HYDROTHERMAL CARBONATES OF THE SCHWARZWALD ORE DISTRICT, SOUTHWESTERN GERMANY: CARBON SOURCE AND CONDITIONS OF FORMATION USING 18O, 13C, 87Sr/86Sr, AND FLUID INCLUSIONS

Abstract

Diagenetic carbonates, metamorphic carbonates, primary hydrothermal carbonates, and secondary remobilized carbonates (including sinters) from the Schwarzwald ore district in SW Germany formed in various tectonic settings and hydrothermal environments over a period of almost 300 Ma. They were investigated in order to define sources of carbon, dispersion of carbon during fluid-rock interaction processes and, where possible, to specify geochemical fingerprints for carbonates formed during different processes and in different geochemical and tectonic environments. For this purpose, 335 samples of calcite, ankerite, dolomite, siderite, and strontianite from 92 localities in 46 mining areas in the Schwarzwald were analyzed for their carbon and oxygen stable isotope, radiogenic strontium isotope, and trace element (including REE) concentrations and compared to analyses from all potential carbon sources available in this region. These include graphite and rare marbles of the crystalline basement, Permian calcrete from redbed sedimentary rocks (Rotliegend) overlying the crystalline basement, and Triassic carbonates from sediments of higher stratigraphic levels (Muschelkalk). Hydrothermal carbonates mostly formed due to fluid-mixing of hot ascending brines with cool sediment-sourced formation water. Fluid inclusions record temperatures of formation between 100 and 150 °C for most primary calcites. The mixed fluid from which they formed was a highly saline brine of around 25 wt.% salinity, containing NaCl and CaCl 2 in similar proportions. Before mixing, the deep brine was in equilibrium with graphite of the basement and contained, as main carbon species, H 2 CO 3 of very low C-isotopic values [around −16‰, Vienna Pee Dee Belemnite (V-PDB)], whereas the sediment-sourced formation water contained HCO 3 − with higher C-isotopic values (around +2‰, V-PDB). We find that graphite and Triassic carbonates in variable proportions (which are mainly related to variations during the fluid mixing process) are the carbon sources for primary calcite, dolomite of the Permian calcrete for primary ankerite, and the Triassic carbonate sediments for the primary ankerite mineralization of the area between Waldkirch and Feldberg. At some localities, remobilization and reprecipitation appears to have taken place without addition of external solutes, as Sr and C show no difference in their isotopic composition between primary and secondary carbonates. The oxygen isotopic composition of secondary carbonates is invariably more positive than that of primary ones, reflecting lower formation temperatures. One very conspicuous type of secondary calcite, which forms olive-green stubby scalenohedra, was dated for the first time using the U-Pb isochron method. Its Neogene age represents uplift and erosional denudation of the Schwarzwald and corresponds well with its remobilized C and O isotope signature. The carbonates in the Schwarzwald hence reflect discontinuous addition of carbon from surface sediments to the crystalline basement through time involving fluid-rock interaction and fluid mixing processes.