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A reconnaissance geochemical drainage survey of the Harlech Dome, north Wales

By D.C. Cooper, P.J. Bide, D.G. Cameron, N. Bell and P.M. Allen

Abstract

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

Topics: Earth Sciences
Publisher: British Geological Survey
Year: 1985
OAI identifier: oai:nora.nerc.ac.uk:11798

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