Control of organic matter on metal mineralisation in uranium-rich black shale “Kupferschiefer”

H.-M. Schulz, S. Bernard, F. Perssen, J.A. Stammeier; H.-M. Schulz; F. Perssen; J.A. Stammeier; S. Bernard

by admin | Feb 28, 2025 | mainpost, vol34

TY  - JOUR
T1  - Control of organic matter on metal mineralisation in uranium-rich black shale “Kupferschiefer”
JO  - Geochemical Perspectives Letters
VL  - 34
SP  - 17
EP  - 22
PY  - 2025
AU  - H.-M. SchulzAU  - S. BernardAU  - F. PerssenAU  - J.A. StammeierDO  - https://doi.org/10.7185/geochemlet.2508
UR  - https://www.geochemicalperspectivesletters.org/article2508
AB  - The natural formation of economically important metal sulfides in the geological “Kupferschiefer” system in Central Europe (i.e. its mineralisation) results from a complex and poorly understood interplay of various processes involving organic matter (OM). Here, we demonstrate that transport of aqueous fluids rich in metal chlorides was limited by laminated, non-porous, and non-fluorescent solid OM in the TOC- and uranium-rich shale T1 of the Kupferschiefer system. These organic laminae exhibit XANES spectra consistent with irradiated type I kerogen or pyrobitumen and can thus be interpreted as remains of fossilised algal mats. The laminae not only enhanced sealing integrity against metal-rich aqueous fluids, but also acted as reductant for metal chloride. Moreover, retained paraffinic oil clogged pore space and reduced permeability. Similar to processes in petroleum systems, metal-rich chloride brines migrated into a shale reservoir with calcite porosity which was self sealed by internal, impermeable organic laminae. Our results highlight that the fate and behaviour of OM and its degradation products are key for the understanding of a variety of organic-inorganic interactions as they bridge the gap between different geochemical sinks.

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Control of organic matter on metal mineralisation in uranium-rich black shale “Kupferschiefer”

H.-M. Schulz¹,

¹GFZ German Research Centre for Geosciences, Department 3: Geochemistry, D-14473 Potsdam, Germany

S. Bernard²,

²Muséum National d’Histoire Naturelle, Institut de Minéralogie, Physique des Matériaux et Cosmochimie, CNRS UMR 7590, Sorbonne Université, CNRS, F-75005 Paris, France

F. Perssen¹,

¹GFZ German Research Centre for Geosciences, Department 3: Geochemistry, D-14473 Potsdam, Germany

J.A. Stammeier¹

¹GFZ German Research Centre for Geosciences, Department 3: Geochemistry, D-14473 Potsdam, Germany

Affiliations | Corresponding Author | Cite as | Funding information

Geochemical Perspectives Letters v34 | https://doi.org/10.7185/geochemlet.2508
Received 15 November 2024 | Accepted 27 January 2025 | Published 28 February 2025

Published by the European Association of Geochemistry
under Creative Commons License CC BY 4.0

Keywords: black shale, organic matter, metal, petroleum, uranium



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Abstract

The natural formation of economically important metal sulfides in the geological “Kupferschiefer” system in Central Europe (i.e. its mineralisation) results from a complex and poorly understood interplay of various processes involving organic matter (OM). Here, we demonstrate that transport of aqueous fluids rich in metal chlorides was limited by laminated, non-porous, and non-fluorescent solid OM in the TOC- and uranium-rich shale T1 of the Kupferschiefer system. These organic laminae exhibit XANES spectra consistent with irradiated type I kerogen or pyrobitumen and can thus be interpreted as remains of fossilised algal mats. The laminae not only enhanced sealing integrity against metal-rich aqueous fluids, but also acted as reductant for metal chloride. Moreover, retained paraffinic oil clogged pore space and reduced permeability. Similar to processes in petroleum systems, metal-rich chloride brines migrated into a shale reservoir with calcite porosity which was self sealed by internal, impermeable organic laminae. Our results highlight that the fate and behaviour of OM and its degradation products are key for the understanding of a variety of organic-inorganic interactions as they bridge the gap between different geochemical sinks.

Figures

Figure 1 (a) Geographic map of wells, Eisleben shaft, and Lieth outcrop (black star symbol) in Germany where samples were taken (details in Table S-1). Blue colour indicates today’s distribution of the Kupferschiefer T1 (modified after Paul, 2006; Borg et al., 2012). Grey shaded areas indicate mineralised areas (combined for areas >0.2 % Cu, Pb, and Zn; Wedepohl and Rentzsch, 2006). (b) Schematic sketch showing the concept of lateral to vertical metal zonation and fluid migration direction of the epigenetic Kupferschiefer system (modified after Borg et al., 2012). Well locations indicate conceptual position relative to Cu, Pb or Zb mineralisation, or where barren Kupferschiefer occurs. White star symbol with “R” indicates samples taken from Rote Fäule in mines (details in Tables S-1, S-2).	Figure 2 (a) Kerogen typing by plotting hydrogen index HI vs. oxygen index OI. Inset: Hydrogen index HI vs. T_max. (b) Uranium content (ppm) vs. hydrogen index HI. Inset: Reflectance of laminated pyrobitumen plotted against uranium content. (c) Uranium content (ppm) vs. relative gas content (C1–C5) of pyrolysate. (d) Uranium content (ppm) vs. aromaticity of pyrolysate (%). Aromaticity is the relative content of o-xylene over the sum of o-xylene and n-nonene. Comparative data in (c) and (d) for Cambro-Ordovician Kolm and Alum Shale samples, the Triassic Yanchang Fm. from the Ordos Basin in China, and the Tertiary Mulga Rock in Australia are from Yang et al. (2020). Dotted green line shows uranium content variations in various wells in the Spremberg deposit after Spieth (2019) whereas the dotted purple line is for the Sangerhausen Basin after Hammer et al. (1990).	Figure 3 (a, b) Organic laminae without ore mineralisation. (a) G019422, ZnS facies, Koenen shaft 2, Nienstedt, depth unknown, well laminated OM layers. (b) G018899, well Allstedt, 946.21 m depth, tightly stacked, well laminated OM layers. (c, d) Organic laminae as reactant for ore mineralisation. (c) G020249, Pb/Zn facies, well Ig-Bottendorf-01-2012, 107.5 m depth; left, fluorescence while UV illumination shows small alginate; right, reflected white light. ZnS precipitation replaces organic laminae. (d) G019422, ZnS facies, Koehnen shaft, Nienstedt, depth unknown. Sphalerite precipitated in dissolved carbonate layers surrounded by laminated OM structures, or replaces reductive OM. (e, f) Organic laminae as seal against upward ore mineralisation. (e) G019422, ZnS facies, Koenen shaft 2, Nienstedt, depth unknown, strong ZnS mineralisation in dissolved carbonate in close association with laminated OM layers as seal. (f) G019421, Cu facies, close to Rote Fäule, Münzer shaft, Sangerhausen, depth unknown. Dense network of organic laminae restricts ore mineralisation.	Figure 4 STXM characterisation of the investigated samples at the carbon K-edge showing C-XANES spectra of different pyrobitumens in Kupferschiefer T1. The spectra are compared with pyrobitumen of the Lower Jurassic Posidonia Shale in northern Germany (Bernard et al., 2012a), pyrobitumen in the Mississippian Barnett Shale (Fort Worth Basin, Texas, USA; Bernard et al., 2012b), and with kerogen type I and III as reference and after irradiation (Le Guillou et al., 2013).
Figure 1	Figure 2	Figure 3	Figure 4

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Introduction

Organic-inorganic interactions in sedimentary basins are important reaction networks which also controlled how metals precipitated in black shales in the geological past (e.g., Bechtel et al., 2002

Bechtel, A., Gratzer, R., Püttmann, W., Oszczepalski, S. (2002) Geochemical characteristics across the oxic/anoxic interface (Rote Fäule front) within the Kupferschiefer of the Lubin-Sieroszowice mining district (SW Poland). Chemical Geology 185, 9–31. https://doi.org/10.1016/S0009-2541(01)00395-3

; Greenwood et al., 2013

Greenwood, P.F., Brocks, J.J., Grice, K., Schwark, L., Jaraula, C.M.B., Dick, J.M., Evans, K.A. (2013) Organic geochemistry and mineralogy. I. Characterisation of organic matter associated with metal deposits. Ore Geology Reviews 50, 1–27. https://doi.org/10.1016/j.oregeorev.2012.10.004

). The Permian Kupferschiefer ore system, a sediment-hosted stratiform copper deposit in northern Central Europe (Oszczepalski, 1999

Oszczepalski, S. (1999) Origin of the Kupferschiefer polymetallic mineralization in Poland. Mineralium Deposita 34, 599–613. https://doi.org/10.1007/s001260050222

; Borg et al., 2012

Borg, G., Piestrzyński, A., Bachmann, G.H., Püttmann, W., Walther, S., Fiedler, M. (2012) An Overview of the European Kupferschiefer Deposits. In: Hedenquist, J.W., Harris, M., Camus, F. (Eds.) Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe. Society of Economic Geologists, Littleton, Special Publication 16, 455–486. https://doi.org/10.5382/SP.16.18

), is a natural laboratory to study the control of organic matter (OM) on mineralisation processes, especially the organic-inorganic interactions involved. The ore system is characterised by an elevated content of copper, zinc and lead sulfides, and occurs in lower Permian sandstones/conglomerates S1, the overlying upper Permian OM-rich black shale unit T1 (hereafter “Kupferschiefer”; Fig. S-1a,b), the carbonate Ca1 above (Fig. S-1a), and occasionally the hanging anhydrite A1 (Borg et al., 2012

Borg, G., Piestrzyński, A., Bachmann, G.H., Püttmann, W., Walther, S., Fiedler, M. (2012) An Overview of the European Kupferschiefer Deposits. In: Hedenquist, J.W., Harris, M., Camus, F. (Eds.) Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe. Society of Economic Geologists, Littleton, Special Publication 16, 455–486. https://doi.org/10.5382/SP.16.18

; Oszczepalski and Chmielewski, 2018

Oszczepalski, S., Chmielewski, A. (2018) Mineralization of the Zechstein Lower Anhydrite in the Fore-Sudetic Monocline. Biuletyn Państwowego Instytutu Geologicznego 472, 135–154. https://doi.org/10.5604/01.3001.0012.7116

). The lower Kupferschiefer black shale T1 of the ore system exhibits high total organic carbon (TOC) concentrations ≥20 wt. %. Such a large amount of OM is believed to have played a crucial role in trapping metals, with redox reactions between reducing OM and oxidising fluids leading to ore formation (Püttmann et al., 1991

Püttmann, W., Fermont, W.J.J., Speczik, S. (1991) The possible role of organic matter in transport and accumulation of metals exemplified at the Permian Kupferschiefer formation. Ore Geology Reviews 6, 563–579. https://doi.org/10.1016/0169-1368(91)90047-B

). Yet, the exact processes involved remain to be understood. Here, we compile published mineralogical and inorganic geochemical data of variously mineralised, altered or barren Kupferschiefer sections from a variety of geological settings in Germany (Fig. 1a), and combine them with new analytical results. The new data was obtained using reflection microscopy, scanning electron microscopy (SEM), scanning transmission X-ray microscopy (STXM), open pyrolysis and thermovaporisation. The results unravel the architecture, type and composition of solid OM, thus clarifying its origin and alteration history. The data shows that OM acted as a physical seal and geochemical trap. Altogether, the Kupferschiefer mineralisation processes share similarities with processes occurring in conventional and unconventional petroleum systems. Similarities also exist with hydrogeochemical redox processes in sedimentary basins when geochemical processes are controlled by OM as reductant. OM reduces soluble U(VI) to U(IV) in uranium roll-fronts, or provides anoxic microenvironments for the reaction of metal chloride to sulfide in black shale-hosted stratiform base metal deposits. Our new findings highlight the critical role of OM in sedimentary ore formation, providing both geochemical reactivity and the physical barrier, underlining the significance of organic-inorganic interactions in other OM-rich systems.

**Figure 1** **(a)** Geographic map of wells, Eisleben shaft, and Lieth outcrop (black star symbol) in Germany where samples were taken (details in Table S-1). Blue colour indicates today’s distribution of the Kupferschiefer T1 (modified after Paul, 2006
Paul, J. (2006) The Kupferschiefer: Lithology, stratigraphy, facies and metallogeny of a black-shale. *Zeitschrift der Deutschen Gesellschaft für Geowissenschaften* 157, 57–76. https://doi.org/10.1127/1860-1804/2006/0157-0057
; Borg *et al*., 2012
Borg, G., Piestrzyński, A., Bachmann, G.H., Püttmann, W., Walther, S., Fiedler, M. (2012) An Overview of the European Kupferschiefer Deposits. In: Hedenquist, J.W., Harris, M., Camus, F. (Eds.) *Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe*. Society of Economic Geologists, Littleton, Special Publication 16, 455–486. https://doi.org/10.5382/SP.16.18
). Grey shaded areas indicate mineralised areas (combined for areas >0.2 % Cu, Pb, and Zn; Wedepohl and Rentzsch, 2006
Wedepohl, K.H., Rentzsch, J. (2006) The composition of brines in the early diagenetic mineralization of the Permian Kupferschiefer in Germany. *Contributions to Mineralogy and Petrology* 152, 323–333. https://doi.org/10.1007/s00410-006-0105-4
). **(b)** Schematic sketch showing the concept of lateral to vertical metal zonation and fluid migration direction of the epigenetic Kupferschiefer system (modified after Borg *et al*., 2012
Borg, G., Piestrzyński, A., Bachmann, G.H., Püttmann, W., Walther, S., Fiedler, M. (2012) An Overview of the European Kupferschiefer Deposits. In: Hedenquist, J.W., Harris, M., Camus, F. (Eds.) *Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe*. Society of Economic Geologists, Littleton, Special Publication 16, 455–486. https://doi.org/10.5382/SP.16.18
). Well locations indicate conceptual position relative to Cu, Pb or Zb mineralisation, or where barren Kupferschiefer occurs. White star symbol with “R” indicates samples taken from Rote Fäule in mines (details in Tables S-1, S-2).

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Kupferschiefer Ore Formation in Brief

A single- or multiple-stage epigenetic hydrothermal formation is generally accepted today (summarised in Borg et al., 2012

Borg, G., Piestrzyński, A., Bachmann, G.H., Püttmann, W., Walther, S., Fiedler, M. (2012) An Overview of the European Kupferschiefer Deposits. In: Hedenquist, J.W., Harris, M., Camus, F. (Eds.) Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe. Society of Economic Geologists, Littleton, Special Publication 16, 455–486. https://doi.org/10.5382/SP.16.18

; Symons et al., 2011

Symons, D.T.A., Kawasaki, K., Walther, S., Borg, G. (2011) Paleomagnetism of the Cu–Zn–Pb-bearing Kupferschiefer black shale (Upper Permian) at Sangerhausen, Germany. Mineralium Deposita 46, 137–152. https://doi.org/10.1007/s00126-010-0319-2

). In such a scenario, metals were transported as chloride species by brines (Brown, 1984

Brown, A.C. (1984) Alternative sources of metals for stratiform copper deposits. Precambrian Research 25, 61–74. https://doi.org/10.1016/0301-9268(84)90024-X

). Yet the source of the metals remains controversial (Borg et al., 2012

Borg, G., Piestrzyński, A., Bachmann, G.H., Püttmann, W., Walther, S., Fiedler, M. (2012) An Overview of the European Kupferschiefer Deposits. In: Hedenquist, J.W., Harris, M., Camus, F. (Eds.) Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe. Society of Economic Geologists, Littleton, Special Publication 16, 455–486. https://doi.org/10.5382/SP.16.18

; Oszczepalski, 1999

Oszczepalski, S. (1999) Origin of the Kupferschiefer polymetallic mineralization in Poland. Mineralium Deposita 34, 599–613. https://doi.org/10.1007/s001260050222

), as well as the timing of the ore mineralisation in different regions (Symons et al., 2011

Symons, D.T.A., Kawasaki, K., Walther, S., Borg, G. (2011) Paleomagnetism of the Cu–Zn–Pb-bearing Kupferschiefer black shale (Upper Permian) at Sangerhausen, Germany. Mineralium Deposita 46, 137–152. https://doi.org/10.1007/s00126-010-0319-2

; Pašava et al., 2010

Pašava, J., Oszczepalski, S., Du, A. (2010) Re–Os age of non-mineralized black shale from the Kupferschiefer, Poland, and implications for metal enrichment. Mineralium Deposita 45, 189–199. https://doi.org/10.1007/s00126-009-0269-8

). Such uncertainties may be due either to alteration and recrystallisation of existing sulfides, or to multistage processes which varied in time and space (Alderton et al., 2016

Alderton, D.H.M., Selby, D., Kucha, H., Blundell, D.J. (2016) A multistage origin for Kupferschiefer mineralization. Ore Geology Reviews 79, 535–543. https://doi.org/10.1016/j.oregeorev.2016.05.007

). Recent investigations by Mohammedyasin et al. (2023

Mohammedyasin, M.S., Magnall, J.M., Gleeson, S.A., Schulz, H.-M., Schleicher, A.M., Stammeier, J.A., Ehling, B.-C. (2023) Diagenetic History and Timing of Cu and Zn-Pb Sulfide Mineralization in the Permian Kupferschiefer System, Saale Subbasin, Eastern Germany. Economic Geology 118, 1467–1494. https://doi.org/10.5382/econgeo.5015

) demonstrated that early diagenetic carbonate (and feldspar) dissolution (Fig. S-1c) created pathways for the infiltrating metal-rich hydrothermal fluids, pinpointing an epigenetic mineralisation. The Kupferschiefer system also exhibits vertical and lateral zonation of metal sulfides (Fig. 1b). This zonation is essentially linked to the hematite-bearing “Rote Fäule” facies, which is thought to represent a dynamic oxidation front (Borg et al., 2012

Borg, G., Piestrzyński, A., Bachmann, G.H., Püttmann, W., Walther, S., Fiedler, M. (2012) An Overview of the European Kupferschiefer Deposits. In: Hedenquist, J.W., Harris, M., Camus, F. (Eds.) Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe. Society of Economic Geologists, Littleton, Special Publication 16, 455–486. https://doi.org/10.5382/SP.16.18

). Zonation is also visible in OM characteristics. Lower hydrogen indices, higher T_max values (from Rock Eval pyrolysis), and stronger aromatisation may occur in and close to the Rote Fäule (Püttmann et al., 1991

Püttmann, W., Fermont, W.J.J., Speczik, S. (1991) The possible role of organic matter in transport and accumulation of metals exemplified at the Permian Kupferschiefer formation. Ore Geology Reviews 6, 563–579. https://doi.org/10.1016/0169-1368(91)90047-B

; Bechtel et al., 2001

Bechtel, A., Gratzer, R., Püttmann, W., Oszczepalski, S. (2001) Variable alteration of organic matter in relation to metal zoning at the Rote Fäule front (Lubin-Sieroszowice mining district, SW Poland). Organic Geochemistry 32, 377–395. https://doi.org/10.1016/S0146-6380(01)00002-X

, 2002

Bechtel, A., Gratzer, R., Püttmann, W., Oszczepalski, S. (2002) Geochemical characteristics across the oxic/anoxic interface (Rote Fäule front) within the Kupferschiefer of the Lubin-Sieroszowice mining district (SW Poland). Chemical Geology 185, 9–31. https://doi.org/10.1016/S0009-2541(01)00395-3

).

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Materials and Methods

Shale samples were collected from the Kupferschiefer T1 interval in several drill cores from Brandenburg, Saxony Anhalt, and Bavaria. Samples were also obtained from now closed mines in Saxony Anhalt (Eisleben) and an outcrop in Schleswig Holstein (Lieth; Fig. 1a). Sediment samples represent deposition in three basin locations with diverse depositional features: (i) basin centre (Lieth outcrop), (ii) distal (Brandenburg, Saxony Anhalt), and (iii) near the southern basin margin (Bavaria). The Kupferschiefer T1 in the Lieth outcrop is not mineralised and serves for comparison with other organic petrographic results (see Fig. S-2). Detailed information for each sample is available in Tables S-1 and S-2. This includes published results (Mohammedyasin et al., 2023

Mohammedyasin, M.S., Magnall, J.M., Gleeson, S.A., Schulz, H.-M., Schleicher, A.M., Stammeier, J.A., Ehling, B.-C. (2023) Diagenetic History and Timing of Cu and Zn-Pb Sulfide Mineralization in the Permian Kupferschiefer System, Saale Subbasin, Eastern Germany. Economic Geology 118, 1467–1494. https://doi.org/10.5382/econgeo.5015

; Poetz et al., 2022

Poetz, S., Liu, Y., Magnall, J.M., Vieth-Hillebrand, A., Yang, S., Göthel, M., Gleeson, S.A., Schulz, H.-M. (2022) Signals of low grade organic matter alteration in the Upper Permian Kupferschiefer (Spremberg area, Eastern Germany) – A by-product of copper mineralization? Organic Geochemistry 169, 104421. https://doi.org/10.1016/j.orggeochem.2022.104421

; Spieth, 2019

Spieth, V. (2019) Zechstein Kupferschiefer at Spremberg and Related Sites: Hot Hydrothermal Origin of the Polymetallic Cu-Ag-Au Deposit. Ph.D. thesis, University of Stuttgart, Germany.

) as well as new data about TOC and U content, facies (Cu/Pb/Zn or barren), hydrogen index (HI), and from open pyrolysis (aromaticity and gas of pyrolysate), and thermovaporisation. Full details about the procedures of the geochemical and imaging analyses are available in the Supplementary Information.

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Results

The OM in the investigated samples is mostly type I/II kerogen with various hydrogen index values (HI), whereas samples from Sangerhausen and Eisleben contain type III kerogen (Fig. 2a). Highest HI values were recorded in the Bottendorf well and the two Spremberg wells 115 and 121. These samples may best reflect the original composition and thus lowest alteration (Fig. 2a). Except for some preserved and fluorescing algal bodies in samples with low U content (<50 ppm, and HI > 200 mg HC/gTOC; e.g., Fig. 2b), the particulate OM in the Kupferschiefer mainly occurs as various forms of a non-fluorescent solid which extends along lamination (e.g., Fig. 3a,b; see also Fig. S-3), forms net-like layers (e.g., Fig. 3f) or encases mineral grains (Fig. 3d,e). These bedding-parallel organic layers exclusively occur at the bottom of the Kupferschiefer. Moreover, the layers lack (nano)porosity (see Fig. S-4) which is typical for a thermal maturity from the onset until peak oil formation. Such a maturity corresponds to the T_max values between 420 and 440 °C (Fig. 2a inset). The reflectance of the solid organic layers falls into two distinct groups (Fig. 2b inset) confirming the presence of type I kerogen (reflectance ≤ 0.3 % R_r) and some other maceral type (reflectance ≥ 1.1 % R_r; cf. Koch, 1997

Koch, J. (1997) Organic petrographic investigations of the Kupferschiefer in northern Germany. International Journal of Coal Geology 33, 301–316. https://doi.org/10.1016/S0166-5162(96)00047-X

; Oszczepalski, 1999

Oszczepalski, S. (1999) Origin of the Kupferschiefer polymetallic mineralization in Poland. Mineralium Deposita 34, 599–613. https://doi.org/10.1007/s001260050222

) in areas enriched in U-bearing species (oxides, phosphates, etc.; Kucha, 2021

Kucha, H. (2021) New minerals and the first mineral occurrences in the Kupferschiefer (U, REE, HgS, chloride minerals PtAs₂, Pt(Sb,Bi)₂, PtBi₂), Poland, and their genetic meaning. Mineralogia 52, 31–42. https://doi.org/10.2478/mipo-2021-0004

). The Kupferschiefer may contain up to 670 ppm U in the Sangerhausen area (Hammer et al., 1990

Hammer, J., Junge, F., Rösler, H.J., Niese, S., Gleisberg, B., Stiehl, G. (1990) Element and isotope geochemical investigations of the Kupferschiefer in the vicinity of “Rote Fäule”, indicating copper mineralization (Sangerhausen basin, G.D.R.). Chemical Geology 85, 345–360. https://doi.org/10.1016/0009-2541(90)90012-V

). Even higher values up to 1.3 wt. % U are observed in the clay-organic matrix and more than 10 wt. % U in thucholite in the Polish Kupferschiefer (Kucha, 2003

Kucha, H. (2003) Geology, mineralogy and geochemistry of the Kupferschiefer, Poland. In: Kelly, J.G., Andrew, C.J., Ashton, J.H., Boland, M.B., Earls, G., Fus-Ciardi, L., Stanley, G. (Eds.) Europe’s Major Base Metal Deposits. Irish Association for Economic Geology, Dublin, 215–238. https://doi.org/10.2113/gsecongeo.102.4.761

; thucholite are grains of bitumen polymerised by the radiolytic action of U and Th). Related to the zoning, highest U concentrations are reported from the Cu zone in direct contact to the Rote Fäule front in the Sangerhausen area and SW Poland (Hammer et al., 1990

Hammer, J., Junge, F., Rösler, H.J., Niese, S., Gleisberg, B., Stiehl, G. (1990) Element and isotope geochemical investigations of the Kupferschiefer in the vicinity of “Rote Fäule”, indicating copper mineralization (Sangerhausen basin, G.D.R.). Chemical Geology 85, 345–360. https://doi.org/10.1016/0009-2541(90)90012-V

; Bechtel et al., 2002

Bechtel, A., Gratzer, R., Püttmann, W., Oszczepalski, S. (2002) Geochemical characteristics across the oxic/anoxic interface (Rote Fäule front) within the Kupferschiefer of the Lubin-Sieroszowice mining district (SW Poland). Chemical Geology 185, 9–31. https://doi.org/10.1016/S0009-2541(01)00395-3

). Within a given location, uranium concentration shows a positive correlation with both the aromaticity and gas content of the pyrolysate, except for wells Allstedt and Bottendorf, which each encompass only two or three samples (Fig. 2c,d). At the same time U concentrations tend to negatively correlate with HI values (Fig. 2b; see also Yang et al., 2020

Yang, S., Schulz, H.-M., Horsfield, B., Schovsbo, N.H., Grice, K., Zhang, J. (2020) Geological alteration of organic macromolecules by irradiation: Implication for organic matter occurrence on Mars. Geology 48, 713–717. https://doi.org/10.1130/G47171.1

). It is important in this context that transition metals like Cu, Pb, or Zn, etc., in Kupferschiefer have no effect on the kind and amount of gaseous compounds generated in hydrous pyrolysis experiments (Lewan et al., 2008

Lewan, M.D., Kotarba, M.J., Więcław, D., Piestrzyński, A. (2008) Evaluating transition-metal catalysis in gas generation from the Permian Kupferschiefer by hydrous pyrolysis. Geochimica et Cosmochimica Acta 72, 4069–4093. https://doi.org/10.1016/j.gca.2008.06.003

). At high uranium concentrations in the investigated samples, the hydrocarbon fragments generated during open pyrolysis are characterised by gaseous compounds like methane, ethane, etc., besides some low molecular, aromatic compounds like benzene, toluene or naphthalene (except for sample G018899 from well Allstedt). At low uranium concentrations higher molecular alkanes occur together with their alkenes (Fig. S-5). Such features correspond to the metal and U zonation in the Kupferschiefer when the gaseous, low molecular aromatic signature is characteristic for the Rote Fäule zone, but also occurs in the U-rich Cu zone. Due to mobilisation by the oxic brines which caused the Rote Fäule facies, U concentrations typically increase in the neighbouring Cu zone, and then decline in the Pb and Zn zone (Hammer et al., 1990

Hammer, J., Junge, F., Rösler, H.J., Niese, S., Gleisberg, B., Stiehl, G. (1990) Element and isotope geochemical investigations of the Kupferschiefer in the vicinity of “Rote Fäule”, indicating copper mineralization (Sangerhausen basin, G.D.R.). Chemical Geology 85, 345–360. https://doi.org/10.1016/0009-2541(90)90012-V

; Bechtel et al., 2002

Bechtel, A., Gratzer, R., Püttmann, W., Oszczepalski, S. (2002) Geochemical characteristics across the oxic/anoxic interface (Rote Fäule front) within the Kupferschiefer of the Lubin-Sieroszowice mining district (SW Poland). Chemical Geology 185, 9–31. https://doi.org/10.1016/S0009-2541(01)00395-3

). The retained hydrocarbons, mainly n-alkanes, released from the samples by thermovaporisation reveal a similar trend: higher molecular alkanes dominate whereas this feature is missing at high U concentrations in the Rote Fäule and the Cu zone where aromatic hydrocarbons dominate (Fig. S-6, again except for sample G018899 in well Allstedt). A qualitative comparison of the data gained by open pyrolysis and thermovaporisation indicates retention of a paraffinic oil fraction (Fig. S-7).

**Figure 2** **(a)** Kerogen typing by plotting hydrogen index HI *vs.* oxygen index OI. Inset: Hydrogen index HI *vs.* T_max. **(b)** Uranium content (ppm) *vs.* hydrogen index HI. Inset: Reflectance of laminated pyrobitumen plotted against uranium content. **(c)** Uranium content (ppm) *vs.* relative gas content (C1–C5) of pyrolysate. **(d)** Uranium content (ppm) *vs.* aromaticity of pyrolysate (%). Aromaticity is the relative content of o-xylene over the sum of o-xylene and n-nonene. Comparative data in **(c)** and **(d)** for Cambro-Ordovician Kolm and Alum Shale samples, the Triassic Yanchang Fm. from the Ordos Basin in China, and the Tertiary Mulga Rock in Australia are from Yang *et al*. (2020
Yang, S., Schulz, H.-M., Horsfield, B., Schovsbo, N.H., Grice, K., Zhang, J. (2020) Geological alteration of organic macromolecules by irradiation: Implication for organic matter occurrence on Mars. *Geology* 48, 713–717. https://doi.org/10.1130/G47171.1
). Dotted green line shows uranium content variations in various wells in the Spremberg deposit after Spieth (2019
Spieth, V. (2019) *Zechstein Kupferschiefer at Spremberg and Related Sites: Hot Hydrothermal Origin of the Polymetallic Cu-Ag-Au Deposit*. Ph.D. thesis, University of Stuttgart, Germany.
) whereas the dotted purple line is for the Sangerhausen Basin after Hammer *et al*. (1990)
Hammer, J., Junge, F., Rösler, H.J., Niese, S., Gleisberg, B., Stiehl, G. (1990) Element and isotope geochemical investigations of the Kupferschiefer in the vicinity of “Rote Fäule”, indicating copper mineralization (Sangerhausen basin, G.D.R.). *Chemical Geology* 85, 345–360. https://doi.org/10.1016/0009-2541(90)90012-V
.

**Figure 3** **(a, b)** Organic laminae without ore mineralisation. **(a)** G019422, ZnS facies, Koenen shaft 2, Nienstedt, depth unknown, well laminated OM layers. **(b)** G018899, well Allstedt, 946.21 m depth, tightly stacked, well laminated OM layers. **(c, d)** Organic laminae as reactant for ore mineralisation. **(c)** G020249, Pb/Zn facies, well Ig-Bottendorf-01-2012, 107.5 m depth; left, fluorescence while UV illumination shows small alginate; right, reflected white light. ZnS precipitation replaces organic laminae. **(d)** G019422, ZnS facies, Koehnen shaft, Nienstedt, depth unknown. Sphalerite precipitated in dissolved carbonate layers surrounded by laminated OM structures, or replaces reductive OM. **(e, f)** Organic laminae as seal against upward ore mineralisation. **(e)** G019422, ZnS facies, Koenen shaft 2, Nienstedt, depth unknown, strong ZnS mineralisation in dissolved carbonate in close association with laminated OM layers as seal. **(f)** G019421, Cu facies, close to Rote Fäule, Münzer shaft, Sangerhausen, depth unknown. Dense network of organic laminae restricts ore mineralisation.

The C-XANES spectra of the laminated OM from the Kupferschiefer (Fig. 4) share similarities with those of pyrobitumens (i.e. products of secondary cracking of bitumen), such as the pyrobitumens from the marine Posidonia Shale (Lower Jurassic, northern Germany) and the Mississippian Barnett Shale (Fort Worth Basin, Texas, USA) (Bernard et al., 2012a

Bernard, S., Horsfield, B., Schulz, H.-M., Wirth, R., Schreiber, A., Sherwood, N. (2012a) Geochemical evolution of organic-rich shales with increasing maturity: A STXM and TEM study of the Posidonia Shale (Lower Toarcian, northern Germany). Marine and Petroleum Geology 31, 70–89. https://doi.org/10.1016/j.marpetgeo.2011.05.010

, b

Bernard, S., Horsfield, B., Schulz, H.-M., Wirth, R., Schreiber, A., Sherwood, N. (2012a) Geochemical evolution of organic-rich shales with increasing maturity: A STXM and TEM study of the Posidonia Shale (Lower Toarcian, northern Germany). Marine and Petroleum Geology 31, 70–89. https://doi.org/10.1016/j.marpetgeo.2011.05.010

). However, irradiation may modify the structure of immature type I or type III kerogens in such a way that it will produce residues exhibiting similar XANES spectra (Le Guillou et al., 2013

Le Guillou, C., Remusat, L., Bernard, S., Brearley, A.J., Leroux, H. (2013) Amorphization and D/H fractionation of kerogens during experimental electron irradiation: Comparison with chondritic organic matter. Icarus 226, 101–110. https://doi.org/10.1016/j.icarus.2013.05.003

), with a rather large peak at 285 eV and a distinctive low slope at 285.8 eV, as is the case here, especially for the organic laminae of the Sangerhausen samples (Fig. 4). Organic laminae from Spremberg, Bottendorf and Sangerhausen may thus be composed of either pyrobitumen or irradiated kerogen (likely type I kerogen in Spremberg and Bottendorf samples and type III kerogen in Sangerhausen samples), the main source of radiation being the high uranium content of the Kupferschiefer.

**Figure 4** STXM characterisation of the investigated samples at the carbon K-edge showing C-XANES spectra of different pyrobitumens in Kupferschiefer T1. The spectra are compared with pyrobitumen of the Lower Jurassic Posidonia Shale in northern Germany (Bernard *et al*., 2012a
Bernard, S., Horsfield, B., Schulz, H.-M., Wirth, R., Schreiber, A., Sherwood, N. (2012a) Geochemical evolution of organic-rich shales with increasing maturity: A STXM and TEM study of the Posidonia Shale (Lower Toarcian, northern Germany). *Marine and Petroleum Geology* 31, 70–89. https://doi.org/10.1016/j.marpetgeo.2011.05.010
), pyrobitumen in the Mississippian Barnett Shale (Fort Worth Basin, Texas, USA; Bernard *et al*., 2012b
Bernard, S., Wirth, R., Schreiber, A., Schulz, H.-M., Horsfield, B. (2012b) Formation of nanoporous pyrobitumen residues during maturation of the Barnett Shale (Fort Worth Basin). *International Journal of Coal Geology* 103, 3–11. https://doi.org/10.1016/j.coal.2012.04.010
), and with kerogen type I and III as reference and after irradiation (Le Guillou *et al*., 2013
Le Guillou, C., Remusat, L., Bernard, S., Brearley, A.J., Leroux, H. (2013) Amorphization and D/H fractionation of kerogens during experimental electron irradiation: Comparison with chondritic organic matter. *Icarus* 226, 101–110. https://doi.org/10.1016/j.icarus.2013.05.003
).

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Origin of Organic Layers and Implications for Mineralisation

The deposition of the 0.4–1.0 m thick and 257.3 ± 1.6 Ma old Kupferschiefer lasted 10–60 kyr, and took place in a water depth between 200–300 m with a low sedimentation rate of 5 mm kyr⁻¹ (Kopp, 2022

Kopp, J.C. (2022) Kupfer-Silber-Lagerstätten im basalen Zechstein (Kupferschiefer und -mergel; Oberes Perm) zwischen Leszczyna (Nordsudetische Mulde, Polen) und Spremberg (Lausitz, Deutschland). Schriftenreihe der Deutschen Gesellschaft für Geowissenschaften, Deutsche Geologische Gesellschaft - Geologische Vereinigun, Hannover, 97. ISBN 978-3-510-49291-6

). The erosion of soils on the basin margins likely enriched the surface waters with nutrients, leading to periodic algal blooming events (primary productivity; Paul, 2006

Paul, J. (2006) The Kupferschiefer: Lithology, stratigraphy, facies and metallogeny of a black-shale. Zeitschrift der Deutschen Gesellschaft für Geowissenschaften 157, 57–76. https://doi.org/10.1127/1860-1804/2006/0157-0057

). However, also methanotrophic microbial mats which formed in anoxic bottom water (as in the Kupferschiefer sea) produced OM laminae (cf. Blumenberg et al., 2012

Blumenberg, M., Thiel, V., Riegel, W., Kah, L.C., Reitner, J. (2012) Biomarkers of black shales formed by microbial mats, Late Mesoproterozoic (1.1 Ga) Taoudeni Basin, Mauritania. Precambrian Research 196–197, 113–127. https://doi.org/10.1016/j.precamres.2011.11.010

). Given the high U concentrations, these organic layers are likely irradiated type I kerogen, implying that the structure of laminae is depositional and not a result from migration. Such irradiation, responsible for the partial amorphisation of these kerogen laminae, has also been regarded as responsible for a loss of hydrogen (cf. Le Guillou et al., 2013

Le Guillou, C., Remusat, L., Bernard, S., Brearley, A.J., Leroux, H. (2013) Amorphization and D/H fractionation of kerogens during experimental electron irradiation: Comparison with chondritic organic matter. Icarus 226, 101–110. https://doi.org/10.1016/j.icarus.2013.05.003

). This process could plausibly explain the increase in δD values towards the U-enriched lower Kupferschiefer T1 (Poetz et al., 2022

Poetz, S., Liu, Y., Magnall, J.M., Vieth-Hillebrand, A., Yang, S., Göthel, M., Gleeson, S.A., Schulz, H.-M. (2022) Signals of low grade organic matter alteration in the Upper Permian Kupferschiefer (Spremberg area, Eastern Germany) – A by-product of copper mineralization? Organic Geochemistry 169, 104421. https://doi.org/10.1016/j.orggeochem.2022.104421

) and the low HI values despite a rather low maturity, as indicated by reflectance values (Fig. 2b). The rare high reflectance values are interpreted as resulting from strong irradiation of organic layers by U decay. This is particularly evidenced by the positive correlation of reflectance values with high U concentrations that occur in the transitional zone of the Kupferschiefer mineralisation (Bechtel et al., 2002

Bechtel, A., Gratzer, R., Püttmann, W., Oszczepalski, S. (2002) Geochemical characteristics across the oxic/anoxic interface (Rote Fäule front) within the Kupferschiefer of the Lubin-Sieroszowice mining district (SW Poland). Chemical Geology 185, 9–31. https://doi.org/10.1016/S0009-2541(01)00395-3

; Oszczepalski et al., 2002

Oszczepalski, S., Nowak, G.J., Bechtel, A., Zák, K. (2002) Evidence of oxidation of the Kupferschiefer in the Lubin-Sieroszowice deposit, Poland: implications for Cu-Ag and Au-Pt-Pd mineralisation. Geological Quarterly 46, 1–23.

), especially at the Sangerhausen and Eisleben sites. In contrast, highly aromatised spherical or granular clusters (thucholite) in the Polish Kupferschiefer were interpreted as charcoal fragments (Syczewski et al., 2024

Syczewski, M.D., Panajew, P., Marynowski, L., Waliczek, M., Borkowski, A., Rohovec, J., Matoušková, Š., Sekudewicz, I., Liszewska, M., Jankiewicz, B., Mukhamed’yarova, A.N., Słowakiewicz, M. (2024) Geochemical implications of uranium-bearing thucholite aggregates in the Upper Permian Kupferschiefer shale, Lubin district, Poland. Mineralium Deposita 59, 1595–1618. https://doi.org/10.1007/s00126-024-01279-y

).

In conclusion, the combined data reveal that organic layers played a crucial role in the mineralisation processes of the Kupferschiefer system:

(1) The upward flow of the metal-laden brine was hindered by the impermeable T1 black shale unit (Figs. S-1b, S-3). Such impermeable nature is interpreted as being directly related to the structuration of non-porous organic layers. As mineralisation also affected the overlying Ca1 carbonate, sealing by organic laminae was not complete. Permeability caused by dissolved carbonate (and feldspar) still prevailed during mineralisation.
(2) Mineralisation of the Kupferschiefer is similar to processes of unconventional and conventional petroleum systems which store fluids and/or gases. The T1 unit served as both a porous and reactive reservoir for sulfide precipitation, and as a cap rock with organic layers ensuring intra-seal integrity (Fig. 3e,f; see also Fig. S-3). Additionally, kerogen swelling and retention of generated high wax oil (see thermovaporisation data in Figs. S-6, S-7) led to clogging pore space by adsorption, thus co-enhancing sealing integrity due to reduced permeability.
(3) The laminated OM offered reductive interfaces, forcing the precipitation of metal sulfides in direct contact to OM, which partly replaced both minerals and OM (Fig. 3c,d). In summary, the ability of OM to act as a reducing agent has significant hydrogeochemical implications for mineralising systems within sedimentary basins. For example, the zonation of metals and ore minerals in the “Kupferschiefer” system exhibits not only significant hydrogeochemical similarities to petroleum systems, but also to uranium roll-front deposits (cf. van Berk and Fu, 2017
van Berk, W., Fu, Y. (2017) Redox Roll-Front Mobilization of Geogenic Uranium by Nitrate Input into Aquifers: Risks for Groundwater Resources. Environmental Science & Technology 51, 337–345. https://doi.org/10.1021/acs.est.6b01569
) where a migrating aqueous phase enriched in soluble U(VI) meets labile OM which reduces the soluble uranium phase so that U(IV) precipitates.

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Acknowledgements

Samples were kindly provided by B. Friedrich (Mansfeld, Germany), M. Göthel (Wünsdorf, Germany), B.-C. Ehling (Halle/Sa., Germany), L. Katzschmann and Sebastian Wagner (Weimar, Germany), K. Poschlod (Augsburg, Germany), A. Ehling (Berlin, Germany), and A. Grube (Hamburg, Germany). We are grateful to K. Hahne, H. Liep, H. Rothe, I. Schäpan, A. Schreiber and R. Wirth from GFZ German Research Centre for Geosciences (Potsdam, Germany) for their technical support. The HERMES beamline (SOLEIL) is supported by the CNRS, the CEA, the Region Ile de France, the Conseil Départemental de l’Essonne and the Region Centre. We thank H. Sanei for advice and input. Finally, we thank M. Blumenberg and an anonymous reviewer for their constructive and helpful feedback, as well as A. Kappler for editorial handling.

Editor: Andreas Kappler

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References

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