In-situ Rb-Sr dating of mica: method refinement and application to mineral resources

In-situ Rb-Sr dating of mica by laser ablation ICP-MS/MS has recently emerged as a new tool to date a range of geological processes including mineralisation events. The majority of age results presented to date are based on Rb/Sr calibration using the widely available MicaMG pressed-powder pellet. However, several studies have reported low accuracy associated with mica Rb-Sr ages using this method or large variations in calculated Rb/Sr for the same mica attributed to the different ablation properties of MicaMG and natural mica. In this work, we first report the results of a systematic comparison between isotope-dilution and in-situ Rb/Sr ages of micas in diamondiferous kimberlites from South Africa and Sierra Leone. We confirm that employment of MicaMG as calibration material may provide inaccurate results and present a new approach to obtain accurate ages, which relies on Rb/Sr calibration using the synthetic glass NIST610 SRM and an in-house mica standard. This updated procedure is then employed to constrain the ages of other diamond deposits and address the formation of Au mineralisation in the Eastern Goldfields (Australia).