Mathias Burisch

Natural fracking and the genesis of five-element veins

Hydrothermal Ag-Co-Ni-Bi-As (five-element vein type) ore deposits show very conspicuous textures of the native elements silver, bismuth, and arsenic indicating formation from a rapid, far-from-equilibrium process. Such textures include up to dm-large tree- and wire-like aggregates overgrown by Co-Ni-Fe arsenides and mostly carbonates. Despite the historical and contemporary importance of five-element vein type deposits as sources of silver, bismuth, and cobalt, and despite of spectacular museum specimens, their process of formation is not yet understood and has been a matter of debate since centuries. We propose, based on observations from a number of classical European five-element vein deposits and carbon isotope analyses, that “natural fracking,” i.e., liberation of hydrocarbons or hydrocarbon-bearing fluids during break up of rocks in the vicinity of an active hydrothermal system and mixing between these hydrocarbons (e.g., methane and/or methane-bearing fluids) and a metal-rich hydrothermal fluid is responsible for ore precipitation and the formation of the unusual ore textures and assemblages. Thermodynamic and isotope mixing calculations show that the textural, chemical, and isotopic features of the investigated deposits can entirely be explained by this mechanism.

Tiefenwässer im Oberen Muschelkalk-Aquifer des Oberrheingrabens und des Südwestdeutschen Molassebeckens

ZusammenfassungAus dem links- und rechtsrheinischen Teil des Oberrheingrabens sowie dem baden-württembergischen Teil des Molassebeckens wurden hydrochemische Daten aus Tiefbohrungen zusammengestellt und ausgewertet. Zielhorizont der Untersuchung war der Obere Muschelkalk-Aquifer. Die Analysendaten gestatten eine hydrochemische Charakterisierung der im Untergrund zu erwartenden Fluide. Anhand der Untersuchung können Informationen über die Herkunft und Genese der Tiefenwässer gegeben werden. Zudem werden die Sättigungszustände der Wässer in Bezug auf verschiedene Minerale und die Einflüsse von Temperatur- und Druckänderungen sowie Änderungen im pH-Wert diesbezüglich diskutiert.AbstractHydrochemical data from deep wells in the Upper Rhine Graben area in France and Germany (Baden-Wuerttemberg) and in the Molasse Basin were collected and examined. Target formations were the Upper and the Lower Muschelkalk aquifer. The hydrochemical data were used to characterize the fluids found in the deep hydrothermal reservoirs and provide information about the origin and development of the deep waters. Saturation states of the deep seated waters are discussed with respect to selected minerals and the influence of temperature and pressure variations on pH and saturation states were determined.