British Geological Survey Mineral Reconnaissance Programme Mineral exploration in. Lewisian supracrustal and basic rocks of the Scottish Highlands and Islands Department of Trade and Industry __- --_L__~__ _I ~ MRP Report 146 Mineral exploration in Lewisian supracrustal and basic rocks of the Scottish Highlands and Islands J S Coats, M H Shaw, A G Gum, K E Rollin and N J Fortey BRITISH GEOLOGICAL SURVEY Mineral Reconnaissance Programme Report 146 Mineral exploration in Lewisian supracrustal and basic rocks of the Scottish Highlands and Islands J S Coats, M H Shaw, A G Gunn, K E Rollin and N J Fortey Compilation, GCOIO~J, Geochemistry and Mitzcralisation J S Coats, BSc, PhD M H Shaw, BSc A G Gunn, BA, MSc BGS, Keyworth, Nottingham Geophysics K E Rollin, BSc Mineralogy N J Fortey, BSc, PhD This report was prepared for the t Department of Trade and Industry Maps and diagrams in this report use topography based on Ordnance Survey mapping Bibliographical reference Coats, J S, Shaw, M H, et al. 1997. Mineral on explorati in Lewisian supracrustal and basic rocks of the Scottish Highlands and Islands. Mineral Reconnaissance Programme Report, British Geologica Survey, No. 146. 0 NERC copyright 1997 Keyworth, Nottingham 1997 BRITISH GEOLOGICAL SURVEY * 0115-936 3100 Telex 378173 BGSKEYG Fax01159363200 The full range of Survey publications is available from the BGS Sales Desk at the Survey headquarters, Keyworth, Nottingham. The Murchison House, West Mains Road, Edinburgh, EH9 3LA more popular maps and books may be purchased from BGS- * 0131-667 1000 Telex 727343 SELSED G approved stockists and agents and over the counter at the Fax0131-6682683 Bookshop, Gallery 37, Natural History Museum, Cromwell Road, (Earth Galleries), London. Sales Desks are also located at the BGS London Information Office at the Natural History Museum, Earth London Information Office, and at Murchison House, Edinburgh. 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Dr D C Cooper Minerals Group British Geological Survey Keyworth Nottingham NG12 5GG Printed in England for the British Geological Survey by Birch Print, Nottingham SUMMARY INTRODUCTION PLANNING AND DEVELOPMENT FRAMEWORK REGIONAL GEOLOGY Geology of the Southern Belt Geology of the basic and ultrabasic intrusive igneous rocks of the Central Belt 8 Regional geophysics GAIRLOCH AREA 11 Geology and mineralisation 11 Geophysics 12 LOCH MAREE AREA 14 Introduction 14 Geology 14 Lithogeochemistry 20 Strathanmore Marble 20 Strathanmore graphitic and mica schists 21 Ben Lair Hornblende Schist 22 Exhalite bands 25 Beinn Airigh Charr and Furnace Mica Schists 29 Inishglass Mica Schist 31 Creag Dharaich Hornblende Schist 32 Letterewe Marble 32 Letterewe Gneiss 32 Mineralised veins 34 Geophysics 34 Conclusions 35 FLOWERDALE FOREST 39 Geology 39 Lithogeochemistry 44 Loch na Beinne Hornblende Schist 44 Metasediments 45 Exhalites 45 Geophysics 47 Mineralogy and petrography 57 Conclusions 60 61 MELVAIG AREA 61 Geology Lithogeochemistry 63 Geophysics 63 Conclusions 66 66 TIREE Geology 66 Previous work 68 Lithogeochemistry 70 Western Belt 70 Central Belt 71 Eastern Belt 72 Rubha Hanais (Locality F) 72 Carnan Mor (Locality G) 72 Mineralogy 72 Discussion 73 COLL 73 Introduction 73 Lithogeochemistry 74 Crossapol 74 Feall Bay 74 Hogh Bay 74 Loch Gortan 74 Discussion and conclusions 75 OUTERHEBRIDES 75 Introduction 75 Benbecula 75 North Uist 77 South Uist 77 Discussion 77 BASIC AND ULTRABASIC ROCKS IN THE CENTRAL BELT 78 Introduction 78 Sampling and analysis 78 Lithogeochemistry 78 Rhiconich area 78 Laxford Shear Belt 79 Foindle 79 Clar Loch 79 Loch na Claise Fearna 79 Gorm Loch - Gorm Chnoc 80 Gorm Loch - Clar Loch Mor 80 Scourie area 80 Loch an Daimh Mor 81 81 Glac Mhor 81 Drumbeg Loch Poll - Lochan Fada 81 Gorm Loch Mor 82 North Kyle&u - Duartmore 82 Ben Strome - Maldie Burn 82 Achmelvich Bay 82 Achiltibuie 82 Gruinard Bay 83 Discussion and conclusions 83 CONCLUSIONS AND RECOMMENDATIONS 83 ACKNOWLEDGEMENTS 85 REFERENCES 86 FIGURES 1 General map of the Lewisian showing location of Gairloch, Loch Maree, Co11and Tiree, Scardroy and Glenelg 3 Geophysical anomalies over Lewisian outcrops of NW Scotland 9 Aeromagnetic anomalies in the Gairloch region 13 Schematic geology of the Loch Wee area 16 Location of rock samples from the Loch Maree area 19 Spidergram of median compositions of Strathanmore, Letterewe marbles and Lewisian dolostones (Rock, 1987) normalised to mean global limestone 20 7 Spidergram of Loch Maree Group mica schists normalised to average shale (Taylor and MacLennan, 1985) 22 8 Spidergram of transitional metals in Loch Maree Group metabasic rocks, chondrite normalised 23 9 Ti vs V variation in Ben Lair (BLHS) and Creag Dharaich Hornblende Schists (CDHS) 24 10 Ba vs Sr variation in Ben Lair and Creag Dharaich Hornblende Schists 24 11 Cu vs Zn variation in Ben Lair and Creag Dharaich Hornblende Schists 25 12 Banded quartz-magnetite rock from the Lower exhalite band, Loch Maree area, showing alternating quartz- and magnetite-rich bands 26 13 Fe/IX vs Mn plot for exhalite bands from Loch Maree. Groups I-V derived by cluster analysis of the chemical data 27 14 Ca vs Ti plot for exhalite bands from Loch Maree 29 15 Fe vs Ti plot for metasediments from the Loch Maree area 30 16 Fe vs Mn plot for me&sediments from the Loch Maree area 30 17 Loch M&e area: ground magnetic data. Total field anomaly relative to a datum of49850 nT 36 18 Loch Maree areas ground magnetic data Detail ofthe anomaly north-east ofFurnace 37 19 Loch Maree area: ground VLF-M field data. In-phase component of the VLF magnetic field 38 20 Location map of the Melvaig, Loch Maree and Flowerdale Forest areas 40 21 Flowrdale Forest showing location of Loch rut Bei.nne Inher 41 22 Loch naBeinne area: detailed topography and rock sample locations 42 23 Fe/Ti vs Mn plot for Flowerdale samples 46 24 Flow&ale Forest: location of geophysical (MAG-VLF) traverse lines 48 25 Flowrdale Forest: ground magnetic data around Germ-loch na Beinw 49 26 Flower&e Forest: ground magnetic data, detail on west side Germ-loch naBeinne 50 27 Flowerdale Forest: contour map ofground magneticdata over Gorm-loch na Beinne area 51 28 Flowerdale Forest: 2D model across Germ-loch na Beinne hillock 52 29 Flowerdale Forest: ground VLF-M field around Gorm-loch na Beinne. In-phase component ofthe VLF magnetic field 53 30 Flowerdale Forest: ground VLF-M field data, detail on west side of Gorm-loch na Beinne 54 3 1 Srath Lungard ground magnetic data 55 32 Srath Lungard: ground VLF-M field data 56 33 Location map of the Melvaig area. Geology from Bhattacharjee (1968) 62 34 Melvaig area: ground magnetic data 64 3 5 Melvaig area: grounci VLF-M field data 65 36 Geology map of Time 67 37 Map of sbuth-west Time showing location of commercial boreholes 69 38 Location map ofthe Outer Hebrides showing detailed project areas 76 TABLES 1 Simplified Lewisian chronology (after Shihe and Park, 1993) 6 2 Physical properties of the Lewisian rocks of Scotland 8 3 Lithological succession of the Loch Maree Group at Gairloch (after Park, 1964 and Jones et al., 1987) 12 4 Tectonic succession at Loch Maree from south-west (top) to north-east (base), mainly after Femandes (1987) 17 5 Tectono-stratigraphic succession at Loch Maree with major thrusts 18 6 Median compositions of pelitic schists in the Loch Maree area compared to the average shale (Taylor and MacLennan, 1985) 21 7 Summary statistics of the Ben Lair and Creag Dharaich Hornblende Schist Units 22 8 Spearman correlation coefficients for exhalite band samples (N = 5 1, rgg = 0.36) 28 9 Comparison of chlorite schist analyses from the Loch Maree and Gairloch areas 31 10 Comparison between the median compositions of the amphibolites in the Letterewe Gneiss, Scourie and Tollie dykes, and the Ben Lair Hornblende Schist 33 11 Proposed Loch na Beinne succession, west to east 43 12 Summary statistics of the Loch na Beinne, Ben Lair and Creag Dharaich Hornblende Schist Units and the Aundrary Basite 44 13 Comparisona between the Aundrarv Basite.I Melvaie and Ben Lair Hornblende Schist Units H Y 63 SUMMARY This report describes an exploration programme for gold and base metals in the Precambrian Lewisian terrain of north-west Scotland.