COPPER ISOTOPE COMPOSITION OF Pt-Fe MINERALS FROM THE NIZHNY TAGIL MASSIF, MIDDLE URALS: FIRST RESULTS

Abstract

In order to identify the source of copper, for the first time in world practice, the features of the Cu-isotopic composition of Pt–Fe minerals are characterized using the example of the Nizhny Tagil massif, the world standard of Ural-type zoned complexes. The chemical and isotopic composition of Pt-Fe minerals were determined by electron probe microanalysis, chemical sample preparation with selective separation of copper from the solution of the studied sample, and subsequent high-precision determination of the δ65Cu value using multiple-collector inductively coupled plasma mass-spectrometry. The majority of platinum group minerals (PGM) from bedrock deposits of the Nizhny Tagil massif, represented by chromitites of the Alexandrovsky and Krutoy logs, are formed by Pt–Fe minerals, among which high-temperature ferroan platinum (Pt2Fe) with inclusions of Os–Ir alloys and laurite (RuS2) dominates. The concentrations of copper in the studied samples of ferroan platinum vary in the range of 0.4–1.4 wt. % Cu. The δ65Cu values for Cu-bearing PGM vary in the range from –0.37 to 0.26‰. Secondary low-temperature PGM assemblage is represented by solid solutions of tetraferroplatinum (PtFe) – tulameenite (PtFe0.5Cu0.5) 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 isotopic compositions of copper that range from –1.15 to –0.72‰. The lightening of the Cu-isotopic composition in secondary Cu-bearing PGM compared with that in ferroan platinum (δ65Cu = –1.01 ± 0.17, n = 8 and δ65Cu = 0.03 ± 0.23, n = 7, respectively) is consistent with the secondary nature of isotopic variations, due to the evolution of the composition of the ore-forming fluid during the low-temperature formation of the tetraferroplatinum (PtFe) – tulameenite (PtFe0.5Cu0.5) series.