We report highly unusual platinum-group mineral (PGM) assemblages from geologically distinct chromitites (banded and podiform) of the Kraubath massif, the largest dismembered mantle relict in the Eastern Alps. The banded chromitite has a pronounced enrichment of Pt and Pd relative to the more refractory platinum-group elements (PGEs) of the IPGE group (Os, Ir, Ru), similar to crustal sections of ophiolites. On the contrary, the podiform chromitite displays a negatively sloping chondrite-normalised PGE pattern typical of ophiolitic podiform chromitite. The chemical composition of chromite varies from Cr# 73-77 in the banded type to 81-86 in the podiform chromitite. Thirteen different PGMs and one gold-rich mineral are first observed in the banded chromitite. The dominant PGM is sperrylite (53% of all PGMs), which occurs in polyphase assemblages with an unnamed Pt-base metal (BM) alloy and Pd-rich minerals such as stibiopalladinite, mayakite, mertieite II, unnamed Pd-Rh-As and Pd(Pt)-(As,Sb) minerals. This banded type also contains PGE sulphides (about 7%) represented by a wide compositional range of the laurite-erlichmanite series and irarsite (8%). Os-Ir alloy, geversite, an unnamed Pt-PdBi-Cu phase and tetrauricupride are present in minor amounts. By contrast, the podiform chromitite, which yielded 21 different PGMs, is dominated by laurite (43% of all PGMs) which occurs in complex polyphase assemblages with PGE alloys (Ir-Os, Os-Ir, Pt-Fe), PGE sulphides (kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed (Fe,Cu)(Ir,Rh) 2 S 4 , braggite, unnamed BM-Ir and BM-Rh sulphides) and Pd telluride (keithconnite). A variety of PGE sulpharsenides (33%) including irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species have been identified, whereas sperrylite and stibiopalladinite are subordinate (2%). The occurrence of such a wide variety of PGMs from only two, 2.5-kg chromitite samples is highly unusual for an ophiolitic environment. Our novel sample treatment allowed to identify primary PGM assemblages containing all six PGEs in both laurite-dominated podiform chromitite as well as in uncommon sperrylite-dominated banded chromitite. We suggest that the geologically, geochemically and mineralogically distinct banded chromitite from Kraubath characterises the transition zone of an ophiolite, closely above the mantle section hosting podiform chromitite, rather than being representative of the crustal cumulate pile.

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, §

Tarkianite, a new mineral species of ideal composition (Cu,Fe)(Re,Mo) 4 S 8 , was recently discovered in sulfide concentrate from the Hitura Ni-Cu-PGE mine, Nivala, western central Finland. It is associated with pyrrhotite, pentlandite, valleriite, chalcopyrite, cubanite, mackinawite, chromite, and the PGE minerals sperrylite, michenerite, irarsite, froodite and hollingworthite. It is black, opaque with a metallic luster, a black streak, and is brittle with an irregular fracture. VHN 15 is in the range 537-584, which corresponds to a Mohs hardness of 5½ to 6. Under reflected plane-polarized light, the mineral is light brown-gray and isotropic. The CIE color values (illuminant C) are: x 0.314 (air), 0.315 (oil); y 0.321 (air), 0.323 (oil) Y: 38.9 (air), 21.7 (oil); d: 575 (air), 573 (oil); P e %: 2.1 (air), 3.2 (oil). The measured values of reflectance in air and oil for a single grain, respectively, are 38.02, 20.91 (470 nm), 37.87, 21.76 (546 nm), 39.18, 21.84 (589 nm), 39.30, 22.12 (650 nm) and values for 400-700 nm are tabulated. Tarkianite is cubic, F43m (by analogy with the synthetic equivalent), with unit-cell parameter refined from powder data: a 9.563(1) Å, V 874.5(1) Å 3 , Z = 4. plex is 1877 ± 2 Ma, determined on zircon from a crosscutting dyke of granitic pegmatite. The ore (Häkli et al. 1976) consists of 1) fine-grained sulfides disseminated in the serpentinite core, 2) medium-grained sulfides disseminated in the serpentinite and amphibole-dominant rock, and 3) high-grade interstitial disseminated sulfides and massive accumulations in an amphibole-dominant rock at the contact zones. In the core of the intrusive body, the main sulfides are pentlandite, mackinawite and valleriite. In the marginal zones, the main ore lodes consist of dense disseminations and net-textured primary intergrowths of pyrrhotite, pentlandite and mackinawite with secondary films of magnetite along cleavage planes of the sulfides. Chalcopyrite and graphite are common, and less common ore minerals include cobaltite, maucherite, nickeline, parkerite, native gold, Au-Ag alloy, native bismuth, pilsenite, tellurobismuthite, galena, altaite, clausthalite, ilmenite, chromite, cassiterite, monazite and uraninite. Tarkianite occurs with the primary sulfides and PGM. The concentrates were investigated for a detailed PGM study, as a follow-up to a previous study by Häkli et al. (1976). This earlier study showed that sperrylite is the only Ptbearing mineral, and that the Pd-bearing minerals include michenerite, froodite, Pd-bearing irarsite and an undefined Pd-bearing Bi-Ni telluride resembling melonite. Iridarsenite, irarsite and hollingworthite were reported as Ir-and Rh-bearing minerals.

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A new method of heavy mineral (HM) separation and assessment of gold grade was compared with the results of conventional AAS analysis. Sixteen gold micronuggets and a number of particles of native metal and metal alloys (brass, tin, bismuth, lead) were extracted from 100 g of till fines (< 50 µm). From the size, number, and composition of micronuggets, the total gold grade (58 ppb) of till fines was evaluated. The assessments agree well with the results of AAS analysis (57 ppb). A slightly lower value (44 ppb) was obtained by Flame Atomic Absorption Analysis with Fire Assay (FAAS FA) method of the extracted HM. Mineralogical investigations allow identification of two types of gold micronuggets thus revealing a complex origin for the geochemical anomaly. The association of brass-pyroxene (Mg# = 80-82) with complex gold-brass-lead-tin intergrowths indicates that some gold in till is derived from ultramafic rocks.

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