Metallogenesis of early Paleozoic graptolite shales from the Graefenthal Horst (northern Bavaria-Federal Republic of Germany)

Abstract

The Graefenthal horst, situated in the northern part of Bavaria, forms part of the Saxothuringian geotectonic domain of the mid-European Variscides. Psammo-pelitic sediments of Ordovician through lower Carboniferous age were folded during the Sudetian movement (transition from lower Carboniferous to upper Carboniferous). Within this sequence the Silurian lower graptolite shales (about 30 m thick) and Lower Devonian upper graptolite shales (10-20 m thick) are of widespread occurrence. These bioliths (phosphorites, alum shales, and carbonaceous matter-bearing chert) are of potential economic interest because of their metal content (maximum value, V = 9,500 ppm, U = 4,800 ppm). They formed under reducing conditions in the outer shelf, in the presence of abundant H 2 S. The terrigenous input into that basin may be disregarded, and based on mineralogical as well as chemical studies, any volcanic intercalations within the stratigraphy of that basin may be ruled out. Elements were mainly introduced from underlying strata into the bottom waters enriched in H 2 S and from coastward upwelling along the shelf abundant in phosphorus.Two different stages of diagenesis (early and late) may be distinguished. The late stage is the most significant one by reason of its abundance of sphalerite, chalcopyrite, galena, and tetrahedrite. The black shales were first subjected to very low grade or early stage conditions during Variscan regional metamorphism. Subsequent metamorphism to lower greenschist facies involved shearing processes subparallel to the S planes. During dynamometamorphic mobilization, only rock-forming elements were able to migrate. Minor amounts of Zn were concentrated in ruby sphalerite.Strata-bound, fault-hosted Fe-Cu-Zn-Pb-U mineralization was initiated by subsequent block faulting. Its mineral assemblage (pyrite, marcasite, sphalerite, galena, and sooty pitchblende) points to low-temperature conditions and an element derivation from the enclosing carbonaceous sediments.During the Upper Devonian the Ag content of these black shales was locally upgraded by contact metamorphic processes caused by diabase sills, which were intruded along the boundary of the graptolite shales and tentaculite beds. Impregnations of Ag-enriched galena as well as marmatitic sphalerite are encountered in the diabases near both contacts. Hornblende hornfels facies was achieved during that contact metamorphism. Subsequent supergene alteration during Pliocene to Pleistocene times caused uraniferous Fe-Al phosphates to be concentrated along fractures by decomposition of pyrite of the black shales.These metalliferous black shales may be categorized genetically as syndiagenetic-metamorphic polymetallic low-temperature, fault-hosted deposits formed by mobilization. Though not economically viable at the present time, they may be considered as a future reserve for certain metals (such as vanadium and uranium).