Fractionation of silver isotopes in native silver explained by redox reactions

Abstract

Scant data exist on the silver isotope composition of native silver specimens because of the relative newness of the technique. This study increases the published dataset by an order of magnitude and presents 80 silver new isotope analyses from native silver originating from a diverse set of worldwide deposits (8 deposit types, 33 mining districts in five continents). The measured isotopic range (defined as 5109Ag/107Ag in per mil units compared to NIST 978 Ag isotope standard) is +2.1 to-0 .86%c (2 a errors less than 0.015); with no apparent systematic correlations to date with deposit type or even within districts. Importantly, the data centering on 0%o all come from high temperature hypogene/primary deposits whereas flanking and overlapping data represent secondary supergene deposits. To investigate the causes for the more fractionated valúes, several laboratory experiments involving oxidation of silver from natural specimens of Ag-rich sulfides and precipitation and adsorption of silver onto reagent grade M n 0 2 and FeOOH were conducted. Simple leach experiments demónstrate little Ag isotope fractionation occurred through oxidation of Ag from native Ag (Asoiution-nativeAg = 0.12%c). In contrast, significant fraction ation occurred through precipitation of native Ag onto M n 0 2 (up to Asoiution-Mn02Ag = 0 .68%c, or 0.3amu). Adsorption of silver onto the M n 0 2 and FeOOH did not produce as lar ge fractionation as precipitation (mean valué of A soiu tio n _M n 0 2 109Ag = 0.10%c). The most likely cause for the isotopic variations seen relates to redox effects such as the reduction of silver from Ag (I) to Ag° that occurs during precipitation onto the mineral surface. Since many Ag deposits have halos dominated by M n 0 2 and FeOOH phases, potential may exist for the silver isotope composition of ores and surrounding geochemical haloes to be used to better understand ore génesis and potential exploration applications. Aside from the Mn oxides, surface fluid silver isotope compositions might provide information about geochemical reactions relevant to both environmental and hydrometallurgical applications.

The Isotopic Composition of Silver in Ore Minerals

“…We present here the first comprehensive study of the isotopic composition of Ag in ore deposits, including 109 Ag/ 107 Ag measurements from a representative selection of Ag-bearing deposits or districts worldwide, many currently and/or previously mined (Figure 1). For the sake of a complete and consistent discussion, the data included here combine our analytical results on 88 native Ag samples previously published in Mathur et al (2018), as well as new data on an additional 77 native Ag or Ag-bearing mineral samples of both hypogene (primary) and supergene (weathering) origin. This study establishes a database of analytical results from samples that address a number of variables, including the mineral host, the type and origin of Ag deposit, and the geographic location, geologic setting, and age of the Ag mineralization.…”

Natural solid-state ion conduction induces metal isotope fractionation

“…In equilibrium fractionation, 107 Ag-S bonds would have a higher vibrational energy because 107 Ag is lighter, and so the isotope exchange reaction would favor the formation of 109 Ag-S bonds, freeing more 107 Ag to enrich the wire silver; moreover, the magnitude of this effect would decrease with increasing temperature (Criss, 1999). Recently, a redox isotope effect was observed by Mathur et al (2018) where precipitation (involving reduction) of native Ag resulted in a shift toward more negative δ 109 Ag values in the newly deposited silver. Because wire silver growth also involves a reduction of Ag + , enrichment in 107 Ag would be expected.…”

“…For this reason, only three hydrothermal synthetic wires were suitable for analysis. Wires were dissolved in 4 ml of 8N HNO 3 and then purified using ion exchange chromatography (Mathur et al, 2018). The U.S. National Institute of Standards and Technology (NIST) standard used had a 107 Ag/ 109 Ag ratio of 1.07638.…”

“…The U.S. National Institute of Standards and Technology (NIST) standard used had a 107 Ag/ 109 Ag ratio of 1.07638. Sample preparation was carried out according to published methods, and mass bias was corrected using Pd (Mathur et al, 2018). Recently, concerns were raised about the reliability of 109 Ag/ 107 Ag ratios when the dissolved system contains Cl resulting in incomplete recovery of Ag (Fujii and Albarede, 2018).…”

Sn-Isotope Fractionation as a Record of Hydrothermal Redox Reactions

“…Fe (Bilenker et al, 2016;Zhu et al, 2018) , Ni (Liu et al, 2018), Cu (Maher et al, 2011;Markl et al, 2006;Mathur et al, 2013), Zn (Gagnevin et al, 2012;Zhou et al, 2014), Mo ( (Greber et al, 2011;Yao et al, 2016), Ag (Mathur et al, 2018) and Te (Fornadel et al, 2014).…”