A reconnaissance drainage geochemical survey was carried
out at a density of about 2 samples per km2 of the
Llanclrindod Wells/ Builth Wells Ordovician inlier in
Powys. The rocks of the area comprise a sequence of
mudstones and shales ranging in age from the Llan\.irn to
the Caradoc containing volcanic horizons and a lrolcanic
complex of acid/intermediate composition overlain in the
south by a further volcanic sequence of more basic composition.
An area of about 65 km2 was sampled and from
each site a wet screened minus BSI 100 mesh (< 150
micrometres) fraction and panned heavy mineral concentratc
samples were obtained. Sie\rcd sediment and concentratc
samples were analysed for B, V, Cr, Mn, Fe,
Co, Ni, Cu, Zn, As, Zr, MO, Ag, Sn, Ba and Pb and Ca,
Ti, Mn, Fe. Ni, Cu, Zn, Sr, Zr, MO, Sn, Sb,Ba, Ce, Pb
and U respecti\.ely. Several Pb and Zn anomalies were
apparent many of which appeared to be associated with
the outcrop of the main volcanic unit of the area and 4 of
these were confirmed by further grab stream sediment
and bank sampling. Drainage sampling was supplemented
by 27 reconnaissance soil tra\rerscs mostl)
across the main outcrops of the \.olcanic- rocks which tencled
to form the most ele\rated ground. The soil samples
were sie\.ed to give a minus 85 BSI mesh (< 180
micrometres) sample for analysis for Cu, Pb, Zn, Ni and
either Mn and Fe or Ag and Co. Samples collected from
an art‘s of‘old lcaci workings in the west of the inlier failed
to pro\ridc c\icience of further vein mineralisation beyond
the limits of the workings. Elswhere the biggest concentration
of‘ Pb and Zn anomalies and the greatest
amplitude of anonlaly (2000 ppm Pb) wcrc located over
the northern outcrop of the the main unit of tuffaceous
volcanic rocks, particularly in the \.icinity of the farm Pen
Rhiw Frank.
A clctailccl geological, geochemical and geophysical
survey of the Pen Rhiw Frank area was carried out using
photogeological interpretation, further soil sampling and
a combination of IP/resistivity, VLF, SP and magnetic
surveys. Geological interpretation indicated that the area
was divided into fault blocks and the geophysical work
re\realecl a zone‘ of high resistivity with roughly coincident
low amplitude chargeability maxima and a VLF crosso\.
t‘r o\c‘r part of the zone. There was also a general
association of this zone with lead in soil anomalies and the
presence of significant amounts of weathered pyrite in the
limited outcrops. The lead in soil anomalies when contoured
in the light of the photogeological interpretation
can by sc‘cbn as broadly following the local strike of the
rocks. Four holes were drilled to in\restiage the source ot
the> geochemical anomalies and to investiage any association
bt~tn~ecn this and the obser\red geophysical
anonlalies. Thy holes wcrc sited to test the down dip extension
of‘ apparently stratabound surface soil anomalies.
l’uff~accous rocks of andcsitic composition with a horizon
of clacitic. tuffs and minor lrcsicular andcsitic 1aL.a wt‘rc intcrsccted
in the holes. Mineralisation occurs in the for-m of
secondary lc~ad minerals within a poorly rcco\rcred soft
clay-rich section 5.8 m thick near the top of ant‘ of the
holes. 1~~1 lc\.els up to 0.52 %I o\‘t‘r 3.4 m were found in
the zone which is located at the interface between a dacitic
tuff unit chcn~icnlly distinct from the rest of the \.olcanic
scqucrlcXX and an andesitic la\,a probably of linlitcd c\;-
tent .‘I’hc (h-ill holes suggest that thcl dip of the strata is
around 5 dcgrccs rather than the 30 degrees d~cluc~d from
th t,xisting gc,ological map of the arca and so the holes
wcr(‘ not sitccl at locations which wc’rc’ likcsly to iI1tc‘rsc‘c.t
any stratabound mineralisation at depth below the zone of
surface Lveathcring. A further consequence of the flat dip
of strata is that by accidents of topography extensive overburden
anomalies could be produced from relatively
minor statabound base-metal enrichments on the hill
tops