CHROMITE DEPOSITS OF THE KEMPIRSAI MASSIF, SOUTHERN URALS, KAZAKHSTAN

The Kempirsai ultramafic massif in the southern Urals, Kazakhstan, con tains large chromite bodies of 'Alpine-type', ophiolitic affinity. Ore bodies reach thicknesses of 150 m and sizes of more than 150 000 000 t chromite ore with average grades of 55-60 wt% Cr2O3. Chromites in the Main ore field are low-Al, high-Cr spinels (Cr number, approximately 80) that exhibit some variation in their Mg-Fe distribution (Mg number , 55-83). This differs markedly from high-Al spinels in peridotite hos t rocks and in narrow chromite pods that occur in the western and nort hern parts of the Kempirsai massif. Chromites carry silicate inclusion s, which are frequently hydrous silicates (e.g. amphibole). Chromites in massive chromitites contain micrometre-size inclusions of platinum- group minerals (PGM), such as laurite, erlichmanite, Os-Ir alloy and N i-Ir sulphide. Composite grains containing two PGM and, occasionally, a hydrous silicate are common. Os-Ir alloys rimmed by PGM sulpharsenid es occur in narrow serpentine veinlets that cut chromite and are evide nce for the reconcentration of PGE concomitant with introduction of As , probably during serpentinization. A variety of PGM, including alloys , sulphides, sulpharsenides and arsenides, have been detected in PGM c oncentrates separated from chromite ores in the dressing plant. Whole- rock PGE contents are highest in massive chromitites of Diamond Pearl mine (totals of 500 ppb). Chondrite-normalized PGE patterns of chromit ites show negative slopes, which are typical for ophiolitic, podiform chromitites in mantle sequences. The PGE patterns of host rocks (perid otites and serpentinites, representing depleted upper mantle tectonite s) are flat and rather depleted compared with those of chondrites and chromitites. Petrological data from unaltered host peridotites suggest a high-temperature overprint leading to sub-solidus re-equilibration of olivine and spinel and to the growth of pargasitic and edenitic amp hiboles.