Copper Isotope Composition of Pt-Fe Minerals from the Nizhny Tagil Massif, Middle Urals: First Results

摘要

In order to provide further insights into the origin of Pt-Fe minerals, this study presents the first copper isotope data for Pt-Fe minerals from the Nizhny Tagil massif, an international standard of the zoned Ural-type complexes. The chemical and isotopic composition of Pt-Fe minerals were determined by electron microprobe analysis, chemical sample preparation with selective chromatographic separation of copper from a solution of the studied sample, followed by high-precision determination of the δ~(65)Cu value using multiple-collector inductively coupled plasma mass-spectrometry. The majority of platinum group minerals (PGM) from chromitites of the Alexandrovsk Log and Krutoy Log deposits within the Nizhny Tagil massif are formed by Pt-Fe minerals, among which high-temperature ferroan platinum (Pt_2Fe) containing subordinate inclusions of Os-Ir alloys and laurite (RuS_2) predominate over other PGM. The concentrations of copper and δ~(65)Cu values in the studied samples of ferroan platinum vary from 0.4 to 1.4 wt % Cu and from -0.37 to 0.26‰, respectively. Secondary low-temperature PGM assemblage is represented by the tetraferro-platinum (PtFe)-tulameenite (PtFe_(0.5)Cu_(0.5)) solid solutions series. The concentrations of copper in these PGM vary in the range of 6.8-11.3 wt %; the values of δ~(65)Cuu are characterized by lighter Cu-isotopic compositions ranging from -1.15 to -0.72‰. The lighter Cu-isotopic composition in secondary Cu-bearing PGM compared to that in ferroan platinum (δ~(65)Cu = -1.01 ± 0.17‰, n = 8 and δ~(65)Cu = 0.03 + 0.23‰, n = 7, respectively) is consistent with a secondary nature of isotopic variations, due to evolved composition of the ore-forming fluid during the low-temperature formation of the tetraferroplatinum-tulameenite solid solution series.