_..._ Natural Environment Research Council -2 Institute of Geological Sciences - -- Mineral Reconnaissance Programme Report c- - _.a - A report prepared for the Department of Industry -- This report relates to work carried out by the British Geological Survey.on behalf of the Department of Trade I-- and Industry. The information contained herein must not be published without reference to the Director, British Geological Survey. I- 0. Ostle Programme Manager British Geological Survey Keyworth ._ Nottingham NG12 5GG I No. 72 I A geochemical drainage survey of the Preseli Hills, south-west Dyfed, Wales I D I_ I BRITISH GEOLOGICAL SURVEY Natural Environment Research Council I Mineral Reconnaissance Programme Report No. 72 A geochemical drainage survey of the I Preseli Hills, south-west Dyfed, Wales Geochemistry I D. G. Cameron, BSc I D. C. Cooper, BSc, PhD Geology I P. M. Allen, BSc, PhD Mneralog y I H. W. Haslam, MA, PhD, MIMM $5 NERC copyright 1984 I London 1984 A report prepared for the Department of Trade and Industry Mineral Reconnaissance Programme Reports 58 Investigation of small intrusions in southern Scotland 31 Geophysical investigations in the 59 Stratabound arsenic and vein antimony Closehouse-Lunedale area mineralisation in Silurian greywackes at Glendinning, south Scotland 32 Investigations at Polyphant, near Launceston, Cornwall 60 Mineral investigations at Carrock Fell, Cumbria. Part 2 -Geochemical investigations 33 Mineral investigations at Carrock Fell, Cumbria. Part 1 -Geophysical survey 61 Mineral reconnaissance at the Highland Boundary with special reference to the Loch Lomond and 34 Results of a gravity survey of the south-west Aberfoyle areas margin of Dartmoor, Devon 62 Mineral reconnaissance in the Northumberland 35 Geophysical investigation of chromite-bearing Trough ultrabasic rocks in the Baltasound-Hagdale area, Unst, Shetland Islands 63 Exploration for volcanogenic sulphide mineralisation at Benglog, North Wales 36 An appraisal of the VLF ground resistivity technique as an aid to mineral exploration 64 A mineral reconnaissance of the Dent-lngleton area of the Askrigg Block, northern England 37 Compilation of stratabound mineralisation in the Scottish Caledonides 65 Geophysical investigations in Swaledale, North Yorkshire 38 Geophysical evidence for a concealed eastern extension of the Tanygrisiau microgranite and its 66 Mineral reconnaissance surveys in the Craven Basin possible relationship, to mineralisation 67 Baryte and copper mineralisation in the 39 Copper-bearing intrusive rocks at Cairngarroch Bay, Renfrewshire Hills, central Scotland south-west Scotland 68 Polymetallic mineralisation in Carboniferous rocks 40 Stratabound barium-zinc mineralisation in Dalradian at Hilderston, near Bathgate, central Scotland schist near Aberfeldy, Scotland; Final report 69 Base metal mineralisation associated with 41 Metalliferous mineralisation near Lutton, Ivybridge, Ordovician shales in south-west Scotland Devon 70 Regional geochemical and geophysical surveys in 42 Mineral exploration in the area around Culvennan the Berwyn Dome and adjacent areas, North Wales Fell, Kirkcowan, south-western Scotland 71 A regional geochemical soil investigation of the 43 Disseminated copper-molybdenum mineralisation Carboniferous Limestone areas south of Kendal near Ballachulish, Highland Region (south Cumbria and north Lancashire) 44 Reconnaissance geochemical maps of parts of 72 A geochemical drainage survey of the Preseli Hills, south Devon and Cornwall south-west Dyfed, Wales 45 Mineral investigations near Bodmin, Cornwall. Part 2-New uranium, tin and copper occurrence in the Tremayne area of St Columb Major 46 Gold mineralisation at the southern margin of the Loch Doon granitoid complex, south-west Scotland 47 An airborne geophysical survey of the Whin Sill between Haltwhistle and Scats’ Gap, south Northumberland On 1 January 1984 the Institute of Geological Sciences 48 Mineral investigations near Bodmin, Cornwall. was renamed the British Geological Survey. It continues Part 3-The Mulberry and Wheal Prosper area to carry out the geological survey of Great Britain and Northern Ireland (the latter as an agency service for the 49 Seismic and gravity surveys over the concealed government of Northern Ireland), and of the surrounding granite ridge at Bosworgy, Cornwall continental shelf, as well as its basic research projects: it 50 Geochemical drainage survey of central Argyll, also undertakes programmes of British technical aid in Scotland geology in developing countries as arranged by the 51 A reconnaissance geochemical survey of Anglesey Overseas Development Administration. 52 Miscellaneous investigations on mineralisation in The British Geological Survey is a component body of the sedimentary rocks Natural Environment Research Council. 53 Investigation of polymetallic mineralisation in Lower Devonian volcanics near Alva, central Scotland 54 Copper mineralisation near Middleton Tyas, North Bibliographic reference Yorkshire Cameron, 0. G., Cooper, C. C., Allen, P. M., and 55 Mineral exploration in the area of the Fore Burn Haslam, H. W. 1984. A geochemical drainage survey igneous complex, south-western Scotland of the Preseli Hills, south-west Dyfed, Wales. Mineral Reconnaissance Programme Rep. Br. Geol. Sun/. , No. 7 2 56 Geophysical and geochemical investigations over the Long Rake, Haddon Fields, Derbyshire 57 Mineral exploration in the Ravenstonedale area, Printed for the British Geological Survey by Four Point Cumbria Printing I I I I CONTEN‘IS SUMMARY 1 INTRODUCTION 1 1 Previous Work 1 GEOLOGY AND MINERALISATION 2 Stratigraphy 2 Cambrian 2 8 Ordovician: Arenig-Llanvirn 2 Ordovician: Llandeilo-Ashgill 4 Environment of deposition 4 Intrusions 5 Structure I Glaciation s Mineralisation 5 8 SAMPLING AND ANALYSIS 5 INTERPRETATION OF RESULTS 5 Frequency distribution 5 Definition of anomalies 6 Mineralogical examination of panned concentrates 7 8 Major sources of element variation 7 . 8 Assessment of anomalous results 11 CONCLUSIONS AND RECOYYENDATlONS 23 ACKNOWLEDGEMENTS 23 8 REFRRENCES 23 APPENDIX 1 Analytical results (pocket) 8 APPENDIX 2 Greyscale regional variation maps (pocket) FIGURES 1 Location of the survey area 2 1 2 Simplified geological map of the Preseli Hills 3 3 Diagrammatic summary of most highly significant inter-element associations 7 I 4 Copper in stream sediment 8 5 Copper in panned concentrate 9 6 Lead in stream sediment 14 7 Lead in panned concentrate 15 8 8 Zinc in stream sediment 17 9 Zinc in panned concentrate 18 10 Barium in stream sediment 19 11 Tin in panned concentrate 20 12 Boron in stream sediment 21 I 13 Sub area location map 22 1 Ordovician stratigraphy 4 Summary of anal&&al-results 6 Threshold levels and class intervals for anomalous results 10 8 Summary of highly significant inter-element correlations 12 8 1 .I SUMMARY INTRODUCTION A geochemical drainage survey at a density of 1 sample A reconnaissance drainage survey of the Preseli Hills in per km2 was carried out across the Preseli Hills, south- Dyfed, South Wales, was carried out as a means of west Dyfed. Stream sediment, water and panned con- assessing the mineral potential of the area and to centrate samples were collected from each of 358 sites, provide baseline geochemical data on the region. The and Cu, Pb, Zn, Ba, Fe, Mn, Co, Ni, V, Cr, B, Zr, As, MO work formed part of a larger project to assess the and Sn were determined in sediment, Cu, Pb, Zn, Ba, Fe, mineral potential of Precambrian and Lower Palaeozoic Mn, Ti, Ni, Sn, As, Ca, Ce, Sr, Sb, Zr, U and MO in rocks of southwest Dyfed (Pembrokeshire). The geo- panned concentrate and Cu and Zn in water. chemical drainage survey was restricted to this area, as From a study of regional variation patterns and previous studies (Allen and others, in prep) showed that multivariate statistical analysis, the main sources of this technique was ineffective in lowland areas, failing geochemical variation in the data were found to be to detect a substantial area of copper enrichment associ- bedrock lithology, mineralisation, contamination and ated with disseminated porphyry style mineralisation. hydromorphic processes. Strong geochemical signatures The survey area (Figure 1) consists of the east-west were shown by dolerite intrusions and by acid volcanics ridge of the Preseli Hills rising to over 500 m, the of the Fishguard Volcanic Group. Dark mudstones of the surrounding lower plateau of 100 to 200 m in height and D. murchisoni Beds and Sealyham Volcanic Series also a small section of coastal platform 50 to 60 m in height. showed characteristic geochemical features whose Most of the area was subject to glacial and periglacial impact was limited by their restricted and sinuous out- processes during the Pleistocene era. The plateau area is crops. A feature of most of the area is the presence of gently rounded in the east, with several fairly broad monazite nodules, generating high levels of rare earth river valleys running north-south, while the west is more elements and uranium in the panned concentrates. These severely incised where sub-glacial channels have cut are particularly high over the Llandeilo-Ashgill sedi- deep narrow gorges in p&es. The coastal platform has mentary rocks. also been cut into by ice-marginal or sub-glacial streams Geological thresholds were established using cumula- forming several deep gorges (Jones, 1965). Most of the tive frequency plots and percentile division. Anomalies lower plateau and coastal platform is farmed, while the due to contamination could be