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A metamorphic mineral source for tungsten in the turbidite-hosted orogenic gold deposits of the Otago Schist, New Zealand

Cave, BJ, Pitcairn, IK, Craw, D, Large, RR, Thompson, JM and Johnson, SC ORCID: 0000-0001-9015-3809 2017 , 'A metamorphic mineral source for tungsten in the turbidite-hosted orogenic gold deposits of the Otago Schist, New Zealand' , Mineralium Deposita, vol. 52, no. 4 , pp. 515-537 , doi: 10.1007/s00126-016-0677-5.

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Abstract

The orogenic gold deposits of the Otago Schist, New Zealand, are enriched in a variety of trace elements including Au, As, Ag, Hg, W and Sb. We combine laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) traverses and images to show that detrital rutile is the most important host mineral for W in the subgreenschist facies rocks. Furthermore, the prograde metamorphic recrystallisation of detrital rutile to titanite releases significant amounts of W (potentially 0.41 g/tonne of rock). Scheelite development closely follows the progression of this W-liberating reaction. Scheelite micrograins form early within the fabric of the rock evolving to locally and regionally sourced scheelite-bearing veins. Scheelite from syn-metamorphic veins at Fiddlers Flat and Lake Hāwea shows distinct differences in composition compared with scheelite from late-metamorphic veins at the Macraes Mine, the latter of which is enriched in REEs, Y and Sr. We suggest that the scheelite at Macraes became enriched due to the liberation of these elements during alteration of the Ca-silicate minerals epidote and titanite by the ore-forming fluid. These results are supportive of recent models for orogenic gold mineralisation in the Otago Schist, whereby prograde metamorphic recrystallisation of diagenetic or detrital metal-rich mineral phases (pyrite to pyrrhotite: Au, As, Ag, Hg and Sb; rutile to titanite: W) releases significant amounts of metals into the concurrently developing metamorphic fluids that can be subsequently focussed into regional structures and form significant tungsten-bearing orogenic gold deposits.

Item Type: Article
Authors/Creators:Cave, BJ and Pitcairn, IK and Craw, D and Large, RR and Thompson, JM and Johnson, SC
Keywords: tungsten, scheelite, orogenic gold, Otago Schist, rutile, titanite, metal source
Journal or Publication Title: Mineralium Deposita
Publisher: Springer-Verlag
ISSN: 0026-4598
DOI / ID Number: 10.1007/s00126-016-0677-5
Copyright Information:

Copyright 2016 Springer-Verlag Berlin Heidelberg

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