Abstract

The Ivrea-Verbano Basic Complex consists of a lower part composed of cyclic units made up of cumulate peridotites, pyroxenites, gabbros, and anorthosites, and an upper part consisting of a huge, almost homogeneous body of gabbro-norite grading upward into diorites. The whole basic sequence is cut by more or less discordant pyroxenitic dikes. Septa of the crystalline basement occur at various positions in the basic complex, locally confirming a multistage origin of the intrusion.The platinum-group elements occur associated with massive and disseminated sulfide concentrations in pyroxenitic layers close to metasediments, with discrete aggregates and stockwork sulfide mineralizations in the pyroxenitic dikes, and as dispersed elements in well-defined ultramafic layers (pegmatoid clinopyroxenites).In the first association the only minerals of the platinum group that have so far been identified are Pd tellurides; Pt is probably contained in solid solution in the sulfides. Although Pt occurs in higher concentrations in the second association, no discrete platinum-group minerals have been detected in any of the samples investigated to date, which indicates that all the platinum-group elements are in solid solution in the sulfides. The third association, in contrast, contains no sulfides, and it would seem that the relatively high Pt values in these rocks are contained in the pyroxenes.

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