Altered Volcanic Rocks.A guide to description and interpretation

288 Pages·44.536 MB·English

by  Cathryn Gifkins, Walter Herrmann, Ross Large.

Erratum
There are several instances of a typographic error in
Figure 4.2 (page 79). The references to 'Y/Zr! in the
caption and 'Y/Zr' in labels on the diagram are
incorrect. They should be 'Zr/Y' in all cases.
Altered Volcanic Rocks
A guide to description and interpretation
Cathryn Gifkins
Walter Herrmann
Ross Large
Published by the Centre for Ore Deposit Research
University of Tasmania, Australia
UTAS
Published by CODES
Centre for Ore Deposit Research,
University of Tasmania,
Private Bag 79,
Hobart, Tasmania, Australia 7001
An ARC Special Research Centre
© Centre for Ore Deposit Research, 2005
National Library of Australia Cataloguing-in-Publication Data
Gifkins, Cathryn.
Altered volcanic rocks : a guide to description and interpretation.
Bibliography.
Includes index.
ISBN 1 86295 219 1.
1. Rocks, Igneous. 2. Hydro thermal alteration. I.
Herrmann, Walter, 1951- . II. Large, Ross R. III.
University of Tasmania. Centre for Ore Deposit Research.
IV. Title.
552.2
another Pongratz Production 2005
Copy editing: Im'press: clear communication
Index: Word Wise and Im'press: clear communication
Printed in Australia by the Printing Authority of Tasmania
Ill
I CONTENTS
PREFACE v.vii
ACKNOWLEDGEMENTS viii
INTRODUCTION ix
1 | ALTERATION IN SUBMARINE VOLCANIC SUCCESSIONS 1
1.1 Submarine volcanic successions 1
Volcanic facies 1
Volcanic facies associations 2
Evidence for submarine environment of emplacement 2
1.2 Alteration in submarine volcanic successions 2
Devitrification 4
Alteration processes 4
Characteristics inherited from volcanic facies 6
1.3 Geology of the Mount Read Volcanics 7
Stratigraphy of the Mount Read Volcanics 9
Submarine facies associations and architecture 10
Post-depositional alteration processes 11
Mineral deposits and prospects 11
1.4 Geology of the Mount Windsor Subprovince 12
Stratigraphy of the Seventy Mile Range Group 12
Submarine facies associations and architecture 13
Post-depositional alteration processes 14
Mineral deposits and prospects 14
2 | DESCRIBING ALTERED VOLCANIC ROCKS 15
2.1 Frequently asked questions 15
2.2 Alteration nomenclature 19
Mineral-based alteration nomenclature 19
Compositional alteration nomenclature 20
Generic alteration nomenclature 20
Descriptive nomenclature — alteration facies 20
2.3 Alteraction facies — the recommended method 22
2.4 Alteration mineral assemblage 23
Tools for mineralogical determination 24
2.5 Alteration intensity 25
Qualitative estimates of alteration intensity 25
Quantitative estimates of alteration intensity 26
An integrated approach to alteration intensity 33
2.6 Alteration data sheets 36
3 | COMMON ALTERATION TEXTURES AND ZONATION PATTERNS 37
3.1 Alteration textures 37
Replacement textures 37
iv | CONTENTS
Infill textures 41
Dissolution textures 41
Static recrystallisation textures 52
Dynamic recrystallisation textures 52
Deformation textures 52
3.2 Pseudotextures 54
Pseudoclastic textures 54
False polymictic texture 63
False matrix-supported texture 63
False coherent textures 63
3.3 Alteration distribution 63
3.4 Alteration zonation patterns 64
Regional diagenetic zones 64
Regional metamorphic zones 64
Regional, deep, semi-conformable altered zones 66
Local contact metamorphic or hydrothermally altered halos 66
Local hydrothermally altered halos around ore deposits 67
Vein and fracture altered halos 67
3.5 Overprinting relationships and timing of alteration 69
Method 70
Overprinting textures 70
4 | GEOCHEMISTRY OF ALTERED ROCKS 73
4.1 Lithogeochemistry 73
Sampling and analytical methods 73
Closure in composition data 78
Chemostratigraphy 79
- Mass transfer techniques 81
Rare-earth-element geochemistry related to alteration 87
4.2 Mineral chemistry 87
Principles 87
Applications 88
4.3 Stable isotopes 92
Theoretical background 92
Isotopic applications in alteration studies 92
5 | SEAFLOOR-AND BURIAL-RELATED ALTERATION 97
5.1 Alteration related to sea-floor processes and burial 97
Physical conditions 98
Definitions 98
5.2 Hydration 98
Palagonite 99
Perlite 100
5.3 Diagenesis (glass to zeolite facies) 102
Diagenetic minerals 102
Diagenetic zones 105
Genesis of diagenetic minerals and zones 108
5.4 Regional metamorphism (zeolite to amphibolite facies) 115
Transition from diagenesis to regional metamorphism 115
Burial metamorphism 115
Burial metamorphic facies 115
Burial metamorphic zones 115
Zeolite facies 116
Genesis 116
5.5 Diagenetic alteration in the Hokuroku Basin 118
Geological setting 118
Alteration facies and zones 119
Genesis of altered zones 120
Data sheets 122
CONTENTS | V
5.6 Diagenetic alteration in the Mount Read Volcanics 128
Geological setting 128
Alteration fades and zonation 128
Genesis of alteration fades 128
Data sheets 133
6 | SYNVOLCANIC INTRUSION-RELATED ALTERATION 139
6.1 The role of intrusions in generating hydrothermal systems 140
Subseafloor regional hydrothermal systems 140
6.2 Regional altered zones assodated with intrusions 141
Recharge zones 141
Discharge zones 141
Deep, semi-conformable altered zones 142
Altered zones as part of a regional hydrothermal system 147
6.3 Altered zones within intrusions 148 '
Deuteric alteration 148
Hydrothermal alteration 148
6.4 Contact altered halos around intrusions 149
Contact altered zones 149
Genesis of contact altered zones 153
6.5 Contact altered zones associated with the Darwin Granite 154
Geological setting 155
Alteration fades and zonation 155
Genesis of the alteration system 156
Data sheets 157
7 | LOCAL HYDROTHERMAL ALTERATION RELATED TO
VHMS DEPOSITS 163
7.1 Common features of VHMS deposits 163
7.2 Hydrothermal alteration halos associated with VHMS deposits 164
Footwall alteration pipes 164
Stratabound altered footwall zones 166
Altered hanging wall zones 167
Chemical reactions and mass changes 168
Alteration box plot trends in altered footwall zones 169
The genesis of footwall alteration pipes 170
Significance of pyrophyllite and kaolinite in VHMS systems 174
Metamorphism of altered zones 174
7.3 The spectrum of volcanic-hosted deposits and associated alteration patterns 174
Hydrothermal alteration related to the spectrum of deposits 176
7.4 Comparisons between Archaean, Palaeozoic and Cainozoic VHMS
alteration systems 178
Australian Palaeozoic VHMS alteration halos 178
Japanese Cainozoic VHMS alteration halos 179
Canadian and Australian Archaean VHMS alteration halos 179
Comparisons : 180
7.5 Hellyer: a massive elongate polymetallic lens 181
Geological setting 182
Alteration fades and zonation 182
Ore genesis 183
Data sheets 184
7.6 Rosebery: a polymetallic sheet-style deposit 194
Geological setting 194
Alteration facies and zonation 195
Genesis of the ore lenses and alteration system 195
Data sheets 196
7.7 Western Tharsis: a hybrid Cu-Au VHMS deposit 202
Geological setting 202
Alteration facies and zonation 202
Vi | CONTENTS
Ore genesis 203
Data sheets 204
7.8 Henty: a volcanogenic gold deposit 212
Geological setting 212
Alteration fades and zonation 212
Ore genesis 213
Data sheets 214
7.9 Thalanga: a polymetallic sheet-style deposit 221
Geological setting 221
Alteration facies and zonation 222
Ore genesis 222
Data sheets 223
7.10 Highway-Reward: a pipe style Cu-Au VHMS deposit 232
Geological setting 232
Alteration facies and zonation 232
Ore genesis 232
Data sheets 233
| FINDING ORE DEPOSITS IN ALTERED VOLCANIC ROCKS : 241
8.1 Principles of discriminating between diagenetic, hydro thermal
and metamorphic alteration facies 241
Diagenetic facies 241
Metamorphic facies 242
Hydrothermal alteration facies 242
8.2 Exploration vectors and proximity indicators 243
Mineral zonation 243
Major element lithogeochemistry 243
Alteration indices 244
Mass change vectors 245
Mineral chemistry vectors 245
Isotopic vectors 246
| REFERENCES 251
| INDEX 271
I vii
PREFACE
Altered volcanic rocks is principally for hands-on geologists, enormous amount of data, case studies and expertise. Some
our fortunate colleagues who practise in mineral exploration of the results of this project have previously been published as
and mining geology, and the students who may in the future a special issue in Economic Geology (Gemmell and Herrmann,
play in those professional fields. eds., A special issue devoted to alteration associated with volcanic-
We began designing and writing this book in mid 2001 hosted massive sulfide deposits, and its exploration significance,
after struggling for several decades to come to terms with a August 2001, v. 96, no. 5).
variety of alteration styles in ancient submarine volcanic We were encouraged by the wide acceptance and success
successions. We realised that although a large number of of the CODES publication by Jocelyn McPhie, Mark
company and research geologists were working on similar Doyle and Rod Allen (1993) Volcanic textures: a guide to the
rocks there was no existing text to help guide us through the interpretation of textures in volcanic rocks, which has been a
complexity of altered volcanic rocks. The so-called volcanic major factor in improving the description and interpretation
rocks we deal with in ancient volcanic successions and around of volcanic facies over the last decade. The advance we have
ore deposits frequently bear little resemblance to their fresh made in Altered volcanic rocks is to integrate observations
counterparts, which are studied in undergraduate igneous and data on volcanic facies and textures with alteration
petrology and volcanology courses. It is typically only with mineralogy and geochemistry at both regional and local scales
long experience that geologists develop the confidence and in order to provide a multidisciplinary method for the study
skills to be comfortable working with altered volcanic rocks, and discrimination of different alteration types: diagenetic,
to interpret the original volcanic facies, unravel complex metamorphic and hydrothermal alteration.
alteration histories and determine their significance in terms We hope that this book will help to equip geologists
of mineral deposit prospectivity, particularly in ancient and working in altered and deformed successions with the skill
deformed successions. and confidence to interpret the original volcanic facies and
The topic and content of the book were inspired by encourage the use of altered rocks as discriminants and
problems that we have faced, and in many cases overcome, vectors in mineral exploration. This book may not provide
while working on industry-related volcanic facies, alteration all the answers, but if it gives readers the courage to tackle the
geochemistry and economic geology research projects, study of altered rocks, embrace the problems and pursue the
particularly in the Mount Read Volcanics. Many of the ideas answers it will have been worthwhile.
presented in this book come from the results of CODES
research projects, which have been run in collaboration
with industry partners and the Australian Research Council
(ARC) over the last 15 years. In particular, AMIRA-ARC Cathryn C. Gifkins
Linkage project P439 (Studies of VHMS-related alteration: Wally Herrmann
geochemical and mineralogical vectors to ore) provided an Ross R. Large
viii |
| ACKNOWLEDGEMENTS
While preparing this book, we were fortunate to have valuable Andrew McNeill very kindly provided a long projection
support, assistance and advice from many people. of the Rosebery ore lenses. Tim Callaghan assisted with core
We extend our sincere thanks to those people whose specimens and whole-rock geochemical data from Mount
discussions and/or reviews of various chapters have helped Julia. Jon Huntington and Melissa Quigley at CSIRO
shape this book. Chapters were peer-reviewed by Stuart Bull, provided HyMap® images of the Mount Lyell field.
David Cooke, Mark Doyle, Kim Denwer, Allan Galley, Bruce We are infinitely grateful for the hard work of the
Gemmell, Anthony Harris, Jocelyn McPhie, Andrew Rae, production team. Karin Orth and Simon Stephens helped with
Mike Solomon and Fernando Tornos. Valuable discussions sample preparation. Mike Blake and Karin Orth assisted with
were also held with Ron Berry, Stuart Bull, Jocelyn McPhie, photography. Rose Pongratz and Izzy von Lichtan prepared
Phil Robinson and Mike Solomon. the bibliography and checked references. June Pongratz
Although samples and photographs used herein are provided expert drafting, design and desktop publishing, and
principally from the authors' collections, we also made use of was incredibly tolerant of the endless revisions. Final editing
hand specimens and thin sections from the School of Earth was by Impress: clear communication and indexing by Word
Sciences rock catalogue at the University of Tasmania, and Wise and Impress: clear communication.
samples and photographs from colleagues. Thank you to those We also appreciate our families, friends and colleagues
people who contributed: Sharon Allen, Stuart Bull, Kate Bull, who have been very understanding of our commitment to
Tim Callaghan, Cari Deyell, Bruce Gemmell, George Hudak, this project over the last three years. Thank you for your
Karin Orth and Jocelyn McPhie. We also thank Izzy von support and patience.
Lichtan, Curator at the School of Earth Sciences, for her help
in finding and returning hundreds of catalogue samples.