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 (<1 km) to the hydrothermal breccias of the Olympic Dam Breccia Complex (ODBC), which host Fe-oxide Cu-Au-(U) mineralisation at Olympic Dam. Microscopic observations and quantitative analyses indicate that a range of feldspar reactions have taken place within the RDG hosting the Olympic Dam deposit. An early phase of igneous plagioclase (∼ An_27–34) is recognised, along with a more abundant, less-calcic plagioclase (∼ An_12–20) 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:	Refereed Article
Keywords:	feldspars, albitisation, iron-oxide copper-gold mineralisation, Olympic dam
Research Division:	Earth Sciences
Research Group:	Geology
Research Field:	Igneous and metamorphic petrology
Objective Division:	Expanding Knowledge
Objective Group:	Expanding knowledge
Objective Field:	Expanding knowledge in the earth sciences
UTAS Author:	Kamenetsky, VS (Professor Vadim Kamenetsky)
ID Code:	116477
Year Published:	2017
Funding Support:	Australian Research Council (LP130100438)
Web of Science® Times Cited:	44
Deposited By:	Earth Sciences
Deposited On:	2017-05-10
Last Modified:	2018-07-30
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