| dc.description.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 andwas 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 describea 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 thatscale. 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 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 | |
