Late Devonian contact metamorphism and a possible upper age to gold mineralisation in the northernmost portion of the Reefton Goldfield

ABSTRACT

The 378–370 Ma Dunphy Granite has contact metamorphosed Greenland Group metasedimentary rocks in the Awarau/Larry River valley in the northernmost portion of the Reefton Goldfield in Westland. Hornblende hornfels facies Greenland Group occurs up to ∼1300 m of the granite contact, followed by albite-epidote hornfels facies assemblages between ∼1300 and 2000 m from the granite, after which the rocks retain Early Paleozoic greenschist facies assemblages. The historic Caledonian goldmine is located on a fault zone within hornfels facies rocks. There is a complex mineralisation paragenesis in samples obtained from the mine area in which quartz, gold, arsenopyrite, pyrite with minor galena and chalcopyrite formed in quartz veins and brecciated Greenland Group. Fractures in the early-formed minerals and rocks contain quartz, minor galena and molybdenite and oligoclase and biotite. Although the poor exposure and collapsed mine workings prohibit firmly establishing the relationship of mineralisation to the contact aureole, hand specimen and optically observed textures indicate that mineralisation is hosted in a brittle fault zone that most likely cuts across the hornfels facies rocks. Mineralisation is therefore interpreted to post-date formation of the Late Devonian contact aureole at 378–370 Ma.

KEYWORDS:

• Greenland Group
• Reefton Goldfield
• contact metamorphism
• gold mineralisation

Acknowledgements

We thank OceanaGold for somewhere to cut the rocks up and the University of Otago Geology Department for supporting the costs of fieldwork. A MBIE grant administered through GNS Science supported this research. Comments from Marco Brenna, Tony Christie, Patti Durance and an anonymous reviewer improved the work.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

James M. Scott http://orcid.org/0000-0001-5185-6261