Geochronology of Coronation Hill uranium, platinum, gold, mineralisation, Northern Territory

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Uraninite geochronology In 2009, uraninite from Coronation Hill was ablated using a 12 m laser beam with the beam rastered along lines at 1 ms-1 to minimise downhole fractionation. Due to the lack of availability of a good uraninite standard isotopic fractionation, mass bias and drift was corrected using the 91500 zircons analysed at the same spot size as the zircons. Experiments were also conducted at smaller spot sizes (4 and 6 m) and showed little or no differences in the U-Pb fractionation at sizes <12 m. Pb-Pb mass fractionation and drift was corrected using NIST610 using a number of line sizes also moving at 1 ms-1. No changes on the Pb isotopes fractionation were observed between different line sizes.  The least disturbed uraninite was re-analysed in 2012 using 13 m laser spots bracketed by uraninite from the Kintyre deposit in northern Australia (83735 Ma Cross et al. 2011; 84020 Ma for the combined U-Pb TIMS and Nd-Sm, Maas pers com. 2013) and the Pleisovice zircons (Slama et al. 2008). NIST 610 was analysed with a 60 m spots size to get high count rates for the Pb-Pb mass bias corrections. Data reduction was performed using the same methods as described for zircons but with the Kintyre uraninite as the primary standard. The best results were obtained in December 2012 with an extra low sensitivity tune on the ICPMS to keep U in pulse mode on the detector to avoid accuracy problems associated with the pulse to analogue transition. The age from both sessions was within the uncertainty of the 2009 analytical run and interestingly the Pleisovice zircons also were close to the recommended age when calculated against the uraninite primary standard (although with a large uncertainty due to the low count rates) suggesting that U-Pb down hole fractionation in zircons is similar in both minerals at small spot sizes. The data is presented as U and Pb isotope ratios for individual data points