A geochemical drainage survey was carried out across
1 050 km’ of the Harlech Dome and adjacent areas at a
mean density of 0.85 sample/km2. Fine (-100 mesh)
stream sediment, panned concentrate and water samples
were collected at every site. Cu, Pb, Zn, MO, As, Ba, Fe,
Mn, Co, Ni, Cr, V and Zr were determined in stream
sediment samples; Cu, Pb, Zn, Ba, Fe, Mn, Ti, Nl, Ce
and Sn in panned concentrates, and Cu, Pb and Zn in
water. Gold was determined in panned concentrates
from part of the area.
The results show that the area is metalhferous,
containing large anomalies for a wide range of metals.
Strong regional patterns are displayed by many elements
and it is demonstrated that these are caused by bedrock
li thology, hydromorphic processes, mineralisation and
contamination. Geochemical signatures characteristic of
the following metallrferous concentrations were identified:
ii) disseminated copper ‘porphyry-style’ mineralisation;
‘gold-belt’ vein-style mineralisation in Cambrian
rocks;
mineralisation in Ordovician volcanic and sedimentary
rocks;
bedded manganese deposits in the Cambrian;
manganese vein-style mineralisation in Ordovician
volcanic rocks;
(ii)
(iii)
iiv)
iv)
(vi)
(vii)
granite-related mineralisation;
dark mudstones.
Rhobell Fawr volcanism and possible associated metasomatism
may also have generated metal anomalies but
the evidence available is inconclusive. Small, bedded iron
ore deposits failed to produce distinct anomalies.
Despite widespread past mining activity eleven areas or
styles of mineralisation have been identified where it is
considered that further work might lead to the recognition
of deposits of economic or supply significance.
These targets include base metal anomalies in
Ordovician volcanic rocks where there is some potential
for volcanogenic stratiform mineralisation; copper and
gold anomalies in Upper Cambrian rocks indicating the
presence of further gold-belt vein-style mineralisation;
arsenic anomalies over Ordovician acid volcanic rocks
whose gold potential merits investigation; manganese
and barium anomalies related to manganese-barium vein
mineralisation in Arenig volcanic rocks and metalliferous
concentrations in dark mudstones marginal to the
Rhobell volcanic centre.
A metallogenetic model is. proposed for the area
involving: syngenetic metalliferous concentration during
Cambrian sedimentation; copper and possibly gold mineralisation
in an island arc-type setting at the close of the
Cambrian; syngenet ic and volcanogenic base-metal
enrichment during the Lower Ordovician, and deep burial
and remobilisation of these metals to form vein-style
deposits during or after the main (end Silurian)
Caledonian erogenic event.
Strong regional trends displayed by elements not
directly involved in mineralisation, such as Zr, Ce and
Cr, can be related to the lithostratigraphy and suggest,
in conjunction with other evidence, significant palaeogeographic
changes at the close of the Middle Cambrian,
involving the derivation of sediment from source areas
of contrasting geochemical character in Lower Cambrian
and Upper Cambrian-Ordovician times. The characteristics
of volcanic rocks show less clearly because of the
rapid alternation of acid and basic lithologies with
respect to the scale of the survey.
INTRODUCTION
This survey .covers about 1100 km2 of western Wales
bounded by the towns of Bala in the east, Tywyn in
south, Blaenau Ffestiniog in the north, and Cardigan Bay
in the west (Figure 1). The area is formed almost
entirely of sedimentary, volcanic and intrusive rocks of
Lower Palaeozoic age, folded into the structure known
as the Harlech Dome. The area lies within the Snowdonia
National Park and 1s mountainous. The most prominent
hills are the Cader Idris, Aran, Arenig and Moelwyn
groups which rise to 907 m on Aran Fawddwy and form a
ring on the periphery of the survey area. The westcentral
part of the area is dominated by another range of
hills, the most prominent of which is Rhinog Fawr.
Drainage is dominated by the catchment of the Afon
Mawddach, which drains the central part of the area.
Acid, peaty soils underlain by variable thicknesses of
glacial deposits and interspersed locally with extensive
areas of rock exposure characterise the high ground.
Lower slopes are blanketed in drift deposits and have
less acid soils. Streams cut to bedrock except in the
lower reaches of the larger rivers. High ground is
covered by heather or rough grass and used for sheep
grazing; lower ground is mostly grass covered with small
areas of deciduous forest. There is little arable farming.
Extensive coniferous forest plantations cover all except
the highest ground in several areas. Stream contamination
is only a problem in the vicinity of farms, villages
and a disused military training area to the east of
Trawsfynydd.
No detailed geochemical survey data has been
published for this area though surveys have been carried
out by universities and commercial companies. The area
is included in the Wolfson Geochemical 4tlas (Imperial
College, 1978) and a geochemical study of Wales by
Urquidi-Barrau (1973). Both works are based on the same
data and provide useful information on a regional scale.
Mohr (1959) studied the lithogeochemistry of the
Manganese Beds, and more recently geochemical data
were Included in studies of the Rhobell Fawr Volcanic
Group (Kokelaar, 1977) and Aran Volcanic Group in the
type area (Dunkley, 1978). The results of airborne geophysical,
magnetic, electromagnetic and radiometric
surveys of the Harlech Dome and geological, geophysical
and geochemical investigations of twenty six anomalies
arising from them, as well as various orientation studies
carried out in support of these investigations, are
described in an earlier MRP report (Allen and others,
1979). The literature on geology and mineralisation in
the area is extensive and was summarised in the earlier
report (Allen and others, 1979). Recent publications of
most relevance to this study include, on the geology,
Matley and Wilson (1946), Davies (1958), Lynas (1973),
Bassett, Whittington and Williams (1966), Ridgway (lY75,
1976) and BGS 1:50 000 map sheet 135; on mineralisation,
Rice and Sharp (1976), Allen and others (1979),
Cole (1977) and Foster-Smith (1977); and on mining and
the environment, Searle (1975)
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