Diagenetic controls on the sediment-hosted stratiform copper mineralization in the Permian Kupferschiefer district, Saale subbasin, eastern Germany

Abstract

The Kupferschiefer district in the Southern Permian basin of central Europe hosts several sediment-hosted stratiform copper (SCC) deposits. The Cu and Zn-Pb sulfide mineralized rocks in the uppermost part of the terrestrial Rotliegend sandstones (S1), organic matter-rich marine Kupferschiefer mudstones (T1), and Zechstein Limestone (Ca1) units in the Kupferschiefer district are located at a major stratigraphic redox boundary. Various genetic models, including syngenetic, early diagenetic, and late diagenetic to epigenetic, have been proposed to explain the formation of stratabound-stratiform, fine-grained sulfide mineralization in the Kupferschiefer district. Most of these models were constructed based on bulk geochemical and isotopic data interpretations without complimentary detailed petrographic investigations.

In this thesis, work has been focused on samples from three drill holes (Sangerhausen, Allstedt, and Wallendorf) in the Saale subbasin, eastern Germany. All drill holes intersect the S1, T1, and Ca1 in different base metal zones which provides an opportunity to investigate their diagenetic and hydrothermal assemblages. The focus of this thesis is to determine the mineralogy and paragenesis of diagenetic assemblages that controlled fluid flow during ore-stage sulfide mineralization, constrain the processes responsible for the formation and distribution of the calcite cement and assess if hotter mineralizing fluids overprinted its isotopic signature, and constrain the timing between the ore-stage sulfides and pore-filling illite. This has been done using detailed petrography, quantitative mineralogy, bulk rock geochemistry, major element chemistry of carbonates, microanalyses of δ13C and δ18O values of calcite cement using secondary ion mass spectrometry (SIMS), and nanoscale investigations of pore-filling illite adjacent to ore-stage sulfides using ultrahigh-resolution transmission electron microscopy (TEM).

The results of this study showed Cu and Zn-Pb sulfide (bornite, sphalerite, and galena) mineralized rocks in the S1 and T1 formed primarily via the replacement of early diagenetic carbonate cement, mostly predating alteration of detrital clasts. The highest base metal grades were found in the carbonate-rich samples of the T1, deposited under reducing conditions. A minor part of the ore-stage sulfides in the S1 and T1 involved the replacement of detrital feldspar clasts. In the Ca1, the sulfides are dominantly formed in vuggy pores. Vein sulfides in the T1 and Ca1 are also formed as a replacement of calcite veins. The calcite cement in the three units has a similar cathodoluminescence response and major element chemistry, suggesting a common origin. Overlapping δ13C and δ18O values of calcite cement in samples from the S1 and T1 in the Sangerhausen and Wallendorf drill cores suggest that the calcite cement was derived from fluids of similar composition. The δ13C values indicate carbonate alkalinity sourced from seawater-derived fluids and organic matter degradation products. However, the high variability in the δ13C values in the calcite veins in the T1 without significant change in their δ18O values suggests that the alkalinity of the porewater during calcite precipitation in veins was derived from organic matter degradation. The δ18O values suggest the influence of alteration of detrital clasts, with a minor contribution from meteoric water and evaporitic fluids. Furthermore, in pores surrounding the ore-stage bornite, Cu chloride nanoparticles (CuCl NPs) are intergrown with pore-filling illite, which indicates illite precipitation during sulfide mineralization, and limited availability of aqueous reduced sulfur in certain microenvironments and steep chemical gradients between these pores.

Overall, this thesis demonstrates that the reduced nature of the T1 provided the necessary redox gradient for sulfide precipitation and the dissolution of carbonate cement was the main control on the lateral migration of mineralizing fluids in the Saale subbasin. This thesis also provides new constraints that the mineralizing fluid flux or temperature was not sufficient to overprint the isotopic composition of the calcite cement. Furthermore, the results in this study demonstrate that Cu was soluble as chloride complexes, in low-temperature brines that infiltrated the host units during burial diagenesis when both illite formation and organic maturation were active processes.