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

Abstract

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±50Ma (2σ, n=48, MSWD=1.16, initial 186Os/ 188Os=0.119819±0.000006). While this is 59Myr 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 2024Ma±101Ma 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±106Ma and 2042±102Ma, respectively. A Pt-Os isochron derived from all Bushveld units gives an age of 2012±47Ma (2σ, 186Os/ 188Os i=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. © 2011 Elsevier B.V.