Detection of noble metal depletion in layered mafic intrusions: a potential aid to exploration for platinum-group element deposits

In layered mafic intrusions, economically important platinum-group element (PGE) mineralisation can result from collection of PGE and Au from the silicate melt by Ni ( ± Cu) S droplets, which settle to form PGE- and Au-enriched layers. An important consequence is that the PGE and Au are depleted in the remaining silicate fraction. The ability to recognise PGE and Au depletion in rocks crystallised from a depleted melt would be useful in exploration for discriminating between favourable (containing PGE- and Au-depleted silicates) and unfavourable layers (those that had not experienced NiS collection of PGE and Au). Drychlorination was applied to drill core samples of the Fox River Sill (Manitoba, Canada) in order to remove metallic minerals. The remaining silicate fractions were digested by microwave heating in a mixture of HFHClHNO3, evaporated to near dryness, re-digested in aqua regia, and re-constituted in distilled de-ionized water. The solutions were analysed by ICP-MS for PGE and Au. The PGE and Au contents of the silicate fractions of samples collected from mineralised layers were significantly depleted relative to the PGE and Au contents of silicate fractions of samples collected from a non-mineralised layer. Verification of the ICP-MS method for the determination of Pd was carried out by means of DCP-AES. Instrumental neutron activation analysis (INAA) was used to verify the method for Au and Ir.