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 presen t 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 bo dies 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 ligh t REE and have epsilon(Nd)(400) > +6 and epsilon(Sr)(400) similar to + 5. C hromitites 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 southe astern part of the Kempirsai Massif, well-known for its world-class deposit s 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 enr iched in Ir, Os and Ru and depleted in Pd and Pt. gamma(Os)(400) of three c hromitites is uniform and approaches C1 and DMM compositions. In veins and pods postdating crystallization of massive chromite, pargasitic amphibole t ermed in equilibrium with fluid-inclusion-bearing chromite at temperatures close to 1000 degrees C. These amphiboles give Ar-40/Ar-39 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 37 9.3 +/- 1.6 Ma Rb-Sr mineral isochron produced from amphibole and phlogopit e of a pyroxenite vein in the western part of the massif. From these data i t is concluded that parts of the Kempirsai Massif have been pervasively met asomatized by large amounts of fluids and melts derived from a subducted sl ab 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 form ed 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.