Nature and distribution of PGE mineralisation in gabbroic rocks of the Lusatian Block, Saxony, Germany

We have employed quantitative automated mineralogy using a mineral liberation analyser to assess samples of gabbroic dykes of the Lusatian Block. These mafic dykes contain platinum-group elements - locally enriched with Cu and Ni sulphides up to subeconomic concentrations of 0.4 ppm (4PGE+Au). In this study we analysed about 100 polished thin sections and polished blocks with both a mapping method and a search mode for bright phases in BSE images (sparse phase liberation analysis).
The aim of the study was to obtain information regarding the occurrence of platinum-group minerals (PGM) and their relationship to base metal sulphides (BMS). Mineral groups found by sparse phase liberation analysis include several PGE-bearing and non-PGE-bearing tellurides, Pd bismuthides and antimonides, Pt arsenide as well as native gold and native bismuth. Mineral grain sizes of these trace minerals are in general below 10 mu m. The results of the mineral association evaluation show that pyrrhotite is the main host for tellurides, native metals and platinum-group minerals. However, several other minerals show also a high degree of association with the PGM, most notably Ni-Co sulpharsenides, chalcopyrite, hydrothermal feldspar and chlorite. By using quantitative automated mineralogy we can clearly demonstrate that low-alteration, low-BMS gabbroic dyke samples contain no or only small amounts of PGM, whereas intense-alteration, high-BMS gabbroic dyke samples have elevated PGM contents. Furthermore, we show that for PGE concentrations <1 ppm MLA analyses of just one polished thin section per sample show limitations with respect to the representativity of results for calculated element concentration, due to a combination of different limiting factors. Mineral liberation analysis reveals that PGM are much more widespread and abundant in the studied area compared to the results of previous careful light microscopic investigations and single grain electron probe microanalysis that resulted only in very few and isolated PGM grains to be identified.