Exploration for porphyry-style copper mineralisation near Llandeloy

potash con tents whilst in K-feldspar altered rocks a If the results of the elements and ratios which most
reduction in potash and increase in soda occurs.
Comparison of surface and borehole rock results From
the mineral exploration viewpoint the differences
between the surface and borehole results are of great
importance, because surface rock sampling gave no clear
indication of the disseminated mineralisa tion intersected
by the boreholes.
Precise comparisons between the borehole and surface
rock data are hampered by the lack of dioritic rocks in
the surface samples, which in itself is significant. The
closest chemical comparison that can be obtained is
shown in Table 15. Alteration in the borehole samples is
dominantly of weak potassic (biotite) type, overprinted
with variable intensity by late stage propylitic
alteration. In the surface samples alteration is very
variable but frequently of quite intense propylitic type
with quartz-chlorite-epidote-sericite assemblages
developed.
The two groups have virtually the same mean silica
content and similar levels of most elements
concentrated in basic rocks. Cu is the only element to
show a great contrast in the two groups, with the lowest
borehole result only slightly less than the highest level
recorded in the surface rock group. The borehole samples
may also contain higher levels of Pb, Zn, As, Ba and Rb
and lower CaO, Na2O and Zr. As the medians indicate,
some of these differences are greatly emphasised by the
presence of one or two very high results and the
differences may not be significant, but with the
exception of Zr, they are changes which might be
predicted when moving toward the centre of a porphyry
system and changing from propylitic to potassic
alteration zones (e.g. Chaffee, 1976). The higher Zr
content of the surface samples may be related to
weathering.
clearly show variation in the vicinity of the
mineralisation (for example Cu, S, Cu/S, Na/K, Rb/Sr)
are plotted out on a regional basis for the surface rocks
no clear pattern emerges, and most variables show as
great a variation within one outcrop as across the whole
area. Only tenuous differences, such as the absence of
very high soda in rocks collected in the vicinity of the
mineralisation, can be discerned and it is concluded that
no regional pattern useful for exploration purposes can
be discovered from the surface rock sample results, even
with the use of hindsight.
Samples taken closest to the boreholes give no
chemical clue to the nearby mineralisation. This is
surprising considering the extent of the mineralisation
and can only be attributed to chance. The quartz diorite
sample taken from Hoilybush quarry (Table 2, no. 2) was
collected within 200 rn of borehole 5 (tonalite at bedrock
surface with 300 ppm Cu in drill mud) and contains the
highest copper (46 ppm) and Fe (9.05%) content of any
surface sample. It also contains low Zr and Na20, but
these are features entirely consistent with the more
basic composition of this lithology compared with other
surface rocks. Only a slightly elevated Mo content
suggests that it is close to disseminated mineralisation.
It is therefore concluded that inadequate sampling
caused by a lack of surface exposure, together with the
masking effects of regional alteration and the
complicated pattern of alteration associated with the
disseminated mineralisation, are the reasons which have
prevented the detection of extensive disseminated
mineralisation by whole rock analysis of samples from
available exposures in this area. It shows that in such an
area this type of approach, used in isolation, could be a
most misleading prospecting tool.
Table 15 Comparison of surface and borehole analyses of igneous rocks
M~O
CaO
Na2O
K20
p205
Surface Rocks1
Maximum Minimum Median Mean
Major elements quoted as percent of oxide, trace elements as ppm.
Borehole samples2
Mean Median Minimum Maximum
1 Twenty porphyritic m icrotonali tes from outcrops in the Llandeloy-Middle Mill area.
2 Sixteen porphyritic microtonalites and tonalites from boreholes 1, 2 and 5.