Sedimentological and structural processes operative along a metalliferous catena from sandstone-hosted to unconformity-related Pb-Cu-Zn deposits in an epicontinental basin, SE Germany

Abstract

The sandstone-hosted Pb-Zn deposits worldwide, including those in the Middle Triassic series of the epicontinental basin in SE Germany, were targeted upon in a literature review by Bjørlykke and Sangster in 1981, treating each mineralization as a separate entity. A different approach based on structural geology, sediment mineralogy and geochemistry (major and trace elements, C-, O-, S- and Pb isotopes) has been taken to study a “metalliferous catena” from sandstone-hosted to unconformity-related Pb-Cu-Zn deposits. The resultant model encompasses five reference types of base metal mineralization lined up along a NW-SE transect perpendicular to the various facies zones of the basin and covering the entire sequence of mineralizing processes from marine-diagenetic processes in the basin center through the epigenetic mineralization finding an outlet in the unconformity-related vein-type deposits close to the basement boundary fault at the edge of the epicontinental basin.The stratiform “Bleiglanzbank-type” (stage I) is confined to the basin center of the Middle Triassic Keuper Basin. Galena derived from bacterial reduction of sulfate and from Pb originating from a marine source of strongly homogenized average basement Pb. The Pb ore evolved in an extensional geodynamic regime. The marine-diagenetic “Bleiglanzbank-type” mineralization can be characterized as a marker stage for a Pb-enriched basin fill.The sulfidic and non-sulfidic stratabound Zn-Cu-Pb mineralization named “Weiden-type” is still dominated by Pb sulfide with only subordinate amounts of cerussite. The ore developed more landward in the epicontinental basin, in a sabkha to alluvial-fluvial depositional environment. The Pb model ages are significantly higher than those of the “Bleiglanzbank-type “ore. The reason for the chemical and facial differences in the epicontinental basin are accounted for by a barrier resultant from synsedimentary faulting around 230Ma. It is a true dia- (to epigenetic) mineralization whose fluid migration is explained by a flood recharge model. Basin dynamic and structural geology changed from extensional to moderately compressional.Two epigenetic, Pb-dominated sandstone-hosted ore types may be defined in the region, the flexurebound sulfidic and non-sulfidic Pb-(Cu-Zn) mineralization of “Eschenfelden-type” (stage III) is structure-related and superimposed on the afore-mentioned “Weiden-type” and the “Freihung-type” (stage IV). In the latter the flexure no longer existed and has turned into a reverse fault while the Pb mineralization became ore grade. The compressional regime dominated the block faulting in this ore type being located more landward. The maximum horizontal stress exerted from the basement onto the platform sediments was reached during the Lower Cretaceous resultant in a stacked pattern of Mesozoic beds and the development of an unconformity truncating the Jurassic beds. Intraformational recycling of Pb was at a maximum around the reverse faults later than 85Ma when reverse faults conduced to an overstepping of Triassic sediments, containing “Weiden-type” Pb protore (Roll front-like intraformational recycling of Pb). The 206Pb/207Pb model ages of “Weiden-“, “Eschenfeld- “and “Freihung-type” ore mineralization are almost identical. Investigations focused on the temperature of formation yielded 110° to 150°C for the sulfides and 141° to 169°C for the carbonate minerals. In this compressional regime, heat has been generated by friction along the fault planes as a function of dip angle and depth, reducing the grain size and delivering elements to the intrastratal solutions. The mineralized structures tend to strike NW-SE and in plan view are shaped concave towards the basement from which the maximum horizontal pressure was exerted onto the platform sediments.The unconformity-related sulfidic Pb-( Zn) mineralization of “Erbendorf-type” (stage V) formed in an extensional regime creating a strike-slip fault system at temperature between 132°C and 200°C. The Pb isotope signature is different from the ore types dealt with previously as far as the U series is concerned and closely resembles the sulfidic members from the sandstone-hosted Pb deposits which both have a common source for Th in monazite. Intraformational recycling in sandstone-hosted Pb deposits and concentration of Pb in unconformity –related vein-type deposits tapped the same element source and belong to the “metalliferous catena” of Pb ore crossing the basin-basement boundary or unconformity, respectively. Sandstone-hosted and unconformity-related mineral deposits are not separate entities but two sides of the same coin, the geodynamic evolution of an epicontinental basin. They are a function of the position within the “metalliferous catena” from the basin center to its edge.