Giant chromite deposits and their host rocks, Kempirsai, Urals: An integrated study of precious metals, rare earth elements, stable and radiogenic isotopes, and the composition of fluid inclusions in chromite

Sm-Nd, Rb-Sr, Re-Os, Ar-Ar, stable (O, D, C) isotope investigations, and fluid inclusion studies led to a new, two-stage model of the formation of giant ophiolitic-podiform chromite orebodies of the Kempirsai massif, Southern Urals. In a first stage (420 to 395 Ma), fertile mantle was melted in an extensional setting, producing basaltic magma from which small bodies of low-Cr, high-Al chromite precipitated in conduits within the uppermost mantle. In a second stage, oceanic crust of the northwestern part of the massif, covered by sediments, was subducted underneath the southeastern part. Sediments of continental affinity dehydrated under eclogite-facies conditions, inducing a second melting event in the overlying, already depleted mantle wedge. Mg-rich melt and volatiles percolating large parts of this mantle wedge mobilized Cr, Mg, and platinum-group elements. Large bodies of high-Cr chromitite formed after melts and fluids transformed harzburgite to dunite which now mantles chromite orebodies. This process post-dated the first melting event by 15-35 Ma.