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Feldspar evolution in the Roxby Downs Granite, host to Fe-oxide Cu-Au-(U) mineralisation at Olympic Dam, South Australia
journal contribution
posted on 2023-05-19, 04:36 authored by Kontonikas-Charos, A, Ciobanu, CL, Cook, NJ, Ehrig, K, Krneta, S, Vadim KamenetskyThe 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 (∼ An27–34) 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.
Funding
Australian Research Council
BHP Billiton Olympic Dam Corporation
History
Publication title
Ore Geology ReviewsVolume
80Pagination
838-859ISSN
0169-1368Department/School
School of Natural SciencesPublisher
Elsevier Science BvPlace of publication
Po Box 211, Amsterdam, Netherlands, 1000 AeRights statement
© 2016 ElsevierRepository Status
- Restricted