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MPM-04 Platinum-group mineralogy

Native PGE-alloys from alluvial deposits in Finnish Lapland

Ragnar Tornroos, University of Helsinki (Finland)
Kari Kojonen, Geological Survey of Finland (Finland)
Mahmud Tarkian, University of Hamburg (Germany)
 

Alluvial nuggets of PGE alloys from the tributaries of Ivalojoki and Lemmenjoki, northern Finland, have been investigated by SEM/EDS and EPMA. Several nuggets are up to several mm in diameter, but also very small ones are present. Mostly, the grains are well rounded due to transport, but some nuggets are euhedral clearly indicating crystal structure.
The Pt-Fe alloys are most common but Pt-Cu, Pt-, Pt-Pd, Pd-Pt-Cu-Au and Os-Ir-Ru alloys occur as well. The alloys occur both as inclusions in the Pt-Fe alloy and as separate nuggets or crystals. The inclusions in the Pt-Fe nuggets show a marked variation in number and size. The inclusions consist of cooperite, sperrylite, laurite, erlichmanite, holligworthite, irarsite, osarsite, ruarsite and cuproiridsite.
Most of the Pt-Fe nuggets show a chemical composition corresponding to isoferroplatinum Pt₃Fe with some Cu and Au. The Os-Ir-Ru alloys occur either as idiomorphic platy crystals 0.5 - 1.0 mm in size or as zoned lath-formed prismatic or platy inclusions in Pt-Fe alloys. Microprobe analyses show a core of iridosmine gradually enriched in Ir and Ru outward ending at the margin in ruthenosmiridium. The Os-Ir-Ru nuggets are dominated most often by Os but some contain most Ir or Ru in solid solution. Examples are from Ivalojoki Os_0.60Ir_0.33Ru_0.06, Os_0.45Ir_0.39Ru_0.16, Os_0.4Ir_0.4Ru_0.2; Moberginoja Os_0.6Ir_0.4, Os_0.39Ir_0.38Ru_0.27, Ru_0.38Ir_0.33Os_0.29, Ir_0.45Os_0.25Ru_0.19Hg_0.10, Ir_0.48Os_0.33Ru_0.26, Ir_0.42Os_0.39Ru_0.19, Ir_0.40Os_0.34Ru_0.26; Miessijoki Os_0.46Ir_0.34Ru_0.19, Os_0.44Ir_0.37Ru_0.19, Ir_0.37Ru_0.35Os_0.28). Cuprian platinum occur as nuggets from 0.2 to 2 mm wide, and as exsolution lamellae together with Pd-Au, Pt-Cu-Au and complex Pt-Fe alloys. The Pt to Cu ratio varies from 1.1, hongshiite, to 8:2, the most common being Pt₇Cu. Pd-Au alloys occur as idiomorphic crystals 0.1-0.5 mm wide together with other Pd and Pt minerals and alloys. Cu is always present in small amounts and may build up together with Pd, Au and Pt a series of alloys of very different compositions. Native Pt_0.82Pd_0.18 and pure Pt grains occur as alteration products of braggite and sperrylite, respectively.
The Pt-Fe, Pt-Cu and OsIrRu alloys are probably magmatic in origin whereas the native Pt, Pt-Pd, Au-Pd-Pt-Cu nuggets are often covered with lateritic oxides and hydroxides. The latter nuggets are thought to have been formed in laterite-type weathering.