LAURITE AND RUARSITE FROM PODIFORM CHROMITITES AT KRAUBATH AND HOCHGROSSEN, AUSTRIA: NEW INSIGHTS FROM OSMIUM ISOTOPES

Abstract

The chemical and osmium-isotope composition of platinum-group minerals (PGM) [e.g., laurite–erlichmanite (RuS 2–OsS2), ruarsite–osarsite (RuAsS–OsAsS) series and Os–Ir alloy (Os,Ir)] from variably altered podiform chromitites of the Kraubath and Hochgrossen dunite–harzburgite massifs are reported for the first time. These massifs, the largest dismembered mantle relics in the Eastern Alps of Austria, were interpreted as a strongly metamorphosed ophiolite sequence, which forms part of the Speik Complex. Unaltered podiform chromitites from both localities display negatively sloped chondrite-normalized platinum-group element (PGE) patterns. The highly altered podiform chromitite at Kraubath is dominated by less refractory PGE (PPGE: Pd, Pt, Rh) over refractory PGE (IPGE: Os, Ir and Ru). The chemical composition of chromite varies from a Cr# [100*Cr/(Cr + Al)] of 74 to 87 and a Mg# [100*Mg/(Mg + Fe 2+ )] of 44 to 61, values typical of podiform chromitites from the mantle section of an ophiolite. The PGM assemblage in the unaltered podiform ores is dominated by laurite (43% and 75% of all PGM at Kraubath and Hochgrossen, respectively). Sperrylite, PtAs 2, is the most abundant PGM (61%) in the altered chromitite, whereas minerals of laurite–erlichnmanite series are subordinate (4%). At Kraubath, Os-bearing PGM (laurite, erlichmanite, ruarsite and Os–Ir alloy ) occur as (a) single grains and (b) complex polyphase assemblages. At Hochgrossen, laurite and Os–Ir alloy are present as solita ry grains only. In situ osmium-isotope measurements of 16 PGM grains from bedrock (e.g., laurite and ruarsite) by laser-ablation