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Feldspar evolution in the Roxby Downs Granite, host to Fe-oxide Cu-Au-(U) mineralisation at Olympic Dam, South Australia

Kontonikas-Charos, A, Ciobanu, CL, Cook, NJ, Ehrig, K, Krneta, S and Kamenetsky, VS ORCID: 0000-0002-2734-8790 2017 , 'Feldspar evolution in the Roxby Downs Granite, host to Fe-oxide Cu-Au-(U) mineralisation at Olympic Dam, South Australia' , Ore Geology Reviews, vol. 80 , pp. 838-859 , doi: 10.1016/j.oregeorev.2016.08.019.

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Abstract

The textural relationships and geochemistry of feldspars from least-altered to sericite-hematite altered and mineralised ∼ 1.595 Ga Roxby Downs Granite (RDG) at Olympic Dam, South Australia, were examined. The sample suite is representative of RDG both distal (>5 km) and proximal (27–34) is recognised, along with a more abundant, less-calcic plagioclase (∼ An12–20) both displaying rapakivi and anti-rapakivi textures with alkali feldspar. Alkali feldspars (∼ Or55Ab43An2) record post-magmatic evolution from cryptoperthite to patch perthite. Subsequent patch perthite is overprinted by highly porous, near end-member albite and K-feldspar, while plagioclase undergoes replacement by albite + sericite ± Ba-rich K-feldspar. In sericite-hematite altered and mineralised RDG along the margin of the ODBC, sericite replaces all plagioclase, whereas red-stained, Fe-rich K-feldspar persists. Sulphide-uranium-rare earth element mineralisation is observed in association with hydrothermal feldspars, and increases in abundance with proximity to the orebody. Petrographic observations and whole-rock geochemistry illustrate the transformation of plagioclase and alkali feldspar from igneous to hydrothermal processes, and indicate that hydrothermal albite and K-feldspar formed within the RDG without the need for an external source of alkalis. Feldspar geothermometry indicates a minimum crystallisation temperature of 765 °C at 2.2 kbar for alkali feldspar (pressure estimate obtained using plagioclase-amphibole geobarometry) followed by a range of lower temperature transformations. Late-stage magma mixing/contamination is postulated from supportive temperature and pressure estimates along with feldspar and mafic mineral relationships.

Item Type: Article
Authors/Creators:Kontonikas-Charos, A and Ciobanu, CL and Cook, NJ and Ehrig, K and Krneta, S and Kamenetsky, VS
Keywords: feldspars, albitisation, iron-oxide copper-gold mineralisation, Olympic dam
Journal or Publication Title: Ore Geology Reviews
Publisher: Elsevier Science Bv
ISSN: 0169-1368
DOI / ID Number: 10.1016/j.oregeorev.2016.08.019
Copyright Information:

© 2016 Elsevier

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