Exploration for porphyry-style copper mineralisation near Llandeloy

propyli tic alteration all these elements are immobile,
whilst under the more extreme conditions of potassic
alteration Nb and Y show irregular mobility. No data are
available for Sc whilst Ce and Th results above the
detection limit are insufficient to reach a conclusion.
Ratio plots of the type suggested by Beswick and
Soucie (1978) permit an evaluation of the effects of
metasomatism in remobilising and redistributing
elements in varied lithologies provided that the
unaltered rocks follow common igneous trends. Plots of
this type (Fig. 36) together with simple 'stable1 element
scattergrams (Fig. 37) of the type used by several
authors (e.g. Pearce and Norry, 19791, indicate that
whilst both Ti and Zr are depleted in a few samples (e.g.
borehole 2, 25.25-26.82 m and 27.80-29.56 m) their ratio
only shows moderate variation within a given rock type.
As the silica content of these samples is heavily
depressed compared with less altered rocks of similar
type (due to extensive carbonate veining) it is suspected
that the decrease in Ti and Zr is largely due to dilution.
~orphyhtic microtonalite
Tonalite w
Figure 36 Ratio plot of Ti/Rb v Zr/Rb for igneous rock samples
35
Y shows more erratic behaviour (Fig. 37) with two
samples from borehole 2 depleted whilst one from
borehole 5 is apparently enriched with respect to Zr or
Ti, suggesting Y mobility in rocks which have suffered
po tassic alteration.
Statistics and elemental levels suggest that P and A1
may also have remained relatively immobile during
alteration though small changes in P may be hidden by
the relative imprecision of the analyses with respect to
the total variation in the rocks. The Nb content of all
rocks is low but is below the detection limit in a greater
proportion of the rocks which have suffered K-feldspar
alteration than can be readily attributed to chance, and
it is believed that depletion during potash alteration may
have taken place. These observations are supported by
multivariate statistical analysis of the borehole data
which show negative associations of the type Cu, S, Fe v
Y and K20, Ba, Rb v Nb, Sr, NagO.
Mobile/stable element plots of the type shown in
Figure 36 can be useful indicators of the degree of