Formation of the Wiesloch Mississippi Valley-type Zn-Pb-Ag deposit in the extensional setting of the Upper Rhinegraben, SW Germany |
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Authors: | Katharina Pfaff Ludwig H Hildebrandt David L Leach Dorrit E Jacob Gregor Markl |
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Institution: | 1. Institut für Geowissenschaften, Wilhelmstr. 56, 72074, Tübingen, Germany 2. Büro für Denkmalpflege und Umweltschutz, Im K?pfle 7, 69168, Wiesloch, Germany 3. Centre for Exploration Targeting School of Earth and Environment, The University of Western Australia, 6009, Perth, Australia 4. Institut für Geowissenschaften und Earth System Science Research Centre, Johannes Gutenberg Universit?t, JJ. Becher-Weg 21, 55099, Mainz, Germany
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Abstract: | The Mississippi Valley-type (MVT) Zn–Pb–Ag deposit in the Wiesloch area, Southwest Germany, is controlled by graben-related
faults of the Upper Rhinegraben. Mineralization occurs as vein fillings and irregular replacement ore bodies consisting of
sphalerite, banded sphalerite, galena, pyrite, sulfosalts (jordanite and geocronite), barite, and calcite in the Middle Triassic
carbonate host rock. Combining paragenetic information, fluid inclusion investigations, stable isotope and mineral chemistry
with thermodynamic modeling, we have derived a model for the formation of the Wiesloch deposit. This model involves fluid
mixing between ascending hot brines (originating in the crystalline basement) with sedimentary formation waters. The ascending
brines originally had a near-neutral pH (around 6) and intermediate oxidation state, reflecting equilibrium with granites
and gneisses in the basement. During fluid ascent and cooling, the pH of the brine shifted towards more acidic (around 4)
and the oxidation state increased to conditions above the hematite-magnetite buffer. These chemical characteristics contrast
strongly with those of the pore and fracture fluid residing in the limestone aquifer, which had a pH between 8 and 9 in equilibrium
with calcite and was rather reduced due to the presence of organic matter in the limestone. Mixing between these two fluids
resulted in a strong decrease in the solubility of silver-bearing sphalerite and galena, and calcite. Besides Wiesloch, several
Pb–Zn deposits are known along the Upper Rhinegraben, including hydrothermal vein-type deposits like Badenweiler and the Michael
mine near Lahr. They all share the same fluid origin and formation process and only differ in details of their host rock and
fluid cooling paths. The mechanism of fluid mixing also seems to be responsible for the formation of other MVT deposits in
Europe (e.g., Réocin, Northern Spain; Trèves, Southern France; and Cracow-Silesia, Poland), which show notable similarities
in terms of their age, mineralogy. and mineral chemistry to the MVT deposit near Wiesloch. |
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