Halogen (F, Cl, Br, I) cycling in the critical zone: Formation of primary ore deposits, their supergene weathering and the fate of halogens in soil and along the hydrological flow path

Abstract

The halogens fluorine (F), chlorine (Cl), bromine (Br) and iodine (I) are ubiquitous in nature and are involved in a vast number of processes in terrestrial environments. They are not only important for humans, animals and plants as micro-nutrients, but also play an important role in for example metal transport in fluids or in mobilizing and re-precipitation of toxic trace elements such as lead (Pb) or arsenic (As). Therefore, halogens are in the scope of important environmental interests and a detailed understanding of halogen distribution and behavior in natural environments is essential in order to prevent potentially harmful impacts. This thesis deals with the formation of primary Pb-deposits, the subsequent supergene weathering with a specific focus on the halogen distribution in thereby formed secondary Pb-minerals and the fate of the halogens (F, Cl, Br and I) in the critical zone, including the pedosphere and the hydrological flow path. The Schwarzwald in SW-Germany provides an ideal area since numerous primary formed hydrothermal veins, their supergene weathering zone and well-developed soil profiles are easily accessible. Polymetallic Sb-Pb(-Ag±Au) deposits have been formed by various fluids with specific physico-chemical conditions and were subject to intense remobilization processes. However, the formation of this vein type within the entire Schwarzwald has not been comprehensively investigated in detail. Based on mineral textures, formation temperature, fluid and mineral composition and stability diagrams, this work revealed, that this vein type typically formed over four mineralization stages. The primary mineralization was formed by cooling of high temperature, low salinity fluids. It was overprinted subsequently by at least three further hydrothermal phases, driven by high salinity and medium to low temperature fluids. The results observed showed increased complex mineral associations over time, associated with an increased importance of halogens within the hydrothermal processes. Exposed to near-surface conditions by uplift and erosion, supergene weathering of such primary hydrothermal Pb-deposits leads to the formation of pyromorphite-group minerals (PyGM) in nearly all cases. These minerals incorporate halogens in up to weight percent ranges and immobilize Pb and As. Bulk geochemical mineral analyses showed that halogen incorporation (especially Br and I) is mineral specific and is less related to crystal chemical controls. Spatially resolved analyses revealed variable halogen ratios in different crystal growth zones independent of major element composition. This implies that the halogen composition is not governed by a crystal-chemical control (alone), but more substantially by external factors such as fluid composition. Halogen distribution in natural fluids and soils above a weathering zone has been shown to be influenced by halogen-specific processes. Simultaneously F, Cl, Br and I have been analyzed in ecosystem solutions and vertical soil profiles at one site for the first time, in order to investigate the near-surface halogen cycle. It was depicted that this cycle is controlled by a complex interaction of biotic and abiotic processes and that these processes differed for each halogen. Accumulation of Cl in the topsoil could be shown to be caused by chlorination processes and nutrient uplift by roots. The cycling of F, Br and I was rather controlled by pH-related weathering and high solubility in combination with sorption processes to pedogenic oxides or incorporation into clay minerals. Furthermore, it was shown that also vegetation and soil external processes such as atmospheric halogen input had an influence on halogen composition along the hydrological flow path, due to the wash-off of dry deposition and foliage leaching. Overall, it was shown that the halogen cycling in the critical zone is governed by coupled biological, mineralogical, chemical and physical processes.