Contact aureole formation adjacent to the Dunphy Granite, Westland, and implications for Reefton Goldfield Au mineralisation

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Robinson, Bryce

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The conditions of metamorphism have been determined through a contact aureole within the interbedded argillites and greywackes of the Greenland Group in Larry River and Brown Creek, north Westland, New Zealand. These rocks were metamorphosed at greenschist facies during folding. The emplacement of a granitoid into the Greenland Group rocks, the 378-370 Ma Dunphy Granite, produced a contact aureole that overprints the original regional metamorphic conditions. The highest grade assemblages, from next to the granite, comprises quartz, muscovite, chlorite, oligoclase and biotite. Ti-in-biotite geothermometry suggests peak metamorphism to have been 654 ± 24°C. This represents hornblende hornfels facies. Moving away adjacent to the Dunphy Granite the hornblende hornfels facies grades to albite-epidote facies, which comprises quartz, muscovite, chlorite and albite. Greenland Group rocks at the contact experience metasomatism with the granite. On the Greenland Group side of the contact there is only minor oligoclase, less biotite and an enrichment in apatite, quartz and muscovite. On the granite side there is a lack of oligoclase and K-feldspar, but an enrichment in muscovite and apatite.
The Caledonian Mine in the Larry River valley is located in the north most point of the Reefton Goldfield. Gold is found in recrystallised quartz veins with sulphitic stylolitic veins. There are two mineralising phases identified. The first phase of mineralisation formed hydrothermal quartz veins associated with initial As-Fe-Pb-Cu-Au-S enrichment. Hydrothermal fluid producing gold-bearing quartz veins is assumed to have meteoric origins or have been produced by metamorphic dehydration reactions at depth. The second phase is separated by a brittle fracturing event and involved Pb-Mo-S rich fluids, possibly with magmatic origins. The Caledonian Mine deposit contained significant amounts of gold, but the economic potential is unknown as historic reports suggest a fault truncates mineralisation. The most precise date of mineralisation to date has been inferred, based on field relationships and SEM-EDS analysis, to have occurred after pluton emplacement and contact metamorphism of the host Greenland Group, <370 Ma.

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132 pages A4, 3 maps and sections, SEM-EDS and XRD data in back pocket

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2016Robinson

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MULTIPOLYGON (((171.906889677648 -41.8981119820802,171.985343292508 -41.8988320454063,172.096642805904 -41.94551555725,172.012738375843 -42.1078198899358,171.958609430256 -42.1302174999423,171.866632766476 -42.1179077549911,171.851215815353 -42.0677128784645,171.906889677648 -41.8981119820802)))

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

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Citation

Robinson, Bryce, “Contact aureole formation adjacent to the Dunphy Granite, Westland, and implications for Reefton Goldfield Au mineralisation,” Otago Geology Theses, accessed November 26, 2022, http://theses.otagogeology.org.nz/items/show/640.

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