Abstract:
Rhenium and osmium elemental and isotopic data have been obtained for the two mafic–ultramafic hosted volcanogenic massive sulphide (VMS) deposits of Dergamish and Ivanovka from the south Urals. The associated ophiolitic blocks belong to the Main Uralian Fault (MUF) melange zone considered to represent obducted early Palaeozoic oceanic crust. Despite their close geographical proximity, the two ore bodies are morphologically, mineralogically and isotopically quite different. Sulphides from Ivanovka possess higher Ni and Os and lower Re and Cu relative to those from Dergamish.The Re and Os isotope data for Dergamish define a best-fit line corresponding to a Late Devonian age of 366±2 Ma (2σ) with an MSWD of 4.6. This age is some 40 My younger than the inferred Silurian crystallisation age of the associated mafic–ultramafic rocks, but in good agreement with the previously published Rb–Sr and Ar–Ar ages of 360–380 Ma corresponding to the high-pressure metamorphic age of the adjacent Maksyutov metamorphic complex. These data suggest that Re–Os systematics of the Dergamish sulphide deposit were reset, either by diffusion or recrystallisation, during high-pressure metamorphism or subsequent cooling.The preservation of unradiogenic Os isotopic ratios in some of the Ivanovka samples and the near chondritic initial Os isotopic composition obtained for the Dergamish samples indicates that most of the Os in the massive sulphides was ultimately derived from the mantle. The corresponding tectonic setting equates to an area with submarine high-level mantle rocks. In contrast, sulphides from Ivanovka have experienced continued re-equilibration and have been modified by post-depositional processes at least some of which occurred relatively recently.
