Carbonate alteration associated with lamprophyres and orogenic gold, Southern Alps, New Zealand

257 Pages·2017·14.46 MB·English

by  Katrin Wellnitz

Carbonate alteration associated with lamprophyres
and orogenic gold, Southern Alps, New Zealand

Anne Katrin Wellnitz

A thesis submitted for the degree of
Doctor of Philosophy
at the University of Otago, Dunedin,
New Zealand

April 2017
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Abstract
In this thesis the chemistry and textures of void-filling hydrothermal carbonate and associated altered
metamorphic and lamprophyric rock in western Otago, New Zealand, were studied. With this
information a detailed model of the mobility behaviour of various elements is established. Conduits
for CO -bearing fluids were faults, joints, folds and, in the northernmost part of the study area, volcanic
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diatremes. The southernmost study area lies in the Shotover Valley, where many of the larger fault
zones are auriferous. Throughout the studied area, many larger faults, including the gold-bearing ones,
strike W to NW. In addition to these faults, there are also structures (i.e. veins and fold hinges) that
trend northwards. The formation of most of the structures happened in the Oligocene -Miocene and
was closely connected to the movements along the newly forming Alpine Fault and reverse
movements along the Moonlight Fault.
Adjacent to the fluid conduits, the wall rock has been extensively altered. This alteration is
especially obvious in greenschist and the altered rock has a pale creamy colour due to the replacement
of various metamorphic phases (epidote, chlorite, actinolite) mostly by Fe-bearing carbonate and
phyllosilicates. In quartzo-feldspathic greyschists the same metamorphic minerals as in greenschist are
unstable in response to the incoming CO -bearing fluid. However, as those minerals are less abundant
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in greyschist, the alteration is less obvious.
Textural and chemical data of the individual metamorphic and hydrothermal minerals forming
during replacement were obtained using SEM, microprobe and LA-ICP-MS. The hydrothermal minerals
replacing metamorphic minerals describe a diverse array of mineral textures, which give insight into
relative solubility of the different mineral phases. The replacement reactions also attest to mobility
and immobility of the different major and trace elements. For example, in the breakdown of epidote
in rocks which contain metamorphic muscovite, Al is mobilised potentially in F-OH complexes and
transported away from the original epidote site, whereafter carbonate forms. In cases where there is
no muscovite in the rock, epidote is replaced by muscovite, hence resulting in local loss of Ca. At the
same time, the REE of this epidote are also mobilised on microscopic scale (µm to mm) as the growing
muscovite cannot accommodate the REE in its crystal structure. These REE are then incorporated in
the ankerite replacing the chlorite. On a macroscopic scale (cm to m), Sr, Ba, Rb, K and Cs show the
largest mobility during the hydrothermal alteration (sometimes up to 20 times enrichment in the
altered rock compared to the unaltered rock) and are often brought into the rock by the hydrothermal
fluid. The REE and Al, on the other hand, do not show any signs of mobility at that scale. Overall, of all
elements in alteration-sensitive metamorphic minerals, only titanium is shown to be immobile
throughout, also on µm scale.
In addition to carbonate forming in the hydrothermal alteration halo around fluid conduits,
carbonate is also a common void-filling mineral, such as fractures and vesicles. The chemical
composition of these carbonates shows that the different elements are controlled by various factors.
Contents of Ca, Sr, Mg, Fe, Mn and according ratios show that these elements can travel metres to
tens of metres in the fluid before they are precipitated in carbonate. REE contents and patterns in the
carbonate are the product of the interplay between fluid- and rock-dominated processes; in cases were
only little rock needs to be leached to form the carbonate, the REE patterns are very similar to the wall
rock. In cases where relatively large rock volumes need to be leached to provide the main components
of the void-filling carbonate, the REE content of the carbonate is dominated by fluid-controlled
processes and the REE patterns reflect the relative solubility of the different REE in the fluid.
Radiogenic isotopic compositions (Nd, Sr) of void filling carbonates and wall rock show that Nd
and Sr in the carbonates travel different distances in the fluid conduit; Nd isotopic ratios show that the
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bulk of the LREE are transported for short distances in the fluid passing through the void (cm to dm),
whereas Sr isotopic ratios confirm that Sr can be for transported many meters by the aqueous fluid.
Stable isotope data (C, O) in conjunction with assessing the regional geological and tectonic settings
permitted to reconstruct the history and sources of the fluids in the studied areas; after taking
temperature effects on isotope fractionation and relative sample locations into account, it is concluded
that two main fluid types were present in the studied area. One of these is a mixture of meteoric and
magmatic components, while the other fluid interacted extensively with the metamorphic rock in
Western Otago, but was most likely originally meteoric-derived water. Only in the volcanic diatremes
is there indication that these two fluid types mix. Outside the diatreme, the isotopic composition of
the carbonate give evidence that only the second mentioned fluid type was present, including in the
auriferous structures and there is no indication that the gold-bearing and magmatic system had any
connection to each other

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Acknowledgments

A huge Thank You to:

 Morgan Farrant, for being patient and standing my continuous complaining about my PhD. For
having critical questions and for being able to look at things differently. - You have been
invaluable for the progress of this thesis and for me staying sane-ish.
 James Scott, for helping make this thesis consistent and understandable. For helpful comments,
questions and suggestions. For showing me how to do field work New Zealand-style.
 Mike Palin, for great discussions about chemistry and uncountable hours on the LA-ICP-MS
analysing minerals. For making the most complex Excel spread sheets in the department.
 Candace Martin, for spending hours and hours with and for me in the clean lab. Thanks for sharing
your knowledge with me.
 my examiners -Dave Craw, Travis Horton and Sandra Piazolo- whose critical remarks have helped
immensely in improving this thesis, both scientifically and linguistically.
 Daphne Lee, for being a great convenor.
 Scarlett, Sheepy, Goaty, Monty, Carrot and Kumera, for being the happy animals they are.
 my father, for looking after the dogs and holding the fort.
 Lauren Farmer, for telling me about tungsten and REE and being an entertaining and tidy
flatmate.
 Nichole Moerhuis, for all the fun times at Uni and the great discussions about geochemistry.
 The Governor's Cafe in Dunedin, for all the coffee. Much of this thesis was written on their
premises - thanks for not kicking me out.
 my school, for teaching me how to use Microsoft Excel.
 Brent Pooley, for making countless thin sections for me and for always being extremely helpful.
 Damian Walls, Luke Easterbrook and Dee Roben, for all the support over the years.
 Gemma, Jordan and Madison, for carrying my rocks out of the mountains.
 Robert van Hale and Diane Clark of the Isotrace Lab at Otago University, for analysing all those
carbonate samples.
 Greg Holk, for analysing carbonate and quartz for me.
 Dominik Hezel and Frank Wombacher at Bonn University for letting me use their clean lab and
instruments.

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Table of contents
CHAPTER 1- AN INTRODUCTION ........................................................................................................................... 18
1.1) Hydrothermal systems, gold, and alkaline magmatic rocks ........................................................................... 19
1.2) Regional Geology ........................................................................................................................................... 21
1.2.1- The Otago Schist ..................................................................................................................................... 22
1.2.2- Textural zones ........................................................................................................................................ 22
1.3) Geological history of western Otago ............................................................................................................. 23
1.3.1- The 1st Episode: Metamorphism of the schist and ductile deformation (Rangitata Orogeny) ............... 23
1.3.2-The 2nd Episode: Exhumation of the Otago Schist and marine transgression ......................................... 24
1.3.3- The 3rd Episode: Deformation during the early Kaikoura Orogeny ........................................................ 24
1.3.3.1: The Alpine Fault ............................................................................................................................... 24
1.3.3.2: The Moonlight Fault ........................................................................................................................ 25
1.4) The Alpine Dike Swarm .................................................................................................................................. 26
CHAPTER 2-THE STUDY AREAS AND THEIR GEOLOGICAL FEATURES .................................................................... 28
2.1) Location and overall structure of the study areas ......................................................................................... 28
2.1.1- Study areas ............................................................................................................................................. 28
2.1.1.1: Study area 1-Bullendale and Copper Creek near Mt Aurum in the Shotover Valley ....................... 28
2.1.1.2: Study area 2-Treble Cone and Black Peak area ............................................................................... 28
2.1.1.3: Study area 3-Red Rock ..................................................................................................................... 28
2.1.1.4: Study area 4-Mt Alta and the eastern flank of Niger Peak .............................................................. 29
2.1.2-Methods .................................................................................................................................................. 31
2.1.2.1: Naming of samples .......................................................................................................................... 31
2.1.2.2: Scanning Electron Microscope ........................................................................................................ 32
2.1.2.3: Microprobe ...................................................................................................................................... 33
2.1.2.4: Cathodoluminescence ..................................................................................................................... 33
2.1.2.5: Chemical analyses of whole rock..................................................................................................... 34
2.2) Mineralogy and textures of metamorphic rocks in the Otago Schist ............................................................ 35
2.2.1-Greyschist ................................................................................................................................................ 35
2.2.2-Greenschist .............................................................................................................................................. 36
2.2.3-Chert, marble and ultramafics. ................................................................................................................ 38
2.3) Structures and features without affiliation to magmatic rocks ..................................................................... 39
2.3.1- Cenozoic structures in the footwall of the Moonlight Fault near Mt Aurum ......................................... 39
2.3.1.1: Folds ................................................................................................................................................ 39
2.3.1.2: Veins associated with kink folds ...................................................................................................... 40
2.3.1.3: NW-W- trending faults .................................................................................................................... 44
2.3.1.4: Gold mineralised structures ............................................................................................................ 44
2.3.1.4-1: Bullendale Reef ........................................................................................................................ 44
2.3.1.4-2: Copper Creek Fault .................................................................................................................. 46
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2.3.1.5: Joints and associated veins and faults............................................................................................. 49
2.3.2- Cenozoic structures and features around Treble Cone and Black Peak ................................................. 51
2.3.3- Red Rock-Cenozoic structures west of the Moonlight Fault .................................................................. 52
2.3.4- Discussion of the non-magmatic structures ........................................................................................... 53
2.3.4.1: Silicified breccias ............................................................................................................................. 54
2.3.4.2: Joints ............................................................................................................................................... 55
2.3.4.3: Tectonic model ................................................................................................................................ 56
2.4) Geological features associated with diatremes of the Alpine Dike Swarm ................................................... 59
2.4.1- Niger Diatreme ....................................................................................................................................... 60
2.4.1.1: Lamprophyre dikes ......................................................................................................................... 60
2.4.1.2: The breccia of Niger Diatreme ........................................................................................................ 61
2.4.1.2-1: The components ...................................................................................................................... 61
2.4.1.2-2: Breccia matrix .......................................................................................................................... 64
2.4.2- Mount Alta Diatreme .............................................................................................................................. 65
2.4.2.1: Lamprophyre dikes in the diatreme ................................................................................................ 65
2.4.2.2: Diatreme Breccia ............................................................................................................................. 66
2.4.2.2-1: The components ...................................................................................................................... 66
2.4.2.2-2: Breccia texture (Components + Matrix) .................................................................................. 67
2.4.2.3: Quartz Reef ..................................................................................................................................... 69
2.4.2.4: Veins and other void fillings ............................................................................................................ 70
2.4.2.4-1: Veins and vesicle fillings in lamprophyre ................................................................................. 70
2.4.2.4-2: Veins in the clastic diatreme breccias ...................................................................................... 72
2.4.2.4-3: Veins in greyschist blocks ........................................................................................................ 72
2.4.3-Summary and interpretation of the features associated with the diatremes ......................................... 72
2.4.3.1: Structure of the diatreme breccias ................................................................................................. 72
2.4.3.2: Hydrothermal alteration ................................................................................................................. 73
2.4.3.3: Silicified faults and their structural relationship to the diatremes ................................................. 73
2.4.3.4: Structure and evolution of the diatremes ....................................................................................... 73
2.6) Summary ........................................................................................................................................................ 75
CHAPTER 3: REACTIONS AND TEXTURES ASSOCIATED WITH HYDROTHERMAL ALTERATION OF OTAGO SCHIST 76
3.1) Methods and samples .................................................................................................................................... 77
3.2) Alteration of metamorphic rock -Textures and mineral reactions ................................................................ 78
3.2.1- Hydrothermal alteration in greyschist .................................................................................................... 79
3.2.1.1: Alteration of greyschist associated with kink-folds - the Murdock Creek Transect. ....................... 79
3.2.1.2: Alteration of greyschist associated with silicified fault breccias - the Mt Alta Transect. ................ 81
3.2.2- Alteration of greenschist ........................................................................................................................ 84
3.2.2.1: Alteration of greenschist poor in metamorphic muscovite - the Bullendale Transect ................... 84
3.2.2.2: Alteration of greenschist rich in metamorphic muscovite .............................................................. 90
3.2.2.2-1: The Treble Cone Transect ........................................................................................................ 90
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3.2.2.2-1: TheBlack Peak Transect ........................................................................................................... 95
3.2.2.3: Examples of alteration in greenschist at Red Rock ......................................................................... 97
3.3) Discussion .................................................................................................................................................... 100
3.3.1- Metamorphic minerals and their replacement mineralogy ................................................................. 101
3.3.1.1: Metamorphic muscovite ............................................................................................................... 102
3.3.1.2: Biotite ............................................................................................................................................ 103
3.3.1.3: Actinolite ...................................................................................................................................... 104
3.3.1.4: Chlorite .......................................................................................................................................... 105
3.3.1.5: Epidote .......................................................................................................................................... 106
3.3.1.6: Calcite in leucocratic layers ........................................................................................................... 108
3.3.1.7: Titanite .......................................................................................................................................... 109
3.3.1.8: Sulphides and sulphates ................................................................................................................ 109
3.3.2- Relative tolerance of minerals to CO in the fluids ............................................................................... 110
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3.3.3- Mineral replacement mechanisms ....................................................................................................... 111
3.4) Conclusions .................................................................................................................................................. 111
CHAPTER 4: CHEMICAL CHARACTERISTICS OF GOLD-BARREN ALTERATION HALOS AND HOW THEY COMPARE TO
GOLD-ASSOCIATED ONES .................................................................................................................................... 113
4.1) Methods ....................................................................................................................................................... 114
4.1.1-Whole rock chemistry ............................................................................................................................ 114
4.1.2-Mineral trace element composition of minerals ................................................................................... 114
4.2) Whole rock chemistry .................................................................................................................................. 116
4.2.1- Chemical composition of unaltered metamorphic rock ....................................................................... 116
4.2.2- Volume changes during hydrothermal alteration ................................................................................ 118
4.2.2.2: Volume and mass changes ............................................................................................................ 118
4.2.2.3: Immobile elements and the mass correction factor ..................................................................... 120
4.2.2.4: Values used for comparison .......................................................................................................... 122
4.2.2.5: Outliers .......................................................................................................................................... 123
4.2.3-Changes of rock chemistry across the alteration halo .......................................................................... 124
4.2.3.1: Major elements ............................................................................................................................. 124
4.2.3.2: Rare Earth elements ...................................................................................................................... 126
4.2.3.3: Large ion lithophile elements ........................................................................................................ 126
4.2.3.4: Chalcophile elements and other metals ........................................................................................ 128
4.2.3.4: Volatiles ......................................................................................................................................... 130
4.2.4-Discussion .............................................................................................................................................. 131
4.3) Composition of metamorphic and hydrothermal minerals ......................................................................... 135
4.3.1 - Albite ................................................................................................................................................... 135
4.3.2 - Metamorphic micas- Muscovite and biotite ....................................................................................... 136
4.3.3 - Actinolite ............................................................................................................................................. 138
4.3.4 - Metamorphic chlorite .......................................................................................................................... 139
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4.3.4.1: Ankerite (partly) replacing chlorite ............................................................................................... 140
4.3.5 - Epidote ................................................................................................................................................. 141
4.3.5.1: Hydrothermal muscovite replacing epidote in the Bullendale greenschist ................................. 141
4.3.6 - Metamorphic calcite in leucocratic layers ........................................................................................... 142
4.3.6.1: Ankerite replacing metamorphic calcite in leucocratic layers and other hydrothermal ankerite in
TP7A ........................................................................................................................................................... 144
4.3.7- Poikiloblastic calcite spots and anti-axial veins .................................................................................... 145
4.3.7.1: Ankerite replacing the calcite in the poikiloblastic spots .............................................................. 145
4.3.8- Titanite and its replacement products ................................................................................................. 148
4.3.9 -Discussion ............................................................................................................................................. 148
4.3.9.1: Metamorphic minerals .................................................................................................................. 149
4.3.9.2: Hydrothermal minerals replacing metamorphic minerals ............................................................ 151
4.3.9.2-1: Epidote, chlorite, actinolite and titanite replacement in BUL3C ........................................... 151
4.3.9.2-2: Epidote and chlorite replacement in the Treble Cone Transect ............................................ 153
4.3.9.2-3: Calcite replacement by ankerite ............................................................................................ 154
4.3.9.3: On the Mg:Fe ratio of metamorphic minerals, the replacing ankerite and sulphides ................. 155
4.4) Synthesis of the textural and chemical changes in the studied alteration halos ......................................... 156
4.4.1- Mineralogical and chemical differences between the different Transects .......................................... 156
4.4.2-Chemistry of the fluid ............................................................................................................................ 157
4.4.3 - Comparison to mesothermal gold deposits in the Shotover and in other parts of the world ............ 158
4.5) Conclusions .................................................................................................................................................. 160
CHAPTER 5) MAJOR AND TRACE ELEMENT CHEMISTRY OF HYDROTHERMAL AND MAGMATIC CARBONATE .. 162
5.1) Samples and methods .................................................................................................................................. 162
5.2) Overview of texture and chemistry of carbonate ........................................................................................ 162
5.2.1- Veins and faults associated to kink folds .............................................................................................. 163
5.2.2 - Carbonate in NW dipping fault at Murdoch Creek .............................................................................. 165
5.2.3 - Veins in Greenschist at Bullendale ...................................................................................................... 165
5.2.4 - Veins in Greenschist at Red Rock......................................................................................................... 166
5.2.5-Carbonate at Mt Alta Diatreme ............................................................................................................. 169
5.2.5.1: Veins in several metres big greyschist block in the diatreme breccia .......................................... 169
5.2.5.2: Veins and vesicle fillings in little-altered lamprophyre ................................................................. 170
5.2.5.3: Sparry veins in highly altered lamprophyre and carbonate in their wall rock .............................. 171
5.2.5.4: Clastic fine-grained breccia at Mt Alta .......................................................................................... 174
5.2.6-Silicified fault breccias ........................................................................................................................... 175
5.2.7- Ocelli in lamprophyre ........................................................................................................................... 177
5.2.8-Ankeritic marble component in Niger Diatreme ................................................................................... 178
5.3) Comparison between the analysed hydrothermal carbonates ................................................................... 178
5.3.1 - Major components-Ca, Mg. Fe, Mn ..................................................................................................... 179
5.3.2-Trace elements-Sr, Ba, Pb, Th, U ........................................................................................................... 180
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5.3.3- REE content and patterns ..................................................................................................................... 182
5.4) Discussion .................................................................................................................................................... 185
5.4.2- Abundance of divalent ions (Ca, Mg, Fe, Mn, Sr) ................................................................................. 186
5.4.2.1: Whole rock chemistry and element availability ............................................................................ 186
5.4.2.2: Lithology- associated controls on carbonate chemistry ............................................................... 188
5.4.2.3: Setting-specific carbonate chemistry ............................................................................................ 189
5.4.2.3-1: Carbonate in veins at Mt Alta. ............................................................................................... 190
5.4.2.3-2: Carbonate in and around silicified fault breccias .................................................................. 190
5.4.2.3-3: Carbonate veins at Red Rock ................................................................................................. 191
5.4.3- Pb, Th and U in the hydrothermal carbonate ....................................................................................... 192
5.4.4- Rare Earth Elements in carbonate ........................................................................................................ 192
5.4.4.1: REE availability............................................................................................................................... 193
5.4.4.2: Processes controlling the carbonate's REE content ..................................................................... 195
5.4.4.3: Summary of the REE chemistry of hydrothermal carbonate ........................................................ 198
5.4.4.4: REE in magmatic ocelli ................................................................................................................... 199
5.4.5- Anomalies in REE patterns in carbonate .............................................................................................. 200
5.5) Conclusions .................................................................................................................................................. 201
CHAPTER 6) ISOTOPE GEOCHEMISTRY OF VEINS AND ROCKS ............................................................................ 203
6.1) Methods ....................................................................................................................................................... 203
6.1.1-Radiogenic isotopes ............................................................................................................................... 204
6.1.2- Stable isotopes ..................................................................................................................................... 205
6.2) Radiogenic isotopes of rocks and associated carbonate ............................................................................. 206
6.2.1- Age correction ...................................................................................................................................... 206
6.2.2- Isotope ratios of unaltered rock and metamorphic calcite .................................................................. 207
6.2.3- Variation of 87Sr/86Sr across alteration halos ........................................................................................ 208
6.2.4- Discussion ............................................................................................................................................. 208
6.3) Radiogenic isotope ratios of hydrothermal, metamorphic and magmatic carbonate ................................. 210
6.3.1- Strontium .............................................................................................................................................. 210
6.3.2- Neodymium .......................................................................................................................................... 212
6.3.3- Discussion ............................................................................................................................................. 213
6.4) Stable Isotopes - carbon and oxygen ........................................................................................................... 215
6.4.2- Isotope ratio variations across alteration halos ................................................................................... 215
6.4.3- Stable and radiogenic isotopic ratios of void-filling carbonate ............................................................ 218
6.4.4- Discussion ............................................................................................................................................. 218
6.5) Isotopic composition of the fluid ................................................................................................................. 219
6.5.1- Physical conditions of crystallisation - temperature ............................................................................ 219
6.5.1.1: Temperature of unaltered wall rock ............................................................................................. 219
6.5.1.2: Temperature of vein precipitation ................................................................................................ 220
6.5.2- Isotopic composition of the fluid precipitating the carbonate ............................................................. 220
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