Comparative geochemical study of different hydrothermal vein deposits of the Schwarzwald (Germany) and implications for ore-forming processes
Abstract
The crystalline basement of the Schwarzwald covers an area of approximately 150 x 80 km. Within this area, more than 100 hydrothermal vein deposits of different types occur. In this study, we measured melt and homogenisation temperatures of fluid inclusions, O, H and S isotopes, and REE patterns of fluorite from a total of 22 occurrences of 3 different vein types (Pb-Zn-Cu-bearing fluorite-barite-quartz veins, Co-Ni-Ag-Bi-U-bearing barite-fluorite-quartz-veins and Sb-Ag-bearing quartz veins) from the whole Schwarzwald area in order to compare their physico-chemical conditions of formation. Interestingly, we found in almost all locations and in all different vein types an aqueous hydrothermal fluid of a rather constant total salinity (Tm -21 to -25°C) of meteoric origin which has the characteristics of a continental basinal brine. Sulfur isotopes of galena and chalcopyrite from all deposits fall within a narrow range of -14 to 0 per mille CDT and may indicate a common sulfur source. REE in fluorite show characteristic patterns of remobilised fluorite in all samples. Calculations for fluorite-barite-quartz veins based on the combination of oxygen isotope equilibria with microthermometric data revealed quartz crystallisation temperatures from 120 to 170°C and pressures between 0.25 to 0.5 kbar. Despite of the relatively uniform chemical and isotopic composition of the hydrothermal fluid and of the ore minerals in the different deposits, each vein type also shows some characteristic geochemical features in their REE patterns and oxygen isotopes: fluorite of the barite-fluorite-quartz veins of the Co-Ni-Ag-Bi-U type have no to negative Eu anomalies and a strong depletion of LREE, whereas fluorite of the Pb-Zn-Cu veins have positive Eu anomalies and a lesser depletion of LREE. Quartz oxygen isotopes from all fluorite-barite-bearing veins fall in a narrow range of 12 to 21 per mille SMOW, whereas quartz oxygen isotope compositions from Sb-Ag-quartz veins range from 2.5 to 18 per mille SMOW, recording either a higher formation temperature from the same fluid (at temperatures around 300°C) or a formation from an only rarely preserved hydrothermal fluid of different origin. In the Schwarzwald basement several large scale hydrothermal systems at various times (from the Perm to Tertiary) generated chemically and isotopically almost homogeneous, highly saline fluids from which the various vein assemblages precipitated.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....1059S