Early expulsion of PGMs from magmatic sulphide liquids: A new forensic technique using laser mapping

摘要

1 IntroductionWe all know that platinum-group elements (PGEs) are captured by magmatic sulphide liquids, which then crystallize to make exploration targets for Ni-Cu-PGE mineralisation. So why is it that the sulphide minerals consistently have extremely low contents for some PGEs, most notably Pt? And also, why is it common to find platinum-group minerals (PGMs), most notably Pt-minerals, around the margins of these same sulphides? This work follows on from previous studies using laser ablation ICP-MS (La ICP-MS), which traditionally employ a combination of line and spot analyses on in-situ sulphide minerals. We present results from a new La ICP-MS technique involving 2D laser mapping, which is revealing new information that is helping to answer the above questions. Sulphide-bearing samples from Norils'k and the Merensky Reef were investigated using this technique and the results are startling. In almost every case Pt is almost below detection and there are very few Pt-PGM inclusions locked within the sulphides. The same rocks can contain abundant Pt-PGMs, many of which show a clear spatial relationship to the sulphides. It seems more and more evident that Pt-bearing, and to a lesser extent Pd-bearing, minerals form prior to crystallisation of the sulphide liquids and that these are expelled to their margins. From here the PGMs can be remobilised into the surrounding silicates.