The Isotopic Composition of Silver in Ore Minerals

Abstract

Abstract The study of the isotopic composition of Ag has applications to numerous fields, including earth and planetary sciences, the environment, and archeology. We present a comprehensive survey of the isotopic composition of Ag in ore deposits globally based on measurement of the 109Ag/107Ag values of 158 samples of native Ag and Ag‐bearing minerals from 65 deposits or districts representative of the main types of Ag mineralization in 20 countries on five continents. The narrow range of δ109Ag values (−0.4‰ to 0.4‰) observed for hypogene native Ag and acanthite/argentite (Ag2S) represents the isotopic composition of Ag in the hydrothermal fluids that are responsible for formation of hypogene (primary) Ag deposits worldwide. Our data provide evidence of mass dependent isotopic fractionation: (a) during low‐temperature remobilization of Ag associated with redox reactions in the supergene (weathering) environment, and (b) among Ag‐bearing mineral phases, including Ag‐halides (bromargyrite, chlorargyrite, iodargyrite, boleite, and pseudoboleite) and As‐bearing sulfosalts (e.g., enargite, tennantite, proustite, and polybasite). We find no evidence of fractionation or isotopic variation related to genetic environment of hypogene mineralization, formation age, or the host rock and geologic terrane of an ore deposit. The mean and range of δ109Ag values from different deposit types largely overlap, and the variation in Ag isotopic composition observed within a mineral district can be as large as that for the respective deposit type. Our initial results suggest limitations to a wider use of Ag isotopes as a geochemical tracer (e.g., in archeometry) and the need to conduct detailed, individual deposit/district studies.

Keywords

• Ag isotopes
• Ag deposits
• fractionation
• hypogene
• supergene
• archeometry