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Durham E-Theses A geological investigation of the cobalt fahlbands of the modum area, Norway Gammon, John Bundell How to cite: Gammon, John Bundell (1964) A geological investigation of the cobalt fahlbands of the modum area, Norway, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/9045/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk 2 A GEOLOGICAL liN^ESTIGATION OF THE COBALT - FAHLBANDS OF THE MODm AREA, NORWAY. A thesis sulmitted for the Degree of Doctor of Philosophy in the University of Durham by John Blundell Gammon Grey College Svly I964 17 0CTm< ^ Abstract The cobalt deposits of Modum, Norway, which were worked from 1772 to 1898, are described. The geology of the area is discussed with particular reference to the Modum Formation which forms the host rock for thegreater part of the cobalt mineralisation. The petro• graphy and field relations of this formation suggest that it represents a series of ancient sediments which have been metamorphosed to thee upper amphibolite facies of regional metamorphism. The origin of the word 'fahlband' is discussed and a sugg• ested definition of the term is given. The fahlbands of the original locality of Kongsberg are compared with those present in the Modum area. It is postulated that the fahlband zones represent metamorphosed sulphidic black muds. Olivine gabbro bodies have intruded the Modum Formation; the mineralogical changes occurring on amphibolitisation of the gabbros are discussed. From a review of the field relations, petrography and chemistry of the an^hibolites in the region, it is concluded that some, at least, of these rocks are derived by metamorphosism of basic intru- sives. The cobalt mineralisation occurs as a low grade impregnation in the amphibolites and metasediments. The structural controls of the mineralisation are discussed and a possible zonal arrangement of tie cobalt minerals is recognized. Optical properties and textural relations of the cobalt minerals have been determined. Pdssible genetic hypo• theses are reviewed and it is concluded that the deposits probably represent an occurrence originally of the Cobalt, Ontario type which has undergone high grade metam^orphism. 11 The mineralogy of the main ore mineral cobaltite (CoAsS) is considered with particular reference to an order-disorder transfor- .mation which, occurs in the vicinity of 850°C. This study leads to the conclusion that the crystal structure and type of twinning in cobaltite are useful criteria for the recognition of metamorphism in ore deposits. The chemical and structural relationships of the minerals in the system FeAsS-CoAsS are also discussed and a method for the determination of cobalt in arsenopyrite is given. Ill Introduction The cobalt deposits of Modum are situated in the south-east of Norway in country of considerable natural beauty. The area is at the confluence of several rivers, draining the mountainous region of central Norway, which unite to form the Drammen River before entering the sea by way of the Oslo fjord. The area exhibits a wide variety of geological phenomena, and has been the scene of mining operations for more than three centuries. It has nevertheless received very little attention from geologists. Some reconnaissance studies of the geology have been made> but only the immediate vicinity of the Kongs- berg silver mines has been subjected to really detailed investigation. The principal object of this work is to record the results of a detailed study of the geology of the Modum cobalt deposits. In addition, a coiig>arison is made with similar deposits at Kongsberg and in central Sweden, based upon reconnaissances made by the author, and upon the writings of former workers. New aspects of the mineralogy of cobaltite (CoAsS) are also recorded. The work was carried out during the tenure of a NATO Science Studentship, Part I records the results of field and laboratory investigations into the geology of the Modum cobalt deposits, and concludes with a comparison with similar deposits and possible genetic hypotheses. In Part II the mineralogical studies on cobaltite and related minerals are reported. One full season was spent in the field, from May 1962 to September 1962. In addition, six weeks in the autumn of 1961, and IV two months in the spring of 1963 were spent in Modum. The rest of the 1963 field season was spent on reconnaisance studies at Kongsberg and at Swedish cobalt localities. All the in^rtant cobalt workings in Modum were visited and detailed maps, on a scale of 1:1000 weie made of their environs. Underground mapping, on a scale of 1:500, was carried out at the Skuterud cobalt mines. Structural data was recorded from all localities and a total of 857 hand specimens were collected. The periods between the field seasons were spent in laboratory examination of material from the area, at the University of Durham and at the University Geological Museum, Oslo. Acknowledgments The auth\)r vashes to thank Professor K. C. Dunham, F.R.S., for his advice and encouragement, for the provision of research facilities in this Department, for granting leave to study at the University of Oslo and for critically reading the manuscript. The facilities of the Mineralogisk-Geologisk Museum of the University of Oslo were placed at the author's disposal through the courtesy of the Director, Professor T.F.W. Barth. The Department of Scientific and Industrial Research is i gratefully acknowledged for the provision of a N.A.T.O. Science Studentship. Personal acknowledgment is due to Dr. H. Neumann for sugg• esting the Modum area for study and to Dr. F. M. Yokes and Mr. R. Phillips for their supervision of this study. The author would particularly like to thank Dr. T. E. Ball and IVIrs. J. Kaye for their assistance with the X-ray investigations; Cand. Real. A.' Brunfeldt for instruction in the use of the gamma-roy spectrometer; Tannlaege K. HunstadbrSten for information concerning the history of the Modum Cobalt Mines;; Cand. Real. 0. Jjrfsang for providing copies of his manuscript geological maps of the Modum area; Dr. H. Bj^^rlykke, Director of the Geological Survey of Norway, for permission to use material from the Survey's archives; Dr. A. Hayhurst for arranging to heat a sample of the Modum cobaltite in the research laboratories of Messrs. Pickford, Holland & Co. Ltd.; Miss P. Killing- worth for carrying out an X-ray micro-analysis on a specimen of glaucodot from Modum, and Dr. H. D. Holland, Dr» A. Dunham and Mr. T. G. Elder for mar^^ fruitful discussions. Grateful thanks are due to Ivir. C. Chaplin of this Depart• ment and his technical staff, particularly Mr. G. Dresser, for the production of thin and polished sections, photographs and diagrams. %.wife, Mr. T. J. Preston, Miss D, Glendenning and Miss C. Grogan are sincerely thanked for their help in the final stages of the production of this thesis. VI List of figures Page No. Fig. 1 Location of area studied 2 fi 2 Geology of Southern Norway 6 II 3 Baneralogical variation of paragneisses 23 II 4 Quartz segregation lenses 26 »« 5 Photomicrograph of strained quartz 27 II 6 Photomicrograph of quartz with polygonal boundaries 27 II 7 Muscovite transgressing foliation 29 II 8 Rounded zircon in quartzite 29 II 9 Textural relations of tourmaline 30 II 10 Quartz-sillimanite nodule 32 II 11 Mineralogical banding in siliceous granulite 32 II 12 Quartz replacing biotite 35 II 13 Sillimanitic nodular biotite-schist in KLara Adit 39 11 14 Thin section of sillimanitic nodular biotite- schist 40 II 15 Correlation of biotite optics and chemical composition 48 It 16 Correlation of muscovite optics and chemical composition 55 II 17 PhotomicrografAi of separated tourmaline 58 It 18 Tourmaline in quartzite 58 It 19 Correlation of tourmaline optics with chemical composition 59 II 20 Relation of tourmaline cell-parameters to chemical composition 61 II 21 Oxide weight percent as a function of SiOg content 69 It 22 Plot of tourmaline vol.% vs. ore minerals vol.% 76 It 23 Biotite bleached near ore minerals 82 II 24 Biotite altering to sillimanite 82 II 25 Olivine gabbro at Morud and Hovdekollen 89 vii Page No. Fig.26 "Herring bone" twins in plagioclase 91 II 27 Pericline twins superimposed on albite twins 91 II 28 Zoned plagioclase 92 II 29 Corona of orthopyroxene around olivine 92 11 30 Conjugate corona around olivine 93 II 31 Pleonaste chains 93 II 32 Garnet in actinolite corona 94 II o 33 Conjugate corona around clinopyroxene 94 11 34 Hornblende corona around magnetite 95 II 35 Recrystallisation of orthopyroxene 95 II 36 - Serpentine core to recrystallised orthopyroxene 96 II 37 Recrystallisation of actinolite corona 96 It 38 Appearance of quartz at edge of corona 97 II 39 Garnet flattened in the plane of the foliation 97 11 40 Field relations of amphibolites 102 II 41 Block diagram of the Skuterud area 103 II 42 Thin-section of amphibolite (Specimen 173) 106 11 43 Thin-section of garnetiferous an^hibolite 109 11 44 Textural relations of garnet and biotite 109 It 45 Cross-section of the garnet-amphibelite at Skuterud, showing the chemical variation 112 It 46 Skeletal ilmenite in amphibolite 114 II 47 Ilmenite with gangue inclusions 114 It 48 Ilmenite altering to rutile 115 ti 49 Pyrite altering to goethite 115 II 50 Texture of granite-pegmatite 124 •I.-.