NDLI: Synmetamorphic carbon mobility and graphite enrichment in metaturbidites as a precursor to orogenic gold mineralisation, Otago Schist, New Zealand

Content Provider	Springer Nature Link
Author	Henne, Anicia Craw, Dave
Copyright Year	2012
Abstract	The Macraes orogenic gold deposit is hosted by a graphitic micaceous schist containing auriferous porphyroblastic sulphides. The host rock resembles zones of unmineralised micaceous graphitic pyritic schists, derived from argillaceous protoliths, that occur locally in background pelitic Otago Schist metasediments. This study was aimed at determining the relationship between these similar rock types, and whether the relationship had implications for ore formation. Argillites in the protolith turbidites of the Otago Schist metamorphic belt contain minor amounts of detrital organic matter (<0.1 wt.%) and diagenetic pyrite (<0.3 wt.% S). The detrital organic carbon was mobilised by metamorphic–hydrothermal fluids and redeposited as graphite in low-grade metaturbidites (pumpellyite–actinolite and greenschist facies). This carbon mobility occurred through >50 million years of evolution of the metamorphic belt, from development of sheared argillite in the Jurassic, to postmetamorphic ductile extension in the Cretaceous. Introduced graphite is structurally controlled and occurs with metamorphic muscovite and chlorite as veins and slicken-sided shears, with some veins having >50% noncarbonate carbon. Graphitic foliation seams in low-grade micaceous schist and metamorphic quartz veins contain equant graphite porphyroblasts up to 2 mm across that are composed of crystallographically homogeneous graphite crystals. Graphite reflectance is anisotropic and ranges from ~1% to ~8% (green light). Texturally similar porphyroblastic pyrite has grown in micaceous schist (up to 10 wt.% S), metamorphic quartz veins and associated muscovite-rich shears. These pyritic schists are weakly enriched in arsenic (up to 60 ppm). The low-grade metamorphic mobility and concentration of graphite in micaceous schists is interpreted to be a precursor process that structurally and geochemically prepared parts of the Otago Schist belt for later (more restricted) gold mineralisation. Economic amounts of gold, and associated arsenic, were subsequently introduced to carbonaceous sulphidic schists in the Macraes gold deposit by a separate metamorphic fluid derived from high-grade metaturbidites. Fluid flow at all stages in these processes occurred at metamorphic rates (mm/year), and fluids were broadly in equilibrium with the rocks through which they were passing.
Starting Page	781
Ending Page	797
Page Count	17
File Format	PDF
ISSN	00264598
Journal	Mineralium Deposita
Volume Number	47
Issue Number	7
e-ISSN	14321866
Language	English
Publisher	Springer-Verlag
Publisher Date	2012-01-19
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Macraes mine Graphite Porphyroblast Pyrite Otago Schist New Zealand Arsenic Gold Mineral Resources Geology Mineralogy
Content Type	Text
Resource Type	Article
Subject	Geophysics Economic Geology Geochemistry and Petrology

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Synmetamorphic carbon mobility and graphite enrichment in metaturbidites as a precursor to orogenic gold mineralisation, Otago Schist, New Zealand

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