Galenas from base metal (Pb-Zn-Cu) hydrothermal mineralizations in the northern Eifel and their country rocks as potential sources for the metals have been studied by means of lead isotopes.
The mineralizations occur as (i) vein systems within a thick pile of shales, siltstones and greywackes of the lower Devonian basement, (ii) impregnations of sandstones and conglomerates of the middle Bunter which unconformably overlies the folded Devonian rocks and (iii) small cavity
fillings in carbonates and marls of the Muschelkalk and Keuper. The latter two types of mineralization are found at the margins of the graben-like Triassic Triangle.
In 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pb vs. 206Pb/204Pb diagrams, 46 galena samples from various
occurrences form two very uniform populations of lead isotopic compositions defined by 18.142 < 206Pb/204Pb < 18.160 and 18.303 < 206Pb/204Pb < 18.411. The lead shows characteristics of crustal origin.
Six galenas from small veins hosted by lower Devonian rocks make up the less radiogenic population, whereas all other ore samples plot into the higher radiogenic array. Within this cluster it is not possible to distinguish between galena lead from former economically important vein occurrences and large sandstone-hosted disseminated mineralizations, and carbonate cavity fillings on basis of their isotopic compositions. This suggests strong genetic links between the different types of mineralization. It can therefore be assumed, that the main mineralization took place after Keuper deposition, i.e. it has a maximum age of about 230 Ma.
The high internal uniformity of the two populations
indicates extraction of lead either from very homogeneous sources during short time periods, or homogenization of lead from different source rocks due to the hydrothermal mineralizing processes.
In order to determine possible sources for the lead, U/Pb isotope analyses were carried out on 8 lower Devonian and Triassic rocks which host the different types of mineralization. Since the age of all mineralizations is only poorly constrained, the rocks were corrected for U decay using different assumed mineralization ages back to the Keuper.
A comparison with the galena isotopic composition shows, that the lead of almost all investigated rocks was too radiogenic to contribute to the galena mineralizations even at the time of Keuper sediment deposition.
Only one thin sequence of lower Devonian shales corresponds to the ore lead in its isotopic composition about 120 Ma ago. Because the isotopic uniformity of the higher radiogenic ore lead geographically covers a very large area of about 1700 km2, a selective extraction of lead from these shales is highly improbable.
The results permit to rule out the host rocks of the mineralizations as possible sources for the lead, but rather indicate that the lead was mobilized from remote sources by a large hydrothermal system and was homogenized isotopically during long migration paths.