DISTRIBUTION AND FEATURES OF CHROMITITES IN THE MAFIC-ULTRAMAFIC NURALI MASSIF (SOUTHERN URALS, RUSSIA)

The Uralian chain (Russia) is a 2500 km long linear mid- Paleozoic orogenic zone representing the closure of an ocean basin-island arc system between the European plate to the west and the Siberian plate to the east. As a result, sections of oceanic lithosphere were obducted onto the European plate producing more than 150 ophiolite occurrences with a size range from 200 km to minor smaller occurrences (Savelieva and Nesbitt, 1996). The Nurali massif is a mafic-ultramafic complex located in the Southern Urals and belonging to the belt of small dismembered ophiolites occurring along the Main Uralian Fault, the suture zone of the Ural orogenic belt. The massif crops out over an area of 45 km2 and extends for about 20 km. From west to east the Nurali massif includes a lherzolitic mantle section (lherzolite, harzburgites, dunites, rare chromitites), an ultramafic cumulate sequence (dunites, wehrlites, pyroxenites, chromitite lenses), and an amphibole gabbro cut eastward by a melange zone (andesites, pillow lava, marbles, skarns, mafic and ultramafic rocks within a serpentinite matrix). The geodynamic evolution of the massif has been long debated. While some Authors consider it as an ophiolite related to aborted slow-spreading zone (rift) within continental margin, others consider it as an orogenic lherzolite massif. The chromitite lenses belonging to the Nurali massif show a wide range of textural and chemical features strictly related to the their host rock. The massive chromitite within the mantle portion occurs as a small metric lens, settled in a broadly serpentinized lherzolitic unit whose spinel is rimmed by and partially reacted with plagioclase. The compositions of Cr-spinels from different mantle rocks (lherzolite, harzburgite and dunite) and from the chromitite lens, draw two different parallel trends with negative correlation between xCr and xMg. The first one, marked by the lherzolitic and harzburgitic rocks, shows Mg# and Cr# between 0.40 - 0.60 and 0.35 - 0.50 respectively. The second one, related to dunite and chromitite, shows values of Mg# and Cr# between 0.30 - 0.60 and 0.60 - 0.75 respectively. The PGE analysis of the chromitite shows a general enrichment in Os (65 ppb), Ir (47.5) Ru (50 ppb) and Rh (9.7 ppb).The REE content of clinopyroxenes within the chromitite lens shows a linear pattern with chondrite- normalized values ranging from 1.55 (LaN) to 4.38 (YbN). The ultramafic cumulate sequence consists of a banded dunite-wehrlite-pyroxenite unit cropping out along the eastern flank of the Nurali massif, in a zone about 20 km long and 0.2-1.3 km wide. The sequence was interpreted as formed by multiple injections of melt derived from a depleted mantle source in a spreading geotectonic setting (Pertsev et al., 1997). Five chromitite lenses were found within the cumulate sequence: the first one located 0.3 km to the north of Miass river (North Miass), the second one about 5 km to the south of Miass river (Jalchigulovo), and the last three ones, in a floor to the top section of the cumulate sequence about 0.5 km to the south of Miass river, (South Miass; levels 1 to 3 of Grieco et al., 2004). Chromitite bodies’ size, shape and composition change within the different levels. Level 1 consists of centimetric irregular yet flattened and massive lenses, hosted in a clinopyroxenite; this level has chromites with the highest Cr# (between 0.65 and 0.71) and the lowest Mg# (between 0.48 and 0.50) and with peculiar high Fe3+ content. Level 1 shows intermediate contents of PGE (1449 ppb) and is enriched in Pt and Pd. Level 2 is made up of a 3 _ 1.5 m deformed massive chromitite lens and of a another smaller and disseminate body, both with a orthopyroxene-amphibole-rich matrix and hosted within a spinel-bearing amphibole-rich olivineclinopyroxenite rock layer. This level has the lowest and the most variable Cr#, ranging between 0.33 and 0.54, and a high Mg# (between 0.60 and 0.80). This level shows a strong PGE enrichment (up to 26.754 ppb), mostly in Os, Ir and Ru. Level 3 consists of a decimetric irregular and very massive lens hosted in olivine-enstatite rocks, and it has been correlated to North Miass and Jalchigulovo chromitite bodies by both textural-petrological evidences and geochemical analyses. Level 3 has high Cr# (between 0.62 and 0.72) and the highest Cr2O3 content, ranging between 47 and 52 wt%; this level is depleted in all PGEs compared to the other levels. Within the serpentinite melange four main chromitite localities were investigated, two of which were mined for chromium during the World War II. Here chromitite occurs mainly as metric to decametric massive lenses within serpentinite whose protholith has been completely obliterated. In some cases late veins of quartz and aragonite cut chromitite. Mineralization often has cataclastic textures with rare undeformed crystals. The composition of chromites from Nurali serpentinite melange shows very high Cr# (0.75- 0.85) and highly variable Mg# (0.11-0.71) and chromite grains are usually rimmed by magnetite. TiO2 contents are lower than 0.25 wt%. Low Mg# chromites are associated to green hornblende and Cr-chlorite (kammererite). PGE contents are very low (PGEtot <150ppb), Ru is the highest, (up to 64 ppb), Os and Ir have a intermediate value (33 and 38 ppb) and Pt and Pd are below detection limits ( < 5 ppb). Only two primary Os-Ir alloys ,2-3 µm in size,have been found within chromites.