Platinum-group elements as petrological indicators in mafic-ultramafic complexes of the central and southern Urals: preliminary results

Platinum-group elements (PGE) and gold were analyzed by ICP-MS after the Ni-sulphide fire assay pre-concentration step in 44 rocks from 5 orogenic mafic-ultramafic massifs of the central and southern Urals. They comprise: (a) ring-zoned, Ural-Alaskan type bodies of the 'Pt-bearing belt' (Kachkanar, Tagil, Uktus); and (b) lherzolite-harzburgite complexes overlain by layered wehrlite, gabbro, and diorite (Nurali, Mindyak) supposed to be ophiolite-type suites.

Total PGE and Au contents are generally higher in Ural-Alaskan type complexes compared with lherzolite-harzburgite complexes. Mantle-normalized PGE patterns range from nearly unfractionated in the dunites (Pd/Ir = 0.57-3.68) to mildly and strongly fractionated in wehrlite, clinopyroxenite, and gabbro (Pd/Ir = 9.1-187). Exceptionally high values of Pd/Ir (761-2622) are observed in amphibole-rich lithologies at Kachkanar, possibly indicating a relationship between fractionation of the less refractory Pd and increased fluid activity in the parental magma. Pronounced, positive Pt-anomalies, up to 20 times the mantle estimate, distinguishes me Ural-Alaskan type dunites, and progressively reduces in pyroxenites and gabbros. In the lherzolite-harzburgite complexes of Mindyak and Nurali, layered wehrlites and pyroxenites, as well as rocks from the gabbro-diorite units have fractionated PGE patterns (Pd/Ir — 2.94-92.39) and positive Pt-anomaly as in the Ural-Alaskan complexes. Conversely, the Pt-anomaly is negative in the peridotites, and increases from Iherzolites and harzburgites, to dunites, similarly to residual mantle from alpine-type orogenic massifs. The Pd/Ir (0.71-3.7) is higher than in ophiolites, and resembles that of lherzolite massifs of the western Mediterranean, representing sub-continental mantle.

Variation of the Pt-anomaly versus Pd/Ir in Ural-Alaskan type complexes and in the layered sequences overlaying lherzolite-harzburgite mantle suites is consistent with magmatic fractionation, in which Pt behaves as a compatible element. It reflects a progressive depletion by partial melting in mantle peridotites, the Pt becoming incompatible in the last stage of melting in which Pt-poor residual dunites are formed. The positive Pt-anomaly in Ural-Alaskan type complexes is striking, and would appear as a distinctive feature of the parent magma, probably inherited from its mantle source.