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-OsS 2), ruarsite-osarsite (RuAsS-OsAsS) series and Os-Ir alloy (Os,Ir)] from variably altered podiform chromitites of the Kraubath and Hochgrössen 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 Hochgrössen, 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 Hochgrössen, laurite and Os-Ir alloy are present as solitary grains only. In situ osmium-isotope measurements of 16 PGM grains from bedrock (e.g., laurite and ruarsite) by laser-ablation multiple-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) revealed low 187 Os/ 188 Os and ␥Os t=0 values, indicative of a subchondritic source of the PGE in the mantle. Combined with less radiogenic Os isotopic values measured by negative thermal ionization mass spectrometry (N-TIMS), 187 Os/ 188 Os ranges from 0.11580 to 0.12437, and ␥Os t=0 values, §

Deposits associated with ultramafic–mafic complexes in Mexico: the Loma Baya case

“…The 187 Os/ 188 Os compositions of the PGM from the Loma Baya chromitites, determined in this paper through in-situ LA-MC-ICPMS analyses, are within the range of those reported for "primary" and "secondary" PGM in other podiform chromitites hosted in the mantle sequence of ophiolite complexes (Malitch et al, 2003;Malitch, 2004 Rollinson (2008). (e) Hicky and Frey (1982), and (f) Wilson (1989).…”

Contrasting Platinum-Group Mineral Assemblages From Two Different Podiform Chromitite Localities in the Pindos Ophiolite Complex, Greece

“…Ophiolitic chromitite is known to be enriched in the IPGE, unlike its surrounding host lithologies (Talkington et al 1984, Stockman & Hlava 1984, Augé 1986, Constantinides et al 1980, Ahmed & Arai 2003, Malitch et al 2003, Garuti et al 1999,Uysal et al 2005, Zaccarini et al 2005. In addition to this enrichment in IPGE, significant concentrations of Pt and Pd and Pt-and Pd-bearing PGM have been discovered associated with some ophiolitic chromitite.…”

Closed-system behaviour of the Re–Os isotope system recorded in primary and secondary platinum-group mineral assemblages: Evidence from a mantle chromitite at Harold's Grave (Shetland Ophiolite Сomplex, Scotland)

“…Our current study is directed towards identifying the Os-isotope composition of both primary and 88 secondary laurite-alloy assemblages as revealed in individual composite grains enclosed within or interstitially to 89 chromite, respectively, since these are rarely reported from ophiolite deposits (Malitch et al, 2003a;2014;2016 ; 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4 primary and secondary PGM assemblages, and 2) constrain the long-term Re-Os evolution of the Earth's upper 101 Prichard, 1985). At least two stacked nappes of ophiolitic lithologies are recognized in the Shetland Ophiolite…”