Arsen in niedrig- bis mitteltemperierten Hydrothermalsystemen
Zusammenfassung der Projektergebnisse
The aim of this project was to investigate the mobility and solid arsenic mineral formation mechanisms in natural hydrothermal environments, i. e. in hydrous fluids at temperatures between 30 and 300 ° C, to investigate the fate of arsenic in natural thermal waters and the natural parameters controlling mobility and immobility of the toxic element arsenic. Specifically, this project focused on 4 major objectives: (1) qualitatively and, where possible, quantitatively constraining the formation conditions of arsenic mineralizations such as salinity, temperature and fluid metal concentrations comparing barren and non-barren, but arsenic mineral-free systems with arsenic mineral-bearing systems; (2) to determine the phase stabilities of arsenic-bearing minerals with the use of available thermos-dynamic data and compare these to the natural occurrences; (3) to investigate the mechanisms underlying the formation of arsenic minerals in these environments, and (4) to evaluate and discuss possible reducing agents as reducing environments are required for hypogene arsenic mineral formation. These four objectives were all fulfilled and published. Ad (1). Hundreds of hand specimens were petrologically investigated and a total of more than 1000 fluid inclusions were analyzed by micro-thermometry, some fluid inclusions by LA-ICP-MS, which allowed the determination of metal and arsenic concentrations during ore formation (Scharrer et al., 2021a; Scharrer et al., 2021b). The obtained concentrations in addition to comparison with literature values of typical fluid concentrations paved the way for the development of ore formation hypothesis and for thorough thermodynamic modelling of these systems. Ad (2). To improve internal consistency of the thermodynamic database, database handling and data implementation was achieved with the help of the Thermoddem database (Blanc et al., 2012) management team. Novel estimation methods allowed to constrain the thermodynamic stabilities of Ni-, Co- and Fe arsenides, sulfarsenides as well as complex sulfosalts such as jordanite (Pb14As6S23). Ad (3). Essential for the formation of hypogene arsenic minerals is the presence of a reduced environment. This reduced environment can for example be achieved through a strong reducing agent that is introduced into an oxidized metal bearing hydrothermal fluid. With the consideration of thermodynamic stabilities and the kinetics of sulfate reduction, such a fast reduction during ore formation fully explains observed mineral formation sequence as was shown for an individual case study in the Black Forrest, Germany, and by a thorough literature compilation and evaluation of all occurrences worldwide. Although reducing environments are crucial, the precipitation of arsenic minerals with other ore minerals and/gangue minerals is commonly also associated with an additional precipitation process such as fluid mixing, which produced the arsenic-bearing fluoride and carbonate assemblages, and fluid cooling, which attributes to the arsenic-bearing quartz rich assemblages (Scharrer et al., 2020). Which arsenic mineral forms is largely dependent on redox state of the environment as well as the availability of metals and reduced sulfur. We were able to show the importance of slow kinetics of sulfur reduction on the formation of sulfide free arsenide and native arsenic assemblages and the impact other metals such as Pb and Zn can have to scavenge any available sulfide and allow native arsenic to form. Ad (4). Reducing agents in question in crustal environments are hydrogen, hydrocarbons, graphite and Fe2+ bearing minerals. Thus, not only were we able to geochemically and thermodynamically show the need for a reducing agent, we were also able to link the reduction during ore formation to the presence of mobile reducing agents such as hydrogen or hydrocarbons. All data obtained during the course of this project, evaluation and discussions thereof were published in four publications in internationally recognized journals.
Projektbezogene Publikationen (Auswahl)
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The mineralogical variability of hydrothermal native element-arsenide (five-element) associations and the role of physicochemical and kinetic factors concerning sulfur and arsenic. Ore Geology Reviews, 113, 103025.
Scharrer, Manuel; Kreissl, Stefan & Markl, Gregor
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Formation of native arsenic in hydrothermal base metal deposits and related supergene U6+ enrichment: The Michael vein near Lahr, SW Germany. American Mineralogist, 105(5), 727-744.
Scharrer, Manuel; Sandritter, Katharina; Walter, Benjamin F.; Neumann, Udo & Markl, Gregor
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Basement aquifer evolution and the formation of unconformity-related hydrothermal vein deposits: LA-ICP-MS analyses of single fluid inclusions in fluorite from SW Germany. Chemical Geology, 575, 120260.
Scharrer, Manuel; Reich, Rebekka; Fusswinkel, Tobias; Walter, Benjamin F. & Markl, Gregor
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The formation of (Ni-Co-Sb)-Ag-As ore shoots in hydrothermal galena-sphalerite-fluorite veins. Mineralium Deposita, 57(5), 853-885.
Scharrer, Manuel; Epp, Tatjana; Walter, Benjamin; Pfaff, Katharina; Vennemann, Torsten & Markl, Gregor
