Open Research Online The Open University’s repository of research publications and other research outputs The Genesis of Sulphides in the Shetland Ophiolite Thesis How to cite: Maynard, Jonathan (1994). The Genesis of Sulphides in the Shetland Ophiolite. PhD thesis The Open University. For guidance on citations see FAQs. c 1993 Jonathan Maynard https://creativecommons.org/licenses/by-nc-nd/4.0/ Version: Version of Record Link(s) to article on publisher’s website: http://dx.doi.org/doi:10.21954/ou.ro.0000ff33 Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online’s data policy on reuse of materials please consult the policies page. oro.open.ac.uk The Genesis of Sulphides in the Shetland Ophiolite by Jonathan Maynard, B.Sc.Hons (London) A dissertation submitted for the degree of Doctor of Philosophy D a d ® M l‘^ 9 3 y 0 M > a ^ SeptemberQ^r\fomK^r 11993 OOQ Department of Earth Sciences, The Open University ProQuest Number: C382455 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest C382455 Published by ProQuest LLO (2019). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLO. ProQuest LLO. 789 East Eisenhower Parkway P.Q. Box 1346 Ann Arbor, Ml 48106- 1346 Abstract This thesis describes the sulphide, arsenide and antimonide mineral assemblages and stable isotope geochemistry of each magmatic and alteration lithology of the Shetland ophiolite complex and identifies a new assemblage of Pd and Pt-bearing nickel antimonides. The assemblages have been characterized as follows. Cumulate rocks typically contain a variety of Ni- Fe- and Cu-beaiing opaque (biases including relict sulphides, native metals, arsenides and antimonides. The sulphide assemblages show a strong correlation with the host silicate lithology. Dunite pods in harzburgite and cumulate dunites contain the assemblage pentlandite-heazlewoodite-millerite-godlevskite-chalcopyrite-chalcocite-native Cu ± awaruite ± troilite ± oicelite ± breithauptite/hfi- Cu antimonide. Cumulate wehrlites and pyroxenites contain pentlandite-pyrrhotite-pyrite-chalcopyriie-bomiie-chalcocite-native copper ± orcellite and maucherite. Gabbros and dolerite dykes contain pyrrhotite-pyrite- marcasite-chalcopyrite ± pentlandite. These assemblages developed by the alteration of precursory magmatic sulphides. A technique has been developed and used to measure whole-rock 5^S values to a precision of ±0.5%o. 5 ^ measurements suggest an igneous source for sulphur in sulphide- bearing dunite pods (+4,3%®). cumulate dunites (+3,1%®), cumulate pyroxenites (+2.9%®) and gabbros and dolerite dykes (+4.5%®). Talc-carbonate altered rocks (+0.9%®) from close to the basal Arust may have derived a portion of sulphur from the underlying metasediments (-4.2%®). Sub-massive pyrite mineralization from high-level gabbros (+9 to +14%®) requires a contribution of hydroAermal sulphur, this has been corroborated by He isotope analysis. Hydrogen and oxygen isotope analyses have shown that serpentine minerals in dunites have exchanged water with meteoric fluids, whereas pyroxenites and gabbros retain a hydroAermal seawater signature. A diverse assemblage of PGM are known to be intergrown or intimately associated wiA the base metal suphides and nickel arsenides in dunite pods in harzburgite and cumulate dunites. A new assemblage of Pd ± Pt-bearing Ni and Ni- Cu antimonides are associated wiA Aese PGM and disseminated hfi- Fe-sulphides. The paragenesis of these antimonides is described for a sulphide-bearing dunite pod from the FGE-rich Cliff locality and from a PGE-enriched mineralized horizon in cumulate dunites. At boA localities Ae paragenesis of the antimonides is closely linked to the alteration of the coexisting sulphide assemblage. Acknowledgements Many thanks are due to Drs. H.M. Prichard, R.A. Ixer and Professor C.T. Pillinger for setting up this project and for guidance and ideas throughout the course of this woik. The help of Drs. LP. Wright and A.D. Morse in supervising laboratory experiments and commenting on Aesis drafts is gratefully acknowledged. Dr. R.A. Lord provided many ideas on aspects of this thesis and the help of Drs. D.S. Stevenson, C. Oppenheimer, S.R. Boyd, LA. Franchi proved invaluable in writing this thesis. Dr. P. McConville performed laser probe 5^S measurements and Dr. F. Stuart made He-isotope analyses. Dr. A.G. Undle and Dr. P. Potts are Aanked for help wiA microprobe analyses and Naiomi Williams operated the SEM. I. Chaplin, K. Chambers and B. Ellis produced many thin sections. J. Watson helped wiA XRF analysis and R. Quill and E. Parker provided secretarial skills. TABLE OF CONTENTS CHAPTER 1 SULPHIDES IN MAFIC AND ULTRAMAFIC ROCKS 1 1.1 Introdiictioa: The distributioD of sulphides in ophiolitic oust 1 1.2 The aouroc o i su^ihur in sulfdiidc» ftom ultramafic and mafic rocks 5 1 1 Thm nUramflfir anA mnftr ignprniR mmpli>T am an nphinlttft 8 1.4 Sulphides in the SWtland ophiolite 13 1.5 Diesis organization 16 15.1 Objectives 16 15.2 Sanqile localities 17 15.3 Techniques 17 15.4 Structure of thesis 17 CHAPTER 2 PETROLOGY, STRUCTURE, GEOCHEMISTRY AND MINERALOGY 19 2.1 Introduction 19 2.2 Pttrology and structure of the Shetland ophiolite 19 2%1 Ultrmnafic rocks 19 2.2.2 Mafic rocks . 2 0 2 5 Whole rock geochemistry 21 2.3.1 Geochemistry ofpartiallyalteied mafic and ultramafic rocks 21 2.35 Geochemistry of talc-caibonate rock and highly sheared gabbros 21 2 5 5 Water and carbon content of altered rocks firom the Shetland ophicdite 26 2.4 Frimmy silicate mineralogy 28 14.1 Olivine 28 2.45 Orthppyroxene 29 2.45 Qint^yroxene 29 2 5 Secondary silicate mineralogy 31 25.1 Secondary silicate mineralogy of the ultramafics; previous work 31 25.1.1 X-ray diffiactioo identification of serpentine and cartxinatB 31 minerals 2.5.15 Microprobe analysis of secondary minerals 33 (i) 2 5 5 Secondary mineralogy of the gabbro unit 36 2 5 5 .1 Amphibole 36 2555E pidote 37 2 5 5 3 Plagioclase feldspar 38 2 5 5 .4 Chlorite and biotite 38 2j9 Discussion and conclusions CHAPTER 3 MINERALOGY AND MINERAL CHEMISTRY OF OPAQUE MINERALS 41 3.1 Introduction 41 3 5 Sulphide mineralogy the ma&; porüoo of the ophWte 41 35.1 Gabbros, basic (fykes and plagiogranite 42 3 5 5 Amphibole pegmatite, plagiogranite and mkaogabbro. North Mu Ness prainsula 45 33 Sulphide mineralogy of the ultramafic portion (tf the ophkdite 45 33.1 High-level wehriite and pyroxenite 45 3 3 5 Cumulate wehrlite and pyroxenite 46 3 3 3 Wehrlitic dunite. cumulate dunite and transitional harzburgite 47 33.4 Dunite pods within harzburgite 59 33.5 Dunite pods within hmzburgite with 4ppm ZPt+Pd, Cliff 60 33.6 Podiform diromitite. Cliff 60 33.7 Serpentinites from the basal thrust 64 3.4 Opaque mineral diemistry 64 3.4.1 Sulphide mineral chemistry of the mafic portion of the ophiolite 65 3.45 Opatpie mineral diemistry of the ultramafic portion oi the ophitdiie 70 3.45.1 Ni and M-Fe sufyhides 70 3.45.2 Ni-Fe alloy and native metals 76 3.4 2.3 Nickel arsenides and sulphameoides 78 3.45.4 Fe-Cu and Cu-Fe sulphides 83 3.45.5 Spinel and magnetite 83 Oi) 3 5 Nickel antimonides 85 35.1 Introduction 85 3 5 5 Mineralogy of nickel antimonides 86 3.55.1 Reddish-pink highly anisotrofric breithaiqitite 86 3 5 5 5 Isotropic, Cu-rich, lavender-mauve to {nuk-grey nickel antimonide 99 3 5 5 .3 hR-iich antimonide 99 3 5 5 Paragenesis of nickel antimonides 106 35 Summoy and conclusions CHAPTER 4 SULPHroE PHASE EQUH.IBRIA, AND A MODEL FOR LOW TEMPERATURE SULPHIDE PARAGENESIS 115 4.1 Introductk» 115 4 5 Low temperature equilibrium lelatkms in the Fe-Ni-S system 115 45.1 Conqxisitiooofpentlandite and heazlewoodite derived fiom experimmtal and natural studies 118 4 5 Review of low temperature Cu-Fe-S phase ecjuUibria 119 4.4 M-Fe alloys and native copper 121 4.4.1 Binary phase relatkms in the Fe-Ni system 121 4.45 Native copper 123 4.5 Sufyhide assemblages from the Shdland ophiolite 123 45.1 Gabbros, dykes and plagiogranite 123 4 5 5 High-level pyroxenites 124 45.3 Cumulate pyroxmites 125 45.4 Wehrlitic dunite, cumulate dunite and dunite pods 127 45.5 Basal thrust 129 4.6 Bulk base-metal content of sulphide aggregates 129 4.7 Activity-octivity diagrams and low temperature paragenesis of Ni-Fe- andNi-sufyhides 131 4.7.1 Introduction 131 4.75 Application to dunites from the Shetland ophioUte 134 4.8 Scrpentinization and talc-caibonate alteration; models for the redistribution of su%dmr and metals 136 4.8.1 Setpentinized dunite pods and cumulate dunite 136 (ÎÜ) 4 5 5 Talc-cartXHiate sulphide awrinMay from the basal thnist 137 4.9 Sumnuffy and conclusions 142 CHAPTER 5 TECHNIQUES USED TO MEASURE SULPHUR ISOTOPE RATIOS 145 5.1 A review of the techniques available for the measurement of su^rimr isotope nuios 145 5 5 Die modified 60È2E mass spectmneter 147 55 Reference gases and zero enrichmoit experiments 147 5.4 Techniques used to extract SÜ2 gas from sulphides and native sulphur 149 5.4.1 Sulphur extraction line 150 5.45 Combustion of sulphides using a sealed tube technique 154 5.45.1 Experiments using standard sulphides and native su ^ u r 157 5.4.3