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

Abstract

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 δ65Cu 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 (Pt2Fe) containing subordinate inclusions of Os–Ir alloys and laurite (RuS2) predominate over other PGM. The concentrations of copper and δ65Cu 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 tetraferroplatinum (PtFe)—tulameenite (PtFe0.5Cu0.5) solid solutions series. The concentrations of copper in these PGM vary in the range of 6.8–11.3 wt %; the values of δ65Cu 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 (δ65Cu = –1.01 ± 0.17‰, n = 8 and δ65Cu = 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.