Wellnitzannek2017phd.Pdf (14.46Mb)
Total Page：16
File Type：pdf, Size：1020Kb
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 2 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 CO2-bearing fluids were faults, joints, folds and, in the northernmost part of the study area, volcanic 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 CO2-bearing fluid. However, as those minerals are less abundant 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 3 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 4 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. 5 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