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

• Published in: Contributions to mineralogy and petrology Vol. 173; no. 9; pp. 1 - 17
• Main Authors: Kullerud, Kåre, Kotková, Jana, Šrein, Vladimír, Drábek, Milan, Škoda, Radek
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.09.2018; Springer Nature B.V
• Subjects: Bearing; Chalcopyrite; Composition; Computational fluid dynamics; Earth and Environmental Science; Earth Sciences; Fluids; Fractionation; Geology; Grains; Mineral Resources; Mineralogy; Organic chemistry; Original Paper; Oxidation; Petrology; Removal; Selenium; Silver; Solid solutions; Solutions; Sulfidization; Sulfur; Sulphur; Naumannite; De-sulfidization; Kongsberg silver district; Se-bearing polybasite; Aguilarite; Oxidation; Selenides; Acanthite
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-018-1500-3

Sulfoselenides [Ag 2 (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 (Ag 2 S) and naumannite (Ag 2 Se). For the data presented here, there is no local maximum at the composition Ag 4 SSe attributed to the distinct phase called aguilarite, suggesting that this composition can be considered as one of many possible along the monoclinic Ag 2 S–Ag 2 S 0.4 Se 0.6 solid solution series rather than a specific mineral phase. We present a model explaining the variations in the Se-content of Ag 2 (S,Se) as a result of gradual de-sulfidization of the rock under oxidizing conditions. During this process, sulfur from the Ag 2 S-component of Ag 2 (S,Se) oxidized and dissolved in the fluid phase as SO 4 2− , resulting in the formation of native silver. The activity ratio a S 2 - / a S e 2 - of the system gradually decreased due to the removal of SO 4 2− , which resulted in the stabilization of a sulfoselenide with higher selenium content. As a result of reaction progress, grains of Ag 2 (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 S 2 - / a S e 2 - of the equilibrium fluid, while grains showing high Se reflect the composition of late low a S 2 - / a S e 2 - fluids. Analyses of Se-bearing polybasite show that selenium is preferentially partitioned into Ag 2 (S,Se) compared to polybasite. The model presented here demonstrates how oxidation of sulfoselenides leads to fractionation of sulfur and selenium.