Te-Cu-rich sudburyite ? the first platinum-group-element mineral from the selenide mineralisation at Skrikerum, Sweden

Abstract

The rare palladium mineral sudburyite occurs as a Te-Cu-rich variety [(Pd,Cu)(Sb,Te,Se,S)] in the classic selenide-rich carbonate vein deposit Skrikerum, located in the southernmost extreme of the Bergslagen ore province in south-central Sweden. This is the first observation of a platinum-group element-(PGE)-bearing mineral in this complex Cu-Ag-Tl selenide mineralisation, and to our knowledge, the first occurrence of sudburyite in a selenide vein-type deposit. Here, we report on its assemblage and mineral chemistry, based on optical, scanning electron microscope and field emission electron microprobe studies. The Te-Cu-rich sudburyite occurs at Skrikerum as euhedral to subhedral, often atoll-like, ca. 5-30 mu m-sized crystals in a selenide fracturefilling hosted by vein calcite. Based on electron microprobe analyses, its empirical formula normalised to ideal stoichiometry is (Pd0.915Cu0.132)(Sigma 1.047)(Sb0.795Te0.152Se0.004S0.002)(Sigma 0.953). The complex host assemblage comprises athabascaite (Cu5Se4), berzelianite (Cu2Se), crookesite [Cu-7(Tl,Ag)Se-4], Cu-bearing clausthalite [(Pb,Cu)Se], eucairite (CuAgSe), Se-Cu-bearing hessite [(Ag,Cu)(2) (Te,Se)], sabatierite (Cu6TlSe4) and umangite (Cu3Se2), as well as unidentified Te-Pd-Ag-Cu-bearing, most likely oxidised compounds. The associated hessite represents the first substantiated observation of a telluride in this deposit. Sudburyite from Skrikerum is Cu-, Te-, Se-and S-bearing, but in contrast to the type locality, it does not contain measurable quantities of e.g. Ni and Bi. The Skrikerum sudburyite contains highly variable amounts of Te, from ca. 2.7 wt% to over 15 wt%. Electron microprobe data show a very good correlation between Sb and Te contents (R = 0.95), which is consistent with Te substituting for Sb, forming part of an extensive but seemingly incomplete solid-solution series towards borovskite and merenskyite in the ternary Pd-Sb-Te system. Based on the general paragenesis of the deposit and the studied assemblage, we observe that the comparatively high temperatures of formation for sudburyite suggested from other studies are not applicable here. We also report previously unpublished mineral chemical data of sudburyite from another Swedish locality, Njuggtr skliden.