COMPARISON OF LAM-ICP-MS AND MICRO-PIXE RESULTS FOR PALLADIUM AND RHODIUM IN SELECTED SAMPLES OF NORIL'SK AND TALNAKH SULFIDES

Abstract

Comparisons of PGE measurements by laser-ablation microprobe (LAM–ICP–MS) with micro-PIXE analyses on the same sulfide grains were made in order to assess data quality. Pyrrhotite, pentlandite and chalcopyrite from a few samples of disseminated ores of the Noril’sk I intrusion and massive ore from the Talnakh deposit, Russia, were analyzed for Pd and Rh by micro-PIXE and for all PGE by LAM–ICP–MS. Micro-PIXE analyses employed focused proton beams of 8*15 μm for grains in heavy-mineral separates and 15*20 μm for grains in hand samples. These represent analyzed volumes of about 2,350 μm 3 in pyrrhotite and 5,900μm3 for Pd in pentlandite. LAM analyses were performed with a focused, 266-nm laser beam fired at 0.3 mJ/pulse and 10 Hz, resulting in 35–40μm spots and analyzed volumes of about 85,000μm3 . Fused pyrrhotite standards separately doped with Pd, Ru, and Rh were synthesized for LAM–ICP–MS, and calibrated against sintered pyrrhotite PGE standards of Ballhaus & Sylvester (2000). The level of Pd, Ru and Rh varies by ~4 to 9% (1σ) in the fused pyrrhotite standards at a 50- μm sampling scale. The LAM and micro-PIXE results for the natural sulfides are quite comparable, with the large majority of determinations falling within 33% of each other for both Pd and Rh on the same spot. This agreement is considered encouraging for use of the LAM–ICP–MS for trace PGE analysis, considering the large differences in volumes analyzed, and in probable trace-element heterogeneity in the grains and standards. The reasonable concordance of most Pd and Rh results by LAM–ICP–MS and micro-PIXE indicates the potential of the LAM–ICP–MS technique. LAM analyses of the natural sulfides for Pt, Ir and Os were made using the standards of Ballhaus & Sylvester (2000). Platinum is more enriched in pentlandite ( n = 10) than in pyrrhotite (3.2 ± 1.2 versus 0.73 ± 0.60 ppm) from Noril’sk (MP6–97), but Ir (0.94± 0.32 versus 1.38 ± 0.13 ppm) is only slightly higher in pyrrhotite ( n = 10), whereas Os (~0.5 ppm) is comparably enriched in both minerals. Noril’sk chalcopyrite contains trivial amounts of all PGE except Pd (6.6–15 ppm).