohm metres
but slightly lower values obtained at lOON and higher
values at about 350-450N may be significant above noise.
The magnetic profile shows only man-made anomalies.
Line 150E
On this traverse artificial interference was a severe
problem again, making many readings unobtainable. The
chargeability pseudosection shows one feature which is
probably due to bedrock geology, values to the south of
about 150s being generally higher than those to the
north. Two strong IP anomalies are almost certainly due
to artificial sources: the high along the roadside at the
north of the line and the narrow high across a farm lane
and buried services at about 150s. The negative IP effect
occurring just north of the baseline has no obvious
artificial source. Resistivity, like chargeability , is higher
south of about 150s than on the rest of the line. A broad
resistivity low extends northwards from 150s to about
200N, where values become somewhat higher. The
magnetic profile has minor peaks (about 50nT amplitude)
in the area of the low resistivities and negative IP
effects.
Line 450E
This traverse also suffers from artificial interference,
Chargeabilities are generally 7 to 17 ms, with a minor
high lying at the south end of the line giving values up to
25 ms. Otherwise high chargeability occurs only in areas
of interference: the airfield perimeter at the northern
end of the line and the farmyard at 150-300s. Variations
on the resistivity pseudosec tion probably reflect bedrock
variations. High resistivity occurs at 800N-500N; low
values at 500N to SON, with a minimum at about 100-
200N: moderately high values recur between 50N and
ZOOS, and values are again low from 200s to the housing
estate at 500s. Other than aritificial anomalies there is
little structure on the magnetic profile.
Line 750E
Line 750E had to be offset 100 m to the west at its
northern end to avoid the quarry (Fig. 16). For most of
its length it runs along or close to the steep slope of the
Solfach river gorge, The line has three chargeability
anomalies, of which two are likely to be artificial in
origin. A minor IP high occurs at about 100N, where it
may be related to a small intrusion. The main feature on
this line is a double anomaly centred at 700-850N on the
offset part of the line, immediately west of the quarry,
where chargeability rises to 60 ms. In pseudosection, the
anomaly has a core of low values, indicative either of an
artificial conductor at about 725N or of a shallow flat-
lying slab-like source less than 100 m wide. The absence
of a coincident low resistivity anomaly tends to support
a geological origin for this feature, but its position,
between the quarry and the end of the airfield where
strong anomalies were obtained on the previous line,
strongly suggest an artificial source. A resistivity
feature of interest on this traverse is the low at about
400N, where a fault has been mapped.
Line 1050E
Three chargeability anomalies occur, of which one is
probably due to electrical noise from Middle Mill village
at about 600N, and another to a large watermain at
700s. The third anomaly is the weakest but probably has a
geological source. It is a diffuse near-surface feature on
the baseline, Resistivity varies considerably, with narrow
lows at about 2758 and 500N. The magnetic profile shows
nothing of interest.
Line 1350E
AZ four chargeability anomalies on this line are probably
artificial in origin, the sources being farms at 400s and
600N, cables 2 900~ and fences at 1150N. The first
three also give low resistivity anomalies. The last could
have a geological source, however. All but the weakest
magnetic anomalies are artificial.