Platinum-group elements geochemistry and mineralogy in eastern Alpine ultramafic massifs

Lise Meitner Position M 601 Platinum-group elements Geochemistry and Mineralogy Kreshimir N. MALITCH 09.10.2000 The main aim for one year extension is to evaluate the platinum-group element (PGE) potential of placer deposits associated with the ultramaric Kraubath massif and to constrain time on ultramafic protolith formation of the biggest mantle slice in the Eastern Alps by measuring Os-isotopic composition in platinum group minerals (PGM). Predominant emphasis during first part of the project was to obtain precise PGE geochemical and mineralogical data. A new technique of investigating the PGE mineralogy of different types of chromitites (podiform, stratiform, highly altered varieties) enabled to obtain a unique extensive data base comprising 1300 PGM grains which were thouroughly evaluated. As a result, data set of noble-metal mineralizations for the Kraubath massif was considerably increased. 20 PGM (including 3 new unnamed minerals) were added to the PGE mineralogy of the Kraubath massif by obtaining a total of 35 PGM. In particular, 9 Pd-rich and 4 Au-Ag-Cu minerals, being generally rare in ophiolitic environment, are of particular genetic significance. A high diversity in PGM assemblages documented in all chromitite types implies that a more complicated history of chromitite formation is preserved in the Kraubath massif. Main results are currently being summarized in two contributions which will be submitted for "Mineralogy and Petrology" and "Canadian Mineralogist". On the basis of the results obtained so far from the Kraubath massif, we plan to evaluate: 1) the PGE mineralogy of alluvial sediments closely associated with the Kraubath massif and a detailed investigation of the PGE mineralogy of the chromitites from the HochgrOssen massif; 2) Re-Os systematics of platinum-group minerals from chromitites and heavy mineral concentrates of placer deposits (stream sediments). These goals are achieved by a combination of methods: (a) field work including a stream sediment survey; (b) analytical work including PGE geochemistry of heavy concentrates from placer deposits; (c) mineral concentration by hydroseparation technique; (c) analytical work on PGM including reflected light microscopy, SEM and EMPA; (d) isotope analysis (Re-Os system, NTIMS study) The results on PGE mineralogy and Os-isotopes of Eastern Alpine ultramaric massifs with emphasis on implications for PGE behaviour during mantle melting and subsequent PGE redistribution during later overprinting thermal events would allow for the first time to constrain the nature and precise time of formation of PGE- mineralization and its parent ultramafic protolith.