Redistribution of silver during supergene oxidation of argentiferous galena: A case study from the Schwarzwald, SW Germany
Keim, Maximilian F.; Vaudrin, Rafael; Markl, Gregor
published: Aug 1, 2016
ArtNo. ESP154019303005, Price: 29.00 €
Argentiferous galena is unstable under surface conditions and secondary lead minerals form during its decomposition. This contribution investigates to which extent silver released from weathered hydrothermal galena is retained in the the supergene lead phases cerussite, anglesite, pyromorphite, and mimetite which are texturally related to the weathering process. Therefore, trace element analyses of galena and supergene lead phases from various supergene ore deposits of the Schwarzwald, SW-Germany, were performed by laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS). The analyzed galena contains between 32 and 18,000 ppm silver. The secondary lead minerals contain 124 ppm silver as a maximum (pyromorphite) and in every case they have less than 13 % of the corresponding galena from the same hand specimen. Cerussite and anglesite are virtually silver-free. This leads to the conclusion that silver set free during weathering of argentiferous galena is likely to be removed as soluble ions/complexes or reprecipitated as distinct silver phases. Thermodynamic modelling based on a closed system approach and modern-day conditions suggests that silver released by the weathering of argentiferous galena or other silver-bearing sulfides is likely to be reprecipitated as native silver due to redox processes or depletion of oxygen. The formation of chlorargyrite is only possible in environments with elevated silver and chloride concentrations. Enrichment of silver during weathering is possible if galena has high silver contents or if other silver minerals serve as additional silver source. Enrichment of chloride is possible in geogenic microenvironments or due to anthropogenic input. The field observations agree with the thermodynamic calculations, since secondary native silver and acanthite are widespread and chlorargyrite is very rare to absent within the oxidized parts of the Schwarzwald ore deposits.