The giant chromite deposits at Kempirsai, Urals: constraints from trace element (PGE, REE) and isotope data

The investigation of stable and radiogenic isotopes and of platinum-group (PGE) and rare earth elements (REE) in chromitites and associated ultramafic rocks of the Kempirsai Massif, southern Urals, gives strong evidence for a multistage formation of giant ophiolitic-podiform chromite deposits present in the southeastern part of the massif. The Kempirsai ophiolite massif is divided by a shear zone into two parts: in the northwestern area, small bodies of Al-rich chromite formed from basaltic melts between 420 to 400 Ma, according to Sm-Nd mineral isochrons of harzburgite, pyroxenite, websterite and gabbro. Harzburgites and pyroxenites in this area are enriched in light REE and have epsilon(Nd)(400) > +6 and epsilon(Sr)(400) similar to + 5. Chromitites have scattered PGE distributions (Pd/Ir, 0.4-7.0), being partly enriched in Pd and Pt. gamma(Os)(400) of one chromitite is -4.4. The southeastern part of the Kempirsai Massif, well-known for its world-class deposits of podiform low-Al magnesiochromite, is characterized by harzburgite and dunite enriched in light REE with very low epsilon(Nd)(400) (+4.3 to -17.1) and positive epsilon(Sr)(400) (> +10) values. Chromitites are strongly enriched in Ir, Os and Ru and depleted in Pd and Pt. gamma(Os)(400) of three chromitites is uniform and approaches C1 and DMM compositions. In veins and pods postdating crystallization of massive chromite, pargasitic amphibole termed in equilibrium with fluid-inclusion-bearing chromite at temperatures close to 1000 degrees C. These amphiboles give 40Ar/39Ar stepwise heating ages of 365 to 385 Ma and are characterized by low epsilon(Nd)(400) (+0.6 to -4.6) and general enrichment in REE. The cooling ages correspond to a 379.3 +/- 1.6 Ma Rb-Sr mineral isochron produced from amphibole and phlogopite of a pyroxenite vein in the western part of the massif. From these data it is concluded that parts of the Kempirsai Massif have been pervasively metasomatized by large amounts of fluids and melts derived from a subducted slab composed of oceanic crust and sediments. Subduction occurred at least 15-35 Ma after a melting event that produced a typical ophiolitic sequence in the Paleozoic Sakmara Zone. We conclude that large chromite orebodies formed from second-stage high-Mg melts that interacted with depleted mantle and fluids on their way upward in a suprasubduction zone regime, and in a fore-are position to the Magnitogorsk island are.