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

Author:

Wellnitz, Anne Katrin (Katrin)

Year:

Project type:

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 (

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257 pages A4

Department:

OU geology Identifier:

2017Wellnitz

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OURArchive access level:

Open Access

Location (WKT, WGS84):

MULTIPOLYGON (((169.007357194797 -44.4801048617569,168.852891876841 -44.7153959537916,168.644939480572 -44.8819889426719,168.53059861996 -44.8789917767537,168.652743501904 -44.4707082049925,169.007357194797 -44.4801048617569)))

Abstract PDF File

http://download.otagogeology.org.nz/temp/Abstracts/2017Wellnitz.pdf

Collection

Citation

Wellnitz, Anne Katrin (Katrin), “Carbonate alteration associated with lamprophyres and orogenic gold, Southern Alps, New Zealand,” Otago Geology Theses, accessed September 26, 2017, http://theses.otagogeology.org.nz/items/show/658.