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trials for slate. Analysis of a quartz-veined siltstone from
one of the workings showed no metal enrichment above
average values recorded in siltstones, but a pyritiferous
.shale from the same site contains high levels of Ni, As and
Fe (Table 6, Nos. 1 and 2). Pyritiferous shales and abun-.
dant float blocks of barren quartz were also noted in the
catchment. This and other streams in the area contain
anomalous or high levels of Co, V, Zn and Mn in sediment
and, in the upper reaches, Zn,. The assemblage
suggests that the anomalies are caused by hydrous oxide
precipitation processes in combination with a high
background generated by mudstones and perhaps tuffaceous
horizons within the Mynydd-tarw Group. The _ -_
weak anomalies in panned concentrates are also ascribed
to these causes with the addition of contamination
sites.
at some
To the east of Llanarmon, anomalous levels of Cup, Pb,,
and Sn, in streams draining Llangadwaladr ([SJ 185
3451, [SJ 176 3411, [SJ 169 335)) were shown by
mineralogical study to be entirely the result of contamination.
Weakly anomalous levels of Ba, at these sites
(600- 750 ppm) and high but not anomalous levels of Cu,
were attributed to the background geology. Although not
examined mineralogically, contamination is also believed
to be the most likely cause of anomalies for Sn, Cu and Zn
in the panned concentrate collected south of the village at
[SJ 157 3231, for upstream
are not anomalous.
samples taken above the roads
7 Anomalies in the Iwrch Valley. Anomalous levels of Cu,,
Pb, and Zn, in samples collected near and downstream of
Craig-y-glyn are related to old mine workings hereabouts.
-4 brief examination of the sites indicated that quartzcarbonate
veins with galena and sphalerite within
limestones, calcareous siltstones, ashes and shales of the
Llandeilian Craig-y-glyn Group have been exploited.
Details of the mines are not known, and the workings are
collapsed and at least partly grassed over. Exploitation appears
to have been from a series of levels and shafts along
the Iwrch Valley and the tributary by Pen y Graig farm,
extending from [SJ 122 2901 in the north to [SJ 124 2841
in the south. Collapsed adit directions suggest that one
vein trends SSW from near Pen y Craig farm, but otherwise
the controlling structural directions and style of
mineralisation remain in doubt. The presence of a gravity
low in this area is an interesting feature. It may simply
reflect the relative closeness to basement, but might also
be construed as indicating the presence of an intrusion,
such as a volcanic plug with which the volcanism and
mineralisation might be associated.
Examination of tip material suggests that sulphide
mineralisation is weak, but analysis of a composite tip
sample (Table 6, No. 3) inferred the presence of appreciable
baryte (10 % Ba) in addition to minor galena
(0.9% Pb) and sphalerite (0.3% Zn). With the exception
of Pb, the panned concentrate anomalies are weak and the
absence of a Ba, anomaly suggests that the majority of tip
material is not entering the drainage system. The Ni, and
some part of the Pb, anomalies are related to the high
levels of Ce, and not to mineralisation. All samples taken
in the Iwrch catchment contain high Ce,, Ni, and Pb,,
related to the presence of monazite nodules and coatings.
Several sites in the upper catchment also contain subanomalous
levels of Zn,, Co, and Zn,. These are attributed
to a high background generated by mudstones
and ashes of the Mynydd-tarw Group enhanced by
hydrous oxide precipitates.
.
3191 contains anomalous Cus, Zn, and a very high level
(0.12 ppm) of Cu,. The catchment is mapped as
Mynydd-tarw Group and contains shales, ashy shales,
ashes and sandstones (Wedd and others, 1929) cut by
several northerly-trending faults. A brief examination of
the site revealed abundant float quartz but repeat and ad-
ditional water samples taken upstream failed to yield
anomalous results. The cause of the majority of the
anomalous metal values is probably the same as that
thought to generate anomalies across the interfluve to the
north, i.e. a combination of background geology and
hydrous oxide precipitates, but the possibility of
mineralisation cannot be eliminated at this stage as the
cause of the Cu, anomaly in particular is unresolved.
8 Baryte south of Llandrillo. A panned concentrate from
Nant Crechwyl [SJ 025 3 191 contains an isolated Ba,
anomaly which mineralogical examination related to
barium in shale fragments and rare, rounded grains of
baryte. The site also produced weak Cu, (0.02 ppm) and
Zn, (340 PP m ) anomalies. The source of the baryte is
uncertain. It is possibly derived from fault-associated
veins in the Swch Gorge Tuff Formation, similar to those
discovered in Cwm Rhiwarth.
9 Cobalt and associated anomalies in the upPer Afon Rhaeadr.
Sample sites in first and second order streams close to and
above Tan-y-pistyll contain high or anomalous levels of
Co, Fe, Mn, Cu, Pb, Zn and Ni in stream sediment and
Cu and Zn in water. The sediment anomalies are at-
tributed to a combination of hydrous oxide precipitates
and a high background from the Llyn Lluncaws, and
Cwm Dywyll basic intrusions. This conclusion is sup-
ported by high or weakly anomalous levels of Ti, at these
sites and an analysed rock sample from the margin of the
Llyn Lluncaws intrusion, which contains relatively high
levels of Fe (7.67%), Mn (0.14%), V (221 ppm) and Co
(28 ppm) compared with most of the sedimentary rocks
(Table 6, No. 4). At the two sites near Tan-y-pistyll,
levels may also be enhanced by contributions from the
volcanic rocks. Anomalous levels of Zn, were confirmed
by re-sampling and are attributed to a combination of
acid upland waters and a high available background. At
[SJ 052 3021 an exceptional level of Cu, (0.12 ppm) was
recorded, but resampling failed to reproduce the anomaly
and the site is contaminated. High and anomalous levels
of Ce, Ni, and Pb, are attributed to monazite coatings
and nodules.
Weakly anomalous levels of Zn, and perhaps some of
the other anomalies such as Cu, and Pb, may have yet
another source, for a brief visit to the catchment revealed
the presence of quartz veining and pyrite apparently
associated with both the volcanics and the basic intrusion.
One of the quartz veins, at [SJ 057 3001, had an adit cut
into it but no sulphide besides pyrite was seen. Some of
the volcanics were quite strongly veined, possibly altered,
and merit further investigations.
10 Anomalies associated with base-metal mining centred on
Llangynog. Many of the anomalies in the Rhaeadr,
Tanat and Hirnant valleys are caused by old base-metal
mines. Generalised sites of these workings are shown on
the anomaly maps. Sites known to be contaminated by
old workings were avoided, wherever possible, whilst
maintaining the sampling density, reducing the overall
impact of the known mineralisation on the results. The
old mine workings generate anomalies characterised by
In addition to these anomalies, the site at [SJ 104 ’ high levels of Cu, Pb and Zn in concentrate and lower
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