A Preliminary Study on Silver Isotopic Composition in Polymetallic Ore Deposits in Eastern China

Abstract

A preliminary survey of silver isotopic composition in four polymetallic ores in eastern China shows a larger variation in δ¹⁰⁹Ag from −0.014‰ to +0.983‰, which is within the total ranges for the entire respective ore deposit types worldwide. The diversity of silver isotopic compositions in ore-deposits reported here and previous studies seemed to preclude simple isotopic links to particular sources, but reflected the silver isotope fractionation in transport- and deposit-related processes instead. The δ¹⁰⁹Ag values in supergene samples from the Qixiashan Pb−Zn−Ag polymetallic deposit are more positive, in consistent with the statistical δ¹⁰⁹Ag distribution from -0.4‰ to +2.2‰ in 36 pieces of supergene ore samples around the World, which reflects the diverse controls on silver isotope fractionation from the first-order thermodynamic effect, reduction-mediated reaction, remobilization of silver with surficial low-temperature weathering processes. The hypogene samples in Dazhuangzi orogenic Au−Ag ore deposit, have δ¹⁰⁹Ag values close to 0, which implies that equilibrium partitioning associated with metal sources at the high-temperature does not result in a resolvable difference in silver isotopic compositions. By contrast, the hypogene samples which are dominated by pyrite without visible silver minerals (i.e., skarn iron ore deposit in Edongnan) have shown the largest variation range of δ¹⁰⁹Ag, followed by that from the porphyry copper ore in Zijinshan. It could be concluded that the surface adsorption and/or lattice substitution are important factors to control Ag isotope fractionation in ore-forming processes, especially for skarn deposits with only pyrite. The perspective of silver isotope shows great potentials to understand the processes that lead to the concentrations of metals to economic levels and to constrain the physicochemical conditions during ore-mineralization in metallic ore-deposits.

Keywords

silver isotopic composition / hypogene ore deposits / supergene ore deposits / metal transport / ore deposits

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