Metamorphogenic base metal mineralization at the margin of a nappe complex: the example of Kupferberg, Germany

New data on pyrite chemistry, petrology and most importantly Cu and S isotope data reveal a two-stage genetic model for the Kupferberg Cu-Zn deposit in eastern Germany, with the principal Cu mineralization being related to the expulsion of syn-orogenic fluids from beneath a highgrade metamorphic nappe complex. The deposit consists of several sulfide lenses bound to Early Palaeozoic volcano sedimentary strata at the margin of the allochthonous Munchberg nappe complex. The main Cu ore is texturally late chalcopyrite, mostly within pyrite-rich carbonaceous and calcareous shale in two different tectonostratigraphic units that became juxtaposed during the Variscan Orogeny. Copper isotope analyses yielded a narrow delta Cu-65 range of -0.26 to 0.36 %o for all ore types suggesting a hypogene origin for the principal chalcopyrite mineralization. The delta S-65 values of the ore lenses in the two different tectonostratigraphic units differ distinctly, whereas little difference exists between strongly mineralized ore types and local pyrite-rich beds. This indicates a strata-internal mobilization from pyrite-rich shales that became overthrust by the allochthonous nappe complex. Syn-tectonic fluid expulsion is further indicated by several positive Cu anomalies along the southwestern margin of the nappe complex. The recognition of syn-orogenic Cu mineralization below a high-grade nappe complex raises the possibility of similar, potentially economic base metal deposits to occur at the base of metamorphic nappe complexes also elsewhere in the world.