Kimberlites, Orangeites, and Related Rocks

422 Pages·1995·16.842 MB·English

by  Roger Howard Mitchell (auth.)

KIMBERLITES,
ORANGEITES, AND
RELATED ROCKS
KIMBERLITES,
ORANGEITES, AND
RELATED ROCKS
Roger Howard Mitchell
Lakehead University
Thunder Bay, Ontario
Canada
SPRINGER SCIENCE+BUSINESS MEDIA, LLC
Library of Congress Cataloging-in-Publication Data
On file
ISBN 978-1-4613-5822-0 ISBN 978-1-4615-1993-5 (eBook)
DOI 10.1007/978-1-4615-1993-5
© 1995 by Springer Science+Business Media New York
Originally published by Plenum Press New York in 1995
Softcover reprint of the hardcover 1s t edition 1995
Ali rights reserved
No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any
means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written
permission from the Publisher
This work is dedicated to my wife
VALERIE ANNE DENNISON
in recognition of her support and
encouragement of my studies of
alkaline rocks, and for providing
an environment favorable to the
preparation of this book
What stuff ~is made, whereof it is born, 1 am to learn.
The Merchant of Veil ice, Act I, Sc.1
W. Shakespeare
PREFACE
This is the final volume of a trilogy of monographs devoted to the petrology of primary
diamond-bearing rocks. It should be noted that, in common with the previous volumes,
Kimberlites and Petrology of Lamproites, the book is not about upper mantle xenoliths
or exploration for diamond.
The principal objective of this work is to present a revised terminology for primary
diamond-bearing rocks. To this end orangeites are recognized as a group of rocks distinct
in mineralogy, geochemistry, and petrology from archetypal kimberlites and lamproites.
The name orangeite is used in recognition of the initial discovery of these rocks in
the Orange Free State of South Africa. The name is not a new one: In 1928, Percy Wagner
suggested that the term orangite [sic] be used to describe the rocks that are currently
known as micaceous or group II kimberlites.
Much of this monograph is devoted to summarizing all that is known of the
mineralogy and geochemistry of orangeites. The work incorporates several thousand new
analyses of minerals from orangeites and kimberlites, together with new trace element
geochemical data. These data are used to compare and contrast orangeites with kimber
lites and lamproites. The work also presents a revised textural genetic classification of
kimberlites and reviews some of the advances in kimberlite petrology since 1986.
The work is a critical synthesis of existing data, not merely a summary of received
concepts, although current hypotheses for the genesis of kimberlite and orangeite are
critically reviewed.
These rocks present particular challenges for petrological investigation because of
their mineralogical complexity and the absence of modern equivalents. Curiously, despite
over 100 years of study, we remain far from achieving a complete understanding of the
mineralogy of kimberlites and orangeites. Hence, it is not surprising that hypotheses
regarding the nature and evolution of their parental magmas remain, to this day, highly
speculative.
Although primary diamond-bearing rocks are relatively rare rock types, they have
an economic and petrological importance which far outweighs their relative obscurity
and therefore justifies continued study oftheir petrogenesis. Thus, it is particularly hoped
that the novel ideas and concepts advanced in the work will stimulate much further study
of the character and evolution of primary diamond-bearing rocks.
vii
viii PREFACE
Many people have contributed to the production of this book, and I wish to
acknowledge the following colleagues who have, over the past 15 years, contributed much
discussion, preprints, thin sections, rock samples, and unpublished and/or difficult-to
obtain information: Steve Bergman, Roger Clement, Howard Coopersmith, Barry
Dawson, Alan Edgar, Tony Erlank, Steve Haggerty, Barry Hawthorne, Bram Janse,
Viktoria Komilova, Sergei Kostrovitskii, Henry Meyer, Peter Nixon, Nick Rock, Mike
Skinner, Patricia Sheahan, Simon Shee, Barbara Scott Smith, Andy Spriggs, Ken Tainton,
Larry Taylor, Nikolai Vladykin, Allan Woolley, and Peter Wyllie.
Special thanks go to Henry Meyer, for providing many hours of microprobe time at
Purdue University, and to Mike Skinner, for samples and the opportunity to examine the
Anglo-American Research Laboratory collection of orangeites. Particular thanks are
expressed to Ken Tainton for permission to quote data from his Ph.D. thesis.
Particular gratitude is expressed to Sam Spivak for drafting and photographic work
and to Anne Hammond for preparing many polished thin sections of these difficult rocks.
Their dedication, skills, and attention to detail are greatly appreciated by the author.
Others from Lakehead University who helped materially during the production of this
work include Reino Viitala (thin sections), Alan MacKenzie (electron microscopy), and
Shelley Moogk-Pickard (trace element analysis). Carl Hager is thanked for assistance in
using the Purdue microprobe.
Critical reviews of all or portions of Chapters 1, 2, and 3 were provided by Henry
Meyer, Barbara Scott Smith, lain Downie, Craig Smith, Alan Edgar, and Howard
Coopersmith. Valerie Dennison is thanked for proofreading innumerable drafts of this
manuscript, typing tables, collating the citations, and improving my written English!
The Natural Sciences and Engineering Research Council of Canada, Lakehead
University, and De Beers Consolidated Mines are acknowledged for financial and
logistical assistance during the course of preparation of this work.
Roger H. Mitchell
Thunder Bay
CONTENTS
Chapter 1. Kimberlites and Orangeites . . . . . . . . . . • • . . . . . • . • • 1
1.1. Etymology of Group I and II Kimberlites . . . . . 1
1.2. Definitions of Cryptogenic and Primary Phases . . 5
1.3. The Hybrid Nature of Kimberlites and Orangeites 5
1.4. Philosophy and Principles of Classification . 7
1.4.1. Modal versus Genetic Classifications 7
1.4.2. Petrological Clans. . . . . . . . . . . 8
1.4.3. The Lamprophyre Clan . . . . . . . . 9
1.4.4. Mineralogical-Genetic Nomenclature within
Petrological Clans. . . . . . . . . . . . . . . . 10
1.5. Mineralogical Comparisons between Kimberlites and Orangeites II
1.6. Definitions of Orangeites and Kimberlites . 14
1.6.1. Orangeites . . . . . . . . . . 14
1.6.2. Kimberlites . . . . . . . . . 14
1.7. Age and Distribution of Orangeites 16
1.8. Occurrences of Orangeites . 18
1. 8.1. Finsch . . . . . . . . 18
1.8.2. Barkly West Region. 21
1.8.2.1. Bellsbank. 21
1.8.2.2. Sover . . . 25
1.8.2.3. Newlands. 27
1.8.2.4. Pniel . . . 27
1.8.3. Boshof District . . . 27
1.8.3.1. Roberts Victor 28
1.8.3.2. New Elands 28
1.8.4. Winburg District ... 28
1.8.5. Kroonstad District .. 29
1.8.6. Swartruggens District. 30
1.8.7. Dokolwayo .. 31
1.8.8. Prieska District 32
1.8.9. Summary ... 35
ix
x CONTENTS
1.9. Textural-Genetic Classifications of Petrological Clans . 35
1.9.1. Kimberlites ........ 37
1.9.1.1. Crater Facies . . . 37
1.9.1.2. DiatremeFacies . 41
1.9.1.3. Hypabyssal Facies 48
1.9.1.4. Spatial Relationships between Diatreme and
Hypabyssal Facies Kimberlites. . 51
l.9.2. Orangeites . . . . . . . . . . . . . 58
l.9.3. Melilitite Clan . . . . . . . . . . . . . . . 58
1.10. Petrographic Characteristics of Orangeite . . . . 60
1.11. Petrographic Differences with Respect to Kimberlites . . 74
1.12. Petrographic Differences with Respect to Lamproites . . 79
Chapter 2. Mineralogy of Orangeites • . • • • • . . . • • • • • . • . . • • •• 91
2.1. Mica .................. 91
2.1.1. Paragenesis .......... 91
2.1.2. Composition of Primary Mica. 94
2.1.2.]. Ah03-Ti02 Variation 97
2.1.2.2. Ah03-FeOT Variation 104
2.1.2.3. Macrocrysts versus Microphenocrysts . 109
2.] .2.4. Minor Elements. . . . . . 111
2.1.2.5. Trace Elements. . . . . . 114
2.1.3. Aluminous Mica-Microxenoliths 115
2.1.4. Aluminous Biotite Macrocrysts . . 117
2.1.5. Micas from the Swartruggens Male Lamprophyre 118
2.1.6. Summary of Mica Compositional Variation . 119
2.1.7. Solid Solutions in Orangeite Mica 122
2.1.8. Mica in Kimberlites . . 126
2.1.8.1. Macrocrysts . . . . . . . . 127
2.] .8.2. Primary Micas ...... 128
2.1.8.3. Summary of Kimberlite Mica Compositional
Variation . 155
2.1.9. Mica in Lamproites . . . . . . . . 157
2.1.10. Mica in Minettes. . . . . . . . . . 160
2.1.11. Mica in Ultramafic Lamprophyres 161
2.2. Clinopyroxene . . . . 166
2.2.1. Paragenesis .,. 166
2.2.2. Composition... 166
2.2.2.1. Diopside 166
2.2.2.2. Titanian Aegirine . 171
2.2.2.3. Minor Elements . . ] 72
2.2.3. Pyroxenes in the Swartruggens Male Lamprophyre .. 176
2.2.4. Megacrystal Pyroxenes . . . . . . . . . . . . . . . .. 177
CONTENTS xi
2.2.5. Comparison with Pyroxenes in Kimberlites 178
2.2.6. Comparisons with Pyroxenes in Lamproites 179
2.2.7. Comparisons with Pyroxenes in Ultramafic
Lamprophyres . . . . . . . . . . . . . . . . 180
2.2.8. Comparisons with Pyroxenes from Minettes . 181
2.3. Olivine . . . . . . . 181
2.3.1. Paragenesis . . . . . . . . . . . . . . . . . 181
2.3.2. Composition ................ 183
2.3.3. Comparisons with Olivines in Kimberlites . 185
2.3.4. Comparisons with Olivines in Lamproites . 187
2.4. Spinel . . . . . . . . 188
2.4.1. Paragenesis . . . . . . . . . . . . . . . 188
2.4.2. Composition .............. 189
2.4.3. Comparisons with Kimberlite Spinels . 195
2.4.4. Spinel Compositional Variation in Lamproites and
Lamprophyres . . . . 198
2.5. Potassium Barium Titanates 200
2.5.1. Hollandite . . . . . . 200
2.5 .1.1. Paragenesis . 200
2.5.1.2. Composition 201
2.5.1.3. Comparison with Hollandites from Lamproites,
Kimberlites, and Other Potassic Rocks 207
2.5.2. Potassium Triskaidecatitanate . 213
2.5.3. Barium Pentatitanate 216
2.6. Perovskite ..... 216
2.6.1. Paragenesis . . . . . 216
2.6.2. Composition .... 218
2.6.3. Comparison with Perovskites from Kimberlite 221
2.6.4. Comparison with Lamproite Perovskite 223
2.7. Phosphates . . . . . . . . . 225
2.7.1. Apatite ....... 225
2.7.1.1. Paragenesis. 225
2.7.1.2. Composition 225
2.7.1.3. Comparison with Kimberlite and Lamproite
Apatite . 225
2.7.2. Daqingshanite . . . . 227
2.7.3. Monazite ...... 228
2.7.4. Sr-REE Phosphate . 229
2.8. Amphiboles-Potassium Richterite 229
2.8.1. Paragenesis . . . . . . . . . 229
2.8.2. Composition ........ 230
2.8.3. Comparison with Potassium Richterite in Lamproite and
Other Potassic Rocks 233
2.9. Potassium Feldspar 235
2.10. Ilmenite . . . . . . . . . . 237
xii CONTENTS
2.10.1. Comparison with Groundmass I1menites from
Kimberlites . . . . . . . . . . . . . . . . . . . . 240
2.10.2. Comparison with Ilmenites in Lamproites . . . . 241
2.11. Rutile . . . . . . . . . . . . . 241
2.12. Zirconium Silicates 243
2.12.1. Zircon .. 243
2.12.2. Wadeite .. 243
2.12.3. Zirconium-Bearing Garnet . . . . .. 244
2.12.4. Calcium Zirconium Silicate .... . 245
2.13. Carbonates .... . .......... . 245
2.13.1. Calcite . . . . . . . . . . . . . . . . 245
2.13.2. Dolomite . . . . . . . 245
2.13.3. Other Carbonates. 246
2.14. Other Minerals 247
2.15. Summary ........ . 247
Chapter 3. Geochemistry of Orangeites • • • • • • • • • • • • • • • • • • • • • 249
3.1. Contamination and Alteration . . . . . . . . . 250
3.2. Primary Magma Compositions. . . . . . . . . . . . . . . . . . 252
3.3. Major Element Geochemistry . . . . . . . . . . . . . . . . . . 252
3.3.1. Unevolved Orangeites .............. . 253
3.3.2. Mineralogical Controls on the Major Element
Geochemistry . . . . . . . . . . . . . . 255
3.3.3. Evolved Orangeites . . . . . . . . . . . . . . . 257
3.3.4. Comparison with Kimberlites ........ . 258
3.3.5. Comparison with Lamproites . . . . . . . . . . . . 261
3.4. First-Period Transition Elements. . . . . . . . . . 262
3.5. Incompatible Elements . . . . . . . . . . . . . . . . . . . 264
3.5.1. Alkaline Earths . . . . . . . . . . . . . . . . . . . 265
3.5.2. Second-and Third-Period Transition Elements .. 265
3.5.2.1. Zirconium and Hafnium ......... . 265
3.5.2.2. Niobium and Tantalum ......... . 268
3.5.3. Thorium and Uranium. . . . . . . . . 271
3.5.4. Rare Earth Elements ........... . 272
3.5.5. Alkali Elements . . . . . . . . . . . 277
3.5.6. Lead . . . . . . . . . . . . . . . . . . . . . 279
3.6. Inter-Element Relationships . . . . . . . . . . . . . . . . . . . 280
3.6.1. Extended Incompatible Element Distribution Diagrams 280
3.6.2. CeIY and La/Yb versus Zr/Nb . . . 286
3.7. Peridotite Mixing and Assimilation .... 288
3.8. Radiogenic Isotopes .. . . . . . . . . . . 292
3.8.1. Strontium and Neodymium ..... 292
3.8.2. Lead . . . . . . . . . . . . . . . . . . . . . . . . . . . 296