Structure and mineralisation at Lot's Wife, East Otago, New Zealand

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King, Alastair

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Abstract:

Lot's Wife was first discovered in 1890 and mined sporadically until 1904, pro- ducing 2002 oz Au. The deposit is an orogenic vein system that is hosted in upper greenschist facies, textural zone four (TZ4), Otago Schist and is lo- cated 11 km southeast of OceanaGold (NZ) Ltd0s currently active Macraes gold mine. The vein system consists of three main, subparallel, mineralised quartz veins, the Bonanza No. 1, Bonanza No. 2 and Zedd lode, which were mined historically in a system of shallow pits and at least one shaft sunk to a depth of 45 m on Bonanza No. 1. The quartz veins are hosted within a series of NW-striking normal faults that cut the relatively at, metamor- phic foliation at 50 to 70. Warping of the schist, up to 4 m, next to the faults is consistent with a normal sense of displacement. Although outcrop is poor arsenic soil geochemistry outlines a continuous strike length of at least 4 km. Hydrothermal quartz in the veins and silicified host schist next to the faults is typically euhedral with prismatic faces and locally infills open cavities. Arsenopyrite and pyrite occur along the margins of the veins, often in association with rutile. SEM-EDS analysis of sulphides has not identified any mineralisation in the fractures of grains, indicating only one mineralising generation of hydrothermal uid. Host schist within the fault zone is cataclas- tically deformed and cut by brittle shears with characteristic brown Fe-oxide staining. Shears contain hydrothermal muscovite, sulphides and ankerite, lo- cally cutting the quartz veins and cataclasites. Sulphides extend up to 20 cm from the veins. SEM-EDS analysis failed to find any micro-particulate gold in the limited samples of sulphides, however some micron-size free gold grains were observed in quartz vein material and in micaceous layers in schist next to the veins. Fluid inclusions within quartz crystals shows little variation in size and indicates the mineralising uid did not undergo boiling. Previous work at Bonanza indicate homogenization temperatures in the range 175C to 270C. Data obtained supports conclusions that mineralisation occurred from an immiscible uid at approximately 1 kbar, at a depth of between 3 and 6 km. Mineralisation at Lot's Wife is correlated with the approximate time that Barewood and Nenthorn mineralised veins formed. This was during late stage extensional exhumation of the high grade core of the Otago Schist belt, 112 - 100 Ma.

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85 pages A4, digital appendix-one spreadsheet

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2016King

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MULTIPOLYGON (((170.454624258042 -45.4655465354816,170.492232871253 -45.4665575501279,170.491638928032 -45.483751567299,170.453112926024 -45.4821840430224,170.454624258042 -45.4655465354816)))

Abstract PDF File

http://download.otagogeology.org.nz/temp/Abstracts/2016King.pdf

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Citation

King, Alastair, “Structure and mineralisation at Lot's Wife, East Otago, New Zealand,” Otago Geology Theses, accessed November 13, 2018, http://theses.otagogeology.org.nz/items/show/631.