The Nolans Bore rareearth elementphosphorusuran

Authors: David L Huston Roland Maas Andrew Cross Kelvin J Hussey Terrence P Mernagh Geoff Fraser David C Champion

Publish Date: 2016/01/29

Volume: 51, Issue: 6, Pages: 797-822

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Abstract

Nolans Bore is a rareearth element REEUP fluorapatite vein deposit hosted mostly by the ~1805 Ma Boothby Orthogneiss in the Aileron Province Northern Territory Australia The fluorapatite veins are complex with two stages 1 massive to granular fluorapatite with inclusions of REE silicates phosphates and fluorocarbonates and 2 calciteallanite with accessory REEbearing phosphate and fluorocarbonate minerals that vein and brecciate the earlier stage The veins are locally accompanied by narrow skarnlike garnetdiopsideamphibole wall rock alteration zones SHRIMP ThPb analyses of allanite yielded an age of 1525 ± 18 Ma interpreted as the minimum age of mineralisation The maximum age is provided by a ~1550 Ma SHRIMP UPb age for a pegmatite that predates the fluorapatite veins Other isotopic systems yielded ages from ~1443 to ~345 Ma implying significant postdepositional isotopic disturbance Calculation of initial εNd and 87Sr/86Sr at 1525 Ma and stable isotope data are consistent with an enriched mantle or lower crust source although postdepositional disturbance is likely Processes leading to formation of Nolans Bore began with northdipping subduction along the south margin of the Aileron Province at 1820–1750 Ma producing a metasomatised volatilerich lithospheric mantle wedge About 200 million years later near the end of the Chewings Orogeny this reservoir and/or the lower crust sourced alkaline lowdegree partial melts which passed into the mid and uppercrust Fluids derived from these melts which may have included phosphatic melts eventually deposited the Nolans Bore fluorapatite veins due to fluidrock interaction cooling depressurisation and/or fluid mixing Owing to its size and high concentration of Th 2500 ppm in situ radiogenic heating caused significant recrystallisation and isotopic resetting The system finally cooled below 300 °C at ~370 Ma possibly in response to unroofing during the Alice Springs Orogeny Surface exposure and weathering of fluorapatite produced acidic fluids and intense nearsurface kaolinitised zones that include highgrade supergeneenriched cheraliterich oresThis study is published with permission of the Chief Executive Officer of Geoscience Australia It has benefited from discussions with Lynton Jaques Jo Whelan and Subhash Jaireth Arafura Resources are thanked for allowing access to the deposit and publication of these results Sue Golding at the University of Queensland and Christian Dietz at the University of Tasmania are thanked for facilitating stable isotope measurements This contribution has benefited from reviews by David Lentz Eric Potter Anthony Schofield and John Wilford and is published with permission from the Chief Executive Officer Geoscience Australia

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