Mineral Reconnaissance Programme Report
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Natural Environment Research Councii _- Institute of Geological Sciences - Mineral Reconnaissance Programme Report - A report prepared for the Department of Industry L- This report relates to work carried ouibythe Institute of Geological Sciences on behalf of the Department if Industry. The informatioh contained herein must not be published without reference to the Director, Institute of Geological Sciences D. Ostle Programme Manager Institute of Geological Sciences Keyworth, Nottingham NG12 5GG No. 32 Investigations at Polyphant, near Launceston, Cornwall INSTITUTE OF GEOLOGICAL SCIENCES I Natural Environment Research Council I Mineral Reconnaissance Programme I Report No. 32 Investigations at Polyphant, near I Launceston, Cornwall Geology M. J. Bennett, BSc Geochemistry K. Turton, BA Geophysics K. E. Rollin, BSc I I 0 Crown copyright 7980 London 1980 A report prepared for the Department of Industry I Mineral Reconnaissance Programme Reports 30 Porphyry style copper mineralisation at Black . Stockarton Moor,south-west Scotland 1 The concealed granite roof in south-west Cornwall 31 Geophysical investigations in the 2 Geochemical and geophysical investigations around Closehouse-Lunedale area Garras Mine, near Truro, Cornwall 32 Investigations at Polyphant, near Launceston, 3 Molybdenite mineralisation in Precambrian rocks Cornwall near Lairg, Scotland 4 Investigation of copper mineralisation at Vidlin, Shetland 5 Preliminary mineral reconnaissanceof Central Wales 6 Report on geophysical surveys at Struy, Inverness- shire 7 Investigation of tungsten and other mineralisation associated with the Skiddaw Granite near Carrock Mine, Cumbria 8 Investigation of stratiform sulphide mineralisation in parts of central Perthshire 9 Investigation of disseminated copper mineralisation near Kilmelford, Argyllshire, Scotland 10 Geophysical surveys around Talnotry mine, Kirkcudbrightshire, Scotland 11 A study of the space form of the Cornubian granite batholith and its application to detailed gravity surveys in Cornwall 12 Mineral investigations in the Teign Valley, Devon. Part l-8arytes . 13 Investigation of stratiform sulphide mineralisation at McPhun’s Cairn, Argyllshire 14 Mineral investigations at Woodhall and Longlands in north Cumbria 15 Investigation of stratiform sulphide mineralisation at Meall Mor, South Knapdale, Argyll 16 Report on geophysical and geological surveys at Blackmount, Argyllshire 17 Lead, zinc and copper mineralisation in basal Carboniferous rocks at Westwater, south Scotland 18 A mineral reconnaissance survey of the Doon-Glenkens area, south-west Scotland . 19 A reconnaissance geochemical drainage survey of the Criffel-Dalbeattie granodiorite complex and its environs 20 Geophysical field techniques for mineral exploration 21 A geochemical drainage survey of the Fleet granitic complex and its environs 22 Geochemical and geophysical investigations north- west of Llanrwst, North Wales 23 Disseminated sulphide mineralisation at Garbh Achadh, Argyllshire, Scotland 24 Geophysical investigations along parts of the Dent and Augill Faults 25 Mineral investigations near Bodmin, Cornwall. Part The Institute of Geological Sciences was formed by the 1 -Airborne and ground geophysical surveys incorporation of the Geological Survey of Great Britain and the Geological Museum with Overseas Geological Surveys 26 Stratabound barium-zinc mineralisation in Dalradian and is a constituent body of the Natural Environment schist near Aberfeldy, Scotland: Preliminary report Research Council 27 Airborne geophysical survey of part of Anglesey, North Wales Bibliographical reference 28 A mineral reconnaissance survey of the Bennett, M. J. and others 1980. Investigations at Abington-Biggar-Moffat area, south-central Polyphant, near Launceston, Cornwall. Mineral Scotland Reconnaissance Programme Rep. inst. Geol. Sci., NO. 32 29 Mineral exploration in the Harlech Dome, North Printed in England for the Institute of Geological Sciences Wales by Four Point Printing CONTENTS summary 1 I Introduction 1 Geology of the area 1 Devonian 1 Upper Carboniferous 1 I Intrusive rocks 1 Dolerite 1 Lamprophyre 1 Lavas 1 1 Polyphant ultmbasic complex 1 His to&al significance 1 Geology of the complex 1 I Geochemical Investigations 3 Results 9 Nickel 9 Chromium 9 Cobalt 9 I Copper 9 Lead 9 Zinc 9 I Radiometric survey 9 Geophysical Investigations 9 Background 9 Magnetic surveys 9 I Induced polarisation survey 12 Conclusions 12 Acknowledgements 12 I References 12 APPENDIX I Borehole logs 13 I FIGURES 1 Geology and location of the Polyphant ultrabasic complex 2 I 2 Outline geology and position of soil traverses at Polyphant with nickel populations 4 3 Outline geology and position of soil traverses at Polyphant with chromium populations 5 I ’ 4 Outline geology and position of soil traverses at Polyphant with cobalt populations 6 5 Outline geology and position of soil traverses at Polyphant with anomalous lead band 7 1 6 Outline geology and position of soil traverses at Polyphant with radiometric readings 8 7 Location of geophysical traverses at Polyphant, Cornwail 10 I 8 Ground magnetic profiles (IGS) on Traverses Ml01 and Ml02. 11_ I TABLFS 1 Location and other features of boreholes near Polyphant 3 I SUMMARY The geology of the area is detailed in the works mentioned above and the mining history is described in A geophysical survey was conducted in the vicinity of the Anon (1977). Only a brief summary of the geology is Carrock tungsten mine, Cumbria. The object of the survey given here and the solid geology is shown in Fig. 1. was to establish an optimum geophysical exploration pro- The southern part of the area is underlain by the cedure for the location of the style of mineralisation Lower Ordovician Skiddaw Group sediments, which are known at Carrock. The VLF-EM method recorded only commonly highly deformed. Fold axes have an east- weak or indistinct anomalies over much of the known north-easterly trend. In the north of the area a thick, mineralisation but a weak anomaly coincident with the near-vertical sequence of volcanic rocks strikes east-west Emerson vein was traced northwards for one km. Several and has been correlated with the Borrowdale Volcanic similar linear features were recorded in the area on trends Series. The central part of the area is dominated by two favourable for mineralisation and two, at Poddy Gill in major intrusions. To the north the Lower Palaeozoic the east and Arm o’ Grain in the west, are coincident for Carrock Fell gabbro complex trends roughly along the part of their strike with exposed mineralisation. east-west regional strike and is in steep contact with the Resistivity measurements indicated that most fault surrounding rocks. To the south, separated from the structures have coincident low resistivity zones; a detailed gabbro by a few hundred feet of homfelsed sediments, traverse across the Emerson vein showed a minor high lies the northernmost part of the Caledonian Skiddaw resistivity peak within the low zone. Granite. This has been greisenised in the Grainsgill Beck Induced polarisation, magnetic and self potential area. anomalies were recorded only within the gabbro on the The mineralisation is in the form of quartz veins extrapolated positions of the Smith and Wilson lodes. It is which strike north-south and dip steeply to the west. The concluded that these anomalies are caused by discontin- veins csrry tungsten as wolframite and scheelite, together uous near-surface lenses of pyrrhotite which have little with lesser amounts of arsenopyrite, pyrrhotite and pyrite VLF response. which increase northwards, The mineralisation is closely The VLF method is identified as the best tool for the associated with the greisenisation of the Skiddaw Granite location of structures which may carry mineralisation in and it is considered that both the tungsten mineralisation this environment. Incidentally to the main objectives of and the greisen are the result of circulation of saline fluids the work, three VLF anomalies are identified which through and around the hot granitic intrusion (Appleton appear to warrant investigation by drilling; the Arm o’ and Wadge, op. cit.). The circulation was especially active Grain anomaly, the Emerson vein extension and the on the northern margin of the granite where joints and Poddy Gill feature. fractures in the gabbro and the granite provided channel- ways for the fluids. The Skiddaw Group sediments, in contrast, were hydrologically much “tighter”, so that circulation was limited in the south. INTRODUCTION A study group has been established within the Mineral Reconnaissance Programme to examine well-documented SURVEY DESIGN mineral occurrences (including working mines) in order to evaluate and refine exploration methods and interpreta- The survey area was geographically defined to include the tion techniques as an aid to the discovery and preliminary harder fractured rocks to the immediate north and west of assessment of further mineralisation in the same area or the present workings. The southern area was not tested in similar occurrences elsewhere. detail. The geophysical methods used were selected on the The Carrock Fell tungsten mining area meets the geo- basis of the following considerations: logical requirements of the project. An active mine i The nature of the veins - tight, continuous quartz provides detailed information on the controls and nature lodes of 1.5-3 m average width, with fracture controlled of mineralisation. The geology, geochemistry and mineral orientation - suggested that they