Solid solutions in the system acanthite (Ag2S)–naumannite (Ag2Se) and the relationships between Ag-sulfoselenides and Se-bearing polybasite from the Kongsberg silver district, Norway, with implications for sulfur–selenium fractionation

Abstract

Sulfoselenides [Ag2(S,Se)] and Se-bearing polybasite have been discovered at the Kongsberg silver district. The selenium-bearing minerals occur in two samples from the northern part of the district, forming either single or polyphase inclusions together with chalcopyrite within native silver. The Ag-sulfoselenides show large chemical variations, covering nearly the complete compositional range between acanthite (Ag2S) and naumannite (Ag2Se). For the data presented here, there is no local maximum at the composition Ag4SSe attributed to the distinct phase called aguilarite, suggesting that this composition can be considered as one of many possible along the monoclinic Ag2S–Ag2S0.4Se0.6 solid solution series rather than a specific mineral phase. We present a model explaining the variations in the Se-content of Ag2(S,Se) as a result of gradual de-sulfidization of the rock under oxidizing conditions. During this process, sulfur from the Ag2S-component of Ag2(S,Se) oxidized and dissolved in the fluid phase as SO42−, resulting in the formation of native silver. The activity ratio $${a_{{{\text{S}}^{2 - }}}}/{a_{{\text{S}}{{\text{e}}^{2 - }}}}$$ of the system gradually decreased due to the removal of SO42−, which resulted in the stabilization of a sulfoselenide with higher selenium content. As a result of reaction progress, grains of Ag2(S,Se) became gradually enclosed in newly formed native silver, and therefore isolated from further reactions with the grain-boundary fluid. Grains isolated early during the process show low content of Se reflecting high $${a_{{{\text{S}}^{2 - }}}}/{a_{{\text{S}}{{\text{e}}^{2 - }}}}$$ of the equilibrium fluid, while grains showing high Se reflect the composition of late low $${a_{{{\text{S}}^{2 - }}}}/{a_{{\text{S}}{{\text{e}}^{2 - }}}}$$ fluids. Analyses of Se-bearing polybasite show that selenium is preferentially partitioned into Ag2(S,Se) compared to polybasite. The model presented here demonstrates how oxidation of sulfoselenides leads to fractionation of sulfur and selenium.