Diamonds, native elements and metal alloys from chromitites of the Ray-Iz ophiolite of the Polar Urals

Abstract

Diamonds and a wide range of unusual minerals were originally discovered from the chromitite and peridotite of the Luobusa ophiolite along the Yarlong–Zangbu suture zone in southern Tibet. To test whether this was a unique occurrence or whether such minerals were present in other ophiolites, we investigated the Ray-Iz massif of the early Paleozoic Voikar–Syninsk ophiolite belt in the Polar Urals (Russia) for a comparative study. Over 60 mineral species, including diamond, moissanite, native elements and metal alloys have been separated from ~1500 kg of chromitite collected from two orebodies in the Ray-Iz ophiolite. More than 1000 grains of microdiamond were recovered from the chromitite and, of particular importance, 2 grains were found as in-situ inclusions in chromite grains, proving that these minerals are intrinsic to the rock, and not the result of natural or anthropogenic contamination. The native elements include Cr, W, Ni, Co, Si, Al and Ta, commonly associated with carbides such as SiC and WC, and metallic alloys, such as CrFe, SiAlFe, NiCu, AgAu, AgSn, FeSi, FeP, and AgZnSn also occur. These minerals are accompanied by oxides (wüstite, periclase, eskolaite, rutile, baddeleyite, ilmenite, corundum, chromite, NiO and SnO2) and silicates (kyanite, zircon, garnet, feldspar, and quartz). Sulfides of Fe, Ni, Cu, Mo, Pb, Ab, AsFe, FeNi, CuZn, and CoFeNi are common, as are various iron minerals, such as native Fe, FeO, and Fe2O3. Here, we focus on the composition and character of the diamonds, native elements and metal alloys. All of the minerals discussed here are similar to those reported from chromitites of the Luobusa ophiolite, Tibet, indicating that they are not restricted to one ophiolite or one geographic region; rather, they may be widespread in the oceanic mantle. The diamonds recovered from these ophiolites are completely different from most of those in kimberlites and ultra-high pressure (UHP) metamorphic rocks, and represent a new occurrence of diamonds on Earth. A three-stage model is proposed to explain the source of the diamonds and highly-reduced minerals, their formation, and their preservation in ophiolites.