Authors / CoAuthors
Huston, D. | Maas, R. | Cross, A. | Hussey, K. | Mernagh, T.P. | Fraser, G. | Champion, D.C.
Abstract
Nolans Bore is a REE-U-P deposit (47 Mt grading 2.6% REO, 186 ppm U3O8 and 11% P2O5) hosted by apatite veins and breccias within the ~1805 Ma Boothby Orthogneiss of the Aileron Province, Northern Territory. Allanite SHRIMP U-Pb analyses indicate a vein crystallisation age of 1525±40 Ma, but mineral system processes necessary to the development of the deposit commenced before 1800 Ma and continue today. Processes leading to the formation of Nolans Bore began with north-dipping subduction along the south margin of the Aileron Province at 1820-1750 Ma, producing a metasomatised, volatile-rich lithospheric mantle wedge. About 200 Ma later, towards the end of the Chewings Orogeny, this reservoir became a source of alkaline low-degree partial melts which passed into the mid- and upper-crust. Among these alkaline products was a phosphate-rich magmatic-hydrothermal fluid which deposited the Nolans Bore apatite veins by local fluid-rock interaction and/or fluid mixing at ~400 degrees C. The deposit then became a radiogenic heat source, owing to its size and high concentration of Th, raising the local ambient temperature to ~300 degrees C, above the closure temperature of some mineral isotopic systems. For example, vein apatite U-Pb ages are in the range ~1240 to ~960 Ma, significantly younger than initial emplacement. The system finally cooled below 300 degrees C (the 40Ar-39Ar closure temperature of biotite) at ~370 Ma, possibly in response to unroofing during the Alice Springs Orogeny. Subsequent to surface exposure, weathering of fluorapatite produced acidic fluids and intense, near-surface kaolinitised zones that form high-grade, supergene-enriched cheralite-rich ores. This groundwater-mediated process continues today. The local heat production of Th- and/or U-rich deposits is an important feature that may be partly responsible for the arrays of post-emplacement isotopic ages which characterise such mineral systems. Other physical and chemical processes continue to be generated by the high abundances of reactive and heat-producing elements at Nolans Bore, with significant effects on the economic, isotopic and geochemical characteristics of the deposit and its host, an observation that may apply to other such deposits.
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nonGeographicDataset
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78984
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- External Publication
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- NT
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2014-01-01T00:00:00
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