Ore-forming processes of hydrothermal vein-type deposits, SW Germany

Abstract

This thesis focuses on ore-forming processes and the provenance of involved fluids, solutes and gases of epithermal to mesothermal Pb-Zn-(Cu)-fluorite-quartz and Ag/Bi-Ni-Co-(Fe)-As-(U)-calcite vein-type deposits of the Schwarzwald and the Odenwald, SW Germany. These veins occur across basement-cover unconformities due to mixing of two or more chemically contrasting fluids. A fluid component (fluid-type A), which originates from deeper crustal levels (high salinity; low Cl/Br (by mass)) of the upper crust is invariably recognised in the ore fluids, while the other fluid component is of basinal, sedimentary and/or meteoric origin (fluid-type B). Although extensive data is available for unconformity-related hydrothermal veins, several important aspects are still poorly constrained. These aspects include a precise understanding of the processes that result in the observed compositions of deep-seated crustal fluids, alteration processes in root zones (below actual vein), regional and temporal variations in the composition of the ore fluids of the Schwarzwald and the genesis of the enigmatic five-element veins of the Odenwald, SW Germany. To attain a better understanding of the presented aspects, we carried out leaching experiments on common igneous, metamorphic and sedimentary rocks including their mineral separates at variable temperatures (25 to 350°C), pressures (0.01 to 1.9 kbar), grain-size fractions and fluid/rock ratios. Leachates were analysed by ion chromatography and total reflexion X-ray fluorescence (TXRF). The samples were characterized prior to the experiments by X-ray fluorescence, microthermometry, electron microprobe analyses, pyrohydrolysis and TXRF. Furthermore, numerous hydrothermal veins of the Schwarzwald and the Odenwald were characterized with respect to their ore geology, vein mineralogy, mineral chemistry, fluid inclusion composition (microthermometry, crush-leach, LA-ICPMS and Raman spectroscopy), stable isotopic composition and radiogenic isotopic composition. This comprehensive analytical and experimental approach yields new and profound insights into processes relevant for hydrothermal vein-type deposits. Halogens fractionate during fluid-rock interaction, since they are distributed between two reservoirs in the rock: as highly soluble phases and as low soluble phases. Lowest Cl/Br ratios, similar to natural basement brines are obtained by short leaching of medium- to fine-grained rock. Still, the maximum salinities that can be obtained by selective leaching of the low soluble phases are limited to ~10 wt.%. Consequently, at least an additional chlorine source is required for natural basement fluids having high salinities (~26 wt.%) and low Cl/Br ratios. Furthermore, the experiments confirm that substantial amounts of Pb, Zn, Cu, Ni, As and W are released by alteration of common rock-forming minerals. Time resolved trace element, Rb/Cs and Cl/Br variations of vein-hosted fluid inclusions of the Jurassic Brandenberg vein near Todtnau (Germany) were used to monitor alteration processes that occur in cataclastic root zones below the actual hydrothermal vein. On the other hand, the sedimentary aquifers involved in ore-forming processes and their associated fluids are inhomogeneous in their composition on the scale of the Schwarzwald. The combined study of the regional geology, basement brines, sedimentary brines and the ore fluids (mixture) of different vein-types and ages enabled a reconstruction of the evolution and origin of the involved fluids during the last 300 million years. A new genetic model for five-element veins is proposed, which seems to be universally valid for most occurrences of five-element veins. Five-element veins form where strongly reducing methane or methane-bearing fluids are introduced into an active hydrothermal sulphide system. This is the first genetic model for five-element veins, which is in total agreement with all textural and chemical features that are typical of five-element veins.