The 190Pt–186Os decay system applied to dating platinum-group element mineralization of the Bushveld Complex, South Africa

We evaluate the 190Pt–186Os decay system, measured by laser ablation MC-ICPMS, as a useful geochronometric tool for direct dating of platinum-group minerals (PGM) in large mafic/ultra-mafic intrusions by analysing mineralised rocks from the geochronologically well constrained Bushveld Complex, S. Africa. Mixed PGM (laurite, cooperite, laurite–platarsite, sperrylite and Pt–Fe alloy) from the Merensky Reef yield a Pt–Os isochron age of 1995 ± 50 Ma (2σ, n = 48, MSWD = 1.16, initial 186Os/188Os = 0.119819 ± 0.000006). While this is 59 Myr younger than the U–Pb zircon age for the Merensky Reef (Scoates and Friedman, 2008) it is consistent with recent Re–Os ages for the Platreef. Considering the younger age recorded by both Re–Os and Pt–Os systems, we propose that the Pt–Os isochron age records a late, low temperature hydrothermal event affecting PGM and sulphides in the Platreef and Merensky Reef.Pt-rich phases may also yield single grain 190Pt–186Os model ages since initial 186Os/188Os can be well constrained. A Pt–Os model age of 2024 Ma ± 101 Ma was calculated for a Merensky cooperite; a Pt–Fe alloy from the Onverwacht pipe and a Tweefontein Hill sperrylite give Pt–Os model ages of 2113 ± 106 Ma and 2042 ± 102 Ma, respectively. A Pt–Os isochron derived from all Bushveld units gives an age of 2012 ± 47 Ma (2σ, 186Os/188Osi = 0.119818 ± 0.000006) with an MSWD of 1.19, hence PGE mineralisation in different stratigraphic horizons of the complex cannot be distinguished temporally using the Pt–Os chronometer. Nonetheless, the potential for the Pt–Os system in dating events in PGE mineralised systems is evident and the agreement of single grain model ages in high Pt/Os phases with the Pt–Os and Re–Os isochron ages indicates their usefulness in the geochronology of these systems.

► 190Pt–186Os isotope decay system can be measured by laser ablation MC-ICPMS.
► Pt-Os isochron for Merensky PGM is consistent with Re-Os ages.
► Pt-Os and Re-Os ages suggest that a late hydrothermal event may have occurred, affecting sulphides and PGM in the Merensky Reef and Platreef.