Origin of the Nizhny Tagil Clinopyroxenite–Dunite Massif, Uralian Platinum Belt, Russia: Insights from PGE and Os Isotope Systematics

Abstract

Zoned clinopyroxenite–dunite Uralian–Alaskan-type complexes of the Uralian Platinum Belt are the source of economic platinum deposits. One of the striking features of Uralian–Alaskan-type complexes is a pronounced Pt anomaly, which clearly distinguishes them from cumulate series of ophiolite massifs elsewhere, but there is still uncertainty regarding the nature of the platinum enrichment and its geodynamic setting. We have studied the platinum-group element (PGE) and Os isotope systematics of platinum-group minerals, chromitites and ultramafic rocks from the Nizhny Tagil zoned clinopyroxenite–dunite massif in the Urals. The whole-rock 187Os/188Os ratios of the dunites vary from 0·1160 to 0·1332, averaging 0·1247, with more radiogenic values possibly affected by subduction-related fluids during Tagil island arc development (c. 410 Ma). Laurite and Os–Ir alloys have 187Os/188Os ratios of 0·12245 ± 0·00038, which are close to those of chromitites (187Os/188Os = 0·1215 ± 0·0006) and correspond to an Os model age (TRD) of c. 800 Ma. This model age is c. 400 Myr older than a melt depletion event corresponding to Tagil island arc development, and can be ascribed to a Neoproterozoic mantle melting event under the influence of either a pre-Uralian subduction zone or a superplume, overprinted by younger processes.The Pt/Pd values in the ultramafic rocks show significant variations, increasing towards the chromite-PGE-bearing mineralization zone. The overall primitive mantle normalized PGE patterns are very similar to those reported for sub-arc mantle peridotites, which are characterized by a positive slope and high Pt/Pd ratios, and are distinct from those of typical peridotites and cumulates from Urals ophiolite massifs. Such a similarity may be explained by the melting of metasomatized depleted mantle that had undergone several melt extraction events in a subduction-zone setting. This is also evident from a high oxygen fugacity averaging 2·7 relative to fayalite–magnetite–quartz, which is distinctly more oxidized compared with mid-ocean ridge basalt and ocean island basalt settings. However, the striking similarities (P–T–fO2 and type of parental magma, high Pt/Pd ratio) to other zoned clinopyroxenite–dunite massifs such as Kondyor, situated within the stable Archean shield, allow us to conclude that this type of massif is not exclusive to subduction-zone settings, but may reflect the presence of a specific depleted, fluid-metasomatized type of relatively shallow mantle in the Paleozoic. The systematic Pt/Pd ratio increase in the Nizhny Tagil rocks towards the chromite mineralized zone suggests that Pt–Pd fractionation may be related to the preferential retention of Pt in chromitites as Pt–Fe alloys.