Element Redistribution in Hydrothermal Systems: From Formation to Supergene Weathering

Abstract

The driving force for elemental redistribution in hydrothermal vein-type deposits during their formation and subsequent supergene weathering is the thermodynamic stability of mineral phases. Elemental redistributional processes will not only influence the economic potential of ore deposits but also lead to the mobilization of (toxic) heavy metals. The Schwarzwald in SW-Germany provides an ideal area to study redistribution processes, as numerous polymetallic hydrothermal veins with their primary-, oxidation- and cementation zones are exposed near to the surface. The Clara mine offers a unique, vertical profile of a silver-rich, hydrothermal sulfide mineralization that is exposed over 850 m vertically. This vertical profile has been influenced by redistribution processes several times, but has not yet undergone intensive scientific investigation. Based on mineral textures and stability diagrams, this work reveals, that a large scale silver zonation was caused by different redox potentials of the formation fluids. The original mineralization was overprinted subsequently by at least four further hydrothermal phases and an additional alteration phase, whereby the general silver zonation has been preserved. If the hydrothermal veins are exposed to near-surface environments by uplift and erosion, the sulfides are thermodynamically no longer stable and new, more stable, mineral phases form or the contained elements go into solution and are removed and/or re-precipitate. The redistribution of silver during the weathering of argentiferous galena and bismuth-bearing fahlore as important carriers of precious as well as heavy metals was investigated, since only partial aspects are considered by previous studies. Trace element analysis and stability calculations show that silver in contrast to lead is mobile during the weathering of galena and is re-precipitated in other parts of the vein by redox processes as discrete mineral phases such as native silver and acanthite. Bismuth, on the other hand, is immobile during the weathering of Bi-containing fahlore, whereby the mobility of antimony is controlled by the formation of Bi-Sb phases. Furthermore, it is shown that the mobility of arsenic and the produced weathering textures depend on the oxidation potential of the weathering fluids. Supergene weathering is not only caused by natural processes, but is also influenced by anthropogenic processes. With the help of thermodynamic fluid path modelling, the complex association of basic lead minerals (their formation is related to medieval fire setting), was used to explain their occurrence also in other anthropogenically influenced environments. In addition, weathering experiments have shown that the basic lead minerals are able to control the mobility of lead under oxidizing conditions. How reducing thermal fluids affect an oxidation zone is not yet known. In order to investigate this unusual effect, galena pseudomorphoses after pyromorphite from the Kautenbach mine in Rhineland-Palatinate (Germany) were investigated for the first time, since they have captured the alternation of oxidizing and reducing fluids. Using a thermodynamic model based on new water analyses, it was shown that these pseudomorphs were formed by calcium and fluorine rich fluids at relatively high pH values, in which lead is immobile.